RING CLOSING VERSUS CYCLIC ISOMERIZATION OF 1,6-DIENES BY RUTHENIUM ALLENYLIDENE COMPLEXES

摘要

Treatment of electron rich carbene precursor olefms, L_2~R (R= CH_2CH_2OMe, CH_2Mes), bearing a 2,4,6-trimethylbenzyl group on the N atom(s), with [RuCl_2(η~6-arene)]_2 (arene = p-cymene, hexamethylbenzene) can afford one of two products. Thus, cleavage of the bridge occurs to yield the expected carbene ruthenium(II) complex 5 or further a displacement reaction can occur to give chelated η~6-mesitylene carbene complexes 6 and 7 (Scheme 1). In a separate but complementary research area we also have interests in the rapidly developing chemistry of (η~6-arene)(N-alkylbenzimidazole)ruthenium (II), 8. Compounds 6 and 7 were transformed into the corresponding cationic allenylidene complexes (6 → 6', 7 → 7', 8→8') by AgOTf and propargylic alcohol HC ≡ CCPh_2OH. In the same cases the in situ generated intermediates were found to be active and selective catalysts for ring closing metathesis (RCM) or cycloisomerization reactions of 1,6-dienes.