Page 585 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

566 Comparing the rates of acid-catalyzed ß-elimination of compounds of the type
MCH CH OH yields the reactivity order for ß-substituents IHg ∼ Ph Pb ∼ Ph Sn >
2 2 3 3
CHAPTER 5 Ph Si > H. The relative rates are within a factor of ten for the first three, but these
3
6
Polar Addition are 10 greater than for Ph Si and 10 11 greater than for a proton. There are two
and Elimination 3
Reactions factors involved in these very large rate accelerations. One is bond energies. The
relevant values are Hg−C= 27 < Pb−C= 31 < Sn−C = 54 < Si−C = 60 < H−C =
96kcal/mol. 306 The metal substituents also have a very strong stabilizing effect for
carbocation character at the ß-carbon. This stabilization can be pictured either as a
orbital-orbital interaction in which the carbon-metal bond donates electron density to
the adjacent p orbital, or as formation of a bridged species.
M M
C C or C C

There are a number of synthetically valuable ß-elimination processes involving
organosilicon 307 and organotin 308 compounds. Treatment of ß-hydroxyalkylsilanes or
ß-hydroxyalkylstannanes with acid results in stereospecific anti eliminations that are
much more rapid than for compounds lacking the group IV substituent.

H
CH CH CH 2 OH H H
3
2
H H SO 4
2
) Si
(CH 3 3
CH CH CH 3 CH CH CH 2 CH CH CH 3
2
2
2
2
2
3
Ref. 309
H H
CH 3 OH H + H
H
Sn
Ph 3 CH
CH3 CH 3 3
Ref. 310
ß-Halosilanes also undergo facile elimination when treated with methoxide ion.
Br
NaOCH 3
CH (CH ) CHCHSi(CH ) CH (CH ) CH CHBr
3 3
3
2 3
2 3
3
Br
Ref. 311
306
D. D. Davis and H. M. Jacocks, III, J. Organomet. Chem., 206, 33 (1981).
307 A. W. P. Jarvie, Organomet. Chem. Rev. Sect. A, 6, 153 (1970); W. P. Weber, Silicon Reagents
for Organic Synthesis, Springer-Verlag, Berlin, 1983; E. W. Colvin, Silicon in Organic Synthesis,
Butterworths, London, 1981.
308 M. Pereyre, J. -P. Quintard, and A. Rahm, Tin in Organic Synthesis, Butterworths, London, 1987.
309
P. F. Hudrlick and D. Peterson, J. Am. Chem. Soc., 97, 1464 (1975).
310 D. D. Davis and C. E. Gray, J. Org. Chem., 35, 1303 (1970).
311
A. W. P. Jarvie, A. Holt, and J. Thompson, J. Chem. Soc. B, 852 (1969); B. Miller and G. J. McGarvey,
J. Org. Chem., 43, 4424 (1978).