Density functional theory study on dimerizations of phospholes

Abstract

Dimerization reactions through [4 + 2] cycloadditions on 1H-, 2H-, and 3H-phospholes and cyclopentadiene were examined at the B3LYP level using 6-31G* and 6-311+G** basis sets. Activation energies for all the structures indicate that endo transition states are lower by about 1−4.5 kcal/mol compared to the corresponding exo transition states. While the transition-state stabilizations are controlled by secondary orbital interactions, the dimer stabilities are controlled by a mix of secondary orbital interactions and steric factors. Among the transition-state structures, 1-TS-endo, syn-1P-TS-endo, 2P-TS-endo4, and 3P-TS-endo2 are akin to bis-pericyclic transition-state structures and the rest are regular [4 + 2] cycloaddition transition states. Computational prediction of the facile dimerization of 2H-phospholes involving P···P bond formation is in agreement with Mathey's experimental observation. In comparison to the dimerization of cyclopentadiene, phosphole dimerizations involving the conversion of C=P to C−P are stabilized by about 10 kcal/mol. However, only when conversion of C=C is involved in the case of 1H-phosphole dimerizations, the activation energies are slightly higher. Further reactivities of dimers in the direction of cage formation or oligomerization are explored. The cage formation seems to be very feasible in 3H-phosphole dimers and very unlikely for 1H-phosphole dimers. Similarly, 1H- and 2H-phospholes are not expected to oligomerize further, and 3H-phospholes have a very high probability of oligomerization.