Molecular Orbital Calculations of Tautomerism and Isomerism of Subistituted Pyrrolidines and Phospholanes

Abstract

he relative energies, tautomeric equilibria and geometries of substituted cyclic pyrrolidines and phospholanes have been studied in the gas-phase (at 298K) using semi-empirical molecular orbital calculations at the AM1 level of the theory. For the set of P-methyl and P-phenyl substituted cyclic imidazolines, oxazolines and thiazolines, the exo-cyclic double bond (phosphinidene forms) are computed to be more stable than the phosphino tautomers. Of the two possible (Z and E) isomers of the methyl and phenyl derivatives of phosphinidene-oxazolidine, the Z isomers have the lowest energy, whereas in the methyl and phenyl derivatives of the phosphinidene-thiazolidine the E isomers are relatively more stable. The amino forms of the N-methyl and N-phenyl substituted cyclic azaphospholes, oxaphospholes and thiaphospholes are found to be the most stable isomers. The methyl and phenyl derivatives of the imino-azaphosphole and imino-oxaphosphole, the E isomers have the lowest energies. The Z isomers of the methyl and phenyl derivatives of the imino-thiaphosphole are found to be more stable. The phosphinidenes (exo-cyclic double bond) of the P-methyl and P-phenyl substituted cyclic azaphospholes and oxaphospholes are predicted to be more stable than the phosphino tautomers; such tautomers prefer the E configuration. For the P-methyl and P-phenyl substituted cyclic thiaphosphole the phosphino forms are relatively more stable than the phosphinidene forms which have the Z isomers to be of the lowest energy. The isomerization and tautomerization free energies in the gas-phase were found to be ranging between 0.07 and 75.70 kJmol-1; and 1.84 and 31.64 kJmol-1 respectively. The absolute values of KT depend strongly on the accuracy of the method used for calculation of these free energies. The obtained tautomeric equilibrium constants (KT) of the various species consolidate the relative energy findings. The optimized geometries of the studied molecules show a distinctly non-planar configuration of the cyclic moieties. The most stable structures are stabilized through intramolecular hydrogen bonding with the heteroatoms