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

Abstract

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 [(PPh 3 )RhH] (1) and [(NP 3 )RhH] (2, PP 3 =P(CH 2 CH 2 PPh 2 ) 3 , NP 3 =N(CH 2 CH 2 PPh 2 ) 3 ). The reactions are catalytic under relatively mild conditions (catalyst to substrate ratio 1:100, toluene, 100 o 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=PP 3 , NP 3 ). 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 [(PP 3 )RhMe]. The novel vinylphosphonium complex [(Ph 2 PCH 2 CH 2 ) 2 P(CH 2 CH 2 PPh 2 )Rh{C=C(H)Ph}(O 2 CPh) has been synthesized and fully characterized by IR and 1 H and 34 P{ 1 H} NMR techniques