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, PtCl 2 (P−P) (P−P = ( m- NaSO 3 C 6 H 4 ) 2 PCH 2 CH 2 P( m -C 6 H 4 SO 3 Na) 2, DPPETS, ( m- NaSO 3 C 6 H 4 ) 2 PCH 2 CH 2 CH 2 P( m -C 6 H 4 SO 3 Na) 2, DPPPTS, ( m- NaSO 3 C 6 H 4 ) 2 PCH 2 CH 2 CH 2 CH 2 P( m -C 6 H 4 SO 3 Na) 2, DPPBTS), catalyze the hydration of 3-pentyn-1-ol and 4-pentyn-1-ol. 2-Pentyn-1-ol is slowly polymerized, allowing characterization of the intermediate η 1 -allenyl complexes. For PtCl 2 (DPPBTS) the η 1 -allenyl complex Pt(C(Et)CCH 2 )(X)(DPPBTS), where X is either OH -, OH 2, or Cl -, is formed. However for PtCl 2 (DPPPTS) and PtCl 2 (DPPETS) the extent of formation of the coordinated complex is much less. The stability of the coordinated Pt(η 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 PtCl 2 (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.