Département de chimie moléculaire

The conjugate acids (PHCH+s) of P-heterocyclic carbenes (PHCs) are prepd. by formal [3 + 2] cycloaddn. of a 1,3-diphosphaallyl or 1,3-phosphinophosphenium cation with various nitriles. The effect of the phosphorus substituent on the fate of the cyclization and on that of the counteranion and base in the subsequent deprotonation reaction are reported. Two PHCs that are indefinitely stable in the solid state are described. In soln., one of them, made from acetonitrile, undergoes a facile [3 + 2] cycloreversion, whereas the other, based on di-Me cyanamide, is stable, presumably owing to its zwitterionic structure, which involves a tricoordinate pentavalent phosphorus atom. The reactivity of PHCs is strongly driven by the high electrophilicity of the phosphorus centers, as demonstrated by their reactivity with water and benzaldehyde. Although both PHCs reported in this paper are direct analogs of the least-basic NHCs, their basicity is comparable to those of the more strongly basic NHCs (as detd. by comparison of the carbonyl stretching frequencies of their corresponding cis-[RhCl(CO)2(L)] complexes). [on SciFinder(R)]