Thioketones, protonation

Oxazoline and thiazoline derivatives metalated at the 2-position can also be prepared by the combination of acyclic precursors, namely by the reaction of a-metalated isocyanides with ketones or thioketones (Scheme 141) [76LA183 77AG(E)339 79PAC1347]. Although this route is normally used to produce 2-unsubstituted derivatives via protonation, there is presumably no reason why other electrophiles could not also be utilized instead. 

Complexes with a,/3-unsaturated thioketones as ligands (49) were obtained in addition to other compounds when [(CO)5M=C(OEt)Aryl] (M = Cr, Aryl = Ph, C4H6Ph-o M = W, Aryl = Ph) were treated at -78°C with LiC(S)NMe2 [prepared from HC(S)NMe2 and LiNEt2 at -100°C] and then protonated at ambient temperature with acetic acid [Eq. (10)]. However, the yield of 49 was rather low (3-8%).96... 

Complexes with related chelating thioketone ligands are reversibly formed on protonation of enethiolato complexes of iron.101... 

A series of thioketones have also been protonated to form carbosulfonium ions 358541,542 and studied by 13C NMR spectroscopy.665 It was found that deshielding of the thiocarbonyl carbon is much less pronounced than that of ketones. This shows that there is significant mercaptocarbenium 358b contribution to the overall protonated thioketone structure. However, this is not the case in the case of protonated ketones. 

A third method of access to sulfines is the oxidation of thiocarbonyl compounds. When the starting material is available it is an attractive route. There has been some dispute in the past whether enethiolisable thiocarbonyl derivatives would lead to the corresponding sulfines or to divinyl disulfides [101, 102]. It is now clear from our research that, even if the C=S molecules bear highly acidic a-protons, oxidation occurs on C=S and does not touch the a-protons. There are many examples of this behaviour. The most easily enethiolised compounds are thioketones. We have shown that their reaction with a peroxycarboxylic acid, mCPBA, is very fast at 0°C and quantitatively provides the corresponding sulfines [103]. In many examples the aliphatic sulfines are not very stable and have to be used in subsequent reactions that will be faster than their decomposition (t1/2 from some hours to days). 

It is interesting to note that nonsilylated enethiols have only been prepared as mixtures with their isomeric thioketones.4a,c The first selective synthesis of aliphatic enethiols was performed by Metzner52 by deprotonation of enethiolizable thioke-tones 42 with LDA and reaction of the enethiolates with trimethylsilylchloride. The subsequent methanolysis of silyl vinyl sulfides 43 afforded enethiols devoid of isomeric thioketones. Treatment of the enethiolates with various proton sources afforded instead mixtures of thioketones and enethiols (Scheme 31). 

An example of the formation of 1,2-dithiolium salts by protonation of an alkylidene-l,2-dithiole is the decarboxylation of 1,2-dithiol-3-ylidene malonic acids (14) in the presence of perchloric acid. In this particular case, a strongly acidic reaction medium is necessary for obtaining satisfactory yields because the corresponding 3-aryl-5-methylene-l,2-dithiole is unstable and undergoes an autocondensation reaction leading to an a-(thiopyran-2-ylidene)thioketone (cf. Ref. 1 p. 83). 

Enolisable thioketones (i.e. those possessing an a-hydrogen atom) like thiocyclohexanone (15) are easily methylated by treatment with diazomethane (Scheme 14a). However, non-enolisable thioketones (6) behave differently and yield the episulfides (23) (Scheme 14b). The first reaction (Scheme 14a) is the normal methylation of the acidic enethiol proton by diazomethane. The second reaction (Scheme 14b) involves initial nucleophilic attack by the diazomethane molecule on the electrophilic thiocarbonyl carbon atom... 

The protiodesilylation of an a-trimethylsilyl thioketone 5-oxide with TBAF (the proton presumably comes from the water present) gives the aryl thioaldehye 5-oxide in excellent yield117 (equation 117). 

A number of unsaturated thioketones, bearing only aliphatic substituents, could be isolated [61, 62]. When the products possess protons on the a carbons, they were accompanied by the tautomeric enethiols [62]. When this form was favored, some authors carried out the [3,3] shift in acetic anhydride as a solvent and isolated the corresponding thiolester [63, 64]. Recently, the rearrangement of tung-sten(pentacarbonyl) complexes of sulfides was reported to lead to the corresponding thiocarbonyl derivatives [65]. 

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