TY - JOUR
T1 - Bergman, Aza-Bergman, and protonated Aza-Bergman cyclizations and intermediate 2,5-arynes
T2 - Chemistry and challenges to computation
AU - Cramer, Christopher J.
PY - 1998/7/1
Y1 - 1998/7/1
N2 - 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 various levels of correlated electronic structure theory, as are singlet triplet splittings for intermediate arynes. To be effective in low-symmetry 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 cyclization of 2,5-pyridyne to pent-3-en-1- ynenitrile is predicted to be only 0.9 kcal/mol, which, combined with a predicted singlet - triplet splitting of - 11.6 kcal/mol, suggests that 2,5- pyridynes are poor hydrogen atom abstracting agents. Protonation of nitrogen decreases the singlet-triplet splitting and raises the barrier to retro-aza- Bergman cyclization such that protonated 2,5-pyridynes may be expected to show reactivities similar to all-carbon analogues.
AB - 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 various levels of correlated electronic structure theory, as are singlet triplet splittings for intermediate arynes. To be effective in low-symmetry 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 cyclization of 2,5-pyridyne to pent-3-en-1- ynenitrile is predicted to be only 0.9 kcal/mol, which, combined with a predicted singlet - triplet splitting of - 11.6 kcal/mol, suggests that 2,5- pyridynes are poor hydrogen atom abstracting agents. Protonation of nitrogen decreases the singlet-triplet splitting and raises the barrier to retro-aza- Bergman cyclization such that protonated 2,5-pyridynes may be expected to show reactivities similar to all-carbon analogues.
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U2 - 10.1021/ja9806579
DO - 10.1021/ja9806579
M3 - Article
AN - SCOPUS:0032125865
SN - 0002-7863
VL - 120
SP - 6261
EP - 6269
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 25
ER -