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.