Res. Chem. Intermed.
, Vol. 27, No. 6, pp. 635–641 (2001)
Use of solvent isotope effect to identify an intermediate
carbanion in the
and N-[2-(2-pyridyl)ethyl]quinuclidinium induced
by acetohydroxamate /acetohydroxamic acid buffers
, A. CONTI and R. PALMIZIO ERRICO
Dipartimento di Chimica, Università di Perugia, 06100 Perugia, Italy
Received 16 October 2000; accepted 6 March 2001
Abstract—Solvent isotope effect is a useful technique for identi ng and characterizing an intermedi-
ate carbanion in the base-induced ¯-elimination reaction from N-[2-(4-pyridyl)ethyl]quinuclidinium,
1, and N-[2-(2-pyridyl)ethyl]quinuclidinium, 2. While at high [buffer] k
O/ > k
to the presence of a primary kinetic solvent isotope effect on the reprotonation of the intermediate
carbanion by BD, at low [buffer] no solvent isotope effect is observed, and k
The data are in agreement with a reversible E1cb mechanism in which carbon deprotonation occurs
, the substrate protonated at the nitrogen atom of the pyridine ring. In the absence of sol-
vent isotope effect at low [buffer], and with the similarity of the results obtained with the two isomers,
1 and 2, the signi cance of an intramolecular proton transfer in the intermediate carbanion can be
excluded in these processes.
: Mechanism; elimination; intermediate; carbanion; isotope.
In a previous paper , we reported that the
-elimination reactions induced by
acetohydroxamate/ acetohydroxamic acid buffer from N-[2-(4-pyridyl)ethyl]quinuc-
lidinium, 1, and N-[2-(2-pyridyl)ethyl]quinuclidinium, 2, with the formation of 4-
or 2-vinylpyridine, proceed by an E1cb mechanism according to Scheme 1. The
two isomeric substrates have a basic nitrogen on the pyridine ring, and the p
the conjugated acids are 4.85 for 1 and 3.81 for 2 (H
D 1 M KCl). In
this paper we indicate the unprotonated substrate 1 or 2 as N, and the conjugated
To whom correspondence should be addressed.