OBSERVATION OF NEW INTERMEDIATES IN HYDROGENATION CATALYZED BY WILKINSONS CATALYST, RHCL(PPH(3))(3), USING PARAHYDROGEN-INDUCED POLARIZATION

Through the use of parahydrogen-induced polarization, the chemistry of RhCl(PPh(3))(3), [RhCl(PPh(3))(2)](2), and RhCl(olefin)(PPh(3))(2) with H-2 and olefinic substrates has been investigated. The phenomenon of PHIP makes possible the detection of previously unobserved dihydride species in the reaction system. Specifically, the binuclear complex H2Rh(PPh3)(2)(mu-Cl)(2)Rh(PPh(3))(olefin) for cis- and trans-2-hexene, 2-pentene, methyl methacrylate, styrene, and substituted styrenes has been detected and characterized. While readily observable during hydrogenation catalysis under ca. 1-2 atm of para-enriched hydrogen, H2Rh(PPh(3))(2)(mu-Cl)(2)Rh(PPh(3))(olefin) is not directly in the catalytic cycle; in fact, its appearance suggests reduced activity of the system through the formation of less active binuclear species. A second new species observed with styrene, p-chlorostyrene, and p-methylstyrene as substrates is the mononuclear complex RhH2(olefin)(PPh(3))(2)(Cl), which appears to be an intermediate in hydrogenation catalysis. From PHIP spectra, it is possible to assign this species as having cis phosphines as well as cis hydrides. The study also underscores the crucial role of PPh(3) at low concentration to inhibit formation of binuclear species in the catalytic system. In a separate experiment, strong polarization for RhH2Cl(PMe(3))(3) is observed, consistent with its poor effectiveness as a catalyst.