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)5Mdouble bond; length as m-dashCdouble bond; length as m-dashCdouble bond; length as m-dashC(NMe2)OMe] (M = Cr (1a), W (1b)), react with 1,3-bidentate nucleophiles such as amidines and guanidine, H2N–C(double bond; length as m-dashNH)R (R = Ph, C6H4NH2-4, C6H4NO2-3, NH2), by displacing the methoxy substituent to give exclusively dimethylamino(imino)-allenylidene complexes, [(CO)5Mdouble bond; length as m-dashCdouble bond; length as m-dashCdouble bond; length as m-dashC{Ndouble bond; length as m-dashC(NH2)R}NMe2] (2a–5a, 2b). Treatment of the chromium complexes 2a–5a with catalytic amounts of hydrochloric acid or HBF4 gives rise to an intramolecular cyclization. Addition of the terminal NH2 substituent to the Cα–Cβ 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)5Crdouble bond; length as m-dashCdouble bond; length as m-dashCdouble bond; length as m-dashC(NMe2)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[1,5a]pyrimidinylidene complex 13. Compound 13 is obtained as two isomers differing in the relative position of the N-bound proton (1H or 4H). The related reaction of 10 with thioacetamide yields a thiazinylidene complex and additionally an alkenyl(amino)carbene complex.