DIASTEREOFACIAL SELECTIVITY IN 1,3-DIPOLAR CYCLOADDITIONS TO CYCLOBUTENES .8. HF/3-21G TRANSITION STRUCTURES OF THE REACTIONS OF FORMONITRILE OXIDE WITH CIS-3,4-DICHLOROCYCLOBUTENE AND NORBORNENE

A transition structure for the syn and anti attack of formonitrile oxide on both cis-3,4-dichlorocyclobutene and norbornene was obtained using HF/3-21G method. These calculations correctly predict dominance of the syn attack in the reaction of dichlorocyclobutene and 100% syn selectivity, with respect to the methano bridge, in the reaction of norbornene. Analysis of the activation energy shows that the inherent facial bias of dichlorocyclobutene is reflected, at the transition state, in a complex way in the deformation energy of both dichlorocyclobutene and formonitrile oxide and in the interaction energy between them. With norbornene the out-of-plane deformation energy of the olefinic hydrogens clearly emerges as the major factor in controlling facial selectivity.