t-Butylmagnesium Chloride1

t-BuMgX
(X = Cl)

[672-22-5]  · C4H9ClMg  · t-Butylmagnesium Chloride  · (MW 116.87) (X = Br)

[2259-30-5]  · C4H9BrMg  · t-Butylmagnesium Bromide  · (MW 161.32)

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

Physical Data: NMR observations indicate that t-BuMgCl in Et2O and THF and t-BuMgBr in THF are in fact mixtures of t-BuMgX, t-Bu2Mg, and MgX2.2,3 Although essentially monomeric in THF, t-BuMgCl is mainly dimeric in Et2O.4 A solid of composition t-BuMgBr(Et2O)2, probably having a dimeric structure, has been isolated.5

Solubility: sol Et2O and THF; insol hydrocarbons.

Form Supplied in: solutions of t-BuMgCl in Et2O and THF are commercially available.

Analysis of Reagent Purity: see entry for Methylmagnesium Bromide.

Preparative Methods: 1c,e,6 in spite of the commercial availability of t-BuMgCl solutions, t-BuMgCl3,7 and t-BuMgBr are often prepared from reaction of a t-butyl halide and Magnesium, usually in Et2O or THF.

Handling, Storage, and Precautions: see entry for Methylmagnesium Bromide t-Butyl Grignard reagents do not significantly attack Et2O or THF at normal reaction temperatures.

Preparation.

Side reactions in preparations of t-BuMgX and other tertiary alkyl Grignard reagents tend to be more significant than in preparations of less branched reagents. t-Butyl radicals, intermediates in the formation of the Grignard reagent, couple, disproportionate, and attack solvent molecules. Moreover, reaction of the Grignard reagent and unreacted alkyl halide is particularly significant with t-BuI. Because of side reactions, X generally exceeds t-Bu in the Grignard solutions. Yields generally seem to be improved by slow addition of the halide to the Mg and by use of activated8 Mg (see the entry for Phenylmagnesium Bromide). A considerable excess of t-BuX and Mg often is used to ensure that sufficient t-BuMgX is present for the subsequent reaction. When exact stoichiometry is important, determining the t-BuMg concentration is important.

Structure.

See the entry for Methylmagnesium Bromide.

Workup Procedures.

See the entry for Methylmagnesium Bromide.

Representative Applications.1b-f

Also see the entry for Methylmagnesium Bromide. A t-butyl Grignard reagent is used 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. A generally unwanted reaction, reduction of such substrates by transfer of a b-H of the t-Bu or other tertiary or branched alkyl group, sometimes is more rapid. t-BuMgCl and di-t-butyl ketone, for example, normally furnish only di-t-butylmethanol.9 The generation of a chiral sulfoxide (eq 1) exemplifies addition to a different unsaturated functional group.10 Abstraction of a hydrogen a to unsaturated functions by a Grignard reagent, particularly a hindered one, may also compete with addition. Only abstraction is seen with the particularly acidic (1). Condensation with benzaldehyde (eq 2) does not occur when the anion is formed using an organolithium compound because equilibrium favors the anion and the aldehyde rather than their addition product.11

Related Reagents.

t-Butyllithium.


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. Parris, G. E.; Ashby, E. C. JACS 1971, 93, 1206. Allen, P. E. M.; Hagias, S.; Mair, C.; Williams, E. H. Ber. Bunsenges. Phys. Chem. 1984, 88, 623.
3. Petiaud, R.; Ciaudy, P.; Pham, Q.-T. Eur. Polym. J. 1976, 12, 441.
4. Walker, F. W.; Ashby, E. C. JACS 1969, 91, 3845.
5. Coates, G. E.; Heslop, J. A. JCS(A) 1968, 514.
6. 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.
7. Puntambeker, S. V.; Zoellner, E. A. OSC 1941, 1, 524.
8. Rieke, R. D.; Bales, S. E. JACS 1974, 96, 1775.
9. Singer, M. S.; Salinger, R. M.; Mosher, H. S. JOC 1967, 32, 3821. But see Richey, H. G., Jr.; DeStephano, J. P. JOC 1990, 55, 3281.
10. Evans, D. A.; Faul, M. M.; Colombo, L.; Bisaha, J. J.; Clardy, J.; Cherry, D. JACS 1992, 114, 5977.
11. Mioskowski, C.; Solladie, G. T 1980, 36, 227.

Herman G. Richey, Jr.,

The Pennsylvania State University, University Park, PA, USA



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