Ab initio studies of benzocyclopropenone, benzocyclopropenone-containing [2.2]paracyclophane, its benzyne derivative, and the bridged benzobarrelene formed by intramolecular [4+2]cycloaddition

Ab initio calculations were carried out on cyclopropenone, 1, benzocyclopro penone, 2, the benzocyclopropenone-containing [2.2]paracyclophane derivativ e tetracyclo[8.3.2.(4,7)0(11,13)]heptadeca-1(13),4,6,10,14, 16-hexaen-12-on e, 3, its decarbonylation product tricyclo[8.2.2.2(4,7)]hexadeca-1(12), 4,6 ,10,13-pentaen-15-yne, 5, a benzyne intermediate, and the bridged benzobarr elene derivative, pentacyclo[5.5.2.2.(1,4)0.(4,14)0(10,13)]hexadeca-2,7,9,1 3,15-pentaene, 6. These calculations suggest that benzocyclopropenone-conta ining [2.2]paracyclophane, 3, and highly strained bridged benzobarrelene, 6 , could exist as stable species. Both aryl rings of the benzocyclopropenone derivative 3 are predicted to be distorted from planarity. This distortion relieves some angle strain present in planar benzocyclopropenone due to th e presence of the annulated three-membered ring. Calculations on benzobarre lene, 8, and [2.2]paracyclophane, 4, were performed for comparison to gain a better understanding of the strain borne in: bridged benzobarrelene 6. Th e activation barrier for the intramolecular [4 + 2] cycloaddition of 5 to g ive 6 was estimated at 18.8 kcal/mol while that for the corresponding [2 2] cycloaddition, giving the less stable 9, was 54.5 kcal/mol. The [2 + 2] cycloaddition's transition state was twisted in a manner reminiscent of the conservation of orbital symmetry prediction for an unstrained system.