2.3- Dihydrothiazol-2-ylidenes

Thiazol-2-ylidenes are less well studied than their imidazole and triazol analogues. Only one isolated and structurally characterised example is known in the literature, the 3-(2,6-diis opropylphenyl)thiazol-2-ylidene synthesised by Arduengo et al. in 1997 [2] (see Figure 6.8). The parent compound, 2,3-dihydrothiazol-2-ylidene, was generated in an argon matrix at 10 K from thiazol-2-carboxylic acid as the starting material [33] (see Figure 6.9). 

Photochemical decarboxylation results in the formation of 2,3-dihydrothiazol-2-ylidene which reverts back to its thermodynamically stable isomer thiazol either under prolonged radiation at 250 20 nm or upon warming. 

Figure 6.9 Generation of 2,3-dihydrothiazol-2-ylidene in an argon matrix.

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol 1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ab initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=S02) are best viewed as dihalocarbenc-SO complexes with a carbon-sulfur bond order of approximately zero.14 hi a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]+ ... 

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