Thiophilic addition to thiocarbonyl groups

Whereas the reactions of aromatic and unsaturated organomagnesium compounds with thiocarbonyl compounds are analogous to their reactions with carbonyl compounds (see Chapter 7), saturated alkylmagnesium compounds (apart from methyl) tend to react by thiophilic addition [1 ]  

The product is a new organomagnesium compound, which may be used in situ for further reactions reactions with dithioesters are particularly useful, giving functionalized dithioketals [2]. It is interesting to contrast the following example with the reaction involving carbophilic addition described in the same paper (see p. 148) [3]. 

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This fits with the general tendency for thiophilic addition to be favoured by a strong withdrawing group a to the thiocarbonyl function [328]. 

The radical deoxygenation of secondary alcohols through tri-n-butyltin hydride reduction of their xanthate derivatives was introduced in 1975 [227]. The reaction proceeds by thiophilic radical addition to the thiocarbonyl group of a tin radical followed by collapse of the carbon intermediate, as shown in the accompanying scheme. Hydrogen transfer completes the process. 

Many examples of the nucleophilic attack of organometallic compounds at the sulfur atom of thiocarbonyl groups were reported in the 1970s by Beak, Vialle, Ohno, Schaumann and their co-workers [119, 120, 327-329]. This thiophilic addition opens the way for a new method of preparation of carbanions a to sulfur. 

This "thiophilic" mode of reaction fits well with the other ionic and radical thiophilic additions on a thiocarbonyl group (see [120], and the Barton-McCombie radical thiophilic addition [227] previously discussed in Sections 4.2.2 and 3.2). With thioaldehydes, both C-C and C-S bonds were formed, as in the following case in which 0-pinene was used to trap thiobenzaldehyde [514] ... 

It has already been mentioned that thiocarbonyl compounds are much more reactive than their carbonyl congeners, but, at the same time, due to the low polarity of the C—S unit, they also react much less selectively. Thus, nucleophilic additions may occur either at the carbon (carbophilic addition, Section IV.C.l) or at the sulfur atom (thiophilic addition, Section IV.C.2), as shown in equation 111. This feature is in striking contrast with the behavior of the carbonyl group, which only undergoes nucleophilic attack on the electron-deficient carbonyl carbon, this being the cornerstone of the synthetic applications of oxo... 

Exceptions to the carbophilic addition of heteronucleophiles to thiocarbonyl compounds were demonstrated with thiols and thioaldehydes [160] or dithioesters [36] bearing an a-electron withdrawing group, which undergo selective thiophilic addition. 

A wide variety of thiophilic radicals induce fragmentation of 0-acyl thlohydroxamates by addition to the thiocarbonyl group, and in doing so generate carboxyl radicals (Scheme 1). After decarboxylation the... 

A new class of nucleophiles have been introduced for sulfur addition. Degl Innocenti and his group [145, 146] have shown that allyl or benzylsi-lanes, in the presence of tetra-n-butylammonium fluoride, reacted in a thiophilic fashion and led to allyl sulfides or dithioacetals. It is remarkable that this selective reaction is general for a large variety of thiocarbonyl compounds thioketones [145], dithioesters [146], and even with the normally sluggish trithiocarbonates [145]. With substituted allyl silanes retention of configuration of the allyl chain is observed. It is noteworthy that the possible [2,3] sigmatropic shift of the intermediate anionic species was not observed. 

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