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

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journal contribution
posted on 2018-07-26, 20:03 authored by Raphel A. E. Enow, Wei-Chih Lee, Travis D. Cournoyer, Travis L. Sunderland, Brian J. Frost
A series of water-soluble and air-stable E-enamine derivatives of 1,3,5-triaza-7-phosphaadamantane (PTA), PTAC­(R)­NH2, 14, 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 14 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 (58), were observed as ∼55/45 mixtures of E and Z isomers. Kinetic data on the E–Z isomerization is reported. Upon coordination of 14 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)42-P,N-PTA-CRNH)], 912. 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.