Page 586 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Fluoride-induced ß-elimination of silanes having leaving groups in the ß-position 567
are important processes in synthetic chemistry, as, for example, in the removal of
ß-trimethylsilylethoxy groups. SECTION 5.10
Elimination Reactions
+ –
RCO CH CH Si(CH ) + R N F RCO 2 – + CH 2 CH 2 + FSi(CH )
3 3
4
2
2
2
3 3
Ref. 312
These reactions proceed by alkoxide or fluoride attack at silicon that results in C−Si
bond cleavage and elimination of the leaving group from the ß-carbon. These reactions
are stereospecific anti eliminations.
Nu:
)Si
(CH 3 3
) SiNu + RCH CHR + X –
(CH 3 3
RCH CHR
X
ß-Elimination reactions of this type can also be effected by converting a ß-hydroxy
group to a better leaving group. For example, conversion of ß-hydroxyalkylsilanes to
the corresponding methanesulfonates leads to rapid elimination. 313

CH SO Cl
) SiCH CR 3 2 H C CR
(CH 3 3 2 2 2 2
OH

-Trimethylsilylalkyl trifluoroacetates also undergo facile anti elimination. 314
The ability to promote ß-elimination and the electron-donor capacity of the
ß-metalloid substituents can be exploited in a very useful way in synthetic chemistry. 315
Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting interme-
diates then undergo elimination of the stannyl or silyl substituent, so that the net effect
is replacement of the stannyl or silyl group by the electrophile. The silyl and stannyl
substituents are crucial to these reactions in two ways. In the electrophilic addition
step, they act as electron-releasing groups that promote addition and control the regio-
chemistry. A silyl or stannyl substituent strongly stabilizes carbocation character at
the ß-carbon atom and thus directs the electrophile to the -carbon.

E
RCH CHMR' 3 + E + RCHCMR 3 RCH CHE
+

Computational investigations indicate that there is a ground state interaction
between the alkene orbital and the carbon-silicon bond that raises the energy of the

312
P. Sieber, Helv. Chim. Acta, 60, 2711 (1977).
313 F. A. Carey and J. R. Toler, J. Org. Chem., 41, 1966 (1976).
314 M. F. Connil, B. Jousseaune, N. Noiret, and A. Saux, J. Org. Chem., 59, 1925 (1994).
315
T. H. Chan and I. Fleming, Synthesis, 761 (1979); I. Fleming, Chem. Soc. Rev., 10, 83 (1981).