substituent(s) on the R-carbon and cyclopropyl group, as
well as examining several classes of carbonyl compounds
(aliphatic ketones, enones, etc.), the nature of the struc-
ture/reactivity relationships in these systems has been
explored.7-15
Cyclopropylcarbinyl f Homoallyl-Type
Ring Opening of Ketyl Radical Anions.
Structure/Reactivity Relationships and the
Contribution of Solvent/Counterion
Reorganization to the Intrinsic Barrier
J. M. Tanko,* Jason G. Gillmore,‡ Robert Friedline, and
M’hamed Chahma
Department of Chemistry, Virginia Polytechnic Institute and
State University, Blacksburg, Virginia 24061
Briefly, the rates of rearrangement depend on the
importance of charge- and spin-delocalization in the ring-
closed and ring-opened (distonic) forms of the radical ions,
which can be accounted for qualitatively and quantita-
tively in terms of the following structural properties: the
thermodynamic stability of the ring-closed radical anion
as reflected by the reduction potential of the parent
carbonyl compound, the ability of substituents on the
cyclopropyl group (R′) to stabilize the radical portion of
distonic radical anion 3, and the thermodynamic stability
of the enolate portion of radical anion 3.9,15 On the basis
of these principles, a thermochemical cycle can be con-
structed to estimate the free energy for the conversion 2
f 3, which when coupled with the kinetic data, allows
these rearrangements to be quantitatively treated in the
context of a Marcus-based theory pertaining to stepwise
dissociative electron transfer.15
Of course, the successful application of this theory
requires good kinetic data for several substrates which
span a broad range of reactivities. Virtually all of the
kinetic data obtained up until now have been obtained
by using electrochemical methods such as cyclic- or
linear-sweep voltammetry and homogeneous redox ca-
talysis.16 While these methods are incredibly powerful
tools for studying electron-transfer processes, problems
arise in certain situations. Specifically, when the ring-
opening reaction is extremely rapid, kinetic control shifts
to the electron-transfer step and it is impossible to
measure ko (i.e., the rate-determining step occurs before
the reaction of interest).8,9 Consequently, alternative
kinetic methods are needed to study exceptionally rapid
ring-opening reactions.
Received November 23, 2004
Following a protocol developed by Mathivanan, Johnston,
and Wayner (J. Phys. Chem. 1995, 99, 8190-8195), the
radical anions of several cyclopropyl- and oxiranyl-contain-
ing carbonyl compounds were generated in an effort to
measure the rate constants for their ring opening (ko) by
laser flash photolysis. The results of these experiments are
compared to those obtained from earlier electrochemical
studies, and the combined data set is used to rationalize the
kinetics of radical anion ring opening in a general context
by using Save´ant’s theory pertaining to stepwise dissociative
electron transfer (Acc. Chem. Res. 1993, 26, 455-461).
Compared to cyclopropylcarbinyl f homoallyl rearrange-
ments of neutral free radicals, at comparable driving force,
the radical anion ring openings are slightly slower. The small
difference in rate is attributed to the contribution of an
additional, approximately 2 kcal/mol, solvent reorganization
component for the radical anion rearrangements. The sol-
vent reorganization energy for ring opening of these radical
anions is believed to be small because the negative charge
does not move appreciably in the progression reactant f
transition state f product.
A means of photochemically generating ketyl anions
that avoids direct excitation of the ketone was reported
In direct analogy to the chemistry of cyclopropyl-
carbinyl neutral free radicals,1-6 radical anions generated
from the one-electron reduction of cyclopropyl ketones
and related moieties undergo unimolecular rearrange-
ment, 2 f 3, as depicted in eq 1. By varying the
(7) Ingold, K. U.; Maillard, B.; Walton, J. C. J. Chem. Soc., Perkin
Trans. 2 1981, 970-974.
(8) Phillips, J. P.; Gillmore, J. G.; Schwartz, P.; Brammer, L. E.,
Jr.; Berger, D. J.; Tanko, J. M. J. Am. Chem. Soc. 1998, 120, 195-
202.
(9) Stevenson, J. P.; Jackson, W. F.; Tanko, J. M. J. Am. Chem. Soc.
2002, 124, 4271-4281.
(10) Tanko, J. M.; Drumright, R. E. J. Am. Chem. Soc. 1990, 112,
5362-5363.
‡ Current address: Hope College, Holland, MI 49422.
(1) Beckwith, A. L. J.; Moad, G. J. Chem. Soc., Perkin Trans. 2 1980,
1083-1092.
(11) Tanko, J. M.; Drumright, R. E. J. Am. Chem. Soc. 1992, 114,
1844-1854.
(2) Effio, A.; Griller, D.; Ingold, K. U.; Beckwith, A. L. J.; Serelis,
A. K. J. Am. Chem. Soc. 1980, 102, 1734-1736.
(3) Kochi, J. K.; Krusic, P. J.; Eaton, D. R. J. Am. Chem. Soc. 1969,
91, 1877-1879.
(12) Tanko, J. M.; Drumright, R. E.; Suleman, N. K.; Brammer, L.
E., Jr. J. Am. Chem. Soc. 1994, 116, 1785-1791.
(13) Tanko, J. M.; Brammer, L. E., Jr.; Hervas, M.; Campos, K. J.
Chem. Soc., Perkin Trans. 2 1994, 1407-1409.
(14) Tanko, J. M.; Phillips, J. P. J. Am. Chem. Soc. 1999, 121, 6078-
6079.
(15) Chahma, M.; Li, X.; Phillips, J. P.; Schwartz, P.; Brammer, L.
E.; Wang, Y.; Tanko, J. M. J. Phys. Chem. A 2005, 109, 3372-3382.
(16) Andrieux, C. P.; Hapiot, P.; Save´ant, J.-M. Chem. Rev. 1990,
90, 723-738.
(4) Kochi, J. K.; Krusic, P. J.; Eaton, D. R. J. Am. Chem. Soc. 1969,
91, 1879-1881.
(5) Mathew, L.; Warkentin, J. J. Am. Chem. Soc. 1986, 108, 7981-
7984.
(6) Newcomb, M.; Glenn, A. G. J. Am. Chem. Soc. 1989, 111, 275-
277.
10.1021/jo047917r CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/06/2005
4170
J. Org. Chem. 2005, 70, 4170-4173