Memory effects in carbocation rearrangements: structural and dynamic study of the norborn-2-en-7-ylmethyl-X solvolysis case.

The solvolysis of two diastereomers may give the same two products, but in different ratios, notwithstanding the fact that the two reaction pathways share an apparently identical intermediate carbocation. This has been dubbed the "memory effect", since the initial carbocation seems to "remember" its origin when undergoing further evolutions through multistep rearrangements. This puzzling result was studied theoretically for the case of the solvolysis of norborn-2-en-7-ylmethyl-X systems by defining the reaction potential energy surface (PES) and then carrying out a dynamical study. The PES shows that upon X(-) loss, multiphase rearrangements concertedly yield the two stablest carbocations, G and L. These carbocations are connected by a transition structure. The carbocation intermediates proposed in the literature do not correspond to any stationary point. The preference for the rearrangement to G or L (the memory effect) is determined by structural and stereoelectronic effects: the competitive interaction between an empty p orbital with a σ orbital or a p/π orbital is guided by geometrical aspects present in the starting carbocations. The dynamical study shows that (1) G and L do not interconvert and (2) the evolving system can switch from one pathway to the other to different extents, thus determining a more or less pronounced memory loss (the leakage).