Regioselectivity in cycloaddition reaction between phosphaacetylene and diazomethane: Anab initio study

Abstract

The [3 + 2] cycloaddition reaction of phosphaacetylene with diazomethane was investigated by means of high level ab initio calculations. It was deduced that the aromatic diazaphosphole is formed via a nonaromatic intermediate. The regiospecificity of the reaction is thus determined by the energy difference between the two transition states that lead to the two possible regioisomeric intermediates. Of the transition states in the concerted pathways, the one leading to the regioisomer with two PC bonds (3) was found to be more stable at all the levels of theory investigated, including coupled-cluster singles doubles (CCSD)(T)/6-311 + G*//Møller-Plessett(MP)2/6-311 + G* (+ basis set superposition, BSSE, correction). The energy difference between the two transition states, however, is always less than 2 kcal/mol. When the free energies in the two reactions are calculated by use of the harmonic frequencies, the energy separation between the two transition structures remains practically unchanged. The free energy of activation ΔG† was 21 kcal/mol at the CCSD(T)/6-311 + G* level of theory and use of the MP2/6-31 + G* frequencies. At the MP2 level, a rather stable complex is obtained in the initial phase of the reaction. However, the stability of the complexes decreases at the CCSD(T) level, and application of the BSSE correction results in unstable complexes. © 1997 by John Wiley & Sons, Inc.