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.