RING-OPENING OF BICYCLO[N.1.0]ALKANONES TO 2-CYCLOALKANONE-1,3-DIYLS - WHY DOES OXYALLYL DIRADICAL FORMATION REQUIRE LESS ENERGY FROM BICYCLO[3.1.0]HEXAN-6-ONES THAN FROM BICYCLO[1.1.0]BUTAN-2-ONES

CASSCF and CASPT2N/6-31G calculations have been performed on the open ing of bicyclo[n.1.0]-alkanones, n = 1-3 (1-3), to the corresponding 2 -cycloalkanone-1,3-diyls (4-6). In agreement with the failure to obser ve 1,4-dimethylbicyclo[2.1.0]pentan-5-one (2b) experimentally, ring-op ened 2-cyclopentanone-1,3-diyl diradicals (5) are calculated to be low er in energy than the corresponding bicyclo[2.1.0]pentan-5-ones (2). A lso, in agreement with kinetic experiments on di-tert-butyl derivative s 1c and 3c, bicyclo[3.1.0]hexan-6-ones (3) are calculated to undergo ring opening more easily than bicycio[1.1.0]butan-2-ones (1). This res ult is surprising since bicyclo[1.1.0]butane (7a) is both calculated a nd found to have a higher strain energy than bicyclo[3.1.0]hexane (9a) . Isodesmic reactions are used to show that the comparative reluctance of bicydo[1.1.0]butan-2-ones (1) to undergo ring opening to 2 cyclobu tanone-1,3-diyls (4) is primarily due to the stabilization of 1 by a s trong interaction between the bent bond between the bridgehead carbons , C-1 and C-3, and the carbonyl group at C-2. Ab initio calculations o f the energies of isodesmic reactions are also used to show that methy l substituents provide considerable stabilization for oxyallyl diradic als 4b-6b, and DFT calculations reveal that steric interactions betwee n the tert-butyl groups in 3c play a minor role in reducing the energy required for its ring opening to 6c, relative to that required for op ening of 1c to 4c.