Boron Tribromide1

[10294-33-4] · BBr3 · Boron Tribromide · (MW 250.52)

(Lewis acid used for deprotection of OH and NH groups; cleaves ethers or esters to alkyl bromides; bromoborates allene and alkynes)

Physical Data: mp -45 °C; bp 91.7 °C; d 2.650 g cm-3.

Form Supplied in: colorless, fuming liquid; a 1.0 M solution in dichloromethane and hexane; BBr3.Me2S complex is available as either a white solid or a 1.0 M solution in dichloromethane.

Purification: by distillation.

Handling, Storage, and Precautions: BBr3 is highly moisture sensitive and decomposes in air with evolution of HBr. Store under a dry inert atmosphere and transfer by syringe or through a Teflon tube. It reacts violently with protic solvents such as water and alcohols. Ether and THF are not appropriate solvents.

Removal of Protecting Groups. BBr3 is highly Lewis acidic. It coordinates to ethereal oxygens and promotes C-O bond cleavage to an alkyl bromide and an alkoxyborane that is hydrolyzed to an alcohol during workup (eq 1).2

BBr3 has been widely used to cleave ethers because the reaction proceeds completely under mild conditions. In a special case, BBr3 has been used to cleave acetals that cannot be deprotected by usual acidic conditions.3 Because alkyl aryl ethers are cleaved at the alkyl-oxygen bond to give ArOH and alkyl bromides, BBr3 has been most generally used for the demethylation of methyl aryl ethers,2,4 for example as the final step of zearalenone synthesis (eq 2).5 Problems are sometimes encountered in attempts to deprotect more than one nonadjacent methoxy group on one aromatic ring, and when stable chelates are formed.6 The presence of a carbonyl substituent facilitates the selective deprotection of polymethoxyaryl compounds (eq 3).7

BBr3

The cleavage of mixed dialkyl ethers occurs at the more substituted carbon-oxygen bond. Methyl ethers of secondary or tertiary alcohols give methanol and secondary or tertiary alkyl bromides selectively by the reaction with BBr3,8 although the addition of Sodium Iodide and 15-Crown-5 ether can change this selectivity (eq 4).9 In contrast, methyl ethers of primary alcohols are generally cleaved at the Me-O bond, as demonstrated in Corey's prostaglandin synthesis (eq 5).10

BBr3 has been also used for the deprotection of carbohydrate derivatives11 and polyoxygenated intermediates in the synthesis of deoxyvernolepin,12 vernolepin,13 and vernomenin.13 Although one of the model compounds is deprotected cleanly (eq 6),14 application of BBr3 to more highly functionalized intermediates leads to cleavage of undesired C-O bonds competitively (eq 7).12,13

For the complete cleavage, 1 mol of BBr3 is required for each ether group and other Lewis-basic functional groups. Sometimes it is difficult to find reaction conditions for the selective cleavage of the desired C-O bond. Recently, modified bromoboranes such as B-Bromocatecholborane,15 dialkylbromoboranes,16 Bromobis(isopropylthio)borane,17 and 9-Bromo-9-borabicyclo[3.3.1]nonane,18 have been introduced to cleave C-O bonds more selectively under milder conditions. BBr3.SMe2 is also effective for ether cleavage and has the advantage of being more stable than BBr3. It can be stored for a long time and handled easily. However, a two- to fourfold excess of the reagent is necessary to complete the dealkylation of alkyl aryl ether.19

Amino acid protecting groups such as benzyloxycarbonyl and t-butoxycarbonyl groups are cleaved by BBr3. However, the hydrolysis of the ester function also occurs under the same reaction conditions.20 Debenzylation and debenzyloxymethylation of uracils proceed successfully in aromatic solvents, but demethylation is more sluggish and less facile (eq 8).21

Substitution Reactions.

BBr3 reacts with cyclic ethers to give tris(ο-bromoalkoxy)boranes which provide ο-bromoalkanols or ο-bromoalkanals when treated with MeOH or Pyridinium Chlorochromate, respectively (eq 9).22 Unfortunately, unsymmetrically substituted ethers such as 2-methyltetrahydrofuran are cleaved nonregioselectively. Generally, ester groups survive under the reaction conditions for ether cleavage, but the ring opening of lactones occurs under mild conditions to give ο-halocarboxylic acids in good yields (eq 10).23

In the reaction with methoxybenzaldehyde, bromination of the carbonyl group takes place more rapidly than demethylation; therefore benzal bromide formation is generally observed in the reaction with aromatic aldehydes.24 Cleavage of t-butyldimethylsilyl ethers or t-butyldiphenylsilyl ethers occurs at the C-O bond to give alkyl bromides.25 Alcohols can be converted to alkyl bromides by this method.

In a special case, BBr3 is used for the bromination of hydrocarbons. Adamantane is brominated by a mixture of Bromine, BBr3, and Aluminum Bromide to give 1,3-dibromoadamantane selectively.26 Tetrachlorocyclopropene27 and hexachlorocyclopentadiene28 are substituted to the corresponding bromides by BBr3 and, in the latter case, addition of AlBr3 and Br2 is effective to improve the result.29

Reduction of Sulfur Compounds.

Alkyl and aryl sulfoxides are reduced by BBr3 to the corresponding sulfides in good yields.30 Addition of Potassium Iodide and a catalytic amount of Tetra-n-butylammonium Iodide is necessary for the reduction of sulfonic acids and their derivatives.31

Transesterification of Esters or Conversion to Amides.

Transesterification reactions of carboxylic esters or conversion into the amides is promoted by a stoichiometric amount of BBr3.32

Removal of Methyl Sulfide from Organoborane-Methyl Sulfide Complexes. Methyl sulfide can be removed from BrBR2.SMe2 or Br2BR.SMe2, which are prepared by the hydroboration reaction of alkenes or alkynes with BrBH2.SMe2 or Br2BH.SMe2, by using BBr3.33 The resulting alkenyldibromoboranes are useful for the stereoselective synthesis of

bromodienes (eq 11).34

Bromoboration Reactions. BBr3 does not add to isolated double bonds, but reacts with allene spontaneously even at low temperature to give (2-bromoallyl)dibromoborane,35 which provides stable (2-bromoallyl)diphenoxyborane by the addition of anisole.36 The diphenoxyborane derivative reacts with carbonyl compounds to give 2-bromohomoallylic alcohols in high yields (eq 12). Bromoboration of 1-alkynes provides (Z)-(2-bromo-1-alkenyl)dibromoboranes stereo- and regioselectively (eq 13),37 which are applied for the synthesis of trisubstituted alkenes,38 α,β-unsaturated esters,39 and γ,δ-unsaturated ketones,40 bromodienes,41 1,2-dihalo-1-alkenes,42 2-bromoalkanals,43 and β-bromo-α,β-unsaturated amides.44

Chiral Bromoborane Reagents. Complexes made from chiral 1-alkyl-2-(diphenylhydroxymethyl)pyrrolidines and BBr3 are effective catalysts for asymmetric Diels-Alder

reactions.45 Bromoboranes prepared from chiral 1,2-diphenyl-1,2-bis(arenesulfonamido)ethanes46,47 are used to prepare chiral allylic boranes,47,48 allenylic borane,49 propargylic boranes,49 and enolates.46,47,50 The B-bromodiazaborolidinene (1), prepared from 1,2-diphenyl-1,2-bis(p-toluenesulfonamido)ethane, is particularly effective in these applications. The reagents prepared from (1) are highly effective for the enantioselective synthesis of homoallylic alcohols (eq 14),48 homopropargylic alcohols (eq 15),49 propadienyl carbinols (eq 16),49 and aldol condensation products (eq 17).46

1. Bhatt, M. V.; Kulkarni, S. U. S 1983, 249. 2. McOmie, J. F. W.; Watts, M. L.; West, D. E. T 1968, 24, 2289. 3. Meyers, A. I.; Nolen, R. L.; Collington, E. W.; Narwid, T. A.; Strickland, R. C. JOC 1973, 38, 1974. 4. (a) Benton, F. L.; Dillon, T. E. JACS 1942, 64, 1128. (b) Manson, D. L.; Musgrave, O. C. JCS 1963, 1011. (c) McOmie, J. F. W.; Watts, M. L. CI(L) 1963, 1658. (d) Blatchly, J. M.; Gardner, D. V.; McOmie, J. F. W.; Watts, M. L. JCS(C) 1968, 1545. 5. (a) Vlattas, I.; Harrison, I. T.; Tökés, L.; Fried, J. H.; Cross, A. D. JOC 1968, 33, 4176. (b) Taub, D.; Girotra, N. N.; Hoffsommer, R. D.; Kuo, C. H.; Slates, H. L.; Weber, S.; Wendler, N. L. T 1968, 24, 2443. 6. (a) Stetter, H.; Wulff, C. CB 1960, 93, 1366. (b) Locksley, H. D.; Murray, I. G. JCS(C) 1970, 392. (c) Bachelor, F. W.; Loman, A. A.;

Snowdon, L. R. CJC 1970, 48, 1554. 7. Schäfer, W.; Franck, B. CB 1966, 99, 160. 8. Youssefyeh, R. D.; Mazur, Y. CI(L) 1963, 609. 9. Niwa, H.; Hida, T.; Yamada, K. TL 1981, 22, 4239. 10. Corey, E. J.; Weinshenker, N. M.; Schaaf, T. K.; Huber, W. JACS 1969, 91, 5675. 11. Bonner, T. G.; Bourne, E. J.; McNally, S. JCS 1960, 2929. 12. Grieco, P. A.; Noguez, J. A.; Masaki, Y. JOC 1977, 42, 495. 13. Grieco, P. A.; Nishizawa, M.; Burke, S. D.; Marinovic, N. JACS 1976, 98, 1612. 14. (a) Grieco, P. A.; Hiroi, K.; Reap, J. J.; Noguez, J. A. JOC 1975, 40, 1450. (b) Grieco, P. A.; Reap, J. J.; Noguez, J. A. SC 1975, 5, 155. 15. (a) Boeckman, Jr., R. K.; Potenza, J. C. TL 1985, 26, 1411. (b) King, P. F.; Stroud, S. G. TL 1985, 26, 1415. 16. (a) Guindon, Y.; Morton, H. E.; Yoakim, C. TL 1983, 24, 3969. (b) Gauthier, J. Y.; Guindon, Y. TL 1987, 28, 5985. (c) Guindon, Y.; Yoakim, C.; Morton, H. E. TL 1983, 24, 2969. (d) Guindon, Y.; Yoakim, C.; Morton, H. E. JOC 1984, 49, 3912. 17. Corey, E. J.; Hua, D. H.; Seitz, S. P. TL 1984, 25, 3. 18. Bhatt, M. V. JOM 1978, 156, 221. 19. Williard, P. G.; Fryhle, C. B. TL 1980, 21, 3731. 20. Felix, A. M. JOC 1974, 39, 1427. 21. Kundu, N. G.; Hertzberg, R. P.; Hannon, S. J. TL 1980, 21, 1109. 22. Kulkarni, S. U.; Patil, V. D. H 1982, 18, 163. 23. Olah, G. A.; Karpeles, R.; Narang, S. C. S 1982, 963. 24. Lansinger, J. M.; Ronald, R. C. SC 1979, 9, 341. 25. Kim, S.; Park, J. H. JOC 1988, 53, 3111. 26. (a) Baughman, G. L. JOC 1964, 29, 238. (b) Talaty, E. R.; Cancienne, A. E.; Dupuy, A. E. JCS(C) 1968, 1902. 27. Tobey, S. W.; West, R. JACS 1966, 88, 2481. 28. West, R.; Kwitowski, P. T. JACS 1968, 90, 4697. 29. Ungefug, G. A.; Roberts, C. W. JOC 1973, 38, 153. 30. Guindon, Y.; Atkinson, J. G.; Morton, H. E. JOC 1984, 49, 4538. 31. Olah, G. A.; Narang, S. C.; Field, L. D.; Karpeles, R. JOC 1981, 46, 2408. 32. Yazawa, H.; Tanaka, K.; Kariyone, K. TL 1974, 15, 3995. 33. (a) Brown, H. C.; Ravindran, N.; Kulkarni, S. U. JOC 1979, 44, 2417. (b) Brown, H. C.; Ravindran, N.; Kulkarni, S. U. JOC 1980, 45, 384. (c) Brown, H. C.; Campbell, Jr., J. B. JOC 1980, 45, 389. 34. Hyuga, S.; Takinami, S.; Hara, S.; Suzuki, A. TL 1986, 27, 977. 35. Joy, F.; Lappert, M. F.; Prokai, B. JOM 1966, 5, 506. 36. Hara, S.; Suzuki, A. TL 1991, 32, 6749. 37. (a) Lappert, M. F.; Prokai, B. JOM 1964, 1, 384. (b) Blackborow, J. R. JOM 1977, 128, 161. (c) Suzuki, A.; Hara, S. Res. Trends Org. Chem. 1990, 77. (d) Suzuki, A. PAC 1986, 58, 629. 38. Satoh, Y.; Serizawa, H.; Miyaura, N.; Hara, S.; Suzuki, A. TL 1988, 29, 1811. 39. Yamashina, N.; Hyuga, S.; Hara, S.; Suzuki, A. TL 1989, 30, 6555. 40. (a) Hara, S.; Hyuga, S.; Aoyama, M.; Sato, M.; Suzuki, A. TL 1990, 31, 247. (b) Aoyama, M.; Hara, S.; Suzuki, A. SC 1992, 22, 2563. 41. Hyuga, S.; Takinami, S.; Hara, S.; Suzuki, A. CL 1986, 459. 42. Hara, S.; Kato, T.; Shimizu, H.; Suzuki, A. TL 1985, 26, 1065. 43. Satoh, Y.; Tayano, T.; Koshino, H.; Hara, S.; Suzuki, A. S 1985, 406. 44. Satoh, Y.; Serizawa, H.; Hara, S.; Suzuki, A. SC 1984, 14, 313. 45. Kobayashi, S.; Murakami, M.; Harada, T.; Mukaiyama, T. CL 1991, 1341. 46. Corey, E. J.; Imwinkelried, R.; Pikul, S.; Xiang, Y. B. JACS 1989, 111, 5493. 47. Corey, E. J.; Kim, S. S. TL 1990, 31, 3715. 48. Corey, E. J.; Yu, C.-M.; Kim, S. S. JACS 1989, 111, 5495. 49. Corey, E. J.; Yu, C.-M.; Lee, D.-H. JACS 1990, 112, 878. 50. Corey, E. J.; Kim, S. S. JACS 1990, 112, 4976.

Akira Suzuki & Shoji Hara

Hokkaido University, Sapporo, Japan

Copyright © 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.

Boron Trichloride1

[10294-34-5] · BCl3 · Boron Trichloride · (MW 117.17)

(Lewis acid capable of selective cleavage of ether and acetal protecting groups; reagent for carbonyl condensations; precursor of organoboron reagents)

Physical Data: bp 12.5 °C; d 1.434 g cm-3 (0 °C).

Solubility: sol saturated and halogenated hydrocarbon and aromatic solvents; solubility in diethyl ether is approximately 1.5 M at 0 °C; stable for several weeks in ethyl ether at 0 °C, but dec by water or alcohols.

Form Supplied in: colorless gas or fuming liquid in an ampoule; BCl3.SMe2 complex (solid) and 1 M solutions in dichloromethane, hexane, heptane, and p-xylene are available.

Handling, Storage, and Precautions: a poison by inhalation and an irritant to skin, eyes, and mucous membranes. Reacts exothermically with water and moist air, forming toxic and corrosive fumes. Violent reaction occurs with aniline or phosphine. All operations should be carried out in a well-ventilated fume hood without exposure to the atmosphere. The gas can be collected and measured as a liquid by condensing in a cooled centrifuge tube and then transferred to the reaction system by distillation with a slow stream of nitrogen.

Cleavage of Ethers, Acetals, and Esters. Like many other Lewis acids, BCl3 has been extensively used as a reagent for the cleavage of a wide variety of ethers, acetals, and certain types of esters.2 Ether cleavage procedures involve addition of BCl3, either neat or as a solution in CH2Cl2, to the substrate at -80 °C. The vessel is then stoppered and allowed to warm to rt. Whereas the complexes of BCl3 with dimethyl ether and diethyl ether are rather stable at rt, they decompose to form ROBCl2 or (RO)2BCl with evolution of alkyl chloride upon heating to 56 °C.1 Diaryl ethers are unreactive. Mixed dialkyl ethers are cleaved to give the alkyl chloride derived from C-O bond cleavage leading to the more stable carbenium ion. The transition state is predominantly SN1 in character, as evidenced by partial racemization of chiral ethers1,2 and the rearrangement of allyl phenyl ethers to o-allylphenols.3 BCl3 can be used for the deprotection of a variety of methoxybenzenes including hindered polymethoxybenzenes and peri-methoxynaphthalene.1,2,4 When methoxy groups are ortho to a carbonyl group, the reaction is accelerated by the formation of a chelate between boron and the carbonyl oxygen atom (Scheme 1).4a-c

BCl3

The reagent is less reactive than Boron Tribromide for ether cleavage; however, the type and extent of deetherification can be more easily controlled by the ratio of substrate to BCl3 as well as the reaction temperature and time. The transformation of (-)-β-hydrastine (1) to (-)-cordrastine II is efficiently achieved by selective cleavage of the methylenedioxy group in preference to aromatic methoxy groups.5 The demethylation of (-)-2-O-methyl-(-)-inositol in dichloromethane proceeds at -80 °C without cleavage of a tosyl ester group.6 Methyl glycosides are converted into glycosyl chlorides at -78 °C without effecting benzyl and acetyl protecting groups.7

One of the difficulties with the use of BCl3 arises from its tendency to fume profusely in air. The complex of BCl3 with dimethyl sulfide is solid, stable in air, and handled easily. By using a two- to fourfold excess of the reagent in dichloroethane at 83 °C, aromatic methoxy and methylenedioxy groups can be cleaved in good yields.8

Another application of BCl3 is for the cleavage of highly hindered esters under mild conditions. O-Methylpodocarpate (2) and methyl adamantane-1-carboxylate are cleaved at 0 °C.9 The highly selective displacement of the acetoxy group in the presence of other potentially basic groups in 2-cephem ester (3) provides the corresponding allylic chloride. On the other hand, treatment of (3) with an excess of BCl3 results in the cleavage of the acetoxy and t-butyl ester groups.10

Tertiary phosphines are cleaved at the P-C bond to give diphenylphosphine oxides. Workup with Hydrogen Peroxide provides diphenylphosphinic acids (eq 1).11

Condensation Reactions. Boron trichloride converts ketones into (Z)-boron enolates at -95 °C in the presence of Diisopropylethylamine. These enolates react with aldehydes with high syn diastereoselectivity (eq 2).12 A similar condensation of imines with carbonyl compounds also provides crossed aldols in reasonable yields.13 The reaction was extended to the asymmetric aldol condensation of acetophenone imine and benzaldehyde by using isobornylamine as a chiral auxiliary (48% ee).14

(N-Alkylanilino)dichloroboranes (5), prepared in situ from N-alkylanilines and boron trichloride, are versatile intermediates for the synthesis of ortho-functionalized aniline derivatives (eqs 3-5).15 The regioselective ortho hydroxyalkylation can be achieved with aromatic aldehydes.16

The reaction of (5) with alkyl and aryl nitriles and Aluminum Chloride catalyst provides ortho-acyl anilines.16 When chloroacetonitrile is used, the products are ideal precursors for indole synthesis.17 Use of isocyanides instead of nitriles provides ortho-formyl N-alkylanilines.18 Although these reactions with BCl3 are restricted to N-alkylanilines, the use of Phenylboron Dichloride allows the ortho-hydroxybenzylation of primary anilines.19

Analogously, boron trichloride induces ortho selective acylation of phenols at rt with nitriles, isocyanates, or acyl chlorides (eq 6).20 The efficiency and regioselectivity of these reactions are best with BCl3 among the representative metal halides that have been examined. In both the aniline and phenol substitutions the boron atom acts as a template to bring the reactants together, leading to cyclic intermediates and exclusively products of ortho substitution. A similar ortho selective condensation of aromatic azides with BCl3 provides fused heterocycles containing nitrogen.21

Aldehydes and ketones condense with ketene in the presence of 1 equiv of boron trichloride to give α,β-unsaturated acyl chlorides.22 Aryl isocyanates are converted into allophanyl chlorides, which are precursors for industrially important 1,3-diazetidinediones (eq 7).23

Synthesis of Organoboron Reagents.

General method of synthesis of organoboranes consists of the transmetallation reaction of organometallic compounds with BX3.24 Boronic acid derivatives [RB(OH)2] are most conveniently synthesized by the reaction of B(OR)3 with RLi or RMgX reagents, but boron trihalides are more advantageous for transmetalation reactions with less nucleophilic organometallic reagents based on Pb,25 Hg,26 Sn,27 and Zr28 (eqs 8 and 9).

Redistribution or exchange reactions of R3B with boron trihalides in the presence of catalytic amounts of hydride provides an efficient synthesis of RBX2 and R2BX.29 Another convenient and general method for the preparation of organodichloroboranes involves treatment of alkyl, 1-alkenyl, and aryl boronates with BCl3 in the presence of Iron(III) Chloride (3 mol %).30 Organodichloroboranes are valuable synthetic reagents because of their high Lewis acidity, and their utility is well demonstrated in the syntheses of piperidine and pyrrolidine derivatives by the intramolecular alkylation of azides (eq 10)31 or the synthesis of esters by the reaction with Ethyl Diazoacetate.32 The various organoborane derivatives, R3B, R2BCl, and RBCl2, all react with organic azides and diazoacetates. However, especially facile reactions are achieved by using organodichloroboranes (RBCl2).

Dichloroborane and monochloroborane etherates or their methyl sulfide complexes have been prepared by the reaction of borane and

boron trichloride.33 However, hydroboration of alkenes with these borane reagents is usually very slow due to the slow dissociation of the complex. Dichloroborane prepared in pentane from boron trichloride and trimethylsilane shows unusually high reactivity with alkenes and alkynes; hydroboration is instantaneous at -78 °C (eq 11).34

Direct boronation of benzene derivatives with BCl3 in the presence of activated aluminum or AlCl3 provides arylboronic acids after hydrolysis (eq 12).35 Chloroboration of acetylene with boron trichloride produces dichloro(2-chloroethenyl)borane.36 Similar reaction with phenylacetylene provides (E)-2-chloro-2-phenylethenylborane regio- and stereoselectively.37

The syntheses of thioaldehydes, thioketones, thiolactones, and thiolactams from carbonyl compounds are readily achieved by in situ preparation of B2S3 from bis(tricyclohexyltin) sulfide and boron trichloride (eq 13).38 The high sulfurating ability of this in situ prepared reagent can be attributed to its solubility in the reaction medium.

Related Reagents. Bis(tricyclohexyltin) Sulfide-Boron Trichloride.

1. Gerrard, W.; Lappert, M. F. CRV 1958, 58, 1081. 2. (a) Bhatt, M. V.; Kulkarni, S. U. S 1983, 249. (b) Greene, T. W. Protective Groups in Organic Synthesis; Wiley: New York, 1981. 3. (a) Gerrard, W.; Lappert, M. F.; Silver, H. B. Proc. Chem. Soc. 1957, 19. (b) Borgulya, J.; Madeja, R.; Fahrni, P.; Hansen, H.-J.; Schmid, H.; Barner, R. HCA 1973, 56, 14. 4. (a) Dean, R. B.; Goodchild, J.; Houghton, L. E.; Martin, J. A. TL 1966, 4153. (b) Arkley, V.; Attenburrow, J.; Gregory, G. I.; Walker, T. JCS 1962, 1260. (c) Barton, D. H. R.; Bould, L.; Clive, D. L. J.; Magnus, P. D.; Hase, T. JCS(C) 1971, 2204. (d) Carvalho, C. F.; Seargent, M. V. CC 1984, 227. 5. (a) Teitel, S.; O'Brien, J.; Brossi, A. JOC 1972, 37, 3368. (b) Teitel, S.; O'Brien, J. P. JOC 1976, 41, 1657.

6. Gero, S. D. TL 1966, 591. 7. Perdomo, G. R.; Krepinsky, J. J. TL 1987, 28, 5595. 8. Williard, P. G.; Fryhle, C. B. TL 1980, 21, 3731. 9. Manchand, P. S. CC 1971, 667. 10. Yazawa, H.; Nakamura, H.; Tanaka, K.; Kariyone, K. TL 1974, 3991. 11. Hansen, K. C.; Solleder, G. B.; Holland, C. L. JOC 1974, 39, 267. 12. Chow, H-F.; Seebach, D. HCA 1986, 69, 604. 13. Sugasawa, T.; Toyoda, T. Sasakura, K. SC 1979, 9, 515. 14. Sugasawa, T.; Toyoda, T.; TL 1979, 1423. 15. Sugasawa, T. J. Synth. Org. Chem. Jpn. 1981, 39, 39. 16. Sugasawa, T.; Toyoda, T.; Adachi, M.; Sasakura, K. JACS 1978, 100, 4842. 17. Sugasawa, T.; Adachi, M.; Sasakura, K.; Kitagawa, A. JOC 1979, 44, 578. 18. Sugasawa, T.; Hamana, H.; Toyoda, T.; Adachi, M. S 1979, 99. 19. Toyoda, T.; Sasakura, K.; Sugasawa, T. TL 1980, 21, 173. 20. (a) Toyoda, T.; Sasakura, K.; Sugasawa, T. JOC 1981, 46, 189. (b) Piccolo, O.; Filippini, L.; Tinucci, L.; Valoti, E.; Citterio, A. T 1986, 42, 885. 21. (a) Zanirato, P. CC 1983, 1065. (b) Spagnolo, P.; Zanirato, P. JCS(P1) 1988, 2615. 22. Paetzold, P. I.; Kosma, S. CB 1970, 103, 2003. 23. Helfert, H.; Fahr, E. AG(E) 1970, 9, 372. 24. (a) Nesmeyanov, A. N.; Kocheshkov, K. A. Methods of Elemento-Organic Chemistry; North-Holland: Amsterdam, 1967; Vol. 1, pp 20-96. (b) Mikhailov, B. M.; Bubnov, Y. N. Organoboron Compounds in Organic Synthesis; Harwood: Amsterdam, 1984. 25. Holliday, A. K.; Jessop, G. N. JCS(A) 1967, 889. 26. Gerrard, W.; Howarth, M.; Mooney, E. F.; Pratt, D. E. JCS 1963, 1582. 27. (a) Niedenzu, K.; Dawson, J. W. JACS 1960, 82, 4223. (b) Brinkman, F. E.; Stone, F. G. A. CI(L) 1959, 254. 28. Cole, T. E.; Quintanilla, R.; Rodewald, S. OM 1991, 10, 3777. 29. Brown, H. C.; Levy, A. B. JOM 1972, 44, 233. 30. (a) Brindley, P. B.; Gerrard, W.; Lappert, M. F. JCS 1956, 824. (b) Brown, H. C.; Salunkhe, A. M.; Argade, A. B. OM 1992, 11, 3094. 31. (a) Jego, J. M.; Carboni, B.; Vaultier, M.; Carrie', R. CC 1989, 142. (b) Brown, H. C.; Salunkhe, A. M. TL 1993, 34, 1265. 32. Hooz, J.; Bridson, J. N.; Calzada, J. G.; Brown, H. C.; Midland, M. M.; Levy, A. B. JOC 1973, 38, 2574. 33. (a) Brown, H. C. Organic Syntheses via Boranes; Wiley: New York, 1975; pp 45-47. (b) Brown, H. C.; Kulkarni, S. U. JOM 1982, 239, 23. (c) Brown, H. C.; Ravindran, N. IC 1977, 16, 2938. 34. Soundararajan, R.; Matteson, D. S. JOC 1990, 55, 2274. 35. (a) Muetterties, E. L. JACS 1960, 82, 4163. (b) Lengyel, B.; Csakvari, B. Z. Anorg. Allg. Chem. 1963, 322, 103. 36. Lappert, M. F.; Prokai, B. JOM 1964, 1, 384. 37. Blackborow, J. R. JCS(P2) 1973, 1989. 38. Steliou, K.; Mrani, M. JACS 1982, 104, 3104.

Norio Miyaura

Hokkaido University, Sapporo, Japan

Copyright © 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.

Boron Trifluoride1

[7637-07-2] · BF3 · Boron Trifluoride · (MW 67.81)

(strongly oxophilic Lewis acid)

Physical Data: gas, mp -127 °C; bp -100 °C/35 mmHg.1,2

Solubility: very sol water, and forms a mono- and dihydrate; it is only partially hydrolyzed to fluoroboric acid. Also sol most organic solvents such as pentane; forms complexes with compounds that contain heteroatoms with lone pair electrons.

Form Supplied in: gas in cylinders or lecture bottles; widely available.

Preparative Method: made by heating calcium fluoride with boron trioxide or from potassium fluoroborate with concentrated sulfuric acid. The product may be contaminated by silicon tetrafluoride unless the starting materials are free from silicon.1b,2

Purification: via distillation and aromatic addition compound formation.40

Handling, Storage, and Precautions: a monel metal valve or stainless steel regulator is recommended. Extremely moisture sensitive; should be used with carefully dried apparatus. Irritant, slowly hydrolyzes to give HF. All work must be carried out in an efficient fume hood.

Introduction. Boron trifluoride acts as a Lewis acid but its major reactivity relates to the fact that the boron reagent is oxophilic, whereas Aluminum Chloride is mostly known for its reactions with halides.3 Thus many of the reactions of boron trifluoride involve alcohols, ethers, carboxylic acids, and esters; the water which is frequently eliminated coordinates with the boron trifluoride. Esters are formed in the presence of boron trifluoride by the interaction of acids with alcohols,4 acids with ethers,5 and amides with alcohols and phenols.6 Thus phenyl acetate is rapidly formed when phenol and acetamide are treated with boron trifluoride. Propene, in the presence of boron trifluoride, reacts with carboxylic acids to form isopropyl esters,7 and with phenol to form the isopropyl ether.8 When salicylic acid is treated with boron trifluoride and then propene the ester, isopropyl salicylate, is formed rapidly. With an excess of propene, isopropyl 2-hydroxy-3,5-diisopropylbenzoate is formed in an almost quantitative yield.7 Ketones that are enolizable are converted into β-diketones in the presence of carboxylic anhydrides and boron trifluoride.9 Similarly, phenyl alkyl ethers afford alkyl phenols.10,11 Benzidine and Beckmann rearrangements and the rearrangement of phenyl acetate to 4-hydroxyacetophenone have all been studied using boron trifluoride.3 A number of the early investigations have been repeated.

Friedel-Crafts Alkylation Reactions.

Alcohols react with benzene12 or naphthalene13 to give alkylated products. The alkylation of benzene using cyclohexanol (eq 1) gives moderate yields, but no other alcohols give better results; for example, both 1-propanol and 2-propanol give mono- and 1,4-diisopropylbenzene in a combined yield of ca. 38%.12a The reactions of isobutanol with benzene in the presence of boron trifluoride, not

BF3

unexpectedly, give t-butyl- and 1,4-di-t-butylbenzene.12e Similarly, naphthalene is alkylated in good yield using cyclohexanol (eq 2).13

The alkylation of aromatic hydrocarbons using alkyl fluorides has also been investigated. Although the reactivity of boron trifluoride is lower than that of the other boron halides, the yields are usually better when using the fluoride. Methyl, ethyl, propyl, isopropyl, t-butyl, and cyclohexyl fluorides were shown to alkylate aromatic compounds in good yields. For example, benzene and cyclohexyl fluoride give the expected product in 85% yield.12b Normally, alkylation does not occur using alkyl halides other than the fluorides, and mixed halides afford products containing halogen. Although the reaction of cyclohexyl fluoride was successful, an attempted reaction using cyclohexyl bromide with toluene was unsuccessful.12b The preparation of cumene in high yield can be achieved using 2-fluoropropane in a reaction with benzene (eq 3); the lack of reactivity of other halogens is exemplified by the reaction of 1-chloro-3-fluoropropane, in which the product is 1-chloro-2-phenylpropane, and by eq 4, where the bromine is retained using 1-bromo-2-fluoroethane.14

The use of alkenes as sources of the electrophiles involved in Friedel-Crafts alkylations has also been studied. In early examples the yields obtained were not as good as in alkylations using alcohols. For example, the yield of 2-cyclohexylnaphthalene is only 35% using cyclohexene.13 On the other hand, the intramolecular alkylation of 1-(2-tolyl)-(E)-pent-3-ene gives 1,5-dimethyl-1,2,3,4-tetrahydronaphthalene in 95% yield, and the example shown in eq 5 was used in a synthesis of 1,3,6,8-tetramethyltriphenylene.15 Related to this latter reaction, gaseous boron trifluoride has been shown to form a complex with nitromethane, which is particularly effective in catalyzing proton-initiated cascade cyclizations like the one shown in eq 6.16

Diastereoselective alkylation reactions of furans have been studied using diacetoxypyrrolidinones as sources of an acyliminium ion. Good yields and some diastereoselection were observed when using Zinc Bromide or Chlorotrimethylsilane as the Lewis acid but, unfortunately, no diastereoselection was observed using boron trifluoride.17

Friedel-Crafts Acylation Reactions. The acylation of particularly electron-rich aromatic compounds can be achieved by a number of procedures; the number of options available that allow formylations to be carried out are fewer. The Vilsmeier protocols apply only to nucleophilic aromatic substrates, for example indoles and pyrroles. Formylation reactions using carbon monoxide, hydrogen chloride, and aluminum chloride function as if Formyl Chloride were produced. It is known, however, that formyl chloride is unstable at temperatures above -60 °C. On the other hand, Formyl Fluoride, which may be prepared by the interaction of anhydrous hydrogen fluoride with the mixed anhydride of formic and acetic acids, and removed as it is formed (bp -29 °C), has been used in conjunction with boron trifluoride. Alkylbenzenes are formylated at low temperatures. In an alternative procedure, formyl fluoride and boron trifluoride are passed into a solution of the aromatic hydrocarbon in carbon disulfide.18 Thus benzene gives benzaldehyde (56%), mesitylene gives mesitaldehyde (70%), and naphthalene gives a 1:5 mixture of α- and β-naphthaldehydes.

In other acylation reactions, acyl fluorides and boron trifluoride afford ketones with better regioselectivity than that observed with aluminum chloride as the Lewis acid. A reaction of isobutyryl fluoride with 2-methylnaphthalene gives an excellent yield of the product shown in eq 7.19

Boron trifluoride can, however, give rise to difficulties as a result of side-chain acylation, as exemplified in eq 8.20 Carboxylic acids have been used in reactions of phenols21 and aryl ethers together with boron trifluoride. Dealkylation of an ether residue ortho to the introduced acyl group is frequently encountered, as in the synthesis of baeckeol (eq 9).22 Advantage of this effect was also taken in a synthesis of the naturally occurring phenol aurentiacin, as shown in eq 10.23

Carbonyl Group and Related Transformations. The conversion of aldehydes and ketones into the related gem-difluoro compounds is effected by treatment with Molybdenum(VI) Fluoride at rt in the presence of catalytic amounts of boron trifluoride. The yields with ketones are significantly better than with aldehydes.24 The acylation of enolizable carbonyl compounds was mentioned in the introduction.9 A checked procedure for the acetylation of acetone, which affords pentane-2,4-dione in 80-85% yield, has been reported.25 In general, it has been established that better yields of β-diketones are obtained if the intermediate boron difluoride complexes are isolated.26 The boron difluoride complexes, isolated in yields of 75-92%, give the β-diketones in yields of 76-95%; the procedure is exemplified in eq 11. In another study, evidence was presented that implicated two mechanisms for the acetylation of ketones.27 One route involves the formation of the enol acetate and the other involves the direct acetylation of the boron enolate. Thus the enol acetate derived from cyclohexanone is formed and isolated in 22% yield in a reaction in which cyclohexanone is allowed to interact with acetic anhydride in the presence of 14 mol % of boron trifluoride gas. The enol acetate was subsequently shown to afford 2-acetylcyclohexanone in 76% yield when treated with acetic anhydride and boron trifluoride. On the other hand, attempts to form the enol acetate from acetophenone were unsuccessful.

The alkylation of β-keto esters using an alcohol as the source of the electrophile can be effected by using boron trifluoride (eq 12).28

Beckmann rearrangements using boron trifluoride3 have been investigated in more detail. For example, benzophenone oxime affords a 1:1 complex in 98% yield when boron trifluoride is passed into a solution of the oxime in light petroleum. The complex rearranges to N-phenylbenzamide in 83% yield when heated to 140-150 °C. The related benzophenone oxime O-methyl ether-boron trifluoride complex also gives N-phenylbenzamide at a lower temperature but in lower yield.29 The isomerization of syn- to anti-arylaldoximes has also been reported via the boron trifluoride complexes.30

Cation-Assisted Rearrangement Reactions. A number of interesting cyclization reactions leading to naturally occurring polycyclic ring systems have been investigated using boron trifluoride. The cedrane ring system is formed (eq 13) when the enol acetate is treated as shown; reaction of the ketone with Methyllithium then gives racemic cedrol.31 In a detailed study of the reactions of an acetoxymenthadiene (eq 14), it was shown that racemic camphor can be obtained in 90% yield when a 0.1 % solution in wet dichloromethane is treated with boron trifluoride at room temperature for 10 min. Lower yields are obtained when higher concentrations of the diene are used, and boron trifluoride etherate fails to afford any of the required product as do other Lewis acids.32 A Lewis acid-assisted fragmentation followed by a 1,2-methyl shift, driven by the enolate, is involved in the terpene rearrangement leading to the nootkatane skeleton shown in eq 15.33 Rearrangement reactions of glycidic esters have been studied using boron trifluoride and are found to proceed in high yields.34 For example, ethyl β-phenylglycidate gives ethyl phenylpyruvate, isolated in 80% yield as the 2,4-dinitrophenylhydrazone.

Diels-Alder Reactions. Lewis acid-catalyzed Diels-Alder reactions are well known, and a number of examples have been studied using boron trifluoride. The regioselectivity of the reaction of unsymmetrical dienes with unsymmetrically substituted quinones can be directed in favor of either regioisomer depending on the catalyst used. This is exemplified in eq 16.35 The regioselectivity has been explained on the basis that boron is capable of forming a tetracoordinate complex, whereas tin can complex via the more basic oxygen and the adjacent methoxy group. The analogous reaction using piperylene (penta-1,3-diene) shows, as expected, slightly improved regioselectivity. A study of diastereoselective Diels-Alder reactions using acrylate esters of 3,4-O-methylene-β-D-arabinoside has involved the use of a number of Lewis acids, including boron trifluoride.36

Miscellaneous Reactions. The nitration of benzene derivatives using methyl nitrate in nitromethane in the presence of a Lewis acid leads to the formation of significant amounts of chlorinated products when using chlorine-containing Lewis acids. Side-chain nitration is also a problem sometimes. On the other hand, the nitrations of tetra- and pentamethylbenzenes proceed efficiently when boron trifluoride is used. Pentamethylbenzene affords the nitro derivative in an almost quantitative yield and durene gives mononitrodurene in 97% yield. When using a 3:1 excess of methyl nitrate and boron trifluoride, durene converts quantitatively into dinitrodurene.37

The reduction of alcohols using a trialkylsilane in the presence of a protic acid can be complicated by skeletal rearrangement and alkene formation as a result of carbenium ion formation. This problem is significantly reduced when using boron trifluoride as the acid (eq 17).38

The preparation of peptide isosteres (eq 18) and related model compounds (eq 19) is achieved by the reductive elimination of γ-oxygenated-α,β-unsaturated carboxylates using boron trifluoride complexes of alkenylcopper reagents.38

Finally, it is of interest to note that α-difluoroamino fluoroimines fragment to α-difluoroamino fluorides in dichloromethane in the presence of boron trifluoride (eq 20).39

Related Reagents. Boron Trifluoride-Acetic Acid; Boron Trifluoride-Dimethyl Sulfide; Boron Trifluoride-Acetic Anhydride; Boron Trifluoride Etherate; Hydrogen Peroxide-Boron Trifluoride; Iodosylbenzene-Boron Trifluoride; Lithium Dimethylcuprate-Boron Trifluoride; Methylcopper-Boron Trifluoride Etherate; Phenylcopper-Boron Trifluoride Etherate; Tetrafluoroboric Acid.

1. (a) Martin, D. R. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Ed.; Wiley: New York, 1980; Vol. 10, pp 685-693. (b) Bowlus, H.; Nieuwland, J. A. JACS 1931, 53, 3835. (c) Greenwood, N. N.; Thomas, B. S. In Comprehensive Inorganic Chemistry; Trotman-Dickenson, A. F., Ed.; Pergamon: New York, 1973; Vol. 1, pp 956-973. (d) Urry, G. In The Chemistry of Boron and Its Compounds; Muetterties, E. L., Ed.; Wiley: New York, 1967; Chapter 6, pp 325-343. (e) For earlier monographic reviews, see, Booth, H. S.; Martin, D. R. Boron Trifluoride and Its Derivatives; Wiley: New York, 1949. (f) Topchiev, A. V.; Zavgorodnii, S. V.; Paushkin,

Ya. M. Boron Fluoride and Its Compounds as Catalysts in Organic Chemistry; Pergamon: New York, 1959. 2. Krause, E.; Nitsche, R. CB 1921, 54, 2784. 3. Meerwein, H. CB 1933, 66, 411. 4. (a) Hinton, H. D.; Nieuwland, J. A. JACS 1932, 54, 2017. (b) Dorris, T. B.; Sowa, F. J.; Nieuwland, J. A. JACS 1934, 56, 2689. 5. Hennion, G. F.; Hinton, H. D.; Nieuwland, J. A. JACS 1933, 55, 2857. 6. Sowa, F. J.; Nieuwland, J. A. JACS 1933, 55, 5052. 7. Croxall, W. J.; Sowa, F. J.; Nieuwland, J. A. JACS 1934, 56, 2054. 8. Croxall, W. J.; Sowa, F. J.; Nieuwland, J. A. JACS 1935, 57, 1549. 9. Adams, J. T.; Hauser, C. R. JACS 1945, 67, 284. 10. Sowa, F. J.; Hinton, H. D.; Nieuwland, J. A. JACS 1933, 55, 3402. 11. Kolka, A. J.; Vogt, R. R. JACS 1939, 61, 1463. 12. (a) McKenna, J. F.; Sowa, F. J. JACS 1937, 59, 470. (b) Burwell, R. L.; Archer, S. JACS 1942, 64, 1032. (c) Hennion, G. F.; Pieronek, V. R. JACS 1942, 64, 2751. (d) Vermillion, G.; Hill, M. A. JACS 1945, 67, 2209. (e) Hennion, G. F.; Auspos, L. A. JACS 1943, 65, 1603. 13. Price, C. C.; Ciskowski, J. M. JACS 1938, 60, 2499. 14. Oláh, G. A.; Kuhn, S.; Oláh, J. JCS 1957, 2174. 15. Canonne, P.; Regnault, A. CJC 1969, 47, 2837. 16. Harring, S. R.; Livinghouse, T. CC 1992, 502. 17. Ohta, T.; Shiokawa, S.; Iwashita, E.; Nozoe, S. H 1992, 34, 895. 18. Olah, G. A.; Kuhn, S. J. JACS 1960, 82, 2380. 19. Hyatt, J. A.; Raynolds, P. W. JOC 1984, 49, 384. 20. Walker, H. G., Jr.; Sanderson, J. J.; Hauser, C. R. JACS 1953, 75, 4109. 21. (a) Fodor, G.; Kiss, J.; Szekerke, M. JOC 1950, 15, 227. (b) Kindler, K.; Oelschläger, H. CB 1954, 87, 194. 22. Schiemenz, G. P.; Schmidt, U. LA 1976, 1514. 23. Schiemenz, G. P.; Schmidt, U. LA 1982, 1509. 24. Mathey, F.; Bensoam, J. T 1971, 27, 3965. 25. Denoon, C. E., Jr. OSC 1955, 3, 16. 26. Sagredos, A. N. LA 1966, 700, 29. 27. Mao, C.-L.; Frostick, F. C. Jr.; Man, E. H.; Manyik, R. M.; Wells, R. L.; Hauser, C. R. JOC 1969, 34, 1425. 28. Adams, J. T.; Levine, R.; Hauser, C. R. OSC 1955, 3, 405. 29. Hauser, C. R.; Hoffenberg, D. S. JOC 1955, 20, 1482. 30. Hauser, C. R.; Hoffenberg, D. S. JOC 1955, 20, 1491. 31. Corey, E. J.; Girotra, N. N.; Mathew, C. T. JACS 1969, 91, 1557. 32. Fairlie, J. C.; Hodgson, G. L.; Money, T. JCS(P1) 1973, 2109. 33. Caine, D.; Graham, S. L.; TL 1976, 2521. 34. House, H. O.; Blaker, J. W.; Madden, D. A. JACS 1958, 80, 6386. 35. Tou, J. S.; Reusch, W. JOC 1980, 45, 5012. 36. Nouguier, R.; Gras, J.-L.; Giraud, B.; Virgili, A. TL 1991, 32, 5529. 37. Olah, G. A.; Lin, H. C. S 1973, 488. 38. Adlington, M. G.; Orfanopoulos, M.; Fry, J. L. TL 1976, 2955. 39. (a) Stevens, T. E. TL 1967, 3017. (b) Stevens, T. E. JOC 1969, 34, 2451. 40. (a) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon: New York, 1988; p 316. (b) Brown, H. C.; Johannesen, R. B. JACS 1950, 72, 2934.

Harry Heaney

Loughborough University of Technology, UK

Copyright © 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.

Boron Triiodide1

[13517-10-7] · BI3 · Boron Triiodide · (MW 391.52)

(strong Lewis acid with labile, nucleophilic iodine atoms; cleaves ethers,2 esters,3 silanes,4 halides,5 and alcohols to alkyl iodides; reductively cleaves sulfonyl6 and sulfinyl7 groups to disulfides)

Physical Data: mp 50.5 °C; d 3.350 g cm-3.

Solubility: sol hydrocarbons, CH2Cl2, CHCl3, CCl4, CS2; reacts with oxygenated solvents.

Form Supplied in: volatile white solid (frequently has a pinkish color due to traces of I2).

Preparative Methods: BI3 of extremely high purity can be prepared in 25% yield by treating I2 in heptane at 80 °C with KBH4 (which has been recrystallized from H2O).8 Recrystallized NaBH4 can also be used; if traces of chloride or bromide can be tolerated then LiBH4 in hexane can be used.9

Purification: BI3 has a vapor pressure similar to that of I2 and can be purified by sublimation from Cu powder at 60 °C (in vacuo). Commercial material is usually contaminated with traces of other halides. While the presence of small amounts of BI2Br and/or BI2Cl does not usually interfere with most organic reactions, these impurities are not easily removed by sublimation or distillation.

Handling, Storage, and Precautions: use in a fume hood; reacts expositively with H2O; traces of moisture result in the liberation of I2 so the formation of any color indicates improper handling. BI3 is also light sensitive; it photolyzes below 360 nm. It should be stored under an inert atmosphere in the dark.

Ether Cleavages.

All of the boron trihalides, except the fluorides, will cleave ethers with varying degrees of efficacy.10 However, the nucleophilic character of iodine coupled with the strong Lewis acidity of boron makes boron triiodide the most potent of these reagents. It is a powerful reagent for the cleavage of C-O bonds in ethers, esters, and alcohols, resulting in the formation of alkyl iodides under mild conditions. Aryl alkyl ethers are cleaved to phenols (eq 1). Diaryl ethers are unreactive. BI3 reacts at least an order of magnitude faster than Boron Tribromide in ether cleavages.1 This is especially useful in the cleavage of the ethers of higher alkyl groups (eq 2).

BI3

The initial products of ether cleavage are the alkyl halide and a borate ester, (RO)3B. The borate esters are usually inert to further displacement but, because the iodide is more nucleophilic than the other halides, warming the borate esters (60-80 °C) in the presence of BI3 will result in the complete conversion of all the alkyl residues to iodides (eq 3).2

Other Cleavages.

Kabalka3 and co-workers have shown that an attenuated form of BI3, BI3.NEt2Ph, will cleave a variety of compounds containing C-O single bonds at elevated temperatures. Solutions of this reagent are prepared by reacting the commercially available amine-borane complex with I2 in benzene at 80 °C for several hours. The reagent cleaves ethers,11 esters,3 and geminal diacetates.11 Esters3 are cleaved to an activated acyl intermediate RCOX which can be used to prepare acids, other esters, and amides (eq 4).

Sulfinyl and sulfonyl compounds react with BI36 and BI3.NEt2Ph7 to afford disulfides (eq 5).

Sulfoxides are deoxygenated by BI3.NEt2Ph.7 Sulfides are also cleaved by BI3. Methionine reacts to yield a complex mixture of C-S bond cleavage products including homocysteic acid, homoserine, and homoserine lactone.12

Substitution Reactions.

As a powerful electrophile, BI3 will effect aromatic and aliphatic substitution reactions on alkyl and aryl halides and silanes (eq 6).4 Alkyl halides (R-Cl and R-Br) also undergo substitutions reactions in CH2Cl2 and CCl4 solution, producing iodides without alkyl rearrangement. In CCl4 the reaction is second order in alkyl halide.5 At elevated temperatures the ipso substitution of aryl iodides occurs readily, resulting in borinic iodides eq 7).13 The direct borylation of benzene is a photochemical process which most likely involves the formation of I&bdot; and I2B&bdot; radicals (eq 8).14

1. Lansinger, J.; Ronald, R. SC 1979, 341. 2. Povlock, T. TL 1967, 4131. 3. Kabalka, G.; Narayana, C.; Reddy, N. SC 1992, 22, 1793. 4. Jutzi, P.; Krato, B.; Hursthouse, M.; Howes, A. CB 1987, 120, 565. Jutzi, P.; Seufert, A. JOM 1979, 169, 357. 5. Goldstein, M.; Haines, L.; Hemmings, J. JCS(D) 1972, 2260. 6. Olah, G.; Narang, S.; Field, L.; Karpeles, R. JOC 1981, 46, 2408. 7. Narayana, C., Padmanabhan, S.; Kabalka, G. SL 1991, 125. 8. Briggs, A.; Simmons, R. N 1990, 77, 595. 9. Renner, T. AG 1957, 69, 478. 10. Weiberg, E.; Sutterlin, W. Z. Anorg. Allg. Chem. 1931, 202, 22. Benton, F.; Dillon, T. JACS 1942, 64, 1128. 11. Narayana, C.; Padmanabhan, S.; Kabalka, G. TL 1990, 31, 6977. 12. Atassi, M.; Perlstein, M. TL 1972, 1861. 13. Siebert, W. Schafer, K.-J.; Asgarouladi, B. ZN(B) 1974, 29, 642. 14. Bowie, R.; Musgrave, O. JCS(C) 1970, 485.

Rob Ronald

Washington State University, Pullman, WA, USA

Copyright © 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.