Ab Initio Computational Studies of Conformationally Restricted Cope Rearrangements. First Examples of Fully Concerted Allenyl Cope Rearrangements

Results of (8,8)CASPT2/6-31G*//(8,8)CASSCF/6-31G* level calculations on the potential surface for the conformationally restricted allenyl Cope rearrangements of syn-5-propadienylbicylco[2.1.0]pent-2-ene (14) and syn-6-propadienylbicyclo[2.1.1]hex-2-ene (15) are reported. Both are found to proceed through concerted pathways. Also included are the results of (6,6)CASPT2/6-31G*//(6,6)CASSCF/6-31G* level calculations on the Cope rearrangements of syn-5-ethenylbicyclo[2.1.0]pent-2-ene (18), syn-6-ethenylbicyclo[2.1.1]hex-2-ene (19), and syn-7-vinylnorborene (20), which are found to involve diallylic diradical intermediates 26, 30, and 36, respectively. Previous studies have shown that the allenyl Cope rearrangement of 1,2,6-heptatriene (1) to 3-methylene-1,5-hexadiene (2) involves a single transition structure that either proceeds to the monoallylic cyclohexane-1,4-diyl derivative 3 or bypasses 3 to form 2 directly. More recently, the conformationally restricted allenyl Cope rearrangement of syn-7-allenylnorbornene (7) has also been found to involve tricyclic monoallylic cyclohexane-1,4-diyl intermediate 11. The rearrangements of 14 and 15 appear to represent the first reported examples of fully concerted allenyl Cope rearrangements. Concertedness in these cases is ascribed to two parallel factors:  (1) the relative instability of possible tricyclic diradical intermediates 16 and 17, compared to diradical intermediates 3 and 11 formed in the rearrangements of 1 and 7, respectively; and (2) the opportunity that exists to form sp-sp² σ bonds in transition structures 21 and 23 that lead, respectively, to products 22 and 24. By contrast, only weaker sp²-sp² σ bonds could form in unobserved concerted transition structures leading to products 28 and 32, formed in the nonconcerted rearrangements of 18 and 19.