Thiono esters reductive deoxygenation

Scheme 5.9 illustrates some of the conditions that have been developed for the reductive deoxygenation of alcohols. Entries 1 to 4 illustrate the most commonly used methods for generation of thiono esters and their reduction by tri-M-butylstannane. These include formation of thiono carbonates (Entry 1), xanthates (Entry 2), and thiono imidazolides (Entries 3 and 4). Entry 5 is an example of use of dimethyl phosphite as the hydrogen donor. Entry 6 uses r .s-(trimethylsilyl)silane as the hydrogen atom donor. 

Tris(trimethylsilyl)silane is found to be an efficient reducing agent for a variety of functional groups. In particular, the reductions of halides, chalcogen groups, thiono esters (i.e. deoxygenation of secondary alcohols) and isocyanides (i.e. deamination of primary amines) are the most commonly described applications. The efficiency of these reactions is also supported by available kinetic data. 

Chatgilialoglu studied (MesSijsGeH as a new radical hydrogen donor, and found that reduction of chlorides, bromides, and iodides, deoxygenation of secondary alcohols via a thiono ester (Barton-McCombie reaction), deamination of primary amines via isocyanides, removal of PhSe group, and replacement of a tertiary nitro group by hydrogen were extremely effective (Scheme 11.15) [31]. 

Equation (24) has been chosen as an example of the radical deoxygenation of secondary alcohols via thiono esters [58], whereas Eq. (25) represents an example of deamination of primary amines via isocyanides [7, 54]. The reaction mechanism of these reductions is similar to that described for tin hydride, i.e. attack of silyl radical on the C=S or N=C moieties to form a radical intermediate which undergoes -scission to form alkyl radicals. Hydrogen abstraction from the hydride gives the product and the (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Tri-n-butyltin hydride also serves as a hydrogen atom donor in radical-mediated methods for reductive deoxygenation of alcohols. The alcohol is converted to a thiocarbonyl derivative. These thiono esters undergo a radical reaction with tri-n-butyltin hydride. This procedure gives good yields with secondary alcohols and, with appropriate adjustment of conditions, can also be adapted to primary alcohols. Scheme 5.7 illustrates some of the conditions that have been developed for the reductive deoxygenation of alcohols. 

Deoxygenation of primary and secondary alcohols.1 This deoxygenation has been effected by reduction of the thiocarbonyl esters with tributyltin hydride and AIBN as the radial initiator (11, 550). A newer, milder method uses diphenylsilane in a radical chain reaction initiated by triethylborane and air. Even secondary thiono-carbonates, particularly those derived from 4-fluorophenol, are deoxygenated at 25°. 

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