Reactions of the Schiff base rhodium hydride complex RhH(Bu(4)salophen) with olefins. Observations and mechanistic studies

The tetradentate dianionic Schiff base ligand Bu(4)salophenH(2) reacts with [RhCl(C2H4)(2)](2) and NR(4)OH (R = n-Bu, Et) to produce the complexes RkR(Bu(4)salophen) (1, R = n-Bu; 2, R = Et), which undergo photolysis (lambda > 475 nm) under a hydrogen atmosphere to generate RhH(Bu(4)salophen) (3) and the corresponding alkanes. In benzene at room temperature, olefins insert into the metal-hydride bond of 3 to produce Rh(CH(2)CH(2)R)(Bu(4)salophen) (2, R = Et; 4, R = Ph; 5, R = CN). In a competition experiment, insertion of styrene is only slightly favored over that of ethylene. For olefinic substrates capable of radical rearrangement (CH2=CHR where R = CH2OCH2CH=CH2 and c-C3H5), insertion products are formed that do not correspond to radical rearrangements. Specifically, Rh(CH2CH2OCH2CH=CH2)(Bu(4)salophen) (6) is observed as the only product of the reaction of 3 with bis(allyl) ether while Rh(CH2CH2-c-C3H5)(Bu(4)salophen) (7) is seen as the major product in the reaction with vinylcyclopropane. For the vinylcyclopropane reaction, additional products are also identified by 2D NMR methods as Rh(CH2CH2CH=CHCH3)(Bu(4)salophen) (8) and (Bu(4)salophen)Rh(mu-CH2CH2CH=CHCH2)Rh(Bu(4)salophen) (9). Both the poor selectivity for styrene versus ethylene insertion and the absence of radical rearranged product with bis(allyl) ether and vinylcyclopropane indicate that, in the reaction of 3 with olefins, an intermediate containing a pure carbon-based radical is not involved.