Phenylacetylene with OsHCl

The reactivity of OsHCl(CO)(P Pr3)2 toward alkynes depends on the type of alkyne used. Whereas phenylacetylene, propyne, and acetylene react by insertion to give the five-coordinate alkenyl derivatives Os ( >CI I=CHR Cl(CO)(PIPr3)2 (R = Ph, Me, H),31,33 the reaction with methylpropiolate affords a mixture of Os C(=CH2)C(OMe)0 Cl(CO)(P,Pr3)2 and 0s ( )-CH=CHC02Me Cl(C0) (P Pr3)234 (Scheme 3), and tert-butyl acetylene and diphenylacetylene are inert. 

The complex OsHCl(CO)(P Pr3)2 is also an effective catalyst precursor for the selective hydrogenation of benzylideneacetone to 4-phenylbutan-2-one.97 In contrast to the hydrogenation of phenylacetylene, kinetic studies on the hydrogenation of benzylideneacetone by OsHCl(CO)(P Pr3)2 show that the reaction is independent of the pressure of hydrogen and first order with respect to the concentration... 

OsHCl(CO)(P Pr3)2 is also a very active and selective catalyst for the addition of triethylsilane to phenylacetylene at 60°C, to yield trans- and m-PhHC= CH(SiEt3).50 The reaction of OsHCl(CO)(P Pr3)2 with triethylsilane is the ratedetermining step. The catalytic reaction proceeds via a silyl dihydrogen intermediate of the formula Os(SiEt3)Cl(r)2-H2)(CO)(P,Pr3)2 and its dihydride tautomer, OsH2(SiEt3)Cl(CO)(P,Pr3)2 (Scheme 53). 

The intermediacy of the complex [Os(SiEt3)Cl(H2)(CO)(P Pr3)2] in the catalytic hydrosilylation of phenylacetylene was first observed by Oro and co-workers when the neutral hydride [OsHCl(CO)(P Pr3)2] was reacted with triethylsilane and the alkyne [81]. A theoretical study of silane addition across the C-C triple bond has also been reported [82] and the general scope of this reaction has been amply reviewed [17],... 

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