Ab initio calculations on the interconversion of homocub-1(9)-ene and homocub-9-ylidene

Abstract

Calculations which include dynamic electron correlation beyond the CASSCF level have been carried out for the interconversion of homocub-1(9)-ene (1) and homocub-9-ylidene (2). The geometry of the transition state was also located at the (4/4)CASSCF/6-31G* level and was found to di er signi® cantly from the previously published (2/2)CASSCF/3-21G geometry. In agreement with experiment, calculations at the (2/2)CASSDCI+ Q/6-31G* level ® nd 1 and 2 to have free energies at 298 K that di er by only~0 . 1 kcal mol -1 . However, the activation energy for the rearrangement of 1 to 2 of E a = 12 . 9 kcal mol -1 , computed at this level of theory, is considerably higher than the experimental estimate of E a < 5 kcal mol -1 . Additional calculations in which the size of the basis set was increased or in which the active space was expanded to 4 electrons in 4 orbitals suggest that the calculated E a may be reduced to as low as 8 kcal mol -1 . However, there is no indication that higher level calculations would give a value of E a as low as 5 kcal mol -1 . Possible reasons for this apparent disagreement between the calculations and the experiments on the rate of equilibration of 1 and 2 are discussed. IntroductionRearrangements of carbenes to ole® ns usually are irreversible reactions [1]. Nevertheless, bridgehead ole-® ns with su cient torsional strain are capable of undergoing reversion to carbenes [2]. The best studied example of the latter process was discovered by Eaton and Ho mann in the rearrangement of 9-phenylhomocub-1(9)-ene (1a) to 1-phenylhomocub-9-ylidene (2a) [3]. Subsequent investigations in the desphenyl series demonstrated that the parent ole® n (1b) also can be formed from the carbene (2b) [4], and experiments by Jones, Platz and coworkers have found the equilibrium constant for this reaction to be approximately unity at room temperature [5].Jones, Platz, and coworkers started with di erent precursors, which led initially either to 1b or to 2b. Independent of the precursor used, the same mixture of CH 3 OD trapping products of 1b and 2b was formed, provided that the concentration of CH 3 OD was less than 1 m. Therefore, under these conditions equilibration of 1b and 2b appears to be faster than trapping with CH 3 OD.Flash photolysis studies indicated that both the ole® n and carbene react with CH 3 OD within an order of magnitude of the di usion controlled rate. Taken together, these two observations indicate that the rate constant for equilibration of 1b and 2b at room temperature

Substituent Effects in the Interconversion of Phenylcarbene, Bicyclo[4.1.0]hepta-2,4,6-triene, and 1,2,4,6-Cycloheptatetraene

“…While previous coupled cluster, G2(MP2, SVP) 15 and complete active space 16 calculations predict BCT to be slightly lower in energy than PC , our results, in agreement with other density functional theory studies, predict PC to be lower in energy, though only slightly. This discrepancy is likely due to the difficulty that single determinant methods such as B3LYP have in calculating the energy of strained alkenes such as BCT . Despite this shortcoming, DFT methods have been quite successful in the modeling of carbene chemistry and should reproduce energetic trends quite reliably, providing a suitable compromise between the computational accuracy required here and the computing resources necessary to carry out this work for a diverse number of substituents.…”