Thioketones sterically crowded

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 stable, sterically crowded hexafluorothioacetone 5-imide (44) reacts smoothly with aromatic thioketones, but the reaction with 1 requires elevated temperatures. Unexpectedly, the product isolated after 3 h at 100 °C is 1,4,2-dithiazolidine (47). Its formation can be rationalized by the isomerization of 44 to give thiaziridine 45 being in an equilibrium with thionitrone 46, which then traps 1 to yield 47 (eq 19). 

Finally, ylides 52 and 69 were found to react regioselectively with aliphatic thioketones to give the sterically less crowded products of type 117 (36,162,163). 

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