Aqueous organometallic chemistry: The mechanism of catalytic hydrogenations with chlorotris(1,3,5-triaza-7-phosphaadamantane) rhodium(I)
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abstract
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 -acetamidocinnamic acid in D2O led to 45-100% deuteration of the products with 25-100% stereoselectivity towards the -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.
