Insight into the selective room-temperature platinum(II) catalytic hydration of alkynes in water

Three platinum complexes of water-soluble, bidentate phosphine ligands have been prepared and characterized. The complexes, PtCl2(P-P) (P-P = (m-NaSO3C6H4)(2)PCH2CH2P(M-C6H4SO3Na)(2), DPPETS, (m-NaSO3C6H4)(2)PCH2CH2CH2P(M-C6H4SO3Na)(2), DPPPTS, (mNaSO(3)C(6)H(4))(2)PCH2CH2CH2CH2P(M-C6H4SO3Na)(2), DPPBTS), catalyze the hydration of 3-penntyn1-ol and 4-pentyn-1-ol. 2-Pentyn-1-ol is slowly polymerized, allowing characterization of the immediate eta(1)-allenyl complexes. For PtCl2(DPPBTS) the eta(1)-allenyl complex Pt(C(Et)CCH2)(X)(DPPBTS), where X is either OH-, OH2, or Cl-, is formed. However for PtCl2(DPPPTS) and PtCl2(DPPETS) the extent of formation of the coordinated complex is much less. The stability of the coordinated Pt(eta(1)-allenyl)(X)(P-P) complexes shows a significant effect on chelate ring size, with DPPBTS > DPPPTS > DPPETS being the order of decreasing stability. Catalytic hydrations of 4-pentyn-1-ol with PtCl2(P-P) show a major effect of chelate ring size in the opposite direction, DPPETS > DPPPTS > DPPBTS. For 3-pentyn-1-ol catalytic hydration is less affected by the chelate ring size. The effect of excess Cl- is also examined. A mechanism is proposed for the catalytic hydration involving coordination, cyclic attack by the alcohol according to Baldwin's rules, and rearrangement to 5-hydroxypentan-2-one.