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

The tetrahydride complex OsH₄{xant­(P^<i>i</i>Pr₂)₂} (<b>1</b>, xant­(P^<i>i</i>Pr₂)₂ = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) activates an <i>ortho</i>-C–H bond of benzophenone and acetophenone to give the osmaisobenzofuran derivatives OsH­{κ²-C,O-[C₆H₄C­(R)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (R = Ph (<b>2</b>), CH₃ (<b>3</b>)). The reaction of <b>1</b> with perdeuterated benzophenone leads to <b>2</b> partially protiated. The deuterium distribution in the latter suggests that the carbonyl group of the ketone traps the <i>ortho</i>-C–H addition product, which is the most disfavored from a kinetic point of view. The ruthenium counterpart RuH₂(η²-H₂)­{xant­(P^<i>i</i>Pr₂)₂}, generated <i>in situ</i> from the tetrahydrideborate RuH­(η²-H₂BH₂)­{xant­(P^<i>i</i>Pr₂)₂} (<b>4</b>) and 2-propanol, also activates benzophenone and acetophenone to afford the ruthenaisobenzofurans RuH­{κ²-C,O-[C₆H₄C­(R)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (R = Ph (<b>5</b>), CH₃ (<b>6</b>)). 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(P^<i>i</i>Pr₂)₂} (<b>7</b>), OsH­{κ²-C,O-[C₆H₄C­(C₆H₃F₂)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (<b>8</b>), and RuH­{κ²-C,O-[C₆H₃FC­(Me)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (<b>9</b>) have been obtained from the <i>ortho</i>-C–H bond activation of the corresponding substrates. Complex <b>1</b> also promotes the C_β–H bond activation of benzylidenacetone and methyl vinyl ketone to afford the osmafurans OsH­{κ²-C,O-[C­(R)­CHC­(Me)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (R = Ph (<b>10</b>), H (<b>11</b>)). The ruthenafuran counterparts RuH­{κ²-C,O-[C­(R)­CHC­(Me)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (R = Ph (<b>12</b>), H (<b>13</b>)) were similarly generated by using <b>4</b> in the presence of 2-propanol. The analogous reactions with benzylidenacetophenone yield mixtures of OsH­{κ²-C,O-[C₆H₄C­(CHCHPh)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (<b>14</b>) and OsH­{κ²-C,O-[C­(Ph)­CHC­(Ph)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (<b>15</b>) and RuH­{κ²-C,O-[C₆H₄C­(CHCHPh)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (<b>16</b>) and RuH­{κ²-C,O-[C­(Ph)­CHC­(Ph)­O]}­{xant­(P^<i>i</i>Pr₂)₂} (<b>17</b>). While the formation of the osmaisobenzofuran <b>14</b> is slightly favored with regard to that of <b>15</b>, no preference is observed for ruthenium. In contrast, both precursors favor OC–H activation over the cleavage of an <i>ortho</i>-C–H bond in aromatic aldehydes. Thus, their reactions with benzaldehyde yield MH­(Ph)­(CO)­{xant­(P^<i>i</i>Pr₂)₂} (M = Os (<b>18</b>), Ru (<b>19</b>)). The decarbonylation of the substrate is also observed with α,β-unsaturated aldehydes. Thus, the reaction of <b>1</b> with 1-cyclohexene-1-carboxaldehyde gives OsH­(C₆H₉)­(CO)­{xant­(P^<i>i</i>Pr₂)₂} (<b>20</b>). Decarbonylation and dehydrogenation of the aldehyde to form the <i>trans</i>-dihydride OsH₂(CO)­{xant­(P^<i>i</i>Pr₂)₂} (<b>21</b>) take place with cyclohexane carboxaldehyde