Tris silane with alkyl radical

Kinetic data are not yet available for the reaction of tris(alkylthio)silanes with alkyl radicals. However, the bond dissociation energy of the Si-H bond in 10 is ca. 4 kcal/mol higher than that in (TMS)3SiH suggesting that the former may be poorer hydrogen donors [28]. 

Chatgilialoglu and Curran have found that allyltris(triniethylsilyl)silanes react with a variety of alkyl halides to provide allylation products via an Sh2 process mediated by the tris(trimethylsilyl)silyl (TTMSS) radical (Scheme 10.199) [523]. In this system the allylsilanes work as radical-allylating agents and TTMSS radical sources. We have used the reactivity of allyltris(trimefhylsilyl)silanes for allylsilyla-tion of alkenes and alkynes via a radical chain mechanism (Scheme 10.199) [524]. 

Chatgilialoglu and Curran synthesized a variety of allyl tris(trimethylsilyl)silanes bearing substituents at the 2-position (Scheme 26) [70], These allylsilanes underwent reaction with alkyl halides when heated with a radical initiator to give very good yields of allylated products. The reactions were relatively sensitive to electronic effects electrophilic radicals reacted well only with electron-rich allyl silanes and vice versa. One potential drawback of this methodology is that the reactions reported were all carried out at 80 °C or above, suggesting that relatively high temperatures are necessary for efficient reaction. 

The chemistry of )9-(thiocarbonyloxy)alkyl radicals stands in complete contrast to that of the (acyloxy)alkyl radicals, with elimination, while not the rule, being the norm [I]. The difference between the acyloxy and thiocarbonyloxy series is likely a consequence of the much weaker thiocarbonyl bond and the related higher stability of sulfur-centered radicals. The method has been developed in combination with the Barton deoxygenation method (Volume 1, Chapter 1.6) as a means of converting a vicinal diol, via the dixanthate, into an alkene (Scheme 33) [60-62]. Tributyltin hydride has been the reagent of choice for this reaction but it may also be conducted with the triethylsilane/benzoyl peroxide couple [63] and, doubtless, tris(trimethylsilyl)silane. 

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. 

The same research group has further performed radical carbonylation reactions on the same microreactor system [36]. First, alkyl halides were initiated and effectively reacted with pressurized carbon monoxide to form carbonyl compounds. The principle was subsequently successfully extrapolated to the multicomponent coupling reactions. 1-Iodooctane, carbon monoxide and methyl vinyl ketone were reacted in the presence of 2,2 -azobis(2,4-dimethylvaleronitrile) (V-65) as an initiator and tributyltin hydride or tris(trimethylsilyl)silane (TTMSS) as catalyst (Scheme 15). 

The radical-initiated allylation of alkyl halides with allyltris(trimethylsilyl)silanes proceeds via an SH2 process mediated by a tris(trimethylsilyl)silyl radical.225 The radical-allylating agents react with alkenes, alkynes, and aldehydes via a radical chain process to give the corresponding allylsilylation products.226... 

In a similar case, hydrosilylation of a 1,6-diene or 5-en-l-one with tris(trimethylsilyl)silane gives the cyclized products with a preferred as orientation of the alkyl groups7. The intermediate silylalkyl or siloxyalkyl radical, respectively, is trapped intramolecularly to give substituted cyclopentanes. 

The reduction of alkyl hahdes has been important in many syntheses. Sodium cyanoborohydride in HMPA will reduce alkyl iodides, bromides, and tosylates selectively in the presence of ester, amide, nitro, chloro, cyano, alkene, epoxide, and aldehyde groups [118]. Tri-n-butyltin hydride will replace chloro, bromo, or iodo groups with hydrogen via a free radical chain reaction initiated by thermal decomposition of AIBN [119]. Other functionality such as ketones, esters, amides, ethers, and alcohols survive unchanged. The less toxic tris(trimethylsilyl) silane can be used similarly [120]. 

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