Aqueous organometallic chemistry: the mechanism of catalytic hydrogenations with chlorotris(1,3,5-triaza-7-phosphaadamantane) rhodium(I)
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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.
author list (cited authors)
Joó, F., Nádasdi, L., Bényei, A. C., & Darensbourg, D. J.