COUPLING REACTIONS .11. OXIDATIVE COUPLING OF 2-ALKYL-6,6-DIMETHYLPENTAFULVENYL ANIONS

CuCl2-Induced oxidative coupling of 2-(tert-butyl)-6,6-dimethylpentafu lvenyl anion 9 predominantly takes place at C(7) and C(5) to give [7-7 ] and [7-5] coupling products 15 and 16 in 35 and 47% yields, respecti vely (Scheme 3) whose structures are elucidated from ID- and 2D-NMR an alysis. Compared with the product distribution observed for 6,6-dimeth ylpentafulvenyl anion 2 (Scheme 1), no coupling at C(2)/C(3) of 9 is o bserved. This means that, besides electronic effects, steric effects a re also important in oxidative couplings of fulvenyl anions. The same couplings occur in the case of ,3-bis(6,6-dimethylfulven-2-yl)-2,3-dim ethylbutane dianion 10 as well, but, due to electronic as well as conf ormational effects (Scheme 5), intermolecular coupling (to give polyme rs 17, Scheme 4) is strongly favored over intramolecular coupling. Mec hanisms explaining base-catalyzed isomerization 15a reversible 15b rev ersible 15c (Scheme 6) as well as isomerization 16a reversible 16b (Sc heme 7) are proposed.