Research Projects
:: Formation and Stability of Carbocations
As a postdoctoral fellow more twenty years ago I developed a general
method now known as the "azide ion clock" for determination of the
lifetimes of carbocations in aqueous solvents. Over the last twenty years,
members of my group have applied this method to the determination of the
lifetimes of a broad range of carbocations, including strongly-destabilized
and strongly-stabilized carbocations; and so-called "antiaromatic"
carbocations. The data have been analyzed to provide a clear description of
many aspects of carbocation reactivity including:
(1) Studies on the effects of aqueous solvation on carbocation
reactivity has been examined. We have paid special attention to the strong
solvation of fluoride ion which has the effect of entrapping this anion so
that nucleophile addition to the 4-methoxybenzyl carbocation is barely
detectable (Scheme 1). On the other hand, our work has shown that solvation
of alkenes has little effect on the selectivity observed for these
nucleophiles in aqueous solution. See
(a)
Journal of the American
Chemical Society, 120, 10372-10378 (1998) and
(b)
Journal of the American
Chemical Society, 124, 9798-9805 (2002).
(2) A comprehensive characterization of the effect of changing
carbocation structure on rate and equilibrium constants for carbocation
formation. A wide range of substituents has been examined, ranging from
strongly electron-donating (-OCH3) to strongly electron-withdrawing. The
most surprising and revealing result of these studies is that the rate
constants for nucleophilic addition to α-substituted methoxybenzyl
carbocations are largely independent of the α-substituents at the benzylic
carbon. See
Journal of the American Chemical Society, 117, 5198-5205
(1995).
(3) A detailed study of the formation and stability of tertiary
carbocation intermediates of solvolysis at tertiary carbon in water. This
work addressed the controversial question of the importance of nucleophilic
solvation in providing stabilization of the transition state for solvolysis
at tertiary carbon. A simple description for these reactions has been
developed, which leaves minimal scope for controversy. Added insight into
this reaction has been obtained through computational studies in the
laboratory of Jiali Gao. See: Contemporary Reactive Intermediates, R. A.
Moss, M. S. Platz, and M. Jones, Jr. ed. John Wiley and Sons: New York, pp
41 - 68 (2004).
(4) A thorough examination of the effect of changing carbocation
structure on the relative rate constants for carbocation partitioning
between nucleophilic addition of solvent and loss of a proton from α-methyl
carbon to form an alkene. See
Advances in Physical Organic Chemistry, 35,
67-115 (2000).
(5) A thorough characterization of the ambident nucleophilic reactivity
of phenol and phenoxide ion (Scheme 2). See
Journal of the American
Chemical Society, 125, 15455-15466 (2003).
Many of the questions about carbocation reactivity addressed in this
work have been answered, and many new questions have been raised so that
this remains an area in which much exciting work is possible. One problem
where considerable further progress is possibly concerns the
characterization of the effect of sulfur bridging on the kinetic and
thermodynamic stability of α-thioamide substituted carbocations.
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