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

The tetrahydride complex OsH4{xant­(PiPr2)2} (1, xant­(PiPr2)2 = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) activates an ortho-C–H bond of benzophenone and acetophenone to give the osmaisobenzofuran derivatives OsH­{κ2-C,O-[C6H4C­(R)­O]}­{xant­(PiPr2)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(η2-H2)­{xant­(PiPr2)2}, generated in situ from the tetrahydrideborate RuH­(η2-H2BH2)­{xant­(PiPr2)2} (4) and 2-propanol, also activates benzophenone and acetophenone to afford the ruthenaisobenzofurans RuH­{κ2-C,O-[C6H4C­(R)­O]}­{xant­(PiPr2)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{κ2-C,O-[C6H3FC­(Me)­O]}­{xant­(PiPr2)2} (7), OsH­{κ2-C,O-[C6H4C­(C6H3F2)­O]}­{xant­(PiPr2)2} (8), and RuH­{κ2-C,O-[C6H3FC­(Me)­O]}­{xant­(PiPr2)2} (9) have been obtained from the ortho-C–H 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{κ2-C,O-[C­(R)­CHC­(Me)­O]}­{xant­(PiPr2)2} (R = Ph (10), H (11)). The ruthenafuran counterparts RuH­{κ2-C,O-[C­(R)­CHC­(Me)­O]}­{xant­(PiPr2)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{κ2-C,O-[C6H4C­(CHCHPh)­O]}­{xant­(PiPr2)2} (14) and OsH­{κ2-C,O-[C­(Ph)­CHC­(Ph)­O]}­{xant­(PiPr2)2} (15) and RuH­{κ2-C,O-[C6H4C­(CHCHPh)­O]}­{xant­(PiPr2)2} (16) and RuH­{κ2-C,O-[C­(Ph)­CHC­(Ph)­O]}­{xant­(PiPr2)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(PiPr2)2} (M = Os (18), Ru (19)). The decarbonylation of the substrate is also observed with α,β-unsaturated aldehydes. Thus, the reaction of 1 with 1-cyclohexene-1-carboxaldehyde gives OsH­(C6H9)­(CO)­{xant­(PiPr2)2} (20). Decarbonylation and dehydrogenation of the aldehyde to form the trans-dihydride OsH2(CO)­{xant­(PiPr2)2} (21) take place with cyclohexane carboxaldehyde.