Triphenyl imidazol-substituted

Different synthetic routes have been used to prepare these carbenes (Scheme 8.6). The most common procedure is the deprotonation of the conjugate acid. In early experiments, sodium or potassium hydride, in the presence of catalytic amounts of either f-BuOK or the DMSO anion were used. ° Then, Herrmann et al. showed that the deprotonation occurs much more quickly in liquid ammonia as solvent (homogeneous phase), and many carbenes of type IV have been prepared following this procedure. In 1993, Kuhn and Kratz" developed a new and versatile approach to the alkyl-substituted N-heterocyclic carbenes IV. This original synthetic strategy relied on the reduction of imidazol-2(3//)-thiones with potassium in boiling tetrahydrofuran (THF). Lastly, Enders et al." reported in 1995 that the 1,2,4-triazol-5-ylidene (Vila) could be obtained in quantitative yield from the corresponding 5-methoxy-l,3,4-triphenyl-4,5-dihydro-l//-l,2,4-triazole by thermal elimination (80 °C) of methanol in vacuo (0.1 mbar). 

Reactions of a number of l-phenylimidazole-2-carboxamides with chlorine in acetic acid, NCS, or hypochlorite failed to introduce chlorine into the 4- or 5-positions (80JHC409). Chlorination of a variety of 2,4-disubstituted imidazoles, however, was quite facile, Thus, 2,4-diesters [83JCS(P1)809] and 2-amino-4-aryl compounds [80IJC(B)526] were readily 5-chlorinated, and even when both the 4- and the 5-positions were blocked, as in 5-aminoimidazole-4-carboxamide, 2-chlorination with iodine monochloride was possible (89MI5). When all three carbons were substituted (e.g., in 2,4,5-triphenyl-, 2-chloro-4,5-diphenyl-, and 2-trichloromethyl-... 

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