Rhodium-catalyzed silylformylation of acetylenic bonds: Its scope and mechanistic considerations

"Silylformylation" of alkynes, which means the simultaneous introduction of a trialkylsilyl group and a formyl group into a carbon-carbon triple bond to give 3-silyl-2-alkenal, is attained selectively by the interaction of an alkyne with a monohydrosilane in the presence of Rh catalyst under CO pressure (over 10 kg/cm²). The presence of Rh catalyst is crucial for the attainment of this coupling, regardless of the types of precursors: Rh₄(CO)₁₂, RhH(CO)(PPh₃)₃, [Rh(COD)Cl]₂, [Rh(COD)(PPh₃)₂]PF₆, or [Rh(COD)(DPPB)]PF₆. This silylformylation is applicable to both terminal and internal alkynes. In the terminal ones, the terminal sp carbon is selectively silylated. The sp carbon bearing the bulkier substituent is formylated preferentially in internal alkynes, except for those containing a strong electronwithdrawing group. When a 1 mol excess of 1-alkynes is used under silylformylation conditions, the formation of 2-cyclopentenone derivatives is confirmed in addition to the usual silylformylation. All of these cyclopentenones are composed of two molecules of 1-alkyne and one molecule each of hydrosilane and CO. A catalyst precursor of these reactions, Rh₄(CO)₁₂, reacts almost quantitatively with hydrosilane and phenylacetylene to form Rh(CO)₄SiR₃ and Rh₂(CO)₇(phenylacetylene), respectively, under CO pressure in the stoichiometric reaction. Both of these species catalyze silylformylation with a similar efficiency to Rh₄(CO)₁₂. Furthermore, Rh(CO)₄SiR₃ readily reacts with phenylacetylene under a CO atmosphere to give l,5-disilyl-2,4-diphenyl-1(Z),4(Z)-pentadien-3-one and Rh₄(CO)₁₂. On the basis of these results, a plausible pathway for silylformylation is elucidated by the sequence of the insertion of alkyne into the Rh-Si bond to form Rh-vinyl species and the subsequent insertion of CO. The insertion of another 1 mol of alkyne into Rh-vinyl species prior to CO insertion results in cyclopentenone annulations.