Dft Study of Allylic Rearrangements (Cope Rearrangements) of Substituted Hexa-L,5-Dienes: Nbo and Nics Analysis

Ab initio density functional theory (DFT) calculations have been performed on the [3,3] sigmatropic rearrangement of substituted hexa- 1,5-dienes (the Cope rearrangement) in the gas phase. The barrier heights calculated at the B3LYP/6-311G** level of theory were in a good agreement with experimental data. Optimized transition states at the B3LYP/6-311G** level were used to calculate nucleus-independent chemical shifts (NICS) and also natural bond orbital analysis (NBO) at the same level. Our results indicate that the aromaticities of the transition states are controlled by the out-of-plane component and these reactions are controlled kinetically by the aromaticity of the transition states. Based on the NBO analysis, the greater resonance energy for σ →π* and σ →σ* delocalization in R2 (compared to R1 and R3) could facilitate the Cope rearrangement of R2. The greater σ*3-4 anti-bonding orbital occupancy and the lower σ3-4 bonding orbital occupancy in R1 could facilitate the Cope rearrangement of R1 compared to R3. Comparative examination between substituted and un-substituted hexa-1,5- dienes show that in these reactions, steric parameters play no significant rule in the general mechanism of the Cope rearrangement.