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

The tetrahydride complex OsH4{xant(PPr2)2} (1, xant(PPr2)2 = 9,9-dimethyl-4,5bis(diisopropylphosphino)xanthene) activates an ortho-C-H bond of benzophenone and acetophenone to give the osmaisobenzofuran derivatives OsH{κ-C,O-[C6H4C(R)O]}{xant(PPr2)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(ηH2){xant(PPr2)2}, generated in situ from the tetrahydrideborate RuH(η-H2BH2){xant(PPr2)2} (4) and 2-propanol, also activates benzophenone and acetophenone to afford the ruthenaisobenzofurans RuH{κ-C,O-[C6H4C(R)O]}{xant(PPr2)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{κ-C,O-[C6H3FC(Me)O]}{xant(PPr2)2} (7), OsH{κ-C,O[C6H4C(C6H3F2)O]}{xant(PPr2)2} (8), and RuH{κ-C,O-[C6H3FC(Me)O]}{xant(PPr2)2} (9) have been obtained from the ortho-CH bond activation of the corresponding substrates. Complex 1 also promotes the Cβ-H bond activation of benzylidenacetone and methyl vinyl ketone to afford the osmafurans OsH{κ-C,O-[C(R)CHC(Me)O]}{xant(PPr2)2} (R = Ph (10), H (11)). The ruthenafuran counterparts RuH{κ-C,O-[C(R)CHC(Me)O]}{xant(PPr2)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{κ-C,O[C6H4C(CH=CHPh)O]}{xant(PPr2)2} (14) and OsH{κ-C,O-[C(Ph)CHC(Ph)O]}{xant(PPr2)2} (15), and RuH{κ-C,O[C6H4C(CH=CHPh)O]}{xant(PPr2)2} (16) and RuH{κ-C,O-[C(Ph)CHC(Ph)O]}{xant(PPr2)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)2} (M = Os (18), Ru (19)). The decarbonylation of the substrate is also observed with α,βunsaturated aldehydes. Thus, the reaction of 1 with 1-cyclohexene-1-carboxaldehyde gives OsH(C6H9)(CO){xant(PPr2)2} (20). Decarbonylation and dehydrogenation of the aldehyde to form the trans-dihydride OsH2(CO){xant(PPr2)2} (21) take place with cyclohexane carboxaldehyde.