REGIOSELECTIVITY IN CYCLOADDITION REACTION BETWEEN PHOSPHAACETYLENE AND DIAZOMETHANE - AN AB-INITIO STUDY

The [3 + 2] cycloaddition reaction of phosphaacetylene with diazometha ne was investigated by means of high level ab initio calculations. It was deduced that the aromatic diazaphosphole is formed via a nonaromat ic intermediate. The regiospecificity of the reaction is thus determin ed by the energy difference between the two transition states that lea d to the two possible regioisomeric intermediates. Of the transition s tates in the concerted pathways, the one leading to the regioisomer wi th two PC bonds (3) was found to be more stable at all the levels of t heory investigated, including coupled-cluster singles doubles (CCSD)(T )/6-311 + G//Moller-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 energie s in the two reactions are calculated by use of the harmonic frequenci es, the energy separation between the two transition structures remain s practically unchanged. The free energy of activation Delta G(double dagger) 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 stabl e complex is obtained in the initial phase of the reaction. However, t he stability of the complexes decreases at the CCSD(T) level, and appl ication of the BSSE correction results in unstable complexes. (C) 1997 by John Wiley & Sons, Inc.