Transition state characteristics of planar singlet 2,4,6,8-cyclononatetraenylidenes and its halo derivatives, via ab initio

Abstract

Calculations are carried out at HF/STO-3G, HF/6-31G*, MP2/6-31G*, B3LYP/6-31G* and B3LYP/6-311+G* levels of theory, for conversion of three series of cyclic conjugated non-planar allenes: 4-, 5-, and 6-X-1,2,4,6,8-cyclononapentaenes ( 1 X, 2 X, and 3 X) to their mirror images ( 1′ X, 2′ X, and 3′ X) through the corresponding singlet planar conjugated carbenic transition states: 3-, 4-, and 5-X-2,4,6,8-cyclononatetraenylidenes ( 1* X, 2* X, and 3* X ), where X=H, F, Cl and Br. The employed halogens (F, Cl and Br) are chosen not to be directly attached to the allenic centers. For Series 1, the relative ease of racemization of 1 X to 1′ X through 1* X is: 1* H> 1* Br> 1* F ≥1* Cl. For Series 2, the relative ease of racemization of 2 X to 2′ X through 2* X is: 2* H> 2* F≥ 2* Cl≥ 2* Br. Finally, for Series 3, the relative ease of racemization of 3 X to 3′ X through 3* X is: 3* H> 3* F≥ 3* Cl≥ 3* Br. Hence, the lowest racemization energy barrier encountered (26.8 kcal/mol) is for X=H, where 1 H (= 2 H= 3 H) is converted to its mirror image 1′ H (= 2′ H= 3′ H) through 1* H (= 2* H= 3* H) as a transition state.