7-MEMBERED HETERODIMETALLIC RING-SYSTEMS FROM (CONJUGATED DIENE) GROUP-4 METALLOCENE COMPLEXES AND ORGANOALUMINUM REAGENTS

The (s-trans/s-cis-eta4-butadiene)zirconocene reagent adds dimethylalu minium chloride to yield the heterodimetallacyclic main group/transiti on-metal system 2b, containing a seven-membered metallacyclic framewor k that exhibits a ''pi-agostic'' sigma-allyl zirconium interaction. Si milar compounds (2c-f, 5a, 6) are obtained upon reaction of (butadiene )zirconocene with Et2AlI and (butadiene)hafnocene with Me2AlCl or Et2A lI. Addition of Me2AlCl or Et2AlI to (isoprene)zirconocene proceeds re gioselectively to yield the corresponding heterodimetallacycles bearin g the methyl substituent at C-3 (i.e. near to the attached aluminium c enter). The iodide-bridged heterodimetallacycles exhibit dynamic NMR s pectra that indicate a rapid enantiomerization process on the NMR time scale (DELTAG(ent)not-equal almost-equal-to 11-12 kcal mol-1). It is assumed that this automerization reaction proceeds by rate-limiting al uminium-halogen bond cleavage leading to an acyclic mixed metal hetero dimetallic intermediate. This process is markedly inhibited by the pre sence of excess diethylaluminium iodide.