A convenient route to six-membered heterocyclic carbene complexes: Reactions of aminoallenylidene complexes with 1,3-bidentate nucleophiles

Abstract

Pentacarbonyl dimethylamino( methoxy)allenylidene complexes of chromium and tungsten, [(CO) 5M C C C(NMe 2)OMe] (M = Cr ( 1a), W ( 1b)), react with 1,3-bidentate nucleophiles such as amidines and guanidine, H 2N–C( NH)R (R = Ph, C 6H 4NH 2-4, C 6H 4NO 2-3, NH 2), by displacing the methoxy substituent to give exclusively dimethylamino(imino)-allenylidene complexes, [(CO) 5M C C C{N C(NH 2)R}NMe 2] ( 2a– 5a, 2b). Treatment of the chromium complexes 2a– 5a with catalytic amounts of hydrochloric acid or HBF 4 gives rise to an intramolecular cyclization. Addition of the terminal NH 2 substituent to the C α–C β bond of the allenylidene chain affords pyrimidinylidene complexes 6– 9 in high yield. In contrast to the chromium complexes 2a– 5a, the corresponding tungsten complex 2b could not be induced to cyclize due to the lower electrophilicity of the α-carbon atom in 2b. The dimethylamino( phenyl)allenylidene complex [(CO) 5Cr C C C(NMe 2)Ph] ( 10) reacts with benzamidine or guanidine similarly to 1a. However, the second reaction step – cyclization to give pyrimidinylidene complexes – proceeds much faster. Therefore, the formation of an imino(phenyl)allenylidene complex as an intermediate is established only by IR spectroscopy. The analogous reaction of 10 with 3-amino-5-methylpyrazole affords, via a formal [3+3]-cycloaddition, a pyrazolo[1,5 a]pyrimidinylidene complex 13. Compound 13 is obtained as two isomers differing in the relative position of the N-bound proton (1 H or 4 H). The related reaction of 10 with thioacetamide yields a thiazinylidene complex and additionally an alkenyl(amino)carbene complex.