Formation of selenaphosphole isomers from 1,2,4-selenadiphosphole by cycloaddition reaction. A synthetic and ab initio quantum chemical study

The reaction of 1,2,4-selenadiphosphole with dimethyl acetylenedicarboxylat e resulted in a mixture of 1,2-selenaphosphole and 1,3-selenaphosphole at 1 10 degreesC in boiling toluene. The mechanism has been interpreted by quant um chemical calculations. A cycloreversion process of the 1,2,4-selenadipho sphole has been ruled out on the basis of the high energy of the selenaphos phirene intermediate. A [4 + 2] cycloaddition of the acetylene on the selen adiphosphole results in the 1,2-selenaphosphole via an intermediate. A [3 2] cycloaddition via a single transition structure results in the 1,3-sele naphosphole. While the energies of the products differ considerably, transi tion structures on the two cycloaddition routes are predicted to have simil ar energies, explaining the observed 1:1 product ratio.