BERGMAN, AZA-BERGMAN, AND PROTONATED AZA-BERGMAN CYCLIZATIONS AND INTERMEDIATE 2,5-ARYNES - CHEMISTRY AND CHALLENGE TO COMPUTATION

Reaction coordinates are computed for the Bergman cyclizations of hex- 3-en-1,5-diyne and neutral and protonated 3-azahex-3-en-1,5-diyne at v arious levels of correlated electronic structure theory, as are single t-triplet splittings for intermediate arynes. To be effective in low-s ymmetry situations showing high degrees of biradical character, CCSD(T ) calculations benefit from use of Brueckner orbitals. Replacement of a CH fragment by N is predicted to increase the stability of the aryne relative to the iminediyne, and to increase drastically the stability of the isomeric enynenitrile. The barrier for retro-aza-Bergman cycli zation of 2,5-pyridyne to pent-3-en-1-ynenitrile is predicted to be on ly 0.9 kcal/mol, which, combined with a predicted singlet-triplet spli tting of -11.6 kcal/mol, suggests that 2,5-pyridynes are poor hydrogen atom abstracting agents. Protonation of nitrogen decreases the single t-triplet splitting and raises the barrier to retro-aza-Bergman cycliz ation such that protonated 2,5-pyridynes may be expected to show react ivities similar to all-carbon analogues.