INTERMOLECULAR COUPLING REACTIONS OF INVERSELY POLARIZED CARBENE LIGANDS STARTING FROM THE CP-2-ASTERISK-TI=C=CH2 INTERMEDIATE AND CHROMIUMCARBENE COMPLEXES

The titanaallene intermediate [Cp-2Ti=C=CH2] (2), generated thermally from Cp-2Ti(CH=CH2)CH3 (1), reacts with methoxyalkyl carbene complex es (CO)(5)Cr=C(OCH3)R (3) [R = CH3 (a), CD3 (b), Et (c)] by CC-couplin g of the carbene ligands to give the heterodinuclear complexes Cp-2 < (TiCH2C(=C(OCH3)R)C(=Cr(CO)(5))O)over bar> (4) and Cp-2<(TiCH2C(=C(mu -OCH3)R)C(=Cr(CO)(4))O)over bar> (5). An unusual isomerization of the exocyclic double bond, which occurs by thermal conversion 4c --> 5c, c an be explained considering ionic resonance forms. The addition of one CO molecule to 5c leads to a displacement of the methoxy coordination . On the other hand, the aminocarbene complex (CO)(5)Cr=C((NHPr)-Pr-1) CH3 (8), which is less electrophilic than alkoxycarbenes, reacts with 2 to give a dinuclear dititanacyclobutene complex (9). Its blue-violet colour is probably caused by a twist around the single bond C(beta)-C (beta) of the dititanacyclobutene unit. (C) 1998 Elsevier Science S.A.