Si–O Bond Formation Mediated by an Osmium-Polyhydride: Dehydrogenative Silylation of Alcohols and Tandem Hydrosilylation/Dehydrogenative Silylation of Salicylaldehydes

High Resolution Image Download MS PowerPoint Slide OsH 5 (SiHPh 2 )(P i Pr 3 ) 2 ( 1 ) catalyzes the monoalcoholysis of diphenylsilane with a variety of alcohols. Density functional theory (DFT) calculations suggest that the reactions occur via a highly ordered transition state resulting from the nucleophilic attack of the alcohol to the silane coordinated to the osmium center in an η 1 -H–SiHPh 2 fashion. The alcoholysis or aminolysis of the Si–H bond of 1 with 2-hydroxypyridine or 2-aminopyridine affords OsH 3 {κ 2 - Si,N -(SiPh 2 -E-py)}(P i Pr 3 ) 2 (E = O ( 4 ), NH ( 5 )). Analogously, OsH 4 (SiH 2 Ph) 2 (P i Pr 3 ) 2 ( 2 ) reacts with 2-hydroxypyridine and 2-aminopyridine to give OsH 3 {κ 2 - Si,N -(SiPh(Epy)-E-py)}(P i Pr 3 ) 2 (E = O ( 6 ), NH ( 7 )), as a result of the alcoholysis or aminolysis, respectively, of both Si–H bonds of one of the phenylsilyl ligands. Additionally, 1 catalyzes the tandem hydrosilylation/dehydrogenative silylation of salicylaldehydes with diphenylsilane to afford silacycles. DFT calculations suggest that this process happens via an outer-sphere hydrogenation of the aldehyde moiety to give a diol and tetrahydride-silylene OsH 4 (=SiPh 2 )(P i Pr 3 ) 2 . Next, the silylative dehydrogenation of the Ph–OH function affords a silyl-O-functionalized pentahydride, which, upon the nucleophilic intramolecular attack of the benzylic OH group, gives the silacycle and intermediate OsH 4 (η 2 -H 2 )(P i Pr 3 ) 2, which reacts with diphenylsilane, giving H 2 and regenerating OsH 5 (SiHPh 2 )(P i Pr 3 ) 2 .