The effects of bulky substituents in the reactions of alkynes with (η-C₅H₅)M(CO)₂, M = Co or Rh

Abstract

The reactions between (η-C₅H₅)Co(CO)₂ and MeC≡CR (R = Bu, Prⁱ) at 120° gave the cyclopentadienone complexes (η-C₅H₅)Co[C₄Me₂R₂CO] (2) and small amounts of the p-benzoquinone complexes(η-C₅H₅)Co[C₄Me₂R₂(CO)₂] (1); all possible isomers of both complexes were obtained. For R = Bu^t, the only products isolated were two isomers of the cyclopentadienone complex (2). The reactions between (η-C₅H₅)Rh(CO)₂ and MeC=CR at 160° gave the cyclopentadienone complexes(η-C₅H₅)Rh[C₄Me₂R₂CO] (2), and small amounts of the p-benzoquinone complexes (η-C₅H₅)Rh-[C₄Me₂R₂(CO)₂] (1) and of the binuclear complexes (η-C₅H₅)₂Rh₂(MeC₂R)₂CO (3). With R = Bu^t, three isomers of (2), two isomers of (1) and three isomers of (3) were identified. The products obtained with R = Prⁱ were three isomers of (2) and three isomers of (3). With R = Bu^t, three isomers of(2) and a very small amount of one of the isomers of (3) were separated. Comparisons of these reactions have shown that increasing the bulk of R results in a decrease in the total yields of products, a decrease in the range of products, and a change in the distribution of the isomers (M = Co only).