C-H Bond Activation Reactions in Ketones and Aldehydes Promoted by POP-Pincer Osmium and Ruthenium Complexes

The tetrahydride complex OsH4{xant((PPr2)-Pr-i)(2)} (1, xant((PPr2)-Pr-i)(2) = 9,9-dimethy1-4,5-bis(diisopropylphosphino)-xanthene) activates an ortho-C-H bond of benzophenone and acetophenone to give the osmaisobenzofuran derivatives OsH{k(2)-C,O-[C6H4C (R)O]}{xant ((PPr2)-Pr-i)(2)} (R = Ph (2), CH3(3)). The reaction of 1 with perdeuterated benzophenone leads to 2 partially protiated. The deuterium distribution in the latter suggests that the carbonyl group of the ketone traps the ortho-C-H addition product, which is the most disfavored from a kinetic point of view. The ruthenium counterpart RuH2(eta(2)-H-2){xant((PPr2)-Pr-i)(2)}, generated in situ from the tetrahydrideborate RuH(eta(2)-H2BH2){xant((PPr2)-Pr-i)(2)} (4) and 2-propanol, also activates benzophenone and acetophenone to afford the ruthenaisobenzofurans RuH{x(2)-C,O-[C6H4C(R)O]}{xant((PPr2)-Pr-i)(2)} (R= Ph (5), CH3 (6)). Both precursors favor the C-H bond activation over the C-F bond cleavage in fluorinated aromatic ketones. Thus, the fluorinated metalaisobenzofuran derivatives OsH{K-2-C,O-[C6H3FC(Me)O]}{xant((PPr2)-Pr-i)(2)} (7)) OsH{k(2)-C,O-[C6H4C(C6H3F2)O]}{xant((PPr2)-Pr-i)(2)} (8), and RuH{k(2)-C,O-[C6H3FC(Me)O]}{xant((PPr2)-Pr-i)(2)} (9) have been obtained from the ortho-C-H bond activation of the corresponding substrates. Complex 1 also promotes the C-beta-H bond activation of benzylidenacetone and methyl vinyl ketone to afford the osmafurans OsH{k(2)-C,O-[C(R)CHC(Me)O]}{xant((PPr2)-Pr-i)(2)} (R = Ph (10), H (11)). The ruthenafuran counterparts RuH{k(2)-C,O-[C(R)CHC(Me)O]}{xant((PPr2)-Pr-i)(2)} (R = Ph (12), H (13)) were similarly generated by using 4 in the presence of 2-propanol. The analogous reactions with benzylidenacetophenone yield mixtures of OsH{k(2)-C,O-[C6H4C (CH=CHPh)O]}{xant ((PPr2)-Pr-i)(2)} (14) and OsH{k(2)-C,O-[C(Ph)CHC(Ph)O]}{xant((PPr2)-Pr-i)(2)} (15) and RuH{k(2)-C,O-[C6H4C(CH=CHPh)O]}{xant((PPr2)-Pr-i)(2)} (16) and RuH{k(2)-C,O-[C(Ph)CHC(Ph)O]}{xant((PPr2)-Pr-i)(2)} (17). While the formation of the osmaisobenzofuran 14 is slightly favored with regard to that of 15, no preference is observed for ruthenium. In contrast, both precursors favor OC-H activation over the cleavage of an ortho-C-H bond in aromatic aldehydes. Thus, their reactions with benzaldehyde yield MH(Ph)(CO){xant((PPr2)-Pr-i)(2)} (M = Os (18), Ru (19)). The decarbonylation of the substrate is also observed. with alpha,beta-unsaturated aldehydes. Thus, the reaction of 1 with 1-cydohexene-1-carboxaldehyde gives OsH(C6H9) (CO){xant((PPr2)-Pr-i)(2)} (20). Decarbonylation and dehydrogenation of the aldehyde to form the trans-dihydride OsH2(CO){xant((PPr2)-Pr-i)(2)} (21) take place with cyclohexane carboxaldehyde.