WHY IS THE DIELS-ALDER ADDUCT BETWEEN DIFLUOROCYCLOPROPENONE AND 1,3-DIPHENYLISOBENZOFURAN SO REACTIVE - AN AB-INITIO MOLECULAR-ORBITAL STUDY OF THE RING-OPENING OF CIS-2,3-DIFLUOROCYCLOPROPANONE

Reaction of difluorocyclopropenone (5) and diphenylisobenzofuran (2) yields the two isomeric 1:2 cycloadducts 7 and 8 in a ratio of 9:1. The structure of the major isomer was determined by X-ray crystallography and was shown to be the unsymmetrical one. The minor isomer is assigned a symmetrical structure based on spectroscopic data. Attempts to observe the initial 1:1 Diels-Alder adduct 6 were unsuccessful because of the low reactivity of difluorocyclopropenone and the high reactivity of the presumed intermediate difluorocyclopropanone 6. As an aid in the investigation, ab initio calculations were carried out on the ring-opening reaction of the parent cis-2,3-difluorocyclopropanone (11) to the corresponding oxyallyl species 13. The results are compared with previous theoretical and experimental work. cis-2,3-difluorocyclopropanone is calculated to have a much smaller barrier (12-14 kcal/mol) to ring opening than the parent cyclopropanone (22-28 kcal/mol).