Cationic intermediates in trans- to cis- isomerization reactions of allylic systems.: An exploratory ab initio study

Computational analyzes were undertaken to investigate the geometrical isomerization mechanism of a truncated tail-end model (C-1-C-10) of the full lycopene molecule, the products of which are the 5-cis and 7-cisforms. The global conformational minima were identified for the neutral all-trans reactant, the cationic isomerization intermediates and that of these two cis-isomeric products. Energies and stabilities were compared during different stages of the isomerization mechanism of the segments. The C-4 allylic hydride affinity values of the all-trans to the 5-cis, and the 7-cis isomers of this lycopene model, are in the range of 255-260 kcal mol(-1), which are within the expected limits of such hydride affinity values (210 kcal mol(-1) for weak and 360 kcal mol(-1) for strong hydride affinity) at the RHF/3-21G level of theory. The bond lengths involving alternating single and double carbon-carbon bonds roughly coincide with the expected changes along each intermediate of the putative isomerization pathway. The following sequence of stability was observed in the tail-end model of neutral lycopene isomers: 5-cis > all-trans > 7-cis For the cation, the order of stability was different: all-trans > 5-cis > 7-cis (C) 2001 Elsevier Science B.V. All rights reserved.