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1Recent Advance of ‘Combined Acid’ Strategy for AsymmetricCatalysis

Hisashi Yamamoto and Kentaro Futatsugi

Lewis acid can be utilized as a more effective tool for chemical reactions bysophisticated engineering (‘designer acid catalysis’) [1]. The goal of such ‘designerLewis acid’ is to achieve high reactivity, selectivity, and versatility, upon which thefull potential of acid catalysts has not yet been realized.

1.1Combined Acid Catalysis

One possible way to take advantage of such abilities may be to apply a ‘combinedacids system’ [2] to the catalyst design. The concept of combined acids, which canbe classified into Brønsted acid-assisted Lewis acid (BLA), Lewis acid-assisted Lewisacid (LLA), Lewis acid-assisted Brønsted acid (LBA), and Brønsted acid-assistedBrønsted acid (BBA), can be a particularly useful tool for the design of asymmetriccatalysis, because combining such acids will bring out their inherent reactivity byassociative interaction, and also provide more organized structure, which will allowan effective asymmetric environment to be secured.

These combined acid catalysts can be classified as shown in Table 1.1. It should beemphasized that we anticipated a more or less intramolecular assembly of such com-bined systems rather than intermolecular arrangements because the intramolecularassembly should generate a well-organized chiral environment, leading to highstereocontrol. Thus, proper design of the catalyst structure is essential for success.

Since we have already summarized examples of combined acid catalysis [2], onlyrecent representative examples of such catalysis are discussed with emphasis onthe area of asymmetric catalysis in the following sections.

1.1.1Brønsted Acid-Assisted Chiral Lewis Acid (BLA)

Since the first report of chiral oxazaborolidine-based Brønsted acid-assisted chiralLewis acid (BLA) for enantioselective Diels–Alder reactions by Corey and coworkers

Acid Catalysis in Modern Organic Synthesis. Edited by H. Yamamoto and K. IshiharaCopyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31724-0

2 1 Recent Advance of ‘Combined Acid’ Strategy for Asymmetric Catalysis

Table 1.1 The general classifications of combined acid catalysis.

MO

M

MO

HO

HO

M

HO

HO

Brønsted acid-assisted Lewis acid catalyst (BLA)

Lewis acid-assisted Lewis acid catalyst (LLA)

Lewis acid-assisted Brønsted acid catalyst (LBA)

Brønsted acid-assistedBrønsted acid catalyst (BBA)

Enhancement of Lewis acidity by the combination with Brønsted acid O

BO

O

O

OOiPr

OiPr

R

O OH

NB

O

Me

Ph

Ph

H

H3B

Enhancement of Lewis acidity by the combination with Lewis acid

Enhancement of Brønsted acidity by the combination with Lewis acid

Enhancement of Brønsted acidityby the combination with Brønsted acid

Type of combined acid catalysts General structure Examples

O

O

SnCl4

H

H

O

O

O

O

Ar Ar

Ar Ar

H

H

NB

O

Ar

Ar

H

H XN

BO

Ar

Ar

H

HX

2a : Ar = Ph, X = OTf2b : Ar = 3,5-Dimethylphenyl X = OTf3a : Ar = Ph, X = NTf23b : Ar = 3,5-Dimethylphenyl X = NTf2

1a: Ar = Ph1b: Ar = 3,5-Dimethylphenyl

BLA form

Scheme 1.1 Generation of Corey’s BLAs from the corre-sponding chiral oxazaborolidine (1a) or (1b) and strongBrønsted acid.

1.1 Combined Acid Catalysis 3

N B O

OH

Me

H

RR

N B OH

Me

O

RR

HTfO

TfO

−

+

−

+

Fig. 1.1 Proposed pretransition state assemblies.

in 2002 [3a], recent studies by Corey group have demonstrated that these chiral BLAsare exceptionally potent and versatile chiral Lewis acids for catalytic enantioselectiveDiels–Alder reactions [3].

These chiral BLAs ((2a–b), (3a–b)) are generated simply by protonation of thechiral oxazaborolidine of type (1) by strong Brønsted acids such as triflic acid ortriflimide (Scheme 1.1).

The absolute stereochemical outcome of the Diels–Alder reactions via thesecatalysts can be successfully predicted on the basis of the pretransition stateassemblies shown in Figure 1.1.

The synthetic power of Corey’s BLAs has been well demonstrated by theirapplications to the enantioselective syntheses of biologically important complexmolecules through catalytic enantioselective Diels–Alder reactions as key steps(Scheme 1.2) [3g–i, 4]. The key Diels–Alder reactions proceed in excellent chemicalyield with high enantioselectivity in all cases and the absolute configurations of theproduct are consistent with the proposed transition state assembly.

The synthetic utility of Corey’s chiral BLAs is quite obvious from many successfulapplications to other carbon–carbon bond-forming reactions described below.

In 2004, Corey reported highly enantioselective cyanosilylation of aldehydescatalyzed by chiral BLA (3b) (Scheme 1.3) [5]. Under the standard conditions(10 mol% of (3b) and 20 mol% of Ph3PO in toluene at 0 ŽC), a variety of botharomatic and aliphatic aldehydes have been transformed into cyanohydrins with>90% enantiomeric purity.

Noteworthy is that the reaction between trimethylsilyl (TMS) cyanide andtriphenylphosphine oxide seems to generate a new reactive cyanide donor,isocyanophosphorane Ph3P(OTMS)(NDC:) and this species is crucial for highenantioselectivity. The novel process described herein has several other advantagessuch as: (i) predictability of absolute configuration of cyanohydrin products from amechanistic model as described in Scheme 1.3; (ii) easy and efficient recovery ofthe catalytic ligand.

The subsequent study revealed that the chiral BLA (2b) is a potent chiralLewis acid for enantioselective cyanosilylation of methyl ketones promoted byTMS cyanide and diphenylmethyl phosphine oxide as coreactants (to generatePh2MeP(OTMS)(NDC: as a reactive intermediate) (Scheme 1.4) [6]. The rationalfor the observed face selectivity can be explained by a transition state assembly in


MeO

+CH2Cl2

−78 °C , 8 h MeO

H

H

92% yield94% ee

NB

OPh

HH

H

(20 mol%)

CHO

EtO2C

CHO

CO2Et

NTf2

HO

H

H

O

H

(+)-Estrone

CHO

CH2Cl2−95 °C , 2 h

3a (20 mol%)+

CHO

76% yield96% ee

O

Georgyone

Scheme 1.2 Enantioselective total syntheses of biologicallyimportant molecules using Corey’s BLAs.


CO2Et

O

H3N

AcHN

H2PO4

CO2CH2CF3

+ent-3a (10 mol%)

Neat23 °C, 30 h

CO2CH2CF3

97% yield> 97% ee

Tamiflu

OTIPS

CHO

HOTIPS

CHO Toluene−93 °C, 3 h

−78 °C, 10 h

3a (20 mol%)

71–74% yield90% ee

H

OHPalominol

Scheme 1.2 (continued)

Scheme 1.4, which involves the hydrogen bonding between the oxygen atom of thechiral oxazaborolidine and α-hydrogen atom of the methyl ketone.

In 2005, a highly enantioselective [3 + 2]-cycloaddition reaction of 2,3-dihydrofuranwith 1,4-benzoquinones using a chiral BLA (3a) as catalyst was developed, which


H

O

NC

3b (10 mol%)

+ TMSCNPh3PO (20 mol%)

Toluene0 °C, 144 h

2N HCl

(1.1 equiv)CN

OH

NC98% yield97% ee

N B O

OH

R

Me

HNTf2

−

+

Scheme 1.3 Enantioselective cyanosilylation of aldehydes catalyzed by (3b).

O

O2N O2N

TMSO CN2b (10 mol%)

+ TMSCNPh2MePO (11 mol%)

Toluene45 °C, 10 d(1.1 equiv)

N B OH O

H

TfOR H

H

83% yield96% ee

+

−

Scheme 1.4 Enantioselective cyanosilylation of methyl ketones catalyzed by (2b).

allows rapid access to a variety of chiral phenolic tricycles with high enantioselectiv-ity (up to 98% ee). The utility of this new methodology was demonstrated by a shortsynthesis of the important pentacyclic natural product, aflatoxin B2 (Scheme 1.5) [7].

In 2006, catalytic enantioselective Michael addition of ketene silyl acetals to cyclicand acyclic α, β-enones was devised using the chiral BLA (3a) (Scheme 1.6) [8]. Var-ious Michael donors and ketene silyl acetals can be employed for this process. Hereagain, the use of triphenylphosphine oxide is crucial to trap any catalytically activesilyl cation species. In addition, the combined use of triphenylphosphine oxide anda stoichiometric amount of 2,6-diisopropylphenol (which acts as a scavenger of silylcation species) is sometimes required to get both high enantioselectivity and thechemical yield. This enantioselective methodology was applied to the asymmetricsynthesis of a key intermediate of caryophyllene.

An enantioselective total synthesis of the apoptosis-inducing natural product,(–)-rasfonin has been reported by Boeckman Jr. and coworkers in 2006 (Scheme 1.7)[9]. The crucial asymmetric vinylogous Mukaiyama aldol addition was achievedwith high diastereoselectivity with chiral BLA (2b) through Corey’s transition statemodel.


ent-3a (20 mol%)

+CH2Cl2–MeCN

(1 : 1)−78 °C, 2 h

−78 °C to rt, 5 h

O

O

MeO O

(1.5 equiv)

OH

MeO

OOH

65% yield92% ee

H

O

O O

O

O

MeO H

H

Aflatoxin B2

Scheme 1.5 Asymmetric [3 + 2] cycloaddition of 1,4-benzoquinones catalyzed by (3a).

ent-3a (20 mol%)

+Toluene

−20 °C, 16 h

91% yield90% ee

OOSiMe3

MeO

O

MeO2C

Ph3PO (25 mol%)

OH

H

Key intermediate of caryophyllene

Other examples (3a as a chiral Lewis acid)

O

MeO2C

O

MeO2C

O

OMeO2C

O

BuSt

O

99% yield99% ee

86% yield90% ee

87% yield90% ee

94% yield90% ee

Scheme 1.6 Enantioselective Michael addition of ketene silyl acetals catalyzed by (3a).


N B O

OH HTfO

O

TMSO

OHC

2b (0.48 equiv)

OTMSO

+CH2Cl2

−78 °C, 18 h

OO

OTMS

81% yieldthreo:erythro = 20 : 1threo dr > 20 : 1

OO

O

OOH

OH(−)-Rasfonin

−+

Scheme 1.7 Total synthesis of (–)-rasfonin through diastere-oselective vinylogous Mukaiyama aldol reaction promoted bychiral BLA (2b).

A novel catalytic system was developed for the enantioselective synthesis ofβ-lactones from ketene and aldehydes by Corey group in 2006 (Scheme 1.8) [10].

A range of aliphatic aldehydes have been converted to the corresponding chiralβ-lactones by the reaction with ketene in the presence of the chiral oxazaborolidineand tributyltin triflate as catalysts with up to 84% ee.

The authors proposed that the chiral BLA (5), generated by the reaction of thechiral oxazaborolidine (4) and tributyltin triflate, acts as a chiral Lewis acid catalystin this system. Noteworthy is the fact that the absolute configuration of the β-lactonecan be predicted by the transition state assembly in Scheme 1.8, which is analogousto that of well-known CBS reduction.

In 2006, Schaus group reported that chiral BINOL-derived diols catalyze the enan-tioselective asymmetric allylboration of ketones (Scheme 1.9) [11]. The reactionrequires 15 mol% of 3,30-Br2-BINOL (6) as the catalyst and allyldiisopropoxyboraneas the nucleophile. The reaction products are generally obtained in good yieldsand high enantiomeric ratios (up to 99.5 : 0.5). In addition, high diastereoselectiv-ities (dr 98 : 2) and enantioselectivities (er 98 : 2) are obtained in the reactions ofacetophenone with crotyldiisopropoxyboranes through Zimmerman–Traxler tran-sition state model. On the basis of a series of experiments, the authors proposedchiral BLA-type transition state assembly in which the acidic proton coordinates tothe alkoxy ligand of the chiral boronate complex through hydrogen bonding. Tosupport this hypothesis, the reaction using monomethylated analog of (6) resultedin lower yield and virtually no asymmetric induction, highlighting the importanceof the diol functionality for BLA-type activation.


(10 mol%)

+Bu3SnOTf (10 mol%)

CH2Cl2−78 °C, 24 h

(10 equiv) 73% yield81% ee

NB

O

H Ph

Ph

HO O

PhCHO

O

O

Ph

OCH

H

NB

O

H Ph

Ph

HO O Bu3SnOTf

NB

O

H Ph

Ph

HOOSnBu3

BLA form (5)

OCH

HNB

O

H Ph

Ph

HOO

OSnBu3

NB

O

H Ph

Ph

HO

O

Bu3SnO

O R

H

+

RCHO

O

O

R

OTf

OTf

OTf

4

4

−+

−

+

−+

+−

Scheme 1.8 Enantioselective β-lactone formation catalyzed by the chiral oxazaborolidine.

Hall and coworkers developed the novel enantioselective allylboration of aldehy-des catalyzed by chiral Brønsted acid in 2006 (Scheme 1.10) [12]. Moderate to goodenantioselectivities (up to 80% ee) were observed in the reactions of allylboronateand both aromatic and aliphatic aldehydes with 10 mol% of chiral Brønsted acid(7) [13], a Lewis acid-assisted chiral Brønsted acid (chiral LBA) developed by Ya-mamoto group. On the basis of their previous studies on allylboration reactions,the authors proposed that the use of the strong chiral Brønsted acid provides thechiral recognition event through BLA-type cyclic transition state with coordinationto the oxygen atom of the allylboronate.

A highly practical enantioselective allylation of aldehyde or ketone-derivedacrylhydrazones was developed by Leighton group in 2003 [14a] and 2004 [14b](Scheme 1.11). The chiral allylsilane reagent (8) can be prepared easily in bulkand is stable to storage. Mechanistic studies have revealed that the acylhydrazone


OiPr

B OiPr6 (15 mol%)

PhCH3 : PhCF3 (3 : 1) −35 °C, 15 h

+R1 R2

O

OH

OH

Br

Br

R1

OHR2

76 – 93% yield

er 95 : 5 −99.5 : 0.5dr > 98 : 2

R3

R4

R3 R4

Br

Br

OO B O

OH

Chiral BLA-type transition state

OH OHOH

OH OH

(from (E)-crotyl boronate) (from (Z)-crotyl boronate)

91% yielder 96.5 : 3.5

7% yielder 97.5 : 2.5

83% yielder 97 : 3

72% yielddr 98: 2, er 99 : 1

75% yielddr 99 : 1, er 98.5 : 1.5

Scheme 1.9 Catalytic enantioselective allyl and crotylboration of ketones by Schaus group.

reactions operate by nucleophilic displacement of the chloride on the silicon centerby the oxygen of the acylhydrazone. This reaction generates one equivalent ofHCl, which protonates the amino group of the pseudoephedrine chiral controller,resulting in a significant increase in the Lewis acidity of the silicon center.

A similar enantioselective allylation and crotylation system was developed byLeighton group in 2006 (Schemes 1.12 [15] and 1.13 [16]). In these reactions,


O

B O

H A*

OSnCl4

OH

H

O BR

H OR

ORH*A

O

B O

chiral BLA

HA* RCHO

R

OH

*

O

B O

7 (10 mol%) OH

Na2CO3 (20 mol%)Toluene, −78 °C, 12 h

+H

O

Ph Ph

85% yield78% ee

Proposed mechanism:

Scheme 1.10 Catalytic enantioselective allylboration of aldehydes by Hall and coworkers.

Ph

NSi

O

Cl

CH2Cl210 °C, 16 h

86% yield88% ee

(1.5 equiv)

Si

N

N

H

BLA-type transition state

OO

Ph

NR3

R2

R1

Cl

Ph H

NNH

Ph

HNNH

O O(S,S)-8

CHCl340 °C, 24 h

86% yield90% ee

(1.5 equiv)

Ph

NNH

Ph

HNNH

PhO PhO(S,S)-8

R2 = H, R3 = Meor

R2 = alkyl, R3 = Ph

−+

Scheme 1.11 Enantioselective allylation of acylhydrazones by Leighton and coworkers.


Ph

N

H

HO

Ph

HN

HO

93% yield96% ee

(S,S)-8 (1.5 equiv)

CH2Cl223 °C, 16 h

OH N OH HN

Toluenereflux, 6 h

93% yield98% ee

(S,S)-8 (1.5 equiv)

Scheme 1.12 Enantioselective allylation of phenol-derived imines by Leighton group.

NSi

N

Cl

OH O OH OH

Toluene40 °C, 48 h

63% yield93% ee

(1.5 equiv)

Br

Br

Scheme 1.13 Enantioselective allylation of phenol-derived ketones by Leighton group.

BLA-type transition states, similar to those discussed above, are likely to operatethrough the activation of the chiral allylsilane reagent by HCl generated after thenucleophilic displacement of the chloride by the phenolic oxygen atom.

1.1.2Lewis Acid-Assisted Chiral Lewis Acid (LLA)

In 2005, Yamamoto group recognized that a new Lewis acid-assisted chiral Lewisacid (LLA, 9), generated from the corresponding chiral oxazaborolidine and SnCl4,is a highly reactive and enantioselective Diels–Alder catalyst for various classesof substrates (88–99% ee) (Scheme 1.14) [17]. Importantly, the enantioselectivityof Diels–Alder reactions can be preserved even in the presence of a large excessamount of SnCl4, which implicates the high reactivity of the chiral LLA. Further-more, this catalyst system is tolerant of a small amount of moisture, oxygen, andother Lewis bases without any significant loss of enantioselectivity.


9 (1 mol%)

CH2Cl2, −78 °C, 2 h+

(5 equiv)

OH

H

93% yieldendo/exo = 92 : 895% ee

OH

O

99% yieldendo/exo => 99 : 199% ee

CHO

> 99% yieldendo/exo = 32 : 6898% ee (endo),95% ee (exo)

OH

90% yieldendo/exo = 99 : 196% ee (endo)


73% yieldendo/exo = 82 : 1888% ee (exo)

CHO

NB

O

Ph

Ph

Ph

Cl4Sn

CO2EtH

CHO

Other examples (10–20 mol% of 9 was employed)

Scheme 1.14 Enantioselective Diels–Alder reactions catalyzed by LLA (9).

The plausible generation of chiral LLA (9) is considered to be analogous tothat of the CBS reduction system [18–20] and Corey’s chiral BLAs [3–9]. Thus,the coordination of the achiral Lewis acid to the nitrogen atom of the chiraloxazaborolidine should serve to increase the Lewis acidity of boron atom.

In 2007, Corey group has reinvestigated Lewis acid activation of the chiraloxazaborolidine (1a) instead of the use of the strong Brønsted acid triflimide,leading to the identification of AlBr3-(1a) complex (10) as an unusually powerfuland effective chiral Lewis acid catalyst for enantioselective Diels–Alder reactions(Scheme 1.15) [21].

Using the very strong Lewis acid AlBr3, the complete complexation to formchiral LLA (10) was observed with 1 equivalent each of (1a) and AlBr3 by 1H NMRanalysis. Excellent Diels–Alder conversions were generally observed with only4 mol% of (10) using even less reactive substrates and the absolute configurationsof the Diels–Alder adducts were consistent with the transition state model for the


NB

O

Ph

Ph

H

Br3Al

OOCH2CF3

O

OCH2CF3

O

O

H10 (4 mol%)

CH2Cl2, −78 °C, 8 h+

(5 equiv)

O


H

H


OH

O

99% yield99% ee

CHO


OCH2CF3

OH


OCH2CF3

O

O

H


OCH2CF3

OH

98% yield91% ee

+

−

Scheme 1.15 Enantioselective Diels–Alder reactions cat-alyzed by LLA (10) derived from AlBr3 and (1a).

triflimide-activated chiral BLA (3a). These data clearly indicate that LLA (10) isconsiderably more efficient than the corresponding BLA (3a) since 10–20 mol% of(3a) is generally required for the optimum results. The authors concluded that thegreater turnover efficiency of LLA (10) may be the result of greater steric screeningof the catalytic boron site by the adjacent AlBr3 subunit and diminished productinhibition.

In 2007, Hall and coworkers have disclosed a new family of simple and efficientdouble allylating reagents of type (11) as stable bimetallic reagents for stereoselectiveallylation reaction of aldehydes (Scheme 1.16) [22].

Chiral hydroxyl-functionalized allylic silanes were obtained by the reaction ofvarious aldehydes and chiral double allylating reagent (11) with high enantioselec-tivities (up to 98% ee) and excellent E/Z ratio ( > 25 : 1 to > 30 : 1) using 1 equivalentof BF3žOEt2 as a Lewis acid promoter. The resulting chiral allylic silanes can be


BO

O

R1CHOBF3 OEt2 (1 equiv)

CH2Cl2, −78 °C, 12 h

R1

OH

+

63 to 85% yieldE : Z = > 25 : 1 to > 30 : 179 to 98% ee

H

SiMe3

SiMe3

B O R1

H

H

OR

ROMe3Si

F3B

LLA-type transition state

Ph

OH

SiMe3

72% yieldE : Z = > 30 : 1, 95% ee

nC7H15

OH

SiMe3

70% yieldE : Z = > 30:1, 98% ee

OH

SiMe3BnO

70% yieldE : Z = > 25 : 1, 91% ee

11

Scheme 1.16 Asymmetric allylation of aldehydes with a new double-allylation reagent (11).

further transformed to the various compound classes such as propionate units,polysubstituted furans, vinylcyclopropanes, and larger carbocycles. The authorsproposed that the high degree of E/Z selectivity can be explained by an LLA-typetransition state involving coordination of the Lewis acid BF3žOEt2 to a boronateoxygen as described in Scheme 1.16.

A Ti-BINOL based LLA system was developed for asymmetric allylation byYamamoto group in 2004 (Scheme 1.17) [23]. Allylation reaction of benzaldehydewith allyltributyltin has been found to be dramatically accelerated by addition of5 mol% of 4-(trifluoromethyl)boroxine to Ti-(S)-BINOL catalyst.

It is important to note that both reactivity and enantioselectivity increasedwith addition of this boron Lewis acid, demonstrating the power of LLA system.This result can be attributed to the enhancement of Lewis acidity of titanium bycoordination of boron to the oxygen atom of BINOL as in the proposed structure ofthe active catalyst.

Maruoka’s chiral bis-titanium Lewis acid (12) [24] can be regarded as an excellentexample of chiral LLA. In this catalytic system, it is proposed that the Lewis acidityof one titanium center might be enhanced by the intramolecular coordination ofthe oxygen atom of isoproxy group to the other titanium (Scheme 1.18).

Following the first report of catalytic enantioselective allylation reactions with(12) by Maruoka group in 2003 [24a], the synthetic utility of this type of chiral LLAwas recently broadened.

In 2005, Maruoka and coworkers reported asymmetric 1,3-dipolar cycloadditionreaction of nitrones and acrolein catalyzed by (12) (Scheme 1.19) [24c].


O

OTi

O

(OiPr)m

BOiPr n

m = n = 1, or m = 0, n = 2

SnBu3

CF3

Ti-(S)-BINOL (10 mol%)

[4-(CF3)-C6H4BO]3 (5 mol%)

CH2Cl2, −20 °C, 4 h+ Ph

OH

92% yield, 93% ee

Plausible structure of the active catalyst

PhCHO

5% yield, 51% ee (without [4-(CF3)-C6H4BO]3)

Scheme 1.17 Enantioselective allylation of aldehydes cat-alyzed by Ti-BINOL-Boroxine combined system.

O

O

Ti

O

O

TiO

iPrO

OiPr

O

O

Ti

O

O

TiO

iPrO

OiPr

LLA form

12

−

+

Scheme 1.18 Plausible formation of LLA species from chiral bis-titanium Lewis acid (12).

The reactions of various aryl-substituted nitrones and sterically hindered alkyl-substituted nitrones with 10 mol% of (12) as a chiral Lewis acid provided thecorresponding chiral isoxazolidines with high to excellent enantioselectivities(70–97% ee).

Noteworthy is the fact that the use of Ti(IV)-(S)-BINOL complexes resulted in lowyields with moderate enantioselectivities, implicating the importance of enhancedLewis acidity in chiral bis-titanium catalyst (12).

Subsequently, Maruoka group developed an enantioselective 1,3-dipolar cycload-dition reaction between diazoacetates and α-substituted acroleins (Scheme 1.20)[24d]. The reactions of 1.5 equivalent of ethyl or tert-butyl diazoacetate andvarious α-substituted acrolein derivatives afford chiral 2-pyrazolines with high


12 (10 mol%)

CH2Cl2−40 °C, 14–40 h

+

(1.5 equiv)

86% yield97% ee

90% yield97% ee

94% yield93% ee

CHONO

Bn

RO

NR

HO

BnNaBH4

EtOH

62–94% yield70–97% ee

ONPh

HO

Bn

ON

HO

Bn

ON

HO

BnSS

+−

Scheme 1.19 Enantioselective 1,3-dipolar cycloaddition re-actions of nitrones catalyzed by chiral bis-titanium catalyst(12).

12 (5 mol%)

CH2Cl2−40 °C, 3 h

+

(1.5 equiv)

75% yield94% ee

48% yield84% ee

CHOMe N N

EtO2C CHO

Me

N N

ButO2C CHO

Et N N

ButO2C CHO**

52% yield95% ee

N NH

O

O

NH

Br

HO2CMe

Manzacidin A

N2CHCO2Et

H

HH

Scheme 1.20 Enantioselective 1,3-dipolar cycloaddition reac-tions of diazoacetates catalyzed by (12).

enantioselectivities with 5 mol% of (12) as a chiral Lewis acid (84–94% ee).Interestingly, possible side reactions such as 1,2- and 1,4-addition of diazoacetatesto α-substituted acroleins or cyclopropanation were somewhat suppressed underthese reaction conditions.


The synthetic utility of this methodology has been demonstrated by the shortenantioselective total synthesis of manzacidin A in five steps, starting from com-mercially available methacrolein and ethyl diazoacetate.

Related to the concept of Lewis acid-assisted chiral Lewis acid, Shibasaki andcoworkers demonstrated the concept of ‘Lewis acid-Lewis acid cooperative catal-ysis’ through the development of highly enantioselective aza-Michael reactionsof methoxyamine catalyzed by (S,S,S)-YLi3tris(binaphthoxide) (YLB) complex (13)(Scheme 1.21) [25a,b].

13 (1 or 3 mol%)Drierite

THF−20 °C, 42–122 h

+

(1.2 equiv)

O

OO

O

O

O

Li

Li

Li

Y

R1 R2

O

MeONH2 R1 R2

O HNOMe

57–98% yield81–96% ee

Ph Ph

O HNOMe

Ph

O HNOMe

O

Ph

O HNOMe

S

Ph

O HNOMe

N

Ph

O HNOMe

Ph

O HNOMe

Ph

95% yield96% ee

95% yield94% ee

96% yield95% ee

91% yield85% ee

95% yield93% ee

91% yield95% ee

Scheme 1.21 Enantioselective aza-Michael reactions catalyzed by YLB (13).

The working hypothesis of ‘Lewis acid-Lewis acid cooperative catalysis’ is pro-posed by the authors as described in Figure 1.2.


O

OO

O

O

O

Li

Li

Li

Y

R1

R2

O*

*

*

O

O

*

=

O

OO

H2N

Lewis acid 1

Lewis acid 2

Me

Fig. 1.2 Proposed working model of Lewis acid-Lewis acidcooperative catalysis using YLB (13).

The authors hypothesized that two Lewis acidic centers in YLB complex (Y andLi) work cooperatively to control the orientations of both the electrophile and thenucleophile, leading to the highly organized transition state to give rise to highenantioselectivity (Figure 1.2). This hypothesis has been proved by a series ofdetailed mechanistic studies and the effectiveness of this concept is clear from theexcellent results in Scheme 1.21.

In a subsequent study, the synthetic utility of this methodology has been broad-ened by the use of α, β-unsaturated N-acylpyrroles as substrates (Scheme 1.22) [25b].

The resulting 1,4-adducts have been successfully transformed into importantchiral building blocks, including chiral β-amino acid derivatives.

13 (10 mol%)Drierite

THF−30 °C, 38–106 h

+

(1.03 equiv)

N R

O

MeONH2 N R

O HNOMe

84–96% yield83–94% ee

N

O HNOMe

N

O HNOMe

N

O HNOMe

96% yield94% ee

94% yield86% ee

91% yield83% ee

MeO R

O HNBoc

Scheme 1.22 Enantioselective aza-Michael reactions of N-acylpyrroles catalyzed by YLB (13).


1.1.3Lewis Acid-Assisted Chiral Brønsted Acid (LBA)

Chiral pyrogallol (14) derived LBA (15) was developed for enantioselective polyenecyclization reaction as an ‘artificial cyclase’ by Yamamoto, Ishihara and coworkersin 2004 (Scheme 1.23) [26]. Various tricyclic skeletons have been synthesized withgood enantioselectivities (79–85% ee) in the presence of chiral LBA (15).

OO

Ar ArOH SnCl4

14 (Ar = m-xylyl)

H

Toluene, −78 °C, 24 h

90% yield85% ee, 88% trans

TFA, SnCl4 iPrNO2, −78 °C, 24 h

O

H

Br

O

HBr


O

H

O

O O

HO

O


SnCl4 (1 equiv)14 (2 equiv)

Toluene−78 °C, 2.5 d


Toluene−78 °C, 2.5 d


OO

Ar ArOH

SnCl4

Chiral LBA 15

Scheme 1.23 SnCl4-chiral pyrogallol complex (15) as a newLBA for enantioselective polyene cyclization.

Chiral catechol-derived LBA (16) was devised as a new artificial cyclase forpolyprenoids by the same group in 2004 (Scheme 1.24) [27]. The syntheticpower of the new chiral LBA (16) has been well demonstrated by enantios-elective cyclizations of various 2-(polyprenyl)phenol derivatives with good toexcellent enantioselectivities (88–90% ee), leading to the efficient asymmetric syn-theses of (–)-chromazonarol, (C)-8-epi-puupehedione, and (–)-110-deoxytaondiolmethyl ether.


O

OMe

H


TFA, SnCl4 iPrNO2, −78 °C, 24 h

60% yield90% ee, 88% dr

OH

O

H

OO

O

OMeH

H

O

OMe

H

O

H

OO

OMe

O

O FSnCl4

H

(R)-16

OO

62% yield89% ee, 73% dr

40% yield88% ee, 69% dr

((S)-16 was used)

(R)-16


22% yield90% ee, 48% dr

Scheme 1.24 SnCl4-chiral catechol complex (16) as a newLBA for enantioselective synthesis of polyprenoids.

Subsequent study revealed that the chiral LBA (16) can be used as artificial cyclasesfor the asymmetric syntheses of (–)-caparrapi oxide (17) and (C)-8-epicaparrapioxide (18) [28a,b]. (17) and (18) can be diastereoselectively synthesized from (S)-(19)(prepared in three steps from commercially available farnesol) by reagent controlof (R)-16 and (S)-16, respectively, regardless of the chirality of (S)-19 (Scheme 1.25).

In addition, (–)-(17) can be obtained directly from (S)-20 by reagent controlof (R)-LBA 16 overcoming the substrate control, while (–)-18 can be synthesizeddirectly from (R)-20 by the double asymmetric induction (both the reagent control


O

HO

(R)-

16 (

2 eq

uiv)

PrC

l-CH

2Cl 2

= 1

: 1

−80

°C, 4

8 h

74%

yie

ld(S

)-19

(90

% e

e)O

H

BnO

2C

H

BnO

2C

BnO

2C

> 9

9% e

e

21%

ee

81 19to

3 st

eps

O

H (−)-

17

O

H

O(S

)-16

(2

equi

v)

PrC

l-CH

2Cl 2

= 1

: 1

−80

°C, 4

8 h

73%

yie

ld(S

)-19

(90

% e

e)O

H

BnO

2C

H

BnO

2C

BnO

2C

27%

ee

98%

ee

8614 to

O

H (+)-

18

3 st

eps

Sche

me

1.25

Rea

gent

-con

trol

led

dias

tere

osel

ectiv

epo

lyen

ecy

cliz

atio

nof

(S)-

19pr

omot

edby

(R)

or(S

)-16

.


by (R)-LBA (16) and the substrate control) in the diastereoselective polyene cycliza-tion of racemic (20) (Scheme 1.26) [28a,b]. Although the total sequence for theasymmetric synthesis of (–)-(17) is the shortest, the reaction gave only 12% yield ofinseparable mixture of four products.

O

H

O

H

O

H

O

H

(R )-16 (1 equiv)

Toluene−78 °C, 24 h

12% yield

(S)-20

(R)-20

+

O

H

O

H

88 (37)a 12 (63)a

> 99 (63)a < 1 (37)a

:

:

ratio

ratio

a 2-methoxyphenol–SnCl4 complex was used as an achiral LBA.

(−)-17

(−)-18 (+)-18

(+)-17

Scheme 1.26 Diastereoselective polyene cyclization ofracemic nerolidol (20) promoted by (R)-(16).

A highly reactive chiral LBA (21) was prepared from TiCl4 and (R)-2-phenyl-2-methoxyethanol for enantioselective protonation of silyl enol ether by Yamamotogroup in 2006 (Scheme 1.27) [29]. It is interesting to note that the enantioselectivityof the product strongly depends on the preparation conditions of chiral LBA.The authors found that relatively high reaction temperature (0 ŽC, 30 minutes) isrequired to get the optimal enantioselectivity owing to the insufficient generationof the chiral LBA complex (21) caused by the strong aggregation of TiCl4.

O

H

TiCl4O

Me

Ph

Ph

OSiMe3

21 (1.1 equiv)

CH2Cl2−78 °C, 1 h

Ph

OH

> 99% yield68% ee

Scheme 1.27 TiCl4-(R)-2-phenyl-2-methoxyethanol complex(21) as a new LBA for enantioselective protonation.


1.1.4Brønsted Acid-Assisted Chiral Brønsted Acid (BBA)

Since the first discovery of the enantioselective catalysis through hydrogen bondactivation by Rawal group in 2003 [30a], TADDOL (Figure 1.3) has represented a‘privileged’ chiral scaffold for this type of asymmetric catalysis [31]. In addition,TADDOL can be regarded as Brønsted acid-assisted chiral Brønsted acid (BBA) byvirtue of its intramolecular hydrogen bonding ability as shown in Figure 1.3.

As further applications of their concept of ‘hydrogen bond catalysis’, Rawal andcoworkers demonstrated the use of hydrogen bonding catalysis in enantioselectivevinylogous Mukaiyama aldol reactions in 2005 (Scheme 1.28) [30c]. The reaction ofthe silyldienol ether (23) and a range of reactive aldehydes proceeds regiospecifically

O

O

O

OR

R

Ar Ar

Ar Ar22a (R = Me, Ar = 1-Naphthyl)22b (R = –(C5H10)–, Ar = 1-Naphthyl)

H

H

TADDOLs

Fig. 1.3 Representative structures of TADDOL.

22a (20 mol%), Toluene −80 °C, 98 h

1N HCl

73% yield90% ee

O

TBSO

O

OH

+

N

S

O

O

O O

O O

O O

N

SH

22a (20 mol%) iPr2NEt (10 mol%)


1N HCl

60% yield87% ee

TBSO

O

OH

+O

H

O

OEt

O

OEt

23

23

Scheme 1.28 Enantioselective vinylogous Mukaiyama aldolreactions catalyzed by TADDOL (22a).


using 20 mol% of TADDOL (22a), affording the addition products in good yieldwith moderate to high enantiomeric excess (up to 90% ee).

Subsequently, Rawal group disclosed the hydrogen bond catalysis of highlydiastereo- and enantioselective Mukaiyama aldol reactions of O-silyl-N,O-keteneacetals by the use of TADDOL (22b) as a catalyst (Scheme 1.29) [30d]. The reactionis effective for a range of aldehydes and affords the corresponding adducts insynthetically useful yields with excellent enantioselectivities (up to 98% ee).

In this study, the authors have succeeded in obtaining the first crystal structureof a complex between a racemic TADDOL (22a) and p-anisaldehyde. The X-raystructure analysis clearly indicated the formation of BBA through intramolecularhydrogen bonding between the two hydroxyl groups and a single point activationof the carbonyl group through intermolecular hydrogen bonding, thus confirmingthe proposed mode of activation of the carbonyl group by TADDOL (Figure 1.4).

22b (10 mol%), Toluene −78 °C, 48 h

HF/CH3CN

47 to 94% yieldsyn/anti 2 : 1 to > 25 : 187 to 98% ee (syn)

+

O

RH

OTBS

N

(2 equiv)

N

O OH

R

N

O OH

Ph

94% yield, syn/anti 15 : 198% ee (syn), 84% ee (anti)

N

O OH

47% yield, syn/anti 9 : 191% ee (syn)a

N

O OH

S

88% yield, syn/anti 10 : 195% ee (syn)a, 48% ee (anti)

N

O OH OMe

N

O OH

Cl86% yield, syn/anti 20 : 197% ee (syn)a

50% yield, syn/anti 8 : 191% ee (syn)a, 58% ee (anti)

a The absolute configuration of the product was assigned by analogy.

Scheme 1.29 Enantioselective Mukaiyama aldol reactions catalyzed by TADDOL (22b).

O H OO

OH

HO R

H

Fig. 1.4 Proposed mode of activation of the carbonyl group by TADDOL.


O

O

ArAr

ArAr

HH

R

R

F

Ar= (R = Me, 24a)(R = Et, 24b)

Chiral BAMOLs

TBSO

N

H R

O O

O R

24a or 24b (20 mol%)Toluene

−40 or −80 °C, 1 or 2 d

AcCl, CH2Cl2–Toluene−78 ˚C, 30 min

+

(2 equiv)42 to 99% yield84 to > 99 % ee

O

O

75% yield97% ee

∗

O

O

O

ON

O

O

O

O Ph

O

O

O

O

O SPh

76% yield94% ee

67% yield92% ee

42% yield99% ee

84% yield98% ee

96% yield> 99% ee

25

∗ ∗

∗

Scheme 1.30 Enantioselective hetero Diels–Alder reactions catalyzed by chiral BAMOLs.

Again, the intramolecular hydrogen bonding in TADDOL plays an important rolenot only to increase Brønsted acidity of the free alcoholic proton for the effectiveactivation of the carbonyl group, but also to keep well-organized chiral scaffold.

In 2005, Yamamoto, Rawal and coworkers reported that axially chiral 1,10-biaryl-2,20-dimethanol (BAMOL) family of diols (24a) and (24b) is a highly effective catalystfor enantioselective hetero Diels–Alder reactions between aminosiloxydiene (25)and a wide variety of unactivated aldehydes (Scheme 1.30) [32]. The reactionsproceed in useful yields and excellent enantioselectivities (84 to > 99% ee). Thediols function in the same capacity as Lewis acids, by activating the aldehydecarbonyl group through hydrogen bonding. A similar mode of activation of thecarbonyl group with TADDOL was indicated by the X-ray structure analysis of1 : 1 complex of the related BAMOL derivative and benzaldehyde, suggesting theformation of BBA through intramolecular hydrogen bonding between the twohydroxyl groups and a single point activation of the carbonyl group throughintermolecular hydrogen bonding.

In 2005, Yamamoto group developed Brønsted acid catalysis of achiral enam-ines for regio- and enantioselective nitroso aldol synthesis (Scheme 1.31) [33].The exclusive formation of a single regioisomer (N-adduct or O-adduct) has been


22a (30 mol%)+

N

O

ONH

Ph

O

NPhO

NPh Toluene

−78 to −88 °C, 2 h

O

O

O

O

Ar Ar

Ar Ar

(Ar = 1-Naphthyl)

H

HOH

26 (30 mol%)+

O O O O

O OO O

N

O

NPh Et2O

−78 to −88 °C, 12 h

O

O

O

83% yield93% ee

63% yield91% ee

H

H

Scheme 1.31 Regioselective synthesis of N- or O-adduct inenantioselective ‘Nitroso aldol’ reactions.

achieved simply by the choice of chiral Brønsted acid catalysts with high enantiose-lectivities. Although the rationale for the regioselectivity is still unclear, the possiblegeneration of BBAs through intramolecular hydrogen bonding was proposed toexplain high enantioselectivities in both processes by the authors.

Yamamoto group subsequently reported diastereo-and enantioselective synthesisof 2-oxo-3-azabicycloketones catalyzed by chiral Brønsted acid (27) (Scheme 1.32)[34]. The reaction of dienamine (28) and nitrosobenzene proceeded in high yield

27 (30 mol%)

+

N

O

O

NPh

(Ar = m-xylyl)

92% yield90% ee

O

OH

H

SiAr3

SiAr3

pentane–CH2Cl2 = 9 : 1 −78 °C, 12 h

1N HCl/THF, −78 °C ON

O Ph

28 (1.5 equiv)

Scheme 1.32 Enantioselective bicycloketone synthesis withachiral dienamines catalyzed by (27).


29 (

10 m

ol%

)

+P

Et 3

(2

equi

v)T

HF

, −10

°C

, 48

h

86%

yie

ld99

% d

e(2

equ

iv)

OOH

H

OO

OH

SiM

e 2P

hO

SiM

e 2P

hH

OO

HH H

81%

yie

ld98

% d

eBF

3.O

Et 2

CH

2Cl 2

−78

to −

10 °

C

H NN N

N

N

HO

Asm

arin

e A

Sche

me

1.33

Asy

mm

etri

cM

orita

–B

aylis

–H

illm

anre

actio

n/Le

wis

acid

prom

oted

annu

latio

nre

actio

nfo

rth

esy

nthe

sis

ofC

lero

dane

deca

linco

re.

1.2 Super Brønsted Acid Catalyst 29

with excellent enantioselectivity in the presence of 30 mol% of chiral BINOLderivative (27). The authors proposed that the BBA form of (27) is responsible forthe active catalyst in this system and the reaction proceeds in a stepwise fashioninitiated with N-nitroso aldol reaction, followed by Michael addition of the resultinganionic oxygen atom.

Schaus and coworkers have developed a general route to the Clerodane diter-pene core by the use of previously developed Brønsted acid catalyzed asymmetricMorita–Baylis–Hillman (MBH) reaction/Lewis acid mediated ring-annulation pro-cess (Scheme 1.33) [31]. Excellent diastereoselectivity was achieved in the key MBHreaction in the presence of 10 mol% of the chiral BINOL derivative (29), affordingthe key intermediate for the synthesis of Clerodane decalin core.

A similar mode of activation of the carbonyl group through BBA system may beexpected for this reaction since two hydroxyl groups are crucial to obtain high yieldand enantioselectivity.

A new type of Brønsted acid-assisted chiral Brønsted acid (chiral BBA) cata-lyst (30) possessing a bis(triflyl)methyl group was developed for enantioselectiveMannich-type reaction by Yamamoto, Ishihara and coworkers [35]. The authorsproposed the two possible chiral BBA form generated through intramolecularhydrogen bonding as shown in Scheme 1.34.

OH

H

TfTf

O

H

H

TfTf

O

S

TfH

CF3O

O

H

BBA form BBA form30

Scheme 1.34 Possible formations of chiral BBAs from (30)through intramolecular hydrogen bonding.

Chiral BBA (30) has been shown to be effective as a Brønsted acid catalyst forMannich-type reaction of ketene silyl acetals and aldimines and good enantios-electivity was observed (up to 77% ee) using 3.5 or 10 mol% of chiral BBA (30)(Scheme 1.35).

It is important to note that a stoichiometric use of the achiral proton sourceis crucial to obtain high enantioselectivity for this catalytic system to scavenge acationic silicon species. The authors proposed two plausible transition states foreach BBA form (Scheme 1.35). As in the case of TADDOL, BBA formation shouldlead to high enantioselectivity by virtue of their well-organized chiral cavities.

1.2Super Brønsted Acid Catalyst

Acidity of organic compounds is affected by structural features. For example,March’s Advanced Organic Chemistry (fifth edition) summarizes major effects


30 (

10 m

ol%

)

tBuO

H(1

equ

iv)

PrC

l, −7

8 °C

, 24

h

85%

yie

ld76

% e

e

(1.5

equ

iv)

N

H

OS

iMe 3

OM

e

CF

3

O

OM

e

NH

F

FF

FF

O H

TfT

f

HNR

H

OHNR

H

Tf

Tf

H

Tra

nsiti

on s

tate

IT

rans

ition

sta

te II

30 (

3.5

mol

%)

2,6-

Dim

ethy

lphe

nol

(1 e

quiv

)P

rCl,

−78

°C, 4

8 h

> 9

9% y

ield

77%

ee

(1.5

equ

iv)

N

H

OS

iEt 3

OM

e

Ph

CF

3

O

OM

e

NH

Ph

F3C

OHH

Tf

Tf

+ +

Sche

me

1.35

Enan

tiose

lect

ive

Man

nich

-typ

ere

actio

nsca

taly

zed

bych

iral

BB

A(3

0).

1.4 Conclusion 31

S

SF

F

F

F

F

CF3

OO

O

O CF3

H

31

Fig. 1.5 Pentafluorophenyl bis(triflyl)methane as a super Brønsted acid.

that influence the acid strength as follows [36]: (i) field effect, (ii) resonance effect,(iii) periodic table correlations, (iv) statistical effect, (v) hydrogen bonding, and (vi)steric effect.

Basically, the acidity of the compound AH increases by stronger stabilizinginteraction with A– and for this reason, TfOH, Tf2NH, and Tf3CH are a niceset of acid catalysts. Thus, we can expect better reactivity by choosing moreelectron-withdrawing and/or nice resonance stabilizing A– for acid catalysis. Forexample, our previously developed pentafluorophenyl bis(triflyl)methane (Figure1.5) was shown to be an excellent super Brønsted acid catalysis for a number oforganic transformations [37]. In this example, the high reactivity of the catalyst maycome not only from the inductive effects of two Tf and a C6F5 group but also froma relatively large resonance effect.

1.3Combined Super Brønsted Acid and Lewis Acid System

Combination of super Brønsted acid with simple Brønsted acid or Lewis acid wouldbe a new system for acid catalysis in organic synthesis. The example of superBLA would be an interesting tool for selective organic transformations. We havealready shown excellent examples in Scheme 1.3 using the Tf2NH–chiral Lewisacid combined system. Scheme 1.36 exemplifies some additional recent reactionsbased on pentafluorophenyl bis(triflyl)methane and chiral Lewis acid catalyst [38].

1.4Conclusion

In recent years, the power of designer acid catalysis has greatly increased as a resultof the development of catalytic enantioselective versions, as described herein. Yet itis likely that there is much more to learn with regard to new reactivity. The conceptof ‘combined acid catalysis’ is still very much in a state of infancy and detailedmechanistic study of the true nature of this type of catalysis is required.

Since the introduction of our review article of combined acid catalysis in 2005[2], this concept has clearly continued to interpenetrate the area of asymmetriccatalysis and rapidly evolving, as is apparent from the summary described herein.


32 (

5 m

ol%

)

CH

2Cl 2

, −78

°C

, 6 h

+

(3 e

quiv

)

CO

2Et

NB

O

Ph

Ph P

h

H

Tf

Tf

F

F

F

FF

CO

2Et

RR

(Mix

ture

of t

hree

regi

oiso

mer

s)R

= M

e: 9

6% y

ield

, 99%

ee

R =

Ally

l: 98

% y

ield

, 97%

ee

endo

: ex

o >

99

:1

32 (

5 m

ol%

)

CH

2Cl 2

, −78

°C

, 6 h

+

(2.5

equ

iv)

CO

2Et

89%

yie

ld >

99%

de

> 9

9% e

e

+

CO

2Et

O O

−78

°C, 3

h

O

OH

(2.0

equ

iv)

Sche

me

1.36

Supe

rB

LAfo

rse

lect

ive

Die

ls–

Ald

erre

actio

n.

References 33

The ultimate goal is to apparently develop a more reactive, more selective, andmore versatile catalyst. We believe that the realization of such an objective wouldbe a tremendous benefit for the development of organic synthesis including greenchemistry.

References

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