Thioketones General

The mechanism of the Hantzsch s synthesis was studied at a very early stage by several authors. The intermediates were generally assumed to be open-chain a-thioketones, but in a series of papers by Murav eva and Schukina (470, 490) the isolation of hydroxythiazolines from the reaction between a-haloketones and a variety of thioureas was reported. 

The torsional barrier of the amino group in thioamides is generally ca. 2 kcal/ mol higher than in the corresponding amides (26), and this trend is also found in the enamino thioketones (17,23 Table 2). The increased conjugative interaction in the thioamides is reflected in the Ct—C2 barriers, and the larger size of sulfur compared to oxygen affects the EIZ population ratio. 

The preparation of thiiranes is most conveniently performed in solution. However, there are also protocols reported for reaction in the gas and solid phase. By using diazo and thiocarbonyl compounds in ether as solvent, both alkyl and aryl substituted thiiranes are accessible. As indicated earlier, aryl substituents destabilize the initially formed 2,5-dihydro-1,3,4-thiadiazole ring and, in general, thiiranes are readily obtained at low temperature (13,15,35). On the other hand, alkyl substituents, especially bulky ones, enhance the stability of the initial cycloadduct, and the formation of thiiranes requires elevated temperatures (36 1,88). Some examples of sterically crowded thiiranes prepared from thioketones and a macro-cyclic diazo compound have been published by Atzmiiller and Vbgtle (106). Diphenyldiazomethane reacts with (arylsulfonyl)isothiocyanates and this is followed by spontaneous N2 elimination to give thiirane-2-imines (60) (107,108). Under similar conditions, acyl-substituted isothiocyanates afforded 2 1-adducts 61 (109) (Scheme 5.23). It seems likely that the formation of 61 involves a thiirane intermediate analogous to 60, which subsequently reacts with a second equivalent... 

The thiocarbonyl group is a highly reactive dipolarophile and in general this group dominates the reactivity of nonenolisable exocyclic thioketones as illustrated for the systems shown 5-methylene-2-thioxo-l,3-thiazolinin-4-one (260) (161), pyrimidone-2- and -4-thiones (261, 262) (134), pyrazolo[l,5,4-e/][l,5]benzodi-azepin-6-thione (263) (162). 2-Thiono-4-imidazolidinone (264) also gave a C=S cycloadduct as expected but, in the case of the analogue 265 with an additional exocyclic methylene group, the latter proved to be more reactive (163). 

It is very well known that polymers of high commercial value are obtained from formaldehyde by addition polymerization of its carbon-oxygen double bond. Not so well known is the addition polymerization capability of the carbon-sulfur double bond, probably because none of the polymers so obtained has yet become commercially acceptable. However, the polymerization chemistry of the carbon-sulfur double bond has been the subject of a number of studies and these have defined the preparation and properties of polythioformaldehyde, polythio-acetone, polymers from a small number of higher thioketones, and polymers from fluorine analogs of thioaldehydes and thioketones. The monomers have great reactivity beyond polymerization, and their general chemistry has been discussed in earlier reviews (/, 2). 

The conformational properties of thioketones of the five-membered heterocycles 148 and 149 appear strictly similar (67SA(A)2605 68BSF703 69BSF831 84JST(112)85 85RTC9) to those of the corresponding ketones, and the X,S-cis orientation is preferred as a general rule. The mesomeric effect... 

Bis-alkylidene and periluoromethylated 1,3-dithietanes are generally quite stable and only decompose at temperatures exceeding 500 °C, resulting in the formation of monomers, thioketenes and thioketones (Scheme 19) (61JA2589,70JOC3470, 77CB1225). 

Several efforts have been made to develop general rules for the induction of CD in the absorption bands of achiral compounds. Hayward was able to correlate the CD induced in the n->7r transition of a series of 30 oxyketones and thioketones (when these were dissolved in (S)-propylene carbonate) with the structures of these solutes [18],... 

Thiocarbonyl compounds have recently emerged as synthetic tools with specific properties. Although some thioamides and thioketones were prepared as early as the 19th century, general methods are rather recent. Nowadays most, if not all, thiocarbonyl compounds that one can imagine can be prepared, with techniques adapted to the stabilities of the target molecules. 

The absence of a convenient method for the preparation of enethiolisab-le thioketones prompted us to fulfil this need. The development of aliphatic thioketones has so far been mostly restricted to examples with steric protection, such as thiocamphor and adamantanethione. Among the difficulties for a general synthesis of aliphatic and acyclic thioketones were easy enethiolisation (enethiols are stable tautomers with thermodynamic stabilities close to those of the thione form), oligomerisation, and susceptibility to air oxidation. They were solved by the adaptation of a former German reac-... 

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. 

This reaction was intensely investigated by Mloston et al. In a classical, but not general reaction, they observed that heating aromatic or aliphatic thioketones [251, 252], thionoesters [253] and dithioesters [252, 253] with aryl-or benzylazides led to the formation of a carbon-nitrogen double bond. A [3+2] cycloaddition was assumed, with successive elimination of nitrogen and sulfur. 

Thioketones are generally readily available and undergo [4 + 2] cycloadditions under mild thermal conditions. Representative examples of this process are shown in equations (85), (86) and (87). ... 

Mercaptans are generally prepared by displacement reactions. However. secondary or hindered mercaptans are more difficult to obtain. The dithiolane cleavage icaction is a convenient in situ generation of thioketones which are known to be reduced with butyllithium to secondary mercaptans by p-hydrogen transfer Table I shows a number of mercaptans prepared from saturated thioketals in 78-90% yields. The aryl example gives lower yields partly because of ring metalation. 

Reactions of alkyl halides with diethyl phosphite in the presence of ammonium acetate/sulfur and acidic aluminia using microwave irradiation provide a simple and general route to thiophosphates. Two diastereoisomers of 1,3,2-dithiadiphosphetane 2-sulfide (19) have been isolated for the first time via the reaction of thioketones (20) with Lawesson s reagent (Figure 5). ... 

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