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 th e potential surface for the conformationally restricted allenyl Cope rearra ngements of syn-5-propadienylbicylco[2.1.0]pent-2-ene (14) and syn-6-propad ienylbicyclo[2.1.1]hex-2-ene (15) are reported. Both are found to proceed t hrough concerted pathways. Also included are the results of (6,6)CASPT2/6-3 1G*//(6,6)CASSCF/6-31G* level calculations on the Cope rearrangements of sy n-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 diallyl ic diradical intermediates 26, 30, and 36, respectively. Previous studies h ave 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 ei ther proceeds to the monoallylic cyclohexane-1,4-diyl derivative 3 or bypas ses 3 to form 2 directly.(4) More recently, the conformationally restricted allenyl Cope rearrangement of syn-7-allenylnorbornene (7) has also been fo und to involve tricyclic monoallylic cyclohexane-1,4-diyl intermediate 11.( 7) The rearrangements of 14 and 15 appear to represent the first reported e xamples of fully concerted allenyl Cope rearrangements. Concertedness in th ese cases is ascribed to two parallel factors: (I) the relative instability of possible tricyclic diradical intermediates 16 and 17, compared to dirad ical intermediates 3 and 11 formed in the rearrangements of I and 7, respec tively; and (2) the opportunity that exists to form sp-sp(2) sigma bonds in transition structures 21 and 23 that lead, respectively, to products 22 an d 24. By contrast, only weaker sp(2)-sp(2) sigma bonds could form in unobse rved concerted transition structures leading to products 28 and 32, formed in the nonconcerted rearrangements of 18 and 19.