_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The main group elements
(a) HYDROSILATION. The addition of Si-H bonds to alkenes and alkynes is
used industrially in the production of intermediates for silicone manufacture
(p. 115).
X,SiH + H 2 C=CHY --> X,SiCH 2 CH 2 Y
X,SiH + HC=CH --> X,SiCH=CH 2 --> X,SiCH 2 CH 2 SiX,
Addition may occur by thermal reaction under pressure, but proceeds better
under UV irradiation or when catalysed. The thermal, photolytic or peroxide
catalysed reactions involve free-radical chain mechanisms. Radicals are first
produced from the silicon hydride,
which then add to the alkene by various pathways,
Cl,Si' +H 2 C=CH 2 --> CI,SiCH 2 CH 2 --> Cl,SiCH 2 CH 2 CH 2 CH 2
Cl,SiCH 2 CH, +'SiC!,
Chain transfer
.:f;j/ \:;· Chain propagation
Cl,SiCH 2 CH 2 R
Chain termination
The chain propagation step is often not very important as the silyl radicals are
very efficient chain transfer agents.
Catalysis by transition metal complexes such as H 2 PtCl 6 is thought to involve an
alkene complex of Pt and intermediates with covalent Pt-Si bonds. Addition
of transition metal hydrides to olefins and the ro le of such reactions in catalytic
processes are discussed in Chapter 12.
(b) HYDROSTANNATIDN. Addition of organotin hydrides to carbon-carbon
double and triple bonds provides a route to intermediates which are valuable in
organic synthesis (p. 228). When functional groups (E) are present, the Grignard
method is not applicable. Addition is accelerated by free radical initia tors such as
azoisobutyronitrile, or by ultraviolet irradiation.
(E = CONH 2 , CH(OEt) 2 , CH,OH, CN)
Homolytic addition to substituted alkynes is mostly regiospecific. Formation ofthe
E-isomer is favoured, although an E/Z mixture is often produced.
Where a strongly polar substituent is present in the unsaturated compound,
addition by a polar mechanism can occur.
24