Ab initio computational study of the allenyl cope rearrangement of syn-7-allenylnorbornene

Results of (8,8)CASPT2/6-31G*//(8,8)CASSCF/6-31G* level calculations on the potential surface for the conformationally restricted allenyl Cope rearran gement of syn-7-allenylnorbornene (10) to triene 11 are reported. The rearr angement is found to involve two separate transition structures 13 and 15, the former 2.1 kcal/mol higher in enthalpy than the other, that both lead t o a common diradical intermediate (12). These results differ substantially from those previously obtained for the allenyl Cope rearrangement of 1,2,6- heptatriene (1) to 3-methylene-1,5-hexadiene (2),(7) which has been shown t o involve a single transition structure that either proceeds to diradical 3 or bypasses 3 to form 2 directly. The terminal methylene group of 10 is al so shown to rotate in only one direction when passing through the lower-ene rgy transition structure 13, but appears to rotate freely in either directi on when passing through 15. This finding is shown to be remarkably consiste nt with the 90% stereoselectivity observed in the thermal Cope rearrangemen ts of dimethyl allenylnorbornene derivatives racemic-7a and racemic-7b.(10) Furthermore, direct participation of the terminal allenyl ct-bond is obser ved in the 10-->11 rearrangement but not in the 1-->2 rearrangement. This d ifference is evidenced by a comparison of the computed bond lengths and the calculated active space molecular orbitals in the two transition structure s 13 and 15 verses transition structure 17, the latter involved in the 1--> 2/3 pathways. Considering such evidence, it may be concluded that the parti cular restriction in conformational mobility afforded the 1,2,6-heptatriene moiety in 10 appears to force the participation of the terminal allenyl pi -bond, resulting in an augmented Cope process.