An in silico study on the ring-size effect in ring enlargement Bellus-Claisen rearrangement

B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level of theory has been used to investigate the ring size effect on the Bellus-Claisen rearrangement. The results show the reaction has a high asynchronous character and there is no direct relationship between ring size and Ea; for example, the Ea is almost the same amount for three- and five-member rings. Four-member rings however, have an unexpectedly higher Ea. Reactants which have nitrogen as a heteroatom in their structures have lower Ea in comparison to oxygen containing rings. The effect of solvent on the Ea is considerable. In all of the cases, by increasing the polarity of solvents, the amount of Ea decreases. The results show that the effect of solvent on Ea for N as heteroatom is more than that of O. There is an inverse relation between solvent polarity and Ea. This can be interpreted as higher stabilization effect of the polar solvents on TSs’ structures comparing to less polar solvents which leads to lower Ea.