Thiocarbonyl compounds nucleophilic additions

As discussed in Section II, thiocarbonyl compounds differ from their carbonyl counterparts in at least two important characteristics. Because of the higher energy of the sulfur p orbitals, they are much more reactive as electron donors. On the other hand, the C=S bond is also much less polarized than the C=0 bond, due to the smaller difference in electronegativities between carbon and sulfur. The latter fact leads to the reactions of the thiocarbonyl group being less selective than those of the carbonyl group. This happens, for instance, in the case of nucleophilic additions (see Section IV.C), and an enhanced reactivity against dipoles has also been observed (see Section IV.E.3). 

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... 

In the next section the formation of acyl anion equivalents by nucleophilic addition to thiocarbonyl compounds is discussed. A direct and non-classical route to thiocarbonyl anions has been achieved [141]. Treatment of a thiocarbamoyl chloride by lithium powder, in the presence of both naphthalene and the carbonyl compound to which the intermediate will be added, led to a-hydroxy thioamides. 

The thiophilic addition of carbon nucleophiles to thiocarbonyl compounds was a topic of intense investigation during the 1980s. 

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. 

A variety of nucleophiles have been reacted with thiocarbonyl compounds, usually for synthetic purposes exploiting the enhanced reactivity of the 2p-3p double bond between carbon and sulfur atoms. In contrast to the preceding part, most nucleophilic additions reported took place on the carbon atom very few cases of thiophilic addition were evidenced. 

Thiocarbonyl ylides are both nucleophilic and basic compounds (40,41,86). For example, adamantanethione (5)-methylide (52) is able to deprotonate its precursor, the corresponding 2,5-dihydro-1,3,4-thiadiazole, and a 1 1 adduct is formed in a multistep reaction (40,86). Thioxonium ion (56) (Scheme 5.22) was proposed as a reactive intermediate. On the other hand, thiofenchone (S)-methylide (48) is not able to deprotonate its precursor but instead undergoes electrocyclization to give a mixture of diastereoisomeric thiiranes (41,87,88). The addition of a trace of acetic acid changes the reaction course remarkably, and instead of an electrocyclization product, the new isomer 51 was isolated (41,87) (Scheme 5.18). The formation of 51 is the result of a Wagner-Meerwein rearrangement of thioxonium ion 49. 

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. 

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