Zwitterionic adducts from addition of imines to the alpha-carbon atom of pentacarbonyl(vinylidene)chromium and -tungsten complexes

Imines, Im, such as MeN=C(Ph)H (5), 2-methyl 4,5-dihydrothiazole (8a), 2-me thyl 4,5-dihydrooxazole (8b) and MeN=C(OMe)Me (13) add to the alpha -carbon atom of the vinylidene ligand in [(CO)(5)Cr=C=CMe2] (4) to give isolable z witterionic adducts, [(CO)(5)Cr--C(=CMe2)(Im(+))]. The reaction of [(CO)(5) W=C=CPh2] (12) with 13 also yields an adduct, [(CO)(5)W--C(=CPh2){NMe=C(OMe )Me}(+)] (15), whereas from the corresponding reaction of 4 with xanthylide neimine, H-N=C(C6H4)(2)O (16), a carbene complex, [(CO)(5)Cr=C(i-Pr)-N=C(C6 H4)(2)O] (17), is obtained. Complex 17 presumably is formed by initial addi tion of 16 to 4 and subsequently rapid rearrangement. In solution, the addu ct [(CO)(5)Cr--C(=CMe2)(NMe=C(Ph)H)(+)] (6) slowly cyclizes to form the 2-a zetidin-1-ylidene complex [(CO)(5)Cr=C-NMe-C(Ph)H-CMe2] (7). In contrast, w hen solution of those zwitterions are heated that are formed by addition of 4,5-dihydrothiazole or 4,5-dihydrooxazole to 4, no cyclization is observed but rather the formation of 4,5-dihydrothiazole and 4,5-dihydrooxazole com plexes, respectively. The structures of two adducts, [(CO)(5)Cr--C(=CMe2)(I m(+))] (Im = MeN=C(Ph)H, 2-methyl 4,5-dihydrothiazole) and of the substitut ion product [(CO)(5)W(2-methyl 4,5-dihydrothiazole)] have been established by X-ray structural analyses. (C) 2001 Elsevier Science B.V. AU rights rese rved.