Activation of 1-alkynes at tripodal (polyphosphine) rhodium systems. Regioselective synthesis of enol esters from 1-alkynes and carboxylic acids catalyzed by rhodium(I) monohydrides

Article abstract of DOI:10.1021/om00118a040

Regioselective formation of 2-(benzoyloxy)propene results from the addition of benzoic acid to propyne in the presence of the trigonal-bipyramidal Rh(I) monohydrides [(PPh₃)RhH] (1) and [(NP₃)RhH] (2, PP₃ = P(CH₂CH₂PPh₂)₃, NP₃ = N(CH₂CH₂PPh₂)₃). The reactions are catalytic under relatively mild conditions (catalyst to substrate ratio 1:100, toluene, 100°C). A detailed experimental study on the reactions of 1 and 2 with carboxylic acids, 1-alkynes, or carboxylic acid/1-alkyne mixtures has allowed us to draw a catalysis cycle involving the 16-electron fragments [(L)Rh]⁺ as catalysts (L = PP₃, NP₃). The catalytic behavior of the precursors 1 and 2 has been compared and contrasted with those of the isostructural and isoelectronic derivatives [(L)Rh(C≡CPh)], [(L)RhCl], and [(PP₃)RhMe]. The novel vinylphosphonium complex [(Ph₂PCH₂CH₂)₂P(CH ₂CH₂PPh₂)Rh{C≡C(H)Ph}(O₂CPh)] has been synthesized and fully characterized by IR and ¹H and ³¹P{¹H} NMR techniques.