A theoretical study on the conformation of 3-phosphinoxido- and 3-phosphono-1,2,3,4,5,6-hexahydrophosphinine 1-oxides

The title compounds (2 and 4) obtained by the diastereoselective hydrogenation of the corresponding 1,2,3,6-tetrahydrophosphinine oxides (1 and 3) were subjected to a detailed quantum chemical study. The possible chair conformers were calculated at the HF/6-31G* level of theory, according to which, the 1-phenyl-3-P(O)Y-2-substituted products (2) exist in the trans(1) form, in which all substituents are equatorial. At the same time, the 1-ethoxy-3-dialkylphosphono compounds (4) adopt the cis conformations, in which the 1-ethoxy group is axial and the 3-P(O)(OR)(2) moiety is equatorial. The major diastereomer (4-1) is cis(3), in which the 5-methyl group is axial, while the minor one is cis, with an equatorial methyl substituent. It is noteworthy that the rotational position of the exocyclic P(O)Z(2) function affected the energy content of the chair conformer to a high extent. The possibility of the involvement of the twist conformers was also considered. (c) 2005 Wiley Periodicals, Inc.