Tris silane with acyl radical

Generation of 3-indolylacyl radicals from the selenoesters 149, using either /j-Bu3SnH or tris(trimethylsilyl)silane (TTMSS) followed by reaction with various alkenes, offers a route to 3-acylindoles 150. On the other hand, the use of n-Bu Sn2 under irradiation gave cyclopent[6]indole derivatives such as 151 via a cascade involving initial addition of the acyl radical to the alkene, and a subsequent oxidative cyclization at the indole C-2 <02JOC6268>. 

Radical reactions with acyl radicals sometimes involve decarbonylation as side-reactions, especially when stabilized secondary or tertiary radicals can be formed. These side-reactions can be suppressed using low-temperature reaction conditions together with different reducing agents such as tris(trimethylsilyl)silane.254... 

Ryu, Sonoda and coworkers reported that tris(trimethylsilyl)silane is a useful mediator for a three-component coupling reaction [45]. Table 4 summarizes examples of radical carbonylations mediated by (TMS)3SiH. The first example shows a three-component coupling reaction in which hexyl iodide, CO, and acrylonitrile combine to form a P-cymo ketone. The CO addition step is in competition with the addition to the alkene and the hydrogen abstraction from radical mediator. Thus, it is anticipated that a set of less efficient hydrogen donors, such as (TMS)3SiH, and the use of a smaller excess amount of an alkene is most favorable. Indeed, the reaction can be carried out at only 20 30 atm of CO pressure, substantially below the 80-90 atm which is used for carbonylative acyl radical reactions which are mediated by tin hydride, and a nearly stoichiometric amount (1.2 equiv) of acrylonitrile is sufficient. Some other examples, which include vinyl radical carbonylation, are also shown in Table 4. 

Alkyl, alkenyl, aryl and acyl radicals can all be used in cyclization reactions. Acyl radicals can be generated by addition of alkyl radicals to carbon monoxide, or more conveniently from acyl selenides, and undergo a variety of radical reactions. A synthesis of the sesquiterpene (—)-kamausallene made use of the radical cyclization from the acyl selenide 69 (4.61). Tris(trimethylsilyl)silane and triethylborane in air were used to promote the reaction, which is highly selective (32 1) in favour of the cis stereoisomer 70, as expected from a chair-tike transition state. Best yields in the cyclization reactions of acyl radicals are found with electron-deficient alkenes, indicating the nucleophilic character of acyl radicals. 

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