Benzylmagnesium Chloride

(X = Cl)

[6921-34-2]  · C7H7ClMg  · Benzylmagnesium Chloride  · (MW 150.89) (X = Br)

[1589-82-8]  · C7H7BrMg  · Benzylmagnesium Bromide  · (MW 195.34)

(adds to many unsaturated functional groups; benzyl can displace halide and like groups; can function as a strong base and a Lewis acid1b-f)

Physical Data: usually prepared or purchased as a solution and used without isolation. Addition of HMPA to an Et2O solution of BnMgCl causes significant changes in the UV spectrum that indicate ionization to benzyl anions.2

Solubility: BnMgCl and BnMgBr are soluble in Et2O and THF, and insoluble in hydrocarbons.

Form Supplied in: solutions of BnMgCl in Et2O and THF are commercially available.

Analysis of Reagent Purity: see Methylmagnesium Bromide.

Preparative Methods: 1c,e,3 in spite of the commercial availability of BnMgCl solutions, BnMgCl4 and BnMgBr are often prepared from reaction of a benzyl halide and Mg, usually in Et2O or THF.

Handling, Storage, and Precautions: see Methylmagnesium Bromide. Benzyl Grignard reagents do not significantly attack Et2O or THF at normal reaction temperatures

Preparation.

Yields of routine BnMgCl and BnMgBr preparations often exceed 90%, but those of BnMgI preparations are lower due to competing formation of bibenzyl and other dimers. Yields of Grignard reagents from benzyl chlorides and bromides having a-substituents and certain aryl substituents also are lower. In such cases, yields often are significantly improved by using activated Mg that permits preparation at lower temperatures. A simple activation procedure involves stirring the Mg under nitrogen,5 but other convenient procedures6 are available (also see Phenylmagnesium Bromide).

Representative Applications.1b-f

A benzyl Grignard reagent is used most frequently in reactions that result in attachment of its organic group to an electrophilic carbon atom of the substrate, most often of a carbonyl group, nitrile, or other unsaturated function. Some reactions7 of benzyl Grignard reagents, with unhindered carbonyl compounds,8,9 (eq 1) for example,8 furnish ortho-substituted products. Further reaction of the triene (e.g. 2) that probably forms initially can provide a product of double addition (1), sometimes significant even when excess Grignard reagent is used. Ortho- and sometimes small amounts of para-substituted products also are found in displacement reactions of benzyl Grignard reagents with reactive alkyl halides.10 See also Benzyllithium and Methylmagnesium Bromide.


1. (a) Lindsell, W. E. In Comprehensive Organometallic Chemistry; Wilkinson G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: Oxford, 1982; Chapter 4. (b) Wakefield, B. J. In Comprehensive Organometallic Chemistry; Wilkinson G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: Oxford, 1982; Chapter 44. (c) Nützel, K. MOC, 1973, 13/2a, 47. (d) Raston, C. L.; Salem, G. In The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Ed.; Wiley: Chichester, 1987; Vol. 4, Chapter 2. (e) Old but still extremely useful is Kharasch, M.; Reinmuth, O. Grignard Reactions of Nonmetallic Substances; Prentice-Hall: New York, 1954. (f) Ioffe, S. T.; Nesmeyanov, A. N. The Organic Compounds of Magnesium, Beryllium, Calcium, Strontium and Barium; North-Holland: Amsterdam, 1967.
2. Ebel, H. F.; Wagner, B. O. CB 1971, 104, 320.
3. Bickelhaupt, F. in Inorganic Reactions and Methods; Hagen, A. P., Ed.; VCH: New York, 1989; Vol. 10, Section 5.4.2.2.1. FF 1967, 1, 415.
4. Gilman, H.; Catlin, W. E. OSC 1941, 1, 471.
5. Baker, K. V.; Brown, J. M.; Hughes, N.; Skarnulis, A. J.; Sexton, A. JOC 1991, 56, 698.
6. (a) Oppolzer, W.; Schneider, P. TL 1984, 25, 3305. Oppolzer, W.; Cunningham, A. F. TL, 1986, 27, 5467. Bönnemann, H.; Bogdanović, B.; Brinkmann, R.; He, D.-W.; Spliethoff, B. AG(E) 1983, 22, 728. (b) Harvey, S.; Junk, P. C.; Raston, C. L.; Salem, G. JOC 1988, 53, 3134.
7. Ref. 1e, Chapter 17.
8. Benkeser, R. A.; Johnston, T. E. JACS 1966, 88, 2220.
9. Bernardon, C.; Deberly, A. JOC 1982, 47, 463.
10. Benkeser, R. A.; DeTalvo, W. JACS 1967, 89, 2141. Benkeser, R. A.; Johnston, T. E.; Tong, W.-H. JOC 1968, 33, 2203. Reuvers, A. J. M.; van Bekkum, H.; Wepster, B. M. T 1970, 26, 2683.

Herman G. Richey, Jr.

The Pennsylvania State University, University Park, PA, USA



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