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 opening 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 observe 1,4-dimethylbicyclo[2.1.0]pentan-5-one (2b) experimentally, ring-opened 2-cyclopentanone-1,3-diyl diradicals (5) are calculated to be lower in energy than the corresponding bicyclo[2.1.0]pentan-5-ones (2). Also, in agreement with kinetic experiments on di-tert-butyl derivatives 1c and 3c, bicyclo[3.1.0]hexan-6-ones (3) are calculated to undergo ring opening more easily than bicyclo[1.1.0]butan-2-ones (1). This result is surprising since bicyclo[1.1.0]butane (7a) is both calculated and 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 bicyclo[1.1.0]butan-2-ones (1) to undergo ring opening to 2-cyclobutanone-1,3-diyls (4) is primarily due to the stabilization of 1 by a strong interaction between the bent bond between the bridgehead carbons, C-1 and C-3, and the carbonyl group at C-2. Ab initio calculations of the energies of isodesmic reactions are also used to show that methyl substituents provide considerable stabilization for oxyallyl diradicals 4b−6b, and DFT calculations reveal that steric interactions between 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 opening of 1c to 4c.