Thiocarbonyl compounds, nucleophilic reactions

The common method involves deprotonation of a thiocarbonyl compound and reaction of the intermediate enethiolate with an allyl halide (Scheme 9.8). This actually relies on two noticeable features of the sulfur series. (1) The proton located a to a thiocarbonyl group is much more acidic, by 7-10 pKa units, than the one of a carbonyl moiety [39, 41]. This may be related to the strong ability of the sulfur atom (polarizability) to stabilize the negative charge of the enethiolate. Presently, the preferred conditions involve LDA as a base for optimum deprotonation [42-45]. (2) The resulting anionic species are soft ambident nucleophiles. The... 

The complexes are reactive towards nucleophilic as well as electrophilic attack. Attack by alkyl halides yields thioalkylated compounds. Mostly an alkylation at one sulfur atom takes place, but examples of alkylation at both sulfur atoms are known (reactions 1 and 2). The facile sulfur abstraction with PPh3 yielding a thiocarbonyl complex is a nucleophilic reaction (reaction 3). 

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). 

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. 

Bis(trifluoromethyl)diazomethane is a reactive, electrophilic compound. It forms adducts with nucleophiles such as amines and phosphines and adds to olefins, acetylenes, and thiocarbonyl compounds to form heterocycles. It has been used as a source of bis(trifluoromethyl) carbene in reactions with benzene, saturated hydrocarbons, carbon disulfld< , and transition metal (iompounds, and it nndergoe.s a uni< im radicuil chain n aidion with saturat d hydrocarbons l.o form addiict.s that are hydrazonc.s and azines. ... 

The sulfur atom of the thiocarbonyl group is a good nucleophile, and reaction between benzyl bromide and l-(2-thiazolyl)thiourea yields the isothiouronium salt (496). The sulfur atom may also be engaged in a chelate, as exemplified by the Cu chelate of 2-thioureido-4-methylthiazole (491). These chelates with metal ions were thoroughly studied in acidic, neutral, and alkaline media for 66 metal ions in order to define their analytical use. They are formed in the molar ratio of 1 2 for metal II compounds (498). 

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. 

---