Unusual Water-Soluble Imino Phosphine Ligand: Enamine and Imine Derivatives of 1,3,5-Triaza-7-phosphaadamantane (PTA)

A series of water-soluble and air-stable E-enamine derivatives of 1,3,5-triaza-7-phosphaadamantane (PTA), PTA═C(R)NH2, 1–4, are reported along with data on E–Z isomerization and tautomerization to the imine form (PTA-CR═NH). Reaction of 1,3,5-triaza-7-phosphaamantane-6-yl lithium, PTA-Li, with aromatic nitriles afforded E-enamine derivatives of PTA in good yield (49–91%). Phosphines 1–4 are stable toward water and air, and do not appear to isomerize or tautomerize, unless coordinated to a metal or oxidized. The corresponding oxides, O═PTA═C(R)NH2 (5–8), were observed as ∼55/45 mixtures of E and Z isomers. Kinetic data on the E–Z isomerization is reported. Upon coordination of 1–4 to W(CO)4(pip)2, a κ1-P enamine is formed, [W(CO)4(pip)(κ1-P-PTA═CRNH2)]. Enamine–imine tautomerization of the metal bound phosphine was observed resulting in κ2-P,N imine complexes, [W(CO)4(κ2-P,N-PTA-CR═NH)], 9–12. The crystal structures of the κ1-P enamine 11a, κ2-P,N imine 9 and 12, phosphines 1 and 3, as well as phosphine oxide 8a were obtained. DFT calculations on the various isomers of the phosphines and phosphine oxides are also reported.