Silylated Cyclohexadienes in Radical Chain Hydrosilylations

Abstract

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.

Transfer Hydrosilylation

“…5, 6 Amrein and Studer also realized that unsaturated radical acceptors such as X would allow the formal transfer of a hydrosilane from the cyclohexa‐1,4‐diene VII to the π‐system of X (Scheme ) 7. 8 After initiation, the cyclohexadienyl radical VIII transfers the silicon fragment to acceptor X ( VII → VIII → IX ) to yield β‐silicon‐substituted radical XI. XI then acts as the chain carrier, abstracting a hydrogen atom from VII ( VII → VIII ) concomitant with formation of the hydrosilylated acceptor XII ( XI → XII ).…”

“…Radical transfer hydrosilylation coupled with 5‐ exo ‐ trig cyclizations is shown for different transfer reagents in Scheme 7. 8 The level of diastereocontrol is generally low, but the slight preference for the cis relative configuration is explained with the Beckwith–Houk model for these ring closures. The transfer of t BuMe 2 SiH released from 1 proceeds under the previously employed setup (AIBN at 85 °C; 11 → 12 ).…”

“…It is noteworthy that even the transfer of Tamao’s (Et 2 N)Me 2 SiH from 3 works in reasonable yield, thereby providing a handle for further functional group manipulations ( 11 → 14 after treatment with isopropanol). Selected triple bonds also participated in the radical transfer hydrosilylation (not shown) 8…”

t-Butyldimethylchlorosilane

“…68 The 3-TBDMS-2,4-dimethoxy-1,4-cyclohexadiene was found to be a very efficient hydrogen donor reagent in various radical reactions. 67 They were prepared by TBDMSCl interception of the regioselective metallation intermediate of cyclohexadienes, and subsequent methylation (eq 40).…”

Silyl Formates as Surrogates of Hydrosilanes and Their Application in the Transfer Hydrosilylation of Aldehydes

“…To overcome these drawbacks,a no ption consists of the utilization of surrogates of hydrosilanes.I nt his respect, the group of Studer pioneered the use of silylated cyclohexa-1,4dienes (1,, obtained by Birch reduction of arenes with sodium and subsequent lithiation-silylation of the 1,4-CHDN,i nt he free-radical hydrosilylation of alkenes and aldehydes (Scheme 1). [5] This concept of transfer hydrosilylation (THSi)w as recently formalized by Oestreich et al, [6] as ameans to circumvent the use of gaseous Me 3 SiH or SiH 4 in the ionic hydrosilylation of alkenes with B(C 6 F 5 ) 3 , [7] and the authors further extended this methodology to the reduction of alkynes,ketones and imines. [8] In addition to the aforementioned safety concerns,t he formation of hydrosilanes from chlorosilanes involves highly reactive and energy-demanding main-group-based hydrides such as LiAlH 4 .…”