The substitution of a functional group for a hydrogen atom is a very important transformation in synthetic organic chemistry. Reactions such as deoxygenations, deselenations, deaminations, dehalogenations, and decarboxylations fall into this category. Such reactions can be conducted efficiently using free radical chemistry. These defunctionalizations can be combined with C-C-bond forming processes. Radical reactions are generally conducted under very mild conditions. Various sensitive functional groups are tolerated under free radical conditions. Tin hydrides (Bu 3 SnH, Ph 3 SnH, Me 3 SnH) have been successfully employed in radical chemistry over the last 40 years, however there are drawbacks associated with tin-based chemistry. Organotin compounds are toxic and very often problems occur with product purification. Therefore, various attempts have been made to overcome these problems. In the present review article, we summarize the achievements on the development of tin hydride substitutes in reductive radical chain reactions.
Various silylated 1,4-cyclohexadienes are presented as superior tin hydride substitutes for the conduction of various radical chain reductions. Debrominations, deiodinations, and deselenations can be performed using these environmentally benign reagents. Furthermore, Barton-McCombie-type deoxygenations using silylated cyclohexadienes are described. Radical cyclizations, ring expansions, and Giese-type addition reactions with the new tin hydride substitutes are presented. The polymerization of styrene can be regulated using silylated cyclohexadienes. Rate constants for hydrogen atom abstraction from two 1-silyl-cyclohexadienes by primary C-radicals were determined. The effects of the cyclohexadiene substituents on the reaction outcomes are discussed. Finally, qualitative EPR experiments on silyl radical expulsion from silylated cyclohexadienyl radicals are presented.
[reaction: see text] A new method for mild metal-free hydrosilylation is described. Silylated cyclohexadienes are used as radical transfer hydrosilylating reagents for various double and triple bonds. A trialkylsilane is transferred from a cyclohexadiene moiety to an alkene. The hydrosilylation can be combined with a C-C bond formation as shown for the preparation of silylated cycloalkanes from the corresponding dienes.
Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday A new method for the mild radical hydrosilylation of alkenes and alkynes is described. Silylated cyclohexadienes that can be readily prepared on large scale are used as radical hydrosilylating reagents. Nonactivated alkenes and alkynes are hydrosilylated in high yields. The reaction can be combined with CÀC bond formation, as demonstrated for the preparation of silylated cycloalkanes from the corresponding dienes. Furthermore, radical hydrosilylations in combination with b-fragmentation reactions for the synthesis of allylsilanes and hydrosilylations of aldehydes and ketones providing protected alcohols can be readily performed by this strategy.
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