Thermal and electron-impact transformations of cis-1,2-dibbnzoylalkenes

Abstract

Several cis-1,2-dibenzoylalkene derivatives have been prepared in yields ranging between 60-80%, through the Diels-Alder addition of the appropriate dienes to dibenzoylacetylene. These include, 2,3-dibenzoyl-bicyclo [2.2.1]hepta-2,5-diene (10), 2,3-dibenzoylbicyclo[2.2.2]octa-2,5-diene (11), 7-oxa-2,3-dibenzoyl-bicyclo [2.2.1]hepta-2,5-diene (12), 1,4-diphenyl-2,3-dibenzoyl-1,4-epoxynaphthalene (13) and 9,10-dihydro-11,12-dibenzoy1-9, 10-ethenoanthracene (15), formed from cyclopentadiene, cyclohexa-1,3-diene, furan, 1,3-diphenylisobenzofuran and anthracene, respectively. Thermolysis of 2,3-dibenzoylbicyclo[2.2.1]hepta-2,5-diene gave chiefly cyclopentadiene, arising through a retro-Diels-Alder mode of fragmentation. Similar retro-Diels-Alder fragmentations have been observed in the cases of 7-oxa-2,3-dibenzoylbicyclo[2.2.1]hepta-2,5-diene and 9,10-dihydro-11,12-dibenzoyl-9,10-ethenoanthracene. The thermoylsis of 1,4-diphenyl-2,3-dibenzoyl-1,4-epoxynaphthalene, however, gave a mixture of 1,3-diphenylisobenzofuran and 1,2-dibenzoylbenzene. The formation of 1,2-dibenzoylbenzene in this case has been shown to be through the air-oxidation of 1,3-diphenylisobenzofuran. Thermolysis of 2,3-dibenzoylbicyclo[2.2.2]octa-2,5-diene, on the other hand, gave a nearly quantitative yield of 1,2-dibenzoylbenzene, which did not undergo further transformation even on heating around 260° for several hours. In none of these cases, the expected pericyclic transformation, analogous to the conversion of cis-1,2-dibenzoylstilbene (6) to the isomeric 2,2,3,4-tetraphenylbut-3-enolide (9), has been observed under thermal conditions. Treatment of 9,10-dihydro-11,12-dibenzoyl-9,10-ethenoanthracene (15) with phosphorous pentasulphide resulted in the formation of a mixture of 12,14-diphenyl-9, 10(3', 4')furanoanthracene (28) and 12,14-diphenyl-9,10(3',4')thiophenoanthracene (31), arising through the postulated intermediates, 9,10-dihydro-11-benzoyl-12-thiobenzoyl-9,10-ethenoanthracene (26) and 9,10-dihydro-11,12-dithiobenzoyl-9, 10-ethenoanthracene (29), respectively. The electron-impact induced transformations of the cis-1,2-dibenzoylalkenes, 6, 10, 11, 12, 13 and 15 on the other hand, can be rationalized in terms of both retro-Diels-Alder type fragmentations and pericyclic transformations of the dibenzoylalkene components.