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)₂PCH₂CH ₂P-(m-C₆H₄SO₃Na)₂, DPPETS, (m-NaSO₃C₆H₄)₂PCH₂CH ₂CH₂P(m-C₆H₄SO₃Na)₂ , DPPPTS, (m-NaSO₃C₆H₄)₂PCH₂CH ₂CH₂CH₂P(m-C₆H₄SO₃N a)₂, DPPBTS), catalyze the hydration of 3-pentynl-ol and 4-pentyn-l-ol. 2-Pentyn-l-ol is slowly polymerized, allowing characterization of the intermediate η¹-allenyl complexes. For PtCl₂(DPPBTS) the η¹-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(η¹-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-l-ol with PtCl₂(P-P) show a major effect of chelate ring size in the opposite direction, DPPETS > DPPPTS > DPPBTS. For 3-pentyn-l-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.