Mechanisms of solvolytic elimination reactions of tertiary substrates: stereospecific 1,2-elimination reactions

Abstract

Solvolysis of (R,S)-1-chloro-1-(fluoren-9-yl)-2-methylcyclopentane (1-Cl) or the analogous 3,5-dinitrobenzoate ester 1-DNB in largely aqueous solutions yields alkenes 1-(fluoren-9-yl)-2-methylcyclopentene (4) and 1-(fluoren-9-yl)-5-methylcyclopentene (5) as the main products. The chloride 1-Cl gives a product ratio 4∶5 of 35∶65 in 25 vol% acetonitrile in water at 25 °C. The former product is formed by an anti elimination route, which shows that the elimination reaction does not have a concerted unimolecular mechanism. The reaction to give 4 is suggested to occur via a carbocation ion pair in which the leaving chloride ion abstracts the β-hydron. Alternatively, the reaction may have an enforced uncoupled concerted mechanism in which water acts as the hydron-abstracting base. Also, solvolysis of 2-methyl-1-phenylcyclopentyl p-nitrobenzoate (8-PNB) yields the more stable alkene 10 by anti stereochemistry.