Silyl radical half-lives

Values of a(a-29Si) for radicals 6-10 fall in the range 5.56-6.28 mT (Table 1) and are consistent with these radicals being the most planar silyl radicals reported to date. In addition, the steric and electronic properties of the substituents on silicon in structures 6-10 provide for highly persistent silyl radicals. Half-lives of 3 h, 1 day, 1.5 months, 5 days and 1 day at 15 °C have been measured for 6-10, respectively11. 

The reactions are radical chain processes (Scheme 3) and, therefore, the initial silyl radicals are generated by some initiation. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azocompounds are used from time to time depending on the reaction conditions. EtsB in the presence of very small amounts of oxygen is an excellent initiator for lower temperature reactions (down to —78°C). The procedures and examples for reductive removal of functional groups by (TMSlsSiH are numerous and have recently been summarized in the book Organosilanes in Radical Chemistry. ... 

Persistent and stable silyl radicals have attracted considerable attention [42]. Bulky aryl or alkyl groups that generally make carbon-centred radicals persistent [43,44] have a much weaker effect on the silyl radicals. The high reactivity of the Ph3Si radical contrary to the stable Ph3C radical is mentioned above. The decay of the trimesitylsilyl radical at 63°C follows a first-order kinetics with a half-life of 20 s [37]. Tri-tert-butylsilyl radical is also not markedly persistent showing the modest tendency of tert-h Ay groups to decrease pyramidalization... 

The most persistent trialkyl-substituted silyl radical is [(Me3Si)2CH]3Si , which at 20 °C follows a first-order decay with a half-life of 480 s [36]. An exceptionally stable diradical was isolated by reaction of l,l-dilithio-2,3,4,5-tetraphenylsilole with l,l-dichloro-2,3-diphenylcyclopropene, for which the structure 13 was suggested on the basis of EPR data and theoretical calculations... 

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