MECHANISM OF STEREOSPECIFIC ENDO ADDITION OF HYDRIDE TO CYCLOHEXADIENYLMANGANESE AND CYCLOHEXADIENYLRHENIUM COMPLEXES

The addition of hydride donors (NaBH4, LiBEt3H, etc.) to the ring in (cyclohexadienyl)M(CO)(NO)(L)+(M = Mn, Re; L = CO, PMe3) gives the cyclohexadiene complexes with the added hydride situated stereospecifically endo to the metal. IR and NMR show that (6-exo-PhC6Me5H)Re(CO)2NO+(1) reacts with Bu4NBH4in CH3CN at -35 °C to form a neutral formyl intermediate that more slowly converts to the cyclohexadiene complex. The manganese analogue reacts more rapidly with hydride donors than does 1 and does not show spectral evidence for a formyl intermediate; nevertheless, it is probable that a reactive formyl species is formed initially, followed by hydride migration to the cyclohexadienyl ring. With (6-exo-PhC6H6)Mn(NO)(L-L)+(6, L-L = dppe; 7, L-L = dppen), which do not contain a carbonyl ligand, hydride donors react by exo attack at the C-2 carbon to give the novel σ,π-allyl complexes (6-exo-Ph, 1-σ-3-5-η-C6H7)Mn(NO)(L-L); the structure of the σ,π-allyl product with L-L = dppen was verified by X-ray diffraction. The conclusion is reached that endo addition to a coordinated cyclohexadienyl ring requires a carbonyl ligand. For hydride additions to coordinated cyclic π-hydrocarbons in general, it is likely that the ability to form a formyl intermediate is a necessary but not sufficient condition to guarantee that the hydride will be situated endo in the final product. Indirect evidence, based on recent observations that certain organolithium reagents react with (cyclohexadienyl)Mn(CO)(NO)L+via initial carbonyl attack, suggests that the formyl intermediate converts to a metal carbonyl hydride prior to the hydride migration to the dienyl ring to give the ultimate endo-diene product. © 1990, American Chemical Society. All rights reserved.