Conformational stabilization of 1,3-benzodioxole: Anomeric effect by natural bond orbital analysis

The conformation of 1,3-benzodioxole has been examined using ab initio calc ulation and natural bond orbital (NBO) analysis in order to find the origin of its unusual nonplanarity. Geometry optimizations for the planar (C-2v) and flap-puckered (C-s) conformers of 1,3-benzodioxole have been performed at the HF, B3LYP, and MP2 levels, and the results indicate that the flap-pu ckerd conformer is more stable than the planar conformer. High-level electr on correlation treatments with extended-basis sets have also been performed to provide a reliable prediction of the puckering barrier for 1,3-benzodio xole. The calculated puckering barrier appears to be in reasonable agreemen t with the experiment, but the divergent behavior of the Moller-Plesset ser ies suggests that it is impossible with conventional basis sets smaller tha n 400 functions to converge the barrier height. NBO analysis of tile Hartre e-Fock wave functions shows that the conformational preference of the C-s c onformer over the C-2v is the result of a wide variety of hyperconjugative orbital interactions, but the interaction between the oxygen lone pair (n(p )) and the o*(CO) orbital, which is closely associated with the anomeric ef fect, is the most important factor favoring the nonplanar conformation. How ever, 1,3-benzodioxole has a lower puckering barrier to planarity than 1,3- dioxole due to the suppression of the anomeric effect by the benzene ring.