A Theoretical Study on the Conformation of 5- and 6-Membered P-Heterocycles: 1- Substituted 2,3,4,5-Tetrahydro-1H-Phosphole 1-Oxides and 1,2,3,4,5,6-Hexahydrophosphinine 1-Oxides

Background: The conformation of ring compounds including different heterocycles may bear useful information. The conformational situation of P-heterocycles is a less studied field, practically there are only data for compounds with more heteroatoms (O and N beside P) in the ring. Objective: To fill this gap, we wished to evaluate the conformational situation of saturated P-heterocycles, such as 2,3,4,5-tetrahydro-1H-phosphole 1-oxides and 1,2,3,4,5,6-hexahydrophosphinine 1-oxides by theoretical calculations. Method: The B3LYP/6-311++G(2d,2p) method was used during the quantum chemical calculations aiming at the evaluation of the energetics of the possible conformational equilibriums and geometry of the conformers in the gas phase. Results: Theoretical calculations suggested that while the two optically inactive cis diastereomers of 1- substituted 3,4-dimethyl-2,3,4,5-tetrahydro-1H-phosphole 1-oxides exist as balanced conformational equilibria, the racemic trans forms are present as shifted equilibria towards the more stable conformers. At the same time, both the cis and trans racemic diastereomers of the monomethyl analogues may be described by equilibria biased towards the conformers with equatorial methyl groups. The 3-methyl-1,2,3,4,5,6- hexahydrophosphinine oxide diastereomers also involve biased equilibria. The energetics of the conformational interconversions in the gas phase were demonstrated by novel 3D diagrams. Conclusion: It is a surprising experience that in the saturated P-hetero ring in tetrahydrophosphole oxide derivatives, the P-center does not undergo a configurational change during the interconversions. This is, however, not the case with the 3-methyl-1,2,3,4,5,6-hexahydrophosphinine oxide rings, where the P-center is also “flexible”. Within the P(O)OH moiety of the 5-ring phosphinic acids, an intramolecular transfer of the proton is possible only on energy investment. At the same time, in an intermolecular relationship, the proton transfer is free.