Aqueous organometallic chemistry: The mechanism of catalytic hydrogenations with chlorotris(1,3,5-triaza-7-phosphaadamantane) rhodium(I)

Article abstract of DOI:10.1016/0022-328X(95)05946-M

The water-soluble phosphine complex of Rh(I), RhCl(PTA)₃ (1) was shown to be an active catalyst for the hydrogenation of various olefinic and oxo-acids, as well as of allyl alcohol and 4-sulfostyrene in aqueous solution under mild conditions. Detailed kinetic investigations were carried out with crotonic acid and allyl alcohol as substrates. The rate of hydrogenation of both compounds showed a sharp maximum as a function of pH at 4.7. Hydrogenation of itaconic, crotonic and α-acetamidocinnamic acid in D₂O led to 45-100% deuteration of the products with 25-100% stereoselectivity towards the α-carbon atom. These results, together with those of pH-static hydrogenation of complex 1, suggest that water strongly assists the dehydrochlorination of 1 to yield the catalytically active monohydrido species HRh(PTA)₃ (2). Nevertheless, depending on the substrate and the pH of the solution the dihydridic pathway may remain partially operative.