Could ionic γ-elimination be concerted: Clocking the internal displacement across a cyclobutane ring

Uri Habusha, Esther Rozental, Shmaryahu Hoz

Research output: Contribution to journalArticlepeer-review

16 Scopus citations

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 PKa 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 ∼3 x 1010 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 HCI (proton removal and leaving-group expulsion) took place from 3-chlorocyclobutanecarbonitrile in a barrierless process without the formation of any discrete intermediate.

Original languageEnglish
Pages (from-to)15006-15011
Number of pages6
JournalJournal of the American Chemical Society
Volume124
Issue number50
DOIs
StatePublished - 18 Dec 2002

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