Page 648 - Advanced Organic Chemistry Part B - Reactions & Synthesis

up to 170 C. 18 In contrast, the inversion of configuration of primary alkylmagnesium 623
19
halides is very fast. This difference in the primary and secondary systems may be the
result of a mechanism for inversion that involves exchange of alkyl groups between SECTION 7.1
magnesium atoms. Preparation and
Properties of
Organomagnesium
R
and Organolithium
H R X Mg C R Reagents
X H
R C Mg Mg C R H
X
R C Mg X
R H
R
If bridged intermediates are involved, the larger steric bulk of secondary systems
would retard the reaction. Steric restrictions may be further enhanced by the fact that
organomagnesium reagents are often present as clusters (see below).
The usual designation of Grignard reagents as RMgX is a useful but incomplete
representation of the composition of the compounds in ether solution. An equilibrium
exists with magnesium bromide and the dialkylmagnesium. 20
2 RMgX R Mg + MgX 2
2
The position of the equilibrium depends upon the solvent and the identity of the
specific organic group, but in ether lies well to the left for simple aryl-, alkyl-, and
alkenylmagnesium halides. 21 Solutions of organomagnesium compounds in diethyl
ether contain aggregated species. 22 Dimers predominate in ether solutions of alkyl-
magnesium chlorides.

Cl
2 RMgCl R Mg Mg R
Cl
The corresponding bromides and iodides show concentration-dependent behavior and
in very dilute solutions they exist as monomers. In tetrahydrofuran, there is less
tendency to aggregate, and several alkyl and aryl Grignard reagents have been found
to be monomeric in this solvent.
A number of Grignard reagents have been subjected to X-ray structure determi-
23
nation. Ethylmagnesium bromide has been observed in both monomeric and dimeric
24
forms in crystal structures. Figures 7.1a and b show, respectively, the crystal structure
18 E. Pechold, D. G. Adams, and G. Fraenkel, J. Org. Chem., 36, 1368 (1971).
19 G. M. Whitesides, M. Witanowski, and J. D. Roberts, J. Am. Chem. Soc., 87, 2854 (1965);
G. M. Whitesides and J. D. Roberts, J. Am. Chem. Soc., 87, 4878 (1965); G. Fraenkel and D. T. Dix,
J. Am. Chem. Soc., 88, 979 (1966).
20
K. C. Cannon and G. R. Krow, in Handbook of Grignard Reagents, G. S. Silverman and P. E. Rakita,
eds., Marcel Dekker, New York, 1996, pp. 271–289.
21 G. E. Parris and E. C. Ashby, J. Am. Chem. Soc., 93, 1206 (1971); P. E. M. Allen, S. Hagias,
S. F. Lincoln, C. Mair, and E. H. Williams, Ber. Bunsenges. Phys. Chem., 86, 515 (1982).
22
E. C. Ashby and M. B. Smith, J. Am. Chem. Soc., 86, 4363 (1964); F. W. Walker and E. C. Ashby,
J. Am. Chem. Soc., 91, 3845 (1969).
23 C. E. Holloway and M. Melinik, Coord. Chem. Rev., 135, 287 (1994); H. L. Uhm, in Handbook
of Grignard Reagents, G. S. Silverman and P. E. Rakita, eds., Marcel Dekker, New York, 1996,
pp. 117–144; F. Bickelhaupt, in Grignard Reagents: New Developments, H. G. Richey, Jr., ed., Wiley,
New York, 2000, pp. 175–181.
24
L. J. Guggenberger and R. E. Rundle, J. Am. Chem. Soc., 90, 5375 (1968); A. L. Spek, P. Voorbergen,
G. Schat, C. Blomberg, and F. Bickelhaupt, J. Organomet. Chem., 77, 147 (1974).