Interaction of monohydrido complexes of rhodium(I) with 1-alkynes. Experimental study on deceptively simple reactions

Article abstract of DOI:10.1021/om00118a039

The reaction of HC≡CR (R = CO₂Et, Ph, n-C₃H₇, n-C₅H₁₁, SiMe₃) with the Rh(I) monohydrides [(NP₃)RhH] (1) and [(PP₃)RhH] (2) in THF is influenced by a number of factors, including stoichiometry, alkyne substituent, and temperature (NP₃ = N(CH₂CH₂PPh₂)₃; PP₃ = P(CH₂CH₂PPh₂)₃). At room temperature, HC≡CCO₂Et and HC≡CPh react with an equimolar amount of 1, yielding mixtures of the trigonal-bipyramidal complexes [(NP₃)Rh(C≡CR)] and [(NP₃)Rh{(E)-CH≡C(H)R}] (R = CO₂Et, Ph). The σ-acetylide compounds form via C(alkyne)-H oxidative addition to rhodium, followed by H₂ elimination. For R = alkyl and SiMe₃, no reaction occurs even when a 10-fold excess of 1-alkyne is used. In contrast, by treatment of 1 with an excess of HC≡CCO₂Et, the σ-acetylide complex selectively forms together with ethyl acrylate. At reflux temperature, 1 reacts with a 10-fold excess of 1-alkynes to give σ-acetylide derivatives (R = SiMe₃, CO₂Et) or σ-acetylide/σ-alkenyl mixtures (R = alkyl, Ph). A variety of organic products is also formed, including as the major products 1,3,5- and 1,2,4-trisubstituted benzenes, 1,4-butadiynes, and butenynes. The only 1-alkyne that stoichiometrically reacts with 2 in THF at room temperature is HC≡CCO₂Et. As a result, the σ-alkenyl[(PP₃)Rh{gem-C(CO₂Et)=CH₂}] is obtained. The σ-acetylide complexes [(PP₃)Rh(C≡CR)] (R = CO₂Et, Ph) are synthesized by reacting 2 with a 10-fold excess of the corresponding 1-alkyne. No reaction is observed for R = alkyl and SiMe₃. When performed in refluxing THF, the reactions between 2 and an excess of 1-alkyne lead to the formation of σ-acetylide complexes regardless of the alkyne substituent. Again, the reactions are catalytic and cause extensive conversion of 1-alkynes to a variety of linear and cyclic oligomers. The product distribution is essentially similar to that observed for the NP₃ hydride 1. The stereospecific addition of 1-alkynes to either 1 or 2 is interpreted in terms of a concerted mechanism.