CASSCF and CASPT2 calculations on the cleavage and ring inversion of bicyclo[2.2.0]hexane find that these reactions involve formation of a common twist-boat diradical intermediate

(6/6)CASSCF and CASPTU6-31G* calculations have been performed to understand the experimental finding of Goldstein and Benzon (J. Ant. Chern. Sec. 1972 , 94, 5119) that exo-bicyclo[2.2.0]hexane-d(4) (1b) undergoes ring inversio n to form endo-bicyclo[2.2.0]hexane-d(4) (4b) faster than it undergoes clea vage to form cis, trans-1,5-hexadiene-d(4) (3b). Goldstein and Benzon also found that the latter reaction, which must occur via a chairlike transition structure (TS), is much faster than cleavage of Ib to trans, trans-1,5-hex adiene-d(4) (2b) via a boatlike TS. Our calculations reveal that all three of these reactions involve ring opening of 1, through a boat diradical TS ( BDTS), to form a twist-boat diradical intermediate (TBDI). TBDI can reclose to 4 via a stereoisomeric boat diradical TS (BDTS'), or TBDI can cleave, e ither via a half-chair diradical TS (HCDTS) to form 3 or via a boat TS (BTS ) to form 2. The calculated values of DeltaH(double dagger) = 34.6 kcal/mol , DeltaS(double dagger) = -1.6 eu, and DeltaH(double dagger) = 35.2 kcal/mo l, DeltaS(double dagger) = 2.0 eu for ring inversion of 1 to 4 and cleavage of 1 to 3, respectively, are in excellent agreement with the values measur ed by Goldstein and Benzon. The higher value of DeltaH(double dagger) = 37. 6 kcal/mol, computed for cleavage of TBDI to 2, is consistent-with the expe rimental finding that very little 2b is formed when Ib is pyrolyzed. The re lationships between BDTS, HCDTS, and BTS and the chair and boat Cope rearra ngement TSs (CCTS and BCTS) are discussed.