Page 865 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Reactions of tetraallylstannanes with aldehydes catalyzed by SnCl also appear to 841
4
involve a halostannane intermediate. It can be demonstrated by NMR that there is
a rapid redistribution of the allyl group. 174 Reactions with these halostannanes are SECTION 9.3
believed to proceed through a cyclic TS. Organotin Compounds

OX
(n-C H ) SnCl 2
4 9 2
RCH O + CH 2 CHCH Sn(n-C H ) RCHCH CH CH 2
2
4 9 3
2
RCOCl or X RCO or (CH ) Si
) SiCl 3 3
(CH 3 3
9.3.2.3. Reactions Involving Transmetallation. With certain Lewis acids, the reaction
may involve a prior transmetallation. This introduces several additional factors into
the analysis of the stereoselectivity, as the stereochemistry of the transmetallation
has to be considered. Reactions involving halo titanium and halo tin intermediates
formed by transmetallation can proceed through a cyclic TS. When TiCl is used as the
4
catalyst, the stereoselectivity depends on the order of addition of the reagents. When
E-2-butenylstannane is added to a TiCl -aldehyde mixture, syn stereoselectivity is
4
observed. When the aldehyde is added to a premixed solution of the 2-butenylstannane
and TiCl , the anti isomer predominates. 175
4
OH
CH O add
CH CH CHCH SnBu
TiCl 4 + 3 2 3
CH 3
CH O OH
add
4 3 3
TiCl + CH CH CHCH 2 SnBu
CH 3

The formation of the anti stereoisomer is attributed to involvement of a butenyltitanium
intermediate formed by rapid exchange with the butenylstannane. This intermediate
then reacts through a cyclic TS.

CH SnBu + TiCl 4 CH TiCl
2
3
CH 3 CH 3 2 3
OH
R O
Ti R
CH 3 CH 2
H CH 3
Indium chloride in polar solvents such as acetone or acetonitrile leads to
good diastereoselectivity with cyclohexanecarboxaldehyde and other representative
aldehydes. 176

174 S. E. Denmark, T. Wilson, and T. M. Willson, J. Am. Chem. Soc., 110, 984 (1988); G. E. Keck,
M. B. Andrus, and S. Castellino, J. Am. Chem. Soc., 111, 8136 (1989).
175 G. E. Keck, D. E. Abbott, E. P. Boden, and E. J. Enholm, Tetrahedron Lett., 25, 3927 (1984).
176
J. A. Marshall and K. W. Hinkle, J. Org. Chem., 60, 1920 (1995).