Could ionic gamma-elimination be concerted: clocking the internal displacement across a cyclobutane ring.

Abstract

Carbanion 1, obtained by a nucleophilic attack of PhSe- on 3-chlorobicyclobutane-carbonitrile in DME undergoes both protonation and elimination as shown in eq 1. Alcohols of increasing acidity in the following order: t-BuOH, i-PrOH, MeOH, trifluoroethanol (TFE), and hexafluoro2-propanol (HFIP) were used as proton donors. An Eigen-type plot of the log of the product ratio (protonation/elimination) vs the pK(a) of the alcohols, levels off for the two most acidic alcohols, TFE and HFIP which react at a diffusion-controlled rate. The partitioning of the products between protonation and elimination enables, therefore, the determination of the rate constant for the internal elimination as approximately 3 x 10(10) s(-1). Ab initio calculations at the B3LYP/6-31G level show that the elimination from a model carbanion (4, eq 4) occurs in a barrierless process. Simulation of the experimental reaction by including solvation effects using the Onsager model, shows that using the dielectric constant of DME (7.2) stabilizes, as expected, the carbanion and prevents a spontaneous elimination. In the absence of solvation effects, using Me- as a base, a complete elimination of HCl (proton removal and leaving-group expulsion) took place from 3-chlorocyclobutanecarbonitrile in a barrierless process without the formation of any discrete intermediate.