the rhodium(ii)-catalyzed aziridination of olefins with...

The rhodium(II)-catalyzed aziridination of

olefins with

{[(4-nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane

Paul Müller, Corine Baud, and Yvan Jacquier

Abstract: The aziridination of olefins with {(4-nitrophenylsulfonyl)imino}phenyl-λ3-iodane, NsNTIPh (1c), in the presence

of [Rh2(OAc)4] proceeds in yields of up to 85% when the olefin is used in large excess. Under optimized conditions, styrene

(4a) is aziridinated with 1 equiv. of NsNTIPh (1c) in 64% yield with 2 mol% of catalyst. The aziridines derived from

electron-rich olefins undergo ring-opening under the conditions of the aziridination and afford rearrangement products or

pyrrolidines. The aziridination is sterospecific with 1,2-dialkyl- and 1,2-arylalkyl-disubstituted olefins, but nonstereospecific

with stilbene.The ρ-value for aziridination of substituted styrenes is –0.61. No ring-opened products are observed upon

aziridination of vinylcyclopropanes. In the presence of chiral RhII catalysts, the aziridination is enantioselective, affording an

ee of 73% with cis-β-methylstyrene (4k) and Pirrungs [Rh2{(R)-(–)-bnp}4] catalyst. The experimental results are consistent

with a one-step mechanism for transfer of the nitrenoid moiety from the catalyst to the olefin.

Key words: aziridination, nitrene transfer, rhodium catalysis.

Résumé: Lorsqu’on utilise un grand excès d’oléfine, l’aziridination des oléfines par le {(4-nitrophénylsulfonyl)imino}phényl-

λ3-iodane, NsNTIPh (1c), en présence de [Rh2(OAc)4], se produit avec des rendements allant jusqu’à 85%. Dans des

conditions optimales, le rendement de l’aziridination du styrène (4a) par un équivalent de NsNTIPh (1c) et 2 mol% de

catalyseur est de 64%. Les aziridines obtenues à partir d’oléfines riches en électrons subissent des ouvertures de cycle dans les

conditions de l’aziridination et elles conduisent à des produits de réarrangement ou à des pyrrolidines. L’aziridination est

stéréospécifique avec les oléfines disubstituées par des groupes alkyle ou arylalkyle en 1,2, mais elle n’est pas

stéréospécifique avec le stilbène. La valeur de ρ pour l’aziridination des styrènes substitués est de –0,61. On n’observe pas de

produits à cycles ouverts lors de l’aziridination de vinylcyclopropanes. En présence de catalyseurs de RhII chiraux,

l’aziridination est énantiosélective, conduisant à un ee de 73% lors de la réaction du cis-β-méthylstyrène (4k) en présence du

catalyseur de Pirrung [Rh2{(R)-(–)-bnp}4]. Les résultats expérimentaux sont en accord avec un mécanisme de transfer en une

étape de la portion nitrénoïde du catalyseur à l’oléfine.

Mots clés : aziridination, transfert de nitrène, catalyseur au rhodium.

[Traduit par la rédaction]

1. Introduction

The possibility of generating sulfonylnitrenes from {(organo-sulfonyl)imino}-λ3-iodanes has been the subject of differentinvestigations in the past. While thermal reaction in benzeneof the phenyliodonium ylid 1a (MsNTIPh) derived frommethane–sulfonamide provided no nitrene-derived decompo-sition products (1), the tosyl analogue 1b (TsNTIPh) trans-ferred the sulfonylnitrene moiety to acceptors such asthioanisole (100°C, 49%), triphenylphosphine (100°C, 87%),and dimethylsulfoxide (25°C, 100%) (2). The first reportedtransition-metal-catalyzed reactions of sulfonylnitrenes, derivedfrom TsNTIPh (1b), concerned CH-bond insertions, which

occurred in the presence of MnIII- or FeIII-porphyrinate or[Rh2(OAc)4] catalysts (3). In connection with model studies on thetransfer of oxygen by cytochrome P-450 and model iron ormanganese complexes, the group of Mansuy (4) reported theaziridination of olefins with TsNTIPh and MnIII- and FeIII-porphyrinates. These systems were also efficient for formalinsertion of sulfonylnitrene into CH bonds (5). The aziridina-tions and CH-bond insertions exhibited characteristics typicalfor radical reactions.

The metal-catalyzed nitrene transfer from TsNTIPh (1b)to olefins was developed to become an efficient method foraziridination by Evans et al. (6). CuI and CuII complexesproved to be the most efficient catalysts. With chiral bis-oxa-zoline ligands the aziridination became enantioselective, andinductions of up to 97% were reported with cinnamate estersas substrates. With simple olefins such as styrenes the enan-tiomeric excess was, however, only 63% (7). The aziridinationwith Jacobsens’s (8) Cu-based catalysts having benzylidenederivatives of 1,2-diaminocyclohexane as ligands afforded en-antioselectivities near to perfection with 6-cyano-2,2-di-methylchromene, but were less satisfactory with simple olefins(ee’s varying from 58–87%). Applications and extensions ofthe Cu-catalyzed aziridination using TsN = IPh (1b) as nitrene

Can. J. Chem. 76: 738–750 (1998)

This paper is dedicated to Professor Erwin Buncel inrecognition of his contributions to Canadian chemistry.

Received December 10, 1997.

P. Müller,1 C. Baud, and Y. Jacquier.Département de ChimieOrganique, Université de Genève, 30 Quai Ernest Ansermet,CH-1211 Genève 4, Switzerland.

1 Author to whom correspondence may be addressed.Telephone: +41 22 702 6527. Fax: +41 22 328 7396. E-mail:[email protected]

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source have been reported in the literature with achiral (9) aswell as chiral ligands (10). Several metal–salen complexeshave been tried as catalysts for olefin aziridination withTsNTIPh, using FeIII, CoII, CrIII, NiII, MnIII, PdII, and RhII, butonly {Mn(salen)Cl} was effective (11). An efficient chiralsalen ligand for aziridination of styrenes with MnIII has, how-ever, been described recently by Katsuki and co-workers (12).

The observation that dimeric rhodiumII complexes havingchiral carboxamidate ligands are highly efficient for asymmet-ric cyclopropanations of olefins (13) and cyclopropenations ofterminal acetylenes (14) was the starting point of our owninvestigations on RhII-catalyzed asymmetric aziridinations.Owing to the formal analogy between TsNTIPh (1b) and al-kylidene phenyliodonium ylides 2, which are convenient sub-stitutes for diazo compounds 3 in RhII-catalyzed carbenoidreactions (15), and the apparent similarity between carbeneand nitrene transfer, we speculated that catalysis with chiralRhII carboxamidates would afford aziridines with enantiose-lectivities in the range of those reported for cyclopropanations.We therefore initiated a search for an efficient system for RhII-catalyzed generation and transfer of sulfonylnitrenes. Mechanisticaspects of the reaction were investigated, and a series of chiralRhII-based catalysts were tested. Some of the results of thisinvestigation using {[(4-nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane (1c) have been reported previously (16).

2. Results and discussion

2.1. Nitrene precursors for aziridination of styreneThe aziridination of styrene (4a) was selected as model

reaction for several systems, which, on the grounds of litera-ture reports, appeared to have potential as precursors in metal-catalyzed nitrene transfer (17). The decomposition of tosylazide (6a) (18) in the presence of CuI or CuII catalysts andstyrene (4a) afforded none of the desired aziridine 5b; how-ever, a poor 3% yield of 5b was isolated when the reaction wascarried out in the presence of copper powder in acetonitrile at80°C. Similarly, the use of [PdCl2(PhCN)2] (19) or[Rh2(OAc)4] afforded no 5b. Exposure of chloramine-T(TsNCl–Na+) to copper powder in dimethylsulfoxide (DMSO)reportedly affords the corresponding sulfoximine in 80% yield(20), presumably via an intermediate Cu-complexed nitrene.However, application of this and related systems to aziridina-tion of 4a appears to be limited. In our hands, the best resultswere obtained with the tetrabutylammonium salt of TsNCl–

(6b), which afforded a 36% yield of 5b with 1 equiv. of Cu0;[CuCl] (0.1 equiv.) in turn, gave a yield of 29% of 5b, and thehighest yield with [Rh2(OAc)4] was 21%. The sulfonamideTsNH2 (7b) was the major secondary product in these reac-tions. See Table 1 and Scheme 2.

2.2. Optimization of aziridination conditions

Selection of catalyst

Recently, Evans et al. (6, 7) reported the metal-catalyzed az-iridination of styrene (4a) by TsNTIPh (1b) with a large vari-ety of catalysts. The CuI and CuII complexes were the mostsuitable for olefin aziridination, and these catalysts were usedlater by other groups. [Rh2(OAc)4] was also tested, but wasfound much less suitable than Cu catalysts. In view of the low

Scheme 1.

Compd. X, Y Catalyst Conditions Yield of 5b (%)

6a N2 Cu0, 0.3 equiv. MeCN, 80°C 3

6a N2 [CuCl], 0.13 equiv. MeCN, 80oC 0

6a N2 [PdCl2(PhCN)2], 0.1 equiv. MeCN, 80°C 0

6a N2 [Rh2(OAc)4], 0.02 equiv. CH2Cl2, 25°C 0

6b Cl, N(Bu)4 Cu0, 1.0 equiv. Styrene, 40 h, 25°C 36

6b Cl, N(Bu)4 [CuCl], 0.10 equiv. Styrene, 40 h, 25°C 29

6b Cl,N(Bu)4 [Cu(acac)2], 0.10 equiv. Styrene, 40 h, 25°C 16

6b Cl,(NBu)4 [Cu(acac)2], 0.10 equiv. CH2Cl2, 14 h, 25°C 9

6b Cl,N(Bu)4 [PdCl2(PhCN)2], 0.10 equiv. MeCN, 14 h, 25°C 8

6b Cl,(NBu)4 [Rh2(OAc)4], 0.10 equiv. CH2Cl2, 14 h, 25°C 21

6b Cl,(NBu)4 [Rh2(OAc)4], 0.10 equiv. MeCN, 14 h, 25°C 8

a Conditions: slow addition of 1.00 mmol of TsNXY (6) in 5.0 mL of solvent to 5.0 mmol of styrene (4a) and catalyst

(0.02–0.35 mmol) in 10.0 mL of solvent.

Table 1.Catalyzed aziridination of styrene (4a) with TsNXY (6), Scheme 2.a

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yields in our experiments with 6a,b, we investigated the azirid-ination of styrene (4a) with TsNTIPh (1b) with PdII and RhII

catalysts under a variety of conditions, but success was verylimited. In the case of PdII the highest yield of 5b (37%) wasachieved with [PdCl2(PhCN)2] (10%) with a five-fold excessof 4a (Table 2). With [Rh2(OAc)4], reaction in dry dichlo-romethane at 20°C produced the highest yield (43%) with afive-fold excess of 4a and 2% of catalyst. When the excess ofstyrene (4a) was increased to 20-fold, the yield of (5b) in-creased to 59%. Other RhII catalysts, such as [Rh2{(2S)-mepy}4] (mepy = methyl pyrrolidone-2-carboxylate) (21) or[Rh2(pfb)4] (pfb = perfluorobutyrate) (22) resulted in reducedyields of 5b.

Optimization of ylide structureIn the next step of the optimization procedure the nature of thesulfonamide group of the iodonium ylides was varied. Theylides 1a–g were synthesized by the standard procedure de-scribed for TsNTIPh (1b), which consists in the reaction ofthe appropriate sulfonamide or carboxamide, respectively,7a–g with diacetoxy iodobenzene, PhI(OAc)2 in MeOH in thepresence of KOH (2). They were in general amorphous solids

that could not be further purified, but were > 95% pure, exceptthe 4-methoxy derivative 1d, which was isolated as an un-separable 64:36 mixture of ylide and the starting sulfonamide7d. A comparative study was carried out for aziridination ofstyrene with [Rh2(OAc)4] in CH2Cl2, (2% of catalyst) and[Cu(acac)2] in acetonitrile (5–10% of catalyst) and the ylides1a–g (Table 3, Scheme 3).

Efficient aziridine formation occurred only with the io-donium ylides derived from aromatic sulfonamides, whilethose derived from methanesulfonamide and tri-fluoroacetamide were either less effective or failed com-pletely. Among the phenyl derivatives, the 4-nitro-compound(NsNTIPh, 1c) afforded the highest yield of aziridine with[Rh2(OAc)4]. Cu catalysis, in turn, was more efficient forTsNTIPh (1b). A recent more detailed study revealed, how-ever, that NsNTIPh (1c) is also more suitable than TsNTIPh(1b) in Cu-catalyzed aziridinations (23). We expected a stillhigher yield with the 2,4-dinitro derivative 1e, but this ylideturned out to be significantly less suitable than the mono nitroderivative NsNTIPh (1c). The ylide 1f, derived from pen-tafluorophenylsulfonamide 7f, afforded no aziridine witheither of the catalysts.

Scheme 2.

Entry Catalyst Conditions Yield of 5b (%)

1 [PdCl2], 0.10 mmol MeCN, 96 h 26

2 [PdCl2], 0.10 mmol MeCN, 50°C, 3 h 22

3 [PdCl2], 0.10 mol MeCN, 80°C, 3 h 12

4 [PdCl2(PhCN)2], 0.10 mmol MeCN, 96 h 37

5 [PdCl2(PhCN)2], 0.10 mmol MeCN, 50°C, 0.5 h 25

6 [Pd(acac)2], 0.10 mmol MeCN, 20°C, 96 h 10

7 [Rh2(OAc)4], 0.02 mmol PhMe, 15 h 18

8 [Rh2(OAc)4], 0.02 mmol DMF, mol. sieves, 3 h 0

9 [Rh2(OAc)4], 0.02 mmol MeNO2, mol. sieves,b 0.5 h 19

10 [Rh2(OAc)4], 0.02 mmol THF, 18 h 6

11 [Rh2(OAc)4], 0.02 mmol MeCN, 19 h 23

12 [Rh2(OAc)4], 0.02 mmol MeCN, mol. sieves,b 16 h 32

13 [Rh2(OAc)4], 0.02 mmol CHCl3, mol. sieves,b 0.5 h 23

14 [Rh2(OAc)4], 0.02 mmol C2H4Cl2, mol. sieves,b 0.5 h 34

15 [Rh2(OAc)4], 0.02 mmol CH2Cl2, 0.25 h 32

16 [Rh2(OAc)4], 0.02 mmol CH2Cl2, mol. sieves,b 1.0 h 43

17 [Rh2(OAc)4], 0.02 mmol as above, 16 h, 20 equiv. of styrene 59

18 [Rh2(OAc)4], 0.02 mmol as above, 16 h, 50 equiv. of styrene 59

19 [Rh2(OAc)4], 0.02 mmol as above, 40°C, mol. sieves,b 1.2 h 28

20 [Rh2(OAc)4], 0.20 mmol CH2Cl2, mol. sieves, 1.0 h 59

21 [Rh2{(2S)-mepy}4], 0.02 mmol CH2Cl2, mol. sieves, 1.0 h 11

22 [Rh2(pfb)4], 0.02 mmol CH2Cl2, mol. sieves, 1.0 h 20

a Conditions: 1.0 mmol of TsNTIPh (1b) suspended in the appropriate solvent (10.0 mL), stirred with 5.0 mmol of

styrene (4a) and the catalyst at 20°C, unless otherwise indicated.b Activated molecular sieves (6.0 g) added to solvent.

Table 2.Aziridination of styrene (4a) with TsNTIPh (1b) and PdII or RhII catalysts.a

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The reasons for the absence of reactivity of the aliphaticylides was not further investigated. Nevertheless, a differencein behavior was observed. All the ylides were almost or en-tirely insoluble in most of the conventional solvents. However,the ylides derived from arenesulfonamides dissolved inCH2Cl2 upon exposure to the catalyst and the substrate andproduced aziridines or sulfonamides. In contrast, the ylides 1aand 1f, derived from methanesulfonamide (7a) and pen-tafluorobenzenesulfonamide (7f), respectively, remained sus-pended in the solvent and were recovered unchanged. Thesame observation applies to 1g. The mechanism for formationof the sulfonamide 7 as the main secondary reaction producthas yet to be established. In the case of the metallo-porphyrin-catalyzed aziridinations with TsNTIPh (1c) it has been pro-posed that TsNH2 (7b) arises from hydrolysis of theintermediate metal nitrene complex; the resulting metal-oxospecies, in turn, attacks olefins to afford epoxides. However,no epoxides were ever detected in the aziridinations using imi-nophenyliodinanes 1 catalyzed with RhII. In addition, controlexperiments revealed the absence of epoxide formation whentrans-stilbene was exposed to iodosylbenzene in the presenceof [Rh2(OAc)4].

As a precautionary measure, all aziridinations with NsNTIPh(1c) were carried out in the presence of activated molecularsieves in order to avoid hydrolysis of NsNTIPh to NsNH2

(7c). Despite this, sulfonamide formation could not be

suppressed entirely. It became particularly important in theaziridination of electron-deficient or sterically hindered ole-fins. The reason for this is a catalyst-induced decompositionof NsNTIPh (1c), competitive with the aziridination, whichultimately, after work-up, affords NsNH2 (7c). However, thenature of this secondary reaction could so far not be estab-lished.

Solvent effect

The RhII-catalyzed aziridination exhibits a peculiar solvent ef-fect. Best results were obtained with chlorinated solvents suchCH2Cl2. The dipolar nonprotogenic solvents sulfolane orMeNO2, etc., were found inefficient. However, the addition ofsmall quantities of sulfolane or, to a minor degree, MeNO2 toCH2Cl2 resulted in an increase of yield of aziridine. Underoptimized conditions, with a CH2Cl2/sulfolane ratio of99.5:0.5 the aziridination of styrene (4a) with 1.0 equiv. ofNsNTIPh (1c) proceeded in 64% yield. Smaller, but alsolarger, amounts of sulfolane resulted in reduced yields, how-ever (see Table 4).

2.3. Scope and limitations of the aziridinationA series of olefins of different structure was examined in orderto establish the scope of the reaction. The reaction conditionswere optimized for styrene (4a) as follows: NsNTIPh (1c1.0 mmol) was suspended in CH2Cl2 (20 mL) at room tem-perature under N2. After addition of the olefin (20.0 mmol) and

Scheme 3.

No. R Catalyst Aziridine Yield (%)

1a MeSO2 [Cu(acac)2] 5a 7

1a MeSO2 [Rh2(OAc)4] 5a 0

1b 4-MeC6H4SO2 [Cu(acac)2] 5b 95b

1b 4-MeC6H4SO2 [Rh2(OAc)4] 5b 59

1c 4-NO2C6H4SO2 [Cu(acac)2] 5c 71

1c 4-NO2C6H4SO2 [Rh2(OAc)4] 5c 85

1c 4-NO2C6H4SO2 none 5c 0

1d 4-MeOC6H4SO2 [Cu(acac)2] 5d 60

1d 4-MeOC6H4SO2 [Rh2(OAc)4] 5d 0

1e 2,4-(NO2)2C6H3 [Cu(acac)2] 5e 58

1e 2,4-(NO2)2C6H3 [Rh2(OAc)4] 5e 48

1f C6F5SO2 [Cu(acac)2] 5f 0

1f C6F5SO2 [Rh2(OAc)4] 5f 0

1g CF3CO [Cu(acac)2] 5g 0

1g CF3CO [Rh2(OAc)4] 5g 0

a Conditions: at room temperature under N2 and in presence of molecular

sieves (6.0 g). [Cu(acac)2]: 5.0 mmol of styrene in dry MeCN (10.0 mL),

stirred with 1.0 mmol of ylide 1a–g and 0.05–0.10 mmol of catalyst.

[Rh2(OAc)4]: 5.0 mmol of styrene in CH2Cl2 (10.0 mL) containing

molecular sieves, stirred with 1.0 mmol of ylide and 0.02 mmol of catalyst.b References 6 and 7.

Table 3.Aziridination of styrene (4a) with ylides 1a–g and

[Cu(acac)2] or [Rh2(OAc)4], Scheme 3.a

Solvent Temp. (°C) Time

Yield of

aziridine 5c (%)

Sulfolane 25 48 h 0

MeNO2 25 21 h 0

CH2Cl2 25 20 h 9

CH2Cl2/sulfolane 2:1 25 64 h 47



CH2Cl2/sulfolane 99:1 25 2 h 43b

CH2Cl2/sulfolane 99.5:0.5 25 2 h 64

CH2Cl2/sulfolane 99.5:0.5 25 2 h 48b



CH2Cl2/sulfolane 99.5:0.5 –20 2 h 45

CH2Cl2/MeNO2 2:1 25 2 h 9

CH2Cl2/MeNO2 95:5 25 2 h 37

a Conditions: 0.02 mmol of [Rh2(OAc)4] stirred in 10 mL of solvent (v/v)

containing molecular sieves (6.0 g) with 1.0 mmol of styrene and 1.0 mmol

of NsNTIPh (1c).b 2.0 equiv. of 1c.

Table 4.Solvent effect on [Rh2(OAc)4]-catalyzed aziridination of

styrene (4a) with 1 equiv. of NsNTIPh (1c).a

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[Rh2(OAc)4] (0.02 mmol) the mixture was stirred until all ofthe ylide was dissolved (see Table 5, Scheme 4).

Inspection of Table 5 reveals that the reactivity of olefinstowards NsNTIPh (1c) – [Rh2(OAc)4] is determined by elec-tronic and steric factors. Electron-deficient olefins such as 3-nitrostyrene (4g), 4-methoxycarbonylstyrene (4e), or ethylacrylate (4o) afford lower yields of aziridines than the elec-tron-rich substrates. Yields with cis-1,2-disubstituted olefinsare slightly higher than with the trans-isomers (except in thecase of stilbene) (16a). The low reactivity of 3-methylbut-1-ene (4i) and norbornene (4p) is best ascribed to steric hin-drance. Ethyl sorbate (4s) produced no aziridination productwith NsNTIPh (1c) – [Rh2(OAc)4], probably owing to an un-favorable combination of steric and electronic factors, and thesame applies to ethyl cinnamate, which is the best substrate inthe Cu-catalyzed aziridination of Evans et al. (6). An attemptto extend the aziridination of olefins to acetylenes, a reactionwell known in the series of carbenoid additions, failed withhex-1-yne.

Consecutive reactions with electron-rich olefins

With some of the electron-rich olefins complications owing toconsecutive reactions of the originally formed aziridines arose.The isolation of rearranged products upon aziridination of 1,1-diphenylethene and trans-stilbene has been reported pre-viously (16a). Similarly, aziridination of norbornadiene (4r)

afforded none of the expected aziridine 8r but a rearrangedproduct 10, presumably by a mechanism involving 9,analogous to that proposed for reaction of norbornadiene withphenyl azide (24) (see Scheme 5).

The aziridine derived from 4-methoxystyrene (4h) under-went ring-opening under the aziridination conditions, whichwas followed by cycloaddition of the supposed intermediatedipolar species 11 to a second molecule of olefin to afford acis/trans mixture of pyrrolidines 12h. Analogous productshave been isolated upon aziridination of ethyl vinylether (4m)and α-methylstyrene (16a). The structure of the pyrrolidineswas deduced from the mass spectra, which showed the incor-poration of a second olefin molecule in the product, and fromdetailed analysis of the 1H NMR. Reinspection of the NMRdata of the isomers of 12m revealed that they are cis/transisomers and that the previous structural assignement was in-correct (16a). The formation of regioisomeric adducts, whichwould have either C2 or Cs symmetry, could be ruled out onthe grounds of the spectra, but the assignment of relative con-figurations of the pyrrolidines proved impossible (17). Sinceno aziridines could be isolated in these reactions, it is not es-tablished whether the dipolar intermediate 11 is derived fromdirect attack of the NsNTIPh (1c) to the olefin or via ring-opening of the putative aziridines (see Scheme 6).

Intermolecular CH-bond insertion

The aziridination of olefins having alkyl substituents is usuallyaccompanied by formal products of nitrene insertion into CHbonds. In the case of cyclohexene, the aziridine was formed inonly 4% yield, while 70% of the allylic sulfonamide was iso-lated (16a). Subsequent work showed, however, that cyclo-hexene is an exception. Olefins react usually via aziridination,and CH insertion is only a secondary reaction, although it mayoccur with other compounds having activated CH bonds, suchas arylalkanes and ethers. The reaction proceeds with retention

No. Olefin Time (h) No. Aziridine yield (%) Comment

4a Styrene 2 5c 85 Reference 16a

4b 4-Acetoxystyrene 0.75 8b 82

4c 4-Bromostyrene 1.3 8c 82

4d 3-Chlorostyrene 0.30 8d 79

4e 4-Methoxycarbonylstyrene 23 8e 17

4f 4-Methylstyrene 0.50 8f 76 Reference 16a

4g 3-Nitrostyrene 19 8g 46 Reference 16a

4h 4-Methoxystyrene 0.75 8h 0 16% of pyrrolidine 12h4i 3-Methylbut-1-ene 91 8i 0

4j Buta-1,3-diene 1.0 8j 94 200 mmol of olefin

4k cis-β-Methylstyrene 0.5 8k 82 Reference 16a

4l trans-β-Methylstyrene 18 8l 68 Reference 16a

4m Ethyl vinylether 0.5 8m 0 52% of pyrrolidine 12m (16a)

4n Vinyl acetate 4 8n 47 Reference 16a

4o Ethyl acrylate 4.5 8o 7

4p Norbornene 0.7 8p 29

4q Indene 2.5 8q 40

4r Norbornadiene 91 8r 0 50% rearr. to 104s Ethyl sorbate 91 8s 0

a Conditions: 20 mmol of olefin stirred with 1.0 mmol of NsNTIPh (1c) and 0.02 mmol of [Rh2(OAc)4] in 10 mL of CH2Cl2 at

room temperature in the presence of molecular sieve (6.0 g).

Table 5.[Rh2(OAc)4]-catalyzed aziridination of olefins with NsNTIPh (1c), Scheme 4.a

Scheme 4.

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of configuration at the reacting carbon center (25) and mostlikely proceeds via a one-step mechanism (25).

Intramolecular aziridinationSince the presence of a large excess of olefin is necessary forefficient aziridination, an intramolecular aziridination was car-ried out in the hope that generation of the reacting species inthe immediate vicinity of the double bond would compensatefor the reduction of the 20:1 olefin/ylide ratio in the intermo-lecular reaction to 1:1 ratio in the intramolecular version. Sincethe preparation of iminophenyliodinanes from unsaturated ali-phatic sulfonamides by the conventional methods failed, thearomatic derivative 15 was synthesized via metallation–sul-fonation of 2-bromostyrene 13 (26) and treatment of the sul-fonamide 14 with diacetoxyiodobenzene. The ylide wascontaminated with unreacted 14, which could not be separated.Treatment of 15 with [Rh2(OAc)4] afforded 16 in only 20%yield together with much sulfonamide 12. Apparently, theeventual advantage of carrying out the reaction intramolecu-larly is lost owing to a less favorable substitution pattern (ab-sence of nitro substituent) on the aromatic moiety (seeScheme 7).

2.4. Mechanistic investigations

Stereochemistry of aziridinationThe stereochemistry of the RhII-catalyzed aziridination withNsNTIPh (1c) was investigated with the cis- and trans-iso-mers of hex-2-ene, β-methylstyrene, and stilbene. Hex-2-eneand β-methylstyrene reacted stereospecifically, but cis-stil-bene afforded a mixture of cis- and trans aziridines in low

yield. In contrast, trans-stilbene reacted to a mixture of trans-aziridine and a rearranged enamide in 41% and 11% yield,respectively (16a).

The observation of stereospecificity upon aziridination ofβ-methylstyrene contrasts with that observed in the Cu-cata-lyzed aziridination where, with most ligands, the stereospeci-ficity is lost. The loss of stereospecificity suggests a two-stepmechanism for aziridination involving either an intermediateradical 17 or a zwitterionic species 18 in which bond rotationoccurs faster than ring closure. The RhII-catalyzed aziridina-tion, in turn, could be concerted or stepwise, but with the rateof ring closure faster than bond rotation. The loss of stereospe-cificity observed with cis-stilbene appears more consistentwith a two-step mechanism, but it is not clear whether stilbenestands at the extreme end of a mechanistic continuum of ageneral two-step mechanism or whether the general mecha-nism is concerted and stilbene must be considered an exception(see Scheme 8).

Polar substituent effectsAs mentioned above, the two-step mechanism for aziridinationcould involve either a radical intermediate 17or a zwitterionicspecies 18. To eliminate this latter possibility, the polar substi-tuent effect on the reaction was examined by means of compe-tition experiments using pairs of substituted styrenes 4a–g.Relative reactivities of the styrenes were determined from theproduct ratios of the aziridines, as analyzed by NMR (Table 6,Figure 1).

The ρ-value for the aziridination with NsNTIPh(1c)/[Rh2(OAc)4] of –0.61 is close to that measured for the Rh(II)-catalyzed cyclopropanation of styrenes with diazomalonate, and

Scheme 5.

Scheme 6.

Scheme 7.

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also close to the values reported for dichlorocarbene additionto styrene (ρ = –0.62, 80°C) (28) or to α-methylstyrene (ρ =–0.38, 0°C) (29). This is clearly too low for a dipolar intermediatewith development of a positive charge at the benzylic positionin the transition state. On the other hand, it is consistent witha concerted as well as with a radical mechanism. The Hammettρ-constant for the [Cu(acac)2]-catalyzed reaction is slightlylower (–0.49; r = 0.991). The plot of this latter reaction suffersfrom an anomaly, since it does not pass through the origin. Thereasons for this anomaly are not clear, but have not been in-vestigated yet.

Radical clocks in aziridination

The possible intervention of radicals in the aziridination path-way was tested with vinylcyclopropane 19a and its cis-diphenyl-substituted derivative 19b. The corresponding

secondary radicals 20a,b ring-open to homoallylic radicals21a,b with rate constants of > 2 × 107 s–1 and > 2 × 1010 s–1,respectively (30). The olefins were synthesized via Wittig ole-fination of the corresponding aldehydes (31) (see Scheme 9).

Aziridination of vinylcyclopropane (19a) afforded the az-iridine 22a in ca. 43% yield (partial decomposition duringwork-up). The diphenyl derivative 19breacted only sluggishlyto yield the aziridine 22b in 35% yield. No products derivedfrom ring-opening of an intermediate radical could be detectedin the crude reaction mixture. It follows that the aziridinationof mono-substituted olefins should proceed via a concertedmechanism or via a radical that undergoes cyclization with arate constant significantly faster than 1012 s–1. While this latterpossibility cannot be ruled out, it has no consequences on theactual stereospecificity of the aziridination. On the other hand,the significance of this result with respect to the aziridinationmechanism of disubstituted olefins, in particular to stilbene isopen to debate.

Asymmetric aziridination

Styrene (4a) and cis-β-methylstyrene (4j) were aziridinated

Scheme 8.

Compd. Subst. log krel[Rh2(OAc)4] log krel[Cu(acac)2] σ+, σmb

4a H 0 0 0

4b 4-OAc –0.029 0.18

4c 4-Br –0.025 0.122 0.15

4d 3-Cl –0.13 0.017 0.37

4e 4-CO2Me –0.243 0.44

4f 4-Me 0.268 0.367 –0.31

4g 3-NO2 –0.365 –0.123 0.71

a Conditions: see Experimental.b Reference 27.

Table 6.Competitive aziridination of styrenes 4a–g with

NsNTIPh (1c) and [Rh2(OAc)4] or [Cu(acac)2].a

Fig. 1. Hammett plot (vs. σ+) (27) for aziridination of styrenes with

NsNTIPh (1c) and [Rh2(OAc)4] (ρ = –0.613, r = 0.984) or

[Cu(acac)2] (ρ = –0.49, r = 0.999) at 22°C.

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with NsNTIPh (1c) in the presence of the known RhII catalysts[Rh2{(2S)-mepy}4] (22), [Rh2{(4S)-phox}4] (32), [Rh2{(2S)-bepy}4] (33), [Rh2{(4S)-macim}4], [Rh2{(S)-ptpa}4] (34),[Rh2{(R)-(–)-bnp}4] (35), [Rh2{(S)-psp}4] (36), and [Rh2{(–)-mpmt}4] (37), which were prepared according to literature pro-cedures. Two new RhII carboxylate catalysts,[Rh2{(–)-campha}4] (23) and [Rh2{(R)-meba}4] (24), wereprepared via ligand exchange from [Rh2(OAc)4] with cam-phanic acid, and with 2-(R-methoxyethyl)benzoic acid, re-spectively (38). The results are summarized in Table 6. Thechemical yields of the reaction were usually satisfactory, butenantioselectivity has not yet reached levels useful for syn-thetic applications. Typically, the RhII-carboxamidate cata-lysts that have shown exceptional enantioselectivities incarbenoid reactions exhibit only modest selectivity in aziridi-nation. At this time, the best results were obtained with Pir-rungs [Rh2{(R)-(–)-bnp}4] (bisnaphtholphosphate ligand),which afforded an ee of 55% with styrene (4a) and 73% withcis-β-methylstyrene (4j), respectively. No enantioselectivitywas observed in the intramolecular aziridination with 16.

Although these selectivities are still considerably below thedesired level, they show unambiguously that the catalyst is not

only involved in the decomposition of the NsNTIPh (1c), butthat it is intimately associated in the transfer of the nitrenemoiety to the olefin (see Scheme 10, Table 7).

Photochemical nitrene generationJacobsen (8) has shown that Cu catalysts are highly efficient intrapping 4-toluenesulfonylnitrene, generated by photolysis ofthe corresponding azide, TsN3. In the presence of a chiral cata-lyst, aziridination proceeded with the same enantioselectivityas in the aziridination starting with TsNTIPh (1b) and thesame catalyst. Analogous experiments have been carried outwith the NsNTIPh/[Rh2{(R)-(–)-bnp}4] system. The aziridi-nation of styrene (4a) proceeds with 74% yield and 55% ee(Table 7). NsN3 (25) is stable towards the catalyst at roomtemperature. Photolytic decomposition of NsN3 (20 mmol) inthe presence of [Rh2(R)-(–)-bnp)4] (1.0 mmol) afforded the az-iridine 5c in 9% yield and 17% ee. Since photolysis of sulfony-lazides is known to produce sulfonylnitrenes (39), theobservation of enantioselectivity implies that at least part ofthe reaction must proceed via a metal-complexed nitrene 26.The trapping efficiency is, however, much lower than that of100% observed by Jacobsen (8) in the Cu-catalyzed aziridination.

Scheme 9.

Scheme 10.

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In our experiments, about two thirds of the reaction proceeddirectly from attack of the free nitrene 27 to produce racemicaziridine 5c, and only a minor part is delivered via the metal-complexed species (see Scheme 11).

3. Conclusion

The RhII-catalyzed aziridination with NsNTIPh (1c) exhibitsmany similarities to the more established CuI-catalyzed reac-tions with TsNTIPh (1b). In both systems, a metal-complexednitrene is probably involved. Both are stereospecific with al-kyl-substituted olefins, but loss of stereospecificity may occurwhen the olefin carries one (Cu/TsNTIPh) or two(Rh/NsNTIPh) aryl substituents. Both systems suffer fromlimited efficiency, which may be overcome by the use of alarge amount of catalyst (with Cu) or a large excess of olefin(with Rh). Further work will be directed towards a more effi-cient system for nitrene transfer and towards improvement of

the enantioselectivity, in order to achieve a general method forenantioselective catalytic olefin aziridination.

4. Experimental

General(See ref. 32.)

Synthesis of iminophenyl-λ3-iodanes 1a–gThe syntheses of 1a(1), 1b (2), and 1g(4) have been describedin the literature.

Preparation of {(4-nitrophenylsulfonyl)imino}phenyl-λ3-iodane (1c)

Toasolutionofp-nitrophenylsulfonamide6c(10.0mmol)andKOH(25.0 mmol) in MeOH was added at 0°C diacetoxyiodobenzene(10.0 mmol) at once. The mixture was stirred at 0°C until thecolor had changed from colorless to pale yellow (25 min). Thetemperature was allowed to raise to 20°C, and stirring was

Olefin Catalyst Time (h)

Aziridine

Yield (%) ee (%)

Styrene (4a) [Rh2{(2S)-mepy}4] 20 81 21

Styrene (4a) [Rh2{(2S)-bepy}4] 3 71 27

Styrene (4a) [Rh2{(4S)-phox}4] 23 56 22

Styrene (4a) [Rh2{(4S)-macim}4] 2.5 50 <10

Styrene (4a) [Rh2{(S)-ptpa}4] 0.2 92 0

Styrene (4a) [Rh2{(R)-(–)-bnp}4] 1.5 74 55

cis-β-Methylstyrene (4k) [Rh2{(2S)-bepy}4] 27 70 35

cis-β-Methylstyrene (4k) [Rh2{(R)-(–)-bnp}4] 1.0 80 73

cis-β-Methylstyrene (4k) [Rh2{(S)-psp}4] 48 70 <10

cis-β-Methylstyrene (4k) [Rh2{(–)-mpmt}4] 1.5 73 0

cis-β-Methylstyrene (4k) [Rh2{(–)-campha}4] 27.5 71 0

cis-β-Methylstyrene (4k) [Rh2{(R)-meba}4] 1.5 78 15

15b [Rh2{(R)-(–)-bnp}4] 0.75 30 0

a Conditions according to general procedure.b {(2-Ethenylsulfonyl)imino}phenyl-λ3-iodane (15), intramolecular reaction.

Table 7.Asymmetric aziridination of olefins with NsNTIPh (1c) and chiral RhII catalysts.a

Scheme 11.

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continued during 3.0 h. After addition of water (100 mL) theylide 1c precipitated upon standing at 0°C overnight. Afterfiltration, the product was dried in vacuo to afford crude 1c(98%), which was used without further purification. IR(KBr):3100m, 1524vs, 1346s, 1274s, 1125s, 1080s, 878m, 853s, 736s.1H NMR (400 MHz, DMF-d7): 8.18 (d, J = 8.7 Hz, 2H), 7.92 (m,4H), 7.51 (m, 1H), 7.39 (m, 2H). 13C NMR (DMF-d7): 152.4(s), 149.2 (s), 137.9 (d), 135.3 (d), 134.9 (d), 132.2 (d), 128.1(d). MS: 404 (0.4, M+), 204 (63), 186 (61), 122 (62), 77 (100).HR-MS: 403.9288 (C12H9IN2O4S

+; calc. 403.9328).

{(4-Methoxyphenylsulfonyl)imino}phenyl-λ3-iodane (1d)

Same procedure; yield 41%; contaminated with sulfonamide.1H NMR (200 MHz, DMSO-d6): 7.75–7.66 (m, 2H),7.48–7.17 (m, 4H), 6.76 (d, J = 8.9 Hz, 2H), 3.73 (s, 3H).

{(2,4-Dinitrophenylsulfonyl)imino}phenyl-λ3-iodane (1e)

Same procedure; yield 98%. IR (KBr): 3093m, 1539vs, 1351s,1286s, 1131s, 1097s, 880m. 1H NMR (200 MHz, DMF-d7):8.66 (d, J = 2 Hz, 1H), 8.33 (dd, J = 8.7, 2 Hz, 1H), 8.12 (d, J= 8.7 Hz, 1H), 7.99 (d, 7.6 Hz, 2H), 7.56–7.37 (m, 3H). 13CNMR (DMF-d7): 149.1 (s), 148.1 (s), 143.5 (s), 134.5 (d),132.2 (d), 131.8 (d), 131.1 (d), 126.4 (d), 119.5 (d), 118.1 (s).MS: 449 (1, M+), 231 (72), 204 (100), 77 (41). HR-MS:448.9216 (C12H8IN3O6S

+; calc. 448.9168).

{(Pentaflurophenylsulfonyl)imino}phenyl-λ3-iodane (1f)Same procedure; yield 32%. IR (KBr): 1641m, 1333s, 1095s.1H NMR (200 MHz, DMSO-d6): 7.79 (d, J = 7.5 Hz, 2H),7.51–7.43 (tl, 1H), 7.29–7.37 (tl, 2H).

Synthesis and characterization of aziridinesThe experimental details are summarized in the footnotes ofTables 1–6.

[Rh2(OAc)4]-catalyzed aziridination of olefins with{[(4-nitrophenyl)sulfonyl]imino}-λ3-phenyliodane (1c).General procedure

To [Rh2(OAc)4] (9.2 mg, 0.02 mmol) in CH2Cl2 (10 mL) wasadded, under N2, activated molecular sieves 4A (6.0 g) and theolefin (20 mmol), followed by NsNTIPh (1c) (404 mg,1.0 mmol). The suspension was stirred at RT until all of 1chaddissolved. The mixture was filtered through a plug of celite,which was washed exhaustively with CH2Cl2, and the filtratewas evaporated. The excess of olefin was removed by flashdistillation under reduced pressure. The crude aziridine waspurified by flash chromatography with silica gel and a mixtureof hexane–ether as eluant, or by recrystallization.

(N-Methanesulfonyl)-2-phenylaziridine (5a)

IR (CHCl3): 3019w, 1324m, 1154m, 1046w. 1H NMR(400 MHz, CDCl3): 7.38–7.30 (m,5H), 3.73(dd, J = 4.4, 7 Hz, 1H),3.12 (s, 3H), 2.99 (d, J = 7 Hz, 1H), 2.45 (d, 4.4 Hz, 1H). 13CNMR (CDCl3): 134.9 (s), 128.7 (d), 128.6 (d), 126.5 (d), 40.8(d), 39.8 (q), 35.4 (t). MS: 197 (< 1, M+), 118 (53), 91 (100),77 (12). HR-MS: 197.0526 (C9H11NO2S

+; calc. 197.0510).

(N-4-Toluenesulfonyl)-2-phenylaziridine (5b)

See refs. 6 and 7.

(N-4-Methoxyphenylsulfonyl)-2-phenylaziridine (5d)mp 75–77°C. IR (CHCl3): 3020w, 2976m, 1522w, 1423w,1224vs, 1157w, 1046m. 1H NMR (400 MHz, CDCl3): 7.92 (d,J = 8.8 Hz, 2H), 7.30–7.26 (m, 3H), 7.23–7.21 (m, 2H), 7 (d, J= 8.8 Hz, 2H), 3.87 (s, 3H), 3.75 (dd, J = 4.4, 7.4 Hz, 1H), 2.97(d, J = 7 Hz, 1H), 2.38 (d, J = 4.4 Hz, 1H). 13C NMR (CDCl3):163.7 (s), 135.2 (s), 130.2 (d), 129.5 (s), 128.6 (d), 128.3 (d),126.6 (d), 114.4 (d), 55.7 (q), 41.0 (d), 35.9 (d). MS: 289 (23,M+), 171 (12), 118 (84), 107 (18), 91 (89), 77 (29). HR-MS:289.0771 (C15H15NO3S

+; calc. 289.0772).

(N-2,4-Dinitrophenylsulfonyl)-2-phenylaziridine (5e)IR (CHCl3): 3018m, 1557vs, 1350s, 1216s, 1168s. 1H NMR(400 MHz, CDCl3): 8.56–8.52 (m, 2H), 8.46 (d, J = 8.4 Hz,1H), 7.35–7.32 (m, 3H), 7.29–7.26 (m, 2H), 4.11 (dd, J = 4.8,7.4 Hz, 1H), 3.32 (d, J = 7.4 Hz), 2.71 (d, J = 4.8 Hz). 13CNMR (CDCl3): 150.3 (s), 148.7 (s), 137.5 (s), 134.0 (s), 133.0(d), 128.9 (d), 128.8 (d), 126.6 (d), 120.0 (d), 43.7 (d), 38.6 (t).MS: 349 (2, M+), 231 (3), 118 (86), 91 (100). HR-MS:349.0358 (C14H11N3O6S

+; calc. 349.0369).

(N-4-Nitrophenylsulfonyl)-2-(4-acetoxyphenyl)aziridine (8b)mp 142–145°C. IR (CHCl3): 3029m, 1754m, 1608w, 1595vs,1349s, 1312m, 1224vs, 1198vs, 1166vs, 854m. 1H NMR(400 MHz, CDCl3): 8.40 (d, J = 9.0 Hz, 2H), 8.19 (d, J =9.0 Hz, 2H), 7.24 (d, J = 8.4 Hz, 2H), 7.05 (d, J = 8.4 Hz, 2H),3.9 (dd, J = 4.4, 7.2 Hz, 1H), 3.12 (d, J = 7.2 Hz, 1H), 2.49 (d,J = 4.4 Hz, 1H), 2.29 (s, 3H). 13C NMR (CDCl3): 169.3 (s),151.0 (s), 150.8 (s), 131.8 (s), 129.2 (d), 127.6 (d), 124.4 (d),122.1 (d), 41.3 (d), 36.7 (t), 21.1 (q). MS: 362 (1, M+), 176(32), 134 (100), 107 (78), 77 (16). HR-MS: 362.0565(C16H15NO4S

+; calc. 362.0572.

(N-4-Nitrophenylsulfonyl)-2-(4-bromophenyl)aziridine (8c)mp 93–95°C. IR (CHCl3): 3031w, 1609w, 1595vs, 1378s,1216s, 1167vs, 1013m, 822 m. 1H NMR (400 MHz, CDCl3):8.38 (d, J = 8.8 Hz, 2H), 8.18 (d, J = 8.8 Hz, 2H), 7.44 (d, J =8.4 Hz, 2H), 7.10 (d, J = 8.4 Hz, 2H), 3.87 (dd, J = 4.5, 7.1 Hz,1H), 3.10 (d, J = 7.1 Hz, 1H), 2.45 (d, J = 4.5 Hz, 1H). 13CNMR (CDCl3): 150.7 (s), 143,7 (s), 133.3 (s), 131.9 (d), 129.2(d), 128.1 (d), 124.4 (d), 122.8 (s), 41.1 (d), 36.6 (t). MS: 384(3, M+), 382 (3, M+), 198 (100), 196 (100), 171 (95), 169 (97),117 (71), 90 (77), 76 (21). HR-MS: 381.9625(C14H12BrNO2S

+; calc. 381.9628).

(N-4-Nitrophenylsulfonyl)-2-(3-chlorophenyl)aziridine (8d)mp 156–158°C. IR (CHCl3): 3013w, 1555vs, 1348s, 1313m,1223m, 1167vs, 856m. 1H NMR (400 MHz, CDCl3): 8.40 (d,J = 8.8 Hz, 2H), 8.19 (d, J = 8.8 Hz, 2H), 7.31–7.24 (m, 2H),7.20 (sl, 1H), 7.15–7.12 (m, 1H), 3.87 (dd, J = 4.4, 7.3 Hz, 1H),3.10 (d, J = 7.3 Hz, 1H), 2,47 (d, J = 4.4 Hz, 1H). 13C NMR(CDCl3): 152.0 (s), 143.7 (s), 136.3 (s), 134.8 (s), 130.1 (d),129.3 (d), 129.0 (d), 126.5 (d), 124.8 (d), 124.4 (d), 40.9 (d),36.8 (t). MS: 338 (< 1, M+), 152 (93), 125 (100), 75 (12).HR-MS: 338.0168 (C14H12ClNO2S

+; calc. 338.013).

(N-4-Nitrophenylsulfonyl)-2-(4-methoxycarbonylphenyl)-aziridine (8e)

mp 116–118°C. IR (CHCl3): 3017m, 1718s, 1348s, 1284vs,1167vs, 1113m. 1H NMR (400 MHz, CDCl3): 8.39 (d, J =8.8 Hz, 2H), 8.20 (d, J = 8.8 Hz, 2H), 7.99 (d, J = 8.4 Hz, 2H),

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7.31 (d, J = 8.4 Hz, 2H), 3.94 (dd, J = 4.4, 7.2 Hz, 1H), 3.91 (s,3H), 3.15 (d, J = 7 Hz, 1H), 2.51 (d, J = 4.4 Hz, 1H). 13C NMR(CDCl3): 166.4 (s), 150.8 (s), 143.7 (s), 139.2 (s), 130.0 (d),129.2 (d), 126.4 (d), 124,4 (d), 52.2 (q), 41.1 (d), 36.8 (t). MS:362 (< 2, M+), 331 (3), 176 (100). HR-MS: 362.0594(C16H15NO4S

+; calc. 362.0572).

(N-4-Nitrophenylsulfonyl)-2-ethenylaziridine (8j)mp 138°C. IR (CHCl3): 3029w, 1535 vs, 1351s, 1217s, 1092m,936m. 1H NMR (400 MHz, CDCl3): 8.40 and 8.16 (AA′XX′,app. (app. = apparent) d, J = 9.1 Hz, 4H), 5.57–5.48 (m, 2H),5.32–5.29 (m, 1H), 3.42–3.38 (m, 1H), 2.92 (d, J = 6.8 Hz),2.34 (d, J = 4.8 Hz, 1H). 13C NMR (CDCl3): 150.7 (s), 144.1(s), 132.2 (s), 129.1 (s), 124.3 (d), 121.2 (t), 41.9 (q), 34.8 (t).MS: 254 (0.5, M+), 186 (2), 122 (8), 76 (20), 68 (100). HR-MS:186.9935 (C6H4O4NS+; calc. 186.9939).

(N-4-Nitrophenylsulfonyl)-2-ethoxycarbonylaziridine (8o)

mp 96–98°C. IR (CHCl3): 3106w, 3029m, 2927m, 2855w,1745s, 1608m, 1535vs, 1403m, 1350vs, 1312m, 1229s,1209vs, 1169vs, 1093s, 1014m, 934m, 882m, 856m. 1H NMR(400 MHz, CDCl3): 8.42 and 8.19 (AA′XX′, app. d, J =8.8 Hz, 4H), 4.21 (q, J = 7 Hz, 2H), 3.45 (dd, J = 7.2, 4 Hz,1H), 2.88 (d, J = 7 Hz), 2.66 (d, J = 4.4 Hz, 1H), 1.27 (t, J =7 Hz, 3H). 13C NMR (CDCl3): 166.1 (s), 151.0 (s), 143.1(s),129.5 (d), 124.4 (d), 62.4 (t), 36.4 (d), 32.4 (t), 14,0 (t). MS:301(< 1, M++1), 255 (2), 227 (3), 186 (5), 122 (9), 114 (37), 86(100), 58 (33).

exo−(N-4-Nitrophenylsulfonyl)-3-aza-tricyclo[3.2.1.02,4]-octane (8p)

mp 133–136°C. IR (CHCl3): 3105w, 3020m, 2968m, 2878m,1633m, 1607m, 1534vs, 1474w, 1401m, 1350vs, 1330s,1310s, 1159vs, 1127m, 1091s, 1034m, 977s, 913s, 855s. 1HNMR (400 MHz, CDCl3): 8.39 and 8.14 (AA′XX′, app. d, J= 8.8 Hz, 4H), 3.07 (s, 2H), 2.49 (s, 2H), 1.51–1.55 (m, 2H),1.40–1.43 (m, 1H), 1.26–1.31 (m, 2H), 0.82 (d, J = 10.3 Hz,1H). 13C NMR (CDCl3): 155.5 (s), 144.8 (s), 128.8 (d), 124.2(d), 43.0 (d), 35.9 (d), 28.2 (t), 25.5 (t). MS: 294 (1, M+), 265(4), 186 (2), 108 (56), 81 (100), 67 (15). HR-MS: 294.0659(C13H14N2O4S

+; calc. 294.0674).

(N-4-Nitrophenylsulfonyl)-6-aza-benzobicyclo[3.1.0]hexene (8q)mp 139–140°C. IR (CHCl3): 3105w, 3030m, 2920w, 1608m,1534vs, 1476m, 1422m, 1402m, 1350vs, 1336s, 1311s,1161vs, 1090s, 972s, 931s, 877s, 854s. 1H NMR (400 MHz,CDCl3): 8.38 and 8.15 (AA′XX′, app. d, J = 9.2 Hz, 4H), 7.42(d, J = 7.7 Hz, 1H), 7.19–7.31 (m, 3H), 4.43 (d, J = 5.1 Hz,1H), 4.06 (t, J = 4 Hz, 1H), 3.22 (app. ABX, dd, J = 18, 4 Hz,1H), 3.16 (app. ABX, d, J = 18 Hz, 1H). 13C NMR (CDCl3):150.6 (s), 144.4 (s), 143.3 (s), 137.5 (s), 129.1 (d), 129.0 (d),127.0 (d), 125.7 (d), 125.1 (d), 124.3 (d), 51.1 (d), 45.7 (d),34.7 (t). MS: 316 (4, M+), 130 (100), 103 (27), 77 (18). HR-MS: 316.0527 (C15H12N2O4S

+; calc. 316.0518).

Data for rearrangement and cycloaddition productsfrom aziridinations (Schemes 5, 6)

2-(N-4-Nitrophenylsulfonyl)-azabicyclo[3.2.1]octa-3,6-diene(10)

mp 123-125°C. 1H NMR (400 MHz, CDCl3): 8.37 and 8.01

(AA′XX′, app. d, J = 8.9 Hz, 4H), 6.30–6.28 (m, 1H),6.20–6.18 (m, 1H), 5.39–5.35 (m, 1H), 5.24–5.22 (m, 1H),4.80 (sl, 1H), 2.73–2.70 (m, 1H), 1.90–1.80 (m, 1H), 1.37 (d,J = 12 Hz, 1H). 13C NMR (CDCl3): 150.1 (s), 145.8 (s), 140.0(d), 127.9 (d), 124.5 (d), 121.2 (d), 112.4 (d), 60.2 (d), 35.9 (d),35.1 (t). MS: 292 (6, M+), 106 (87), 79 (100). HR-MS:292.0482 (C13H12N2O4S

+; calc. 292.0517).

cis- and trans-(N-4-Nitrophenylsulfonyl)-2,4-di(4-methoxyphenylpyrrolidine (12h)

Purified on aluminum oxide with ether – petroleum ether3:2; separation of isomers by HPLC; first fraction (8%), oil.IR (CHCl3): 3030vw, 2935vw, 2837vw, 1607w, 1531s,1510vs, 1346s, 1248s, 1161m, 1035m, 856w. 1H NMR(400 MHz, CDCl3): 8.17 and 7.63 (AA′XX′, app. d, J =8.8 Hz, 4H), 7.13 (d, J = 8.8 Hz, 2H), 7.11 (d, J = 8.8 Hz, 2H),6.87 (d, J = 8.8 Hz, 2H), 6.74 (d, J = 8.8 Hz, 2H), 4.92 (dd, J= 9.9, 6.6 Hz, 1H), 4.30 (dd, J = 9.5, 7.7 Hz, 1H), 3.80 (s, 3H),3.78 (s, 3H), 3.45 (app. t, J = 10.7 Hz, 1H), 3.31–3.25 (m, 1H),2.76–2.70 (m, 1H), 2.15–2.06 (m, 1H). 13C NMR (CDCl3):159.3 (s), 158.9 (s), 149.6 (s), 145.9 (s), 132.8 (s), 130.6 (s),128.4 (d), 128.1 (d), 128.0 (d), 123.8 (d), 114.2 (d), 113.8 (d),64.3 (q), 55.7 (t), 55.4 (d), 44.3 (t), 43.1 (d). MS: 468 (42, M+),281 (38), 254 (100), 121 (57), 91 (15). HR-MS: 468.1412(C24H24N2O6S

+; calc. 468.1388).

Second fraction (8%), oil. IR (CHCl3): 3030vw, 2935vw,2837vw, 1607w, 1531w, 1510vw, 1346s, 1248s, 1161m,1035m, 856w. 1H NMR (400 MHz, CDCl3): 8.28 and 7.86(AA′XX′, app. d, J = 8.8 Hz, 4H), 7.18 (d, J = 8.8 Hz, 2H),7.03 (d, J = 8.8 Hz, 2H), 6.83 (d, J = 8.8 Hz, 2H), 6.82 (d, J= 8.8 Hz, 2H), 5.05 (dd, J = 2.9, 8.1 Hz, 1H), 4.01 (dd, J = 7,9.2 Hz, 1H), 3.81 (s, 3H), 3.78 (s, 3H), 3.57–3.48 (m, 1H), 3.44(app. t, J = 9.2 Hz, 1H), 2.26–2.16 (m, 2H). 13C NMR(CDCl3): 159.1 (s), 158.8 (s), 149.9 (s), 144.4 (s), 133.9 (s),131.3 (s), 128.4 (d), 127.9 (d), 127.5 (td), 124.1 (d), 114.2 (d),113.9 (d), 63.0 (q), 55.3 (t), 55.3 (d), 42.6 (t), 41.0 (d). MS:468 (42, M+), 281 (38), 254 (100), 121 (57), 91 (15). HR-MS:468.1412 (C24H24N2O6S

+; calc. 468.1388).

cis- and trans-(N-4-Nitrophenylsulfonyl)-2,4-diethoxypyrrolidine (12m)

Separated on silica gel with hexane – ethyl acetate 3:1; firstfraction (36%), oil. IR (CHCl3): 3104w, 2980m, 2942m,2897m, 1606m, 1532vs, 1479w, 1444w, 1401m, 1350vs,1312s, 1169vs, 1110s, 1062s, 1002m, 920m, 899s, 855m. 1HNMR (400 MHz, CDCl3): 8.35 and 8.05 (AA′XX′, app. d, J= 8.8 Hz, 4H), 5.29 (dd, J = 5.7, 1.8 Hz, 1H), 4.22–4.17 (m,1H), 3.78–3.70 (m, 1H), 3.61–3.48 (m, 2H), 3.34–3.27 (m,3H), 2.19 (ddd, J = 13.3, 6.2, 1.4 Hz, 1H), 1.80–1.73 (m, 1H),1.18 (t, J = 7 Hz, 3H), 1.04 (t, J = 7 Hz, 3H). 13C NMR(CDCl3): 150.1 (s), 145.2 (s), 128.8 (d), 124.1 (d), 89.9 (d),76.9 (d), 65.1 (t), 63.8 (t), 51.8 (t), 39.6 (t), 15.2 (q), 15. 0 (q).MS: (M+, absent), 299 (100), 253 (33), 186 (20), 129 (19), 122(32), 86 (55), 76 (25), 58 (28).

Second fraction (16%), oil. 1H NMR (400 MHz, CDCl3):8.35 and 8.10 (AA′XX′, app. d, J = 9.2 Hz, 4H), 5.34 (dd, J= 6.2, 1.8 Hz, 1H), 4.06–3.98 (m, 1H), 3.77–3.25 (m, 6H),2.19–2.12 (m, 1H), 2.06–2.00 (m, 1H), 1.15 (t, J = 7 Hz, 3H),1.14 (t, J = 7 Hz, 3H). MS: 344 (M+, 1), 299 (82), 252 (83),186 (51), 129 (30), 122 (100), 112 (27), 86 (89), 76 (20), 58 (27).

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Intramolecular aziridination

{(2-Ethenylphenylsulfonyl)imino}-phenyl-λ3-iodane (15)2-Ethenylphenylsulfonamide (14)See ref. 25. The ylide was prepared according to the generalprocedure and used without purification and characterization.

1,1a-Dihydro-6-thia-6a-aza-cycloprop[a]inden-6,6-dioxide(16)

The ylide 15 (1.00 mmol) was suspended under N2 in dryCH2Cl2 (10.0 mL) containing [Rh2(OAc)4] (0.02 mmol) andmolecular sieves (6.0 g) and stirred during 2 h. The mixturewas filtered through celite, which was washed out with CH2Cl2(100 mL). After evaporation of the solvent, the crude productwas purified by flash chromatography on silica gel using hex-ane – ethyl acetate 2:1 as eluant to afford 16 as an oil. IR(CHCl3): 3155w, 2927w, 1601w, 1552w, 1470w, 1340s,1178vs, 1052w, 916vs, 738vs. 1H NMR (400 MHz, CDCl3):7.81–7.54 (m, 4H), 4.14 (m, 1H), 2.88 (dd, J = 1.1, 4.8 Hz, 1H),2.37 (dd, J = 1.1, 4 Hz, 1H). 13C NMR (CDCl3), 136.9 (s),133.4 (d), 133.3 (s), 130.4 (d), 125.5 (d), 123.6 (d), 44.2 (t),43.0 (t). MS: 181 (4, M+), 167 (13), 117 (93), 103 (11), 90(100), 76 (29). HR-MS: 167.0022 (C7H5NO2S

+; calc.167.0041).

Relative reactivity of substituted styrenes with respect toNsNTIPh

To a mixture of dry CH2Cl2 (10.0 mL), [Rh2(OAc)4](0.02 mmol), styrene (4a, 5.0 mmol) and substituted styrene(4b–g, 5.0 mmol), and molecular sieves (6.0 g) was addedNsNTIPh (1c, 1.00 mmol). The suspension was stirred atroom temperature until all of the ylide was dissolved. Thesolution was filtered through celite, which was washed withCH2Cl2 (100 mL). The filtrate was evaporated, and the ratio ofthe aziridines was determined by 1H NMR by comparison ofthe integrals of the protons of the aziridines. In the case of4-methoxycarbonylstyrene (4e), 1.0 mmol of 4awas used with5.0 mol of 4eand 0.5 mol of NsNTIPh (1c), and with 3-nitro-styrene (4g) 10.0 mol were used with 2.0 mol of 4a and1.0 mmol of 1c.

The [Cu(acac)2]-catalyzed reactions were carried out by thesame procedure, using 1:1 mixtures of styrenes and 10% of catalyst.

Aziridination of vinylcyclopropanes (radical probes)

(N-4-Nitrophenylsulfonyl)-2-cyclopropylaziridine (22a)The reaction was carried out according to the general proce-dure, using 20 mmol of cyclopropylethene (19a) (30a),1.00 mmol of NsNTIPh (1c), molecular sieves, and0.02 mmol of [Rh2(OAc)4]. After stirring during 10 min. thereaction was worked up as usual; crude yield of aziridine 43%.Part of the product decomposed upon purification (preparativeTLC on silica gel, using hexane–ether 2:1 as solvent); colorlessoil. IR (CHCl3): 3029vw, 1534s, 1350m, 1166m, 856w. 1HNMR (400 MHz, CDCl3): 8.15 and 8.40 (AA′XX′, app. d, J= 8.6 Hz, 4H), 2.88–2.86 (m, 1H), 2.72 (d, J = 7 Hz, 1H), 2.21(d, J = 4.8 Hz, 1H), 0.94–0.90 (m, 1H), 0.55–0.42 (m, 2H),0.25–0.16 (m, 2H). 13C NMR (CDCl3): 151.0(s), 144.0 (s),129.0 (d), 124.0 (d), 43.0 (d), 34.0 (t), 10.0 (t), 3.0 (t), 2.0 (t).MS: 268 (5, M+), 186 (4), 82 (80). HR-MS: 268.0512(C11H12N2O4S

+; calc. 268.0518).

(N-4-Nitrophenylsulfonyl)-2-(2-trans-3-trans-diphenylcyclo-propyl)aziridine (22b)

Same conditions as above with 20 mmol of 2-trans-3-trans-diphenylvinylcyclopropane (19b) (30b). Reaction timethree days; yield of aziridine 35% (oil). IR (CHCl3): 3035m,1604m, 1535vs, 1349s, 1220vs, 1166s, 1092w, 856m. 1HNMR (400 MHz, CDCl3): 8.41 and 8.20 (AA′XX′, app. d, J= 8.8 Hz, 4H), 7.08 (sl, 6H), 6.84–6.80 (m, 4H), 3.30–3.26 (m,1H), 2.88 (d, J = 7.1, 1H), 2.41 (d, J = 4.9 Hz, 1H), 2.38–2.29(m, 2H), 1.96–1.92 (m, 1H). 13C NMR (CDCl3): 151.0 (s),143.9 (s), 136.2 (s), 136.1 (s), 129.3 (d), 128.9 (d), 128.6 (d),127.9 (d), 126.3 (d), 126.2 (d), 124.4 (d), 41.5 (d), 34.2 (t), 29.6(d), 27.7 (d), 24.2 (d). MS: 420 (0.3, M+), 234 (10), 186 (15),91 (100), 77 (20). HR-MS: 234.1277 (C17H16N

+; calc.234.1283).

Synthesis of RhII carboxylate catalysts

[Rh2{(–)campha}4] (23)

In a flask fitted with a Soxhlet extractor containing groundNa2CO3 and sand was heated, under N2 and with stirring, (–)-camphanic acid (2.19 mmol) and [Rh2(OAc)4] (0.21 mmol) inchlorobenzene (50 mL) to reflux during three days. The mix-ture was evaporated to dryness and the residue was purified byflash chromatography on silica gel, using a mixture of ethylacetate – CH2Cl2 1:5 containing 1% of MeOH. Yield 97% ofamorphous powder. 1H NMR (200 MHz, CDCl3): 2.72–2.50(d, 1H), 1.90–1.50 (m, 3H), 1.04 (s, 3H), 0.87 (s, 3H), 0.71 (s,3H). 13C NMR (CDCl3): 186.8 (s), 179.1 (s), 91.5 (s), 54.7 (s),54.0 (s), 30.7 (t), 28.7 (t), 16.9 (q), 16.8 (q), 9.7 (q). MS (elec-trospray): 1017.0 (M+ + Na).

[Rh2{(1R)-meba}4] (24)

Same procedure with 2.19 mmol of 2-((1R)-methoxyethyl)benzoic acid prepared from 2(1R-methoxyethyl)bromobenzene via carbonation of theorganomagnesium derivative (92% ee) and 0.21 mmol of[Rh2(OAc)4]. The crude product was dissolved in CH2Cl2 andextracted three times with 10% aq. NaHCO3. The organic layerwas dried (MgSO4), filtered, and the filtrate was evaporated toafford the product as amorphous powder in 98%. 1H NMR(400 MHz, CDCl3): 7.49–7.45 (m, 2H), 7.38 (d, J = 7.6 Hz,1H), 7.26–7.23 (m, 1H), 4.72 (q, J = 6.4 Hz, 1H), 2.81 (s, 2H),1.15 (d, J = 6.4 Hz, 3H). 13C NMR (CDCl3, 187.9 (s), 143.1(s), 131.6 (d), 131.4 (s), 128.5 (d), 126.8 (d), 125.4 (d), 75.2(q), 55.6 (d), 23.8 (q). MS (electrospray): 945 (M+ + Na).[α]D

22 = +21.3 (CHCl3, c = 0.30).

Photochemical nitrene generationA solution of styrene (4a, 20 mmol), [Rh2{(R)-bnp}4](0.05 mmol), and NsN3 (25, 1.00 mmol) (40) in dry CH2Cl2(15 mL) was irradiated in a Quartz vessel, which was placed ina Rayonet photoreactor (λmax = 254 nm), under N2 and withstirring during 70 h at room temperature. The reaction mixturewas filtered through celite, which was washed with EtOAc.The solvent was evaporated, and excess styrene was removedby flash distillation. The crude product was purified by flashchromatography (silica gel, hexane–ether 3:1) to yield the az-iridine 5c in 9% yield with an ee of 17%.

Müller et al. 749

© 1998 NRC Canada

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Acknowledgments

This work was supported by the Swiss National Science Foun-dation (grant nos. 20-38’907.93 and 20-44’144.95). Theauthors are indebted to Mr. J.-P. Larcinese and P. Nury forexperimental work on aziridinations, to Mrs. J.-P. Saulnier andA. Pinto for the NMR spectra, and Ms. G. Klink for the massspectra.

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the rhodium(ii)-catalyzed aziridination of olefins with...

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