As an example of an organic catalyst participating in [3+2]-cycloaddition with nitrile oxide through the S--O bond, we chose N-sulfinylaniline (Ph-N--S --O). However, unlike SO 2 , which is symmetric in the [3+2]-cycloaddition reaction, PhNSO is asymmetric, leading to the two possible intermediates that are labeled A and B as shown in Figure , where PhNSO is a reactant in pathway A and a catalyst in pathway B. The product arising from the intermediate A is diphenyl carbodiimide (Ph-N--C --N-Ph), while the intermediate B leads to the catalytic conversion of phenylnitrile oxide into phenyl isocyanate. The effective reaction barrier in THF for the pathway A is calculated to be 19 kcal/mol. In contrast, the effective reaction barrier for the pathway B is remarkably higher (23 kcal/mol). Therefore, the simple estimation of the selectivity from the rate constants is given by Eyring-Polanyi equation . The ratio of these constants is 860:1, with reaction pathway A being the preferred one.