"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Tris(trimethylsilyl)silane 53
Thermolysis of tert-butyl perbenzoate [PhC(O)OOBu-t] and di-tert-butyl peroxide
(t-BuOOBu-t), whose t 1=2 are 1 h at 125 8C and 147 8C, respectively, has
been used from time to time.
Photochemically generated radicals in chain reactions are less familiar
to synthetic chemists [8,21]. The above mentioned peroxides have been
used in the presence of light to initiate radical chain reactions at room
or lower temperatures. Azo compounds are also known to decompose photolytically
to afford alkyl radicals. AIBN has rarely been used under such conditions.
Triethylborane in the presence of very small amounts of oxygen is an excellent
initiator for radical chain reactions. For a long time it has been known
that trialkylboranes R3B react spontaneously with molecular oxygen to give
alkyl radicals (Reaction 4.7), but only recently has this approach successfully
been applied as the initiation [22]. The reactions can be run at temperatures
as low as 78 8C, which allow for a better control of stereoselectivity (see
below).
R3B þ O2 !R2BOO: þ R: (4:7)
A correct choice of the initiator is generally decided according to the experimental
conditions. For example, a-cyanoalkyl radicals derived from AIBN are
capable of abstracting a hydrogen atom from (TMS) 3SiH, but not from
Ph2SiH2. Both alkoxyl and aryloxyl radicals are able to abstract a hydrogen
atom efficiently even from Et3SiH, that has one of the strongest Si w H bond
energies. It is advisable to add the initiators slowly during the course of the
reaction, either in solution or portion-wise, with due regard for the half-life of
their decomposition at the operating temperature of the reaction. The direct
addition at once of the thermal initiator together with temperatures much
higher than those of 1 h half-life are generally not the optimal conditions. At
low temperatures, when the thermal initiation is obviously impractical, the
choice of Et3B=O2 initiation is particular useful in combination with
(TMS) 3SiH as reducing agent. Alternatively, photoinitiation using AIBN or
dibenzoyl peroxide could be useful.
4.3 TRIS(TRIMETHYLSILYL)SILANE
The synthetic application of free radical reactions has increased dramatically
since the mid-1980s. Tributyltin hydride is the most commonly used reagent
for the reduction of functional groups. However, it was later shown that organotin
compounds are toxic and environmentally not tolerated, and they are
difficult to remove from non-polar reaction products [1–7]. It becomes now
more evident that it is inappropriate to propose these reagents for application
in medicinal chemistry. Therefore, a great deal of research has been devoted
to finding new reagents with favourable characteristics from toxicological