J. Am. Chem. Soc. 1996, 118, 13-19
13
Absolute Kinetics of Aminium Radical Reactions with Olefins
in Acetonitrile Solution1
Brian D. Wagner,2 Ge´raldine Ruel,3 and Janusz Lusztyk*
Contribution from the Steacie Institute for Molecular Sciences, National Research Council of
Canada, Ottawa, Canada, K1A 0R6
ReceiVed January 17, 1995X
Abstract: Photolysis of N-nitrosamines in acidic acetonitrile produces aminium radical cations via protonation of
the initially-formed aminyl radicals. The kinetics of these species can be monitored by transient UV spectroscopy
via their absorption band which is found at ca. 300 nm in the case of the piperidinium radical, for example. By
measuring the aminium radicals’ lifetimes as a function of the concentration of added olefin, absolute values for the
bimolecular rate constants for the addition reactions were obtained. In the case of the piperidinium radical, these
rate constants varied from <1 × 106 M-1 s-1 for acrylonitrile to 1.1 ( 0.1 × 109 M-1 s-1 for 1,1-diphenylethylene
and generally increased with decreasing ionization potential of the olefin, thus confirming the electrophilic nature of
the piperidinium radical. The rate constants for analogous reactions of diethylaminium radicals were 1.5-25 times
smaller indicating the importance of steric factors in aminium radical additions to olefins. The rate constant for the
intramolecular 1,5-addition of the secondary aminium radical cation to an unactivated double bond is estimated to
be ca. 1 × 106 s-1, but the intramolecular addition rate constant increases to >1 × 108 s-1 upon the phenyl substitution
at the olefinic terminus.
Applications of nitrogen-centered radicals and their protonated
counterparts, aminium cation radicals, in synthesis were inves-
tigated in detail several years ago.4 It was established that
protonation of neutral aminyl radicals affects strongly both their
overall reactivity and the selectivity of their reactions, which
makes aminium radicals considerably more attractive for
synthetic purposes than their neutral counterparts. In contrast
to the rather unreactive, nucleophilic aminyl radicals, aminium
radicals, which are electrophilic, were shown to add readily to
alkenes and arenes and to undergo synthetically useful intramo-
lecular hydrogen atom abstraction reactions to form cyclic
amines (the Hofmann-Lo¨ffler-Freytag reaction).4
The recent development of new methods for generating
aminyl and aminium radicals has produced a renewed interest
in these species as synthetically useful entities in free radical
chain processes.5,6 However, these synthetic applications, unlike
those of carbon-centered radicals, are not augmented by any
detailed knowledge of the kinetic properties of aminium
radicals.7 This lack of quantitative kinetic data is particularly
true for additions of aminium radical cations to olefins. In this
paper we present information obtained via a UV-visible laser
flash photolysis (LFP) technique on the generation and spec-
troscopy of some alkyl and aryl secondary aminium radicals in
acidic acetonitrile solutions and detailed information on the
absolute kinetics of their reactions with olefins. In a recent
article, Newcomb et al. also reported the absolute kinetics for
reactions of some dialkylaminium radicals, using a different
experimental approach.8
Results and Discussion
Generation of Aminium Radicals. Numerous methods have
been reported for the generation of aminium radicals, e.g.,
electrochemistry9,10 and pulse radiolysis.11-14 There are also a
number of photogeneration methods, which are of potential use
in flash photolysis experiments. One method involves the
reduction of sensitizers such as ketone triplets15,16 or dicy-
anobenzene17 by amines in an electron transfer quenching
reaction, which generates the radical anion of the sensitizer and
the radical cation of the amine. Aminium radicals have also
been observed from the electron transfer quenching of aromatic
radical cations.18 All these methods have the disadvantage that
sensitizer radical anion or other transient species is also formed,
X Abstract published in AdVance ACS Abstracts, December 1, 1995.
(1) Issued as NRCC No. 39081.
(2) NRCC Research Associate 1993-1995.
(3) NRCC Summit Fellow 1993-1994.
(8) Martinez, F. N.; Horner, J. H.; Martinez, F. N.; Musa, O. M.;
Newcomb, M.; Shahin, H. J. Am. Chem. Soc. 1995, 117, 11124-11133.
(9) Nelsen, S. F.; Chen, L.-J.; Petillo, P. A.; Evans, D. H.; Neugebauer,
F. A. J. Am. Chem. Soc. 1993, 115, 10611-10620.
(10) Handoo, K. L.; Cheng, J.-P.; Parker, V. D. Acta Chem. Scand. 1993,
47, 626-628.
(4) (a) Wolf, M. E.; Chem. ReV. 1963, 63, 55-64. (b) Neale, R. S.
Synthesis 1971, 1-15. (c) Minisci, F. Synthesis 1973, 1-24. (d) Chow,
Y. L. Acc. Chem. Res. 1973, 6, 354-360. (e) Minisci, F. Acc. Chem. Res.
1975, 8, 165-171. (f) Chow, Y. L.; Danen, W. C.; Nelsen, S. F.;
Rosenblatt, D. H. Chem. ReV. 1978, 78, 243-274. (g) Stella, L. Angew.
Chem., Int. Ed. Engl. 1983, 22, 337-350.
(5) Esker, J. L.; Newcomb, M. AdV. Heterocycl. Chem. 1993, 58, 1-45.
(6) Bowman, W. R.; Clark, D. N.; Marmon, R. J. Tetrahedron Lett. 1991,
32, 6441-6444. Bowman, W. R.; Clark, D. N.; Marmon, R. J. Tetrahedron
1994, 50, 1275-1294 and 1295-1310. Beckwith, A. L. J.; Maxwell, B.
J.; Tsanaktsidis, J. Aust. J. Chem. 1991, 44, 1809-1812. Murphy, J. A.;
Dickinson, J. M. Tetrahedron 1992, 48, 1317-1326. Schwan, A. L.; Refvik,
M. D. Tetrahedron Lett. 1993, 32, 4901-4904. Dekimpe, N.; Desmaele,
D.; Bogaert, P. Synlett 1994, 287-288.
(11) Gebicki, J.; Marcinek, A.; Stradowski, C. J. Phys. Org Chem. 1990,
3, 606-610.
(12) Werst, D. W.; Trifunac, A. J. Phys. Chem. 1991, 95, 1268-1274.
(13) Jonsson, M.; Lind, J.; Eriksen, T. E.; Merenyi, G. J. Am. Chem.
Soc. 1994, 116, 1423-1427.
(14) Jonsson, M.; Lind, J.; Merenyi, G.; Eriksen, T. E. J. Chem. Soc.,
Perkin Trans. 2 1995, 61-65.
(15) Scaiano, J. C.; Stewart, L. C.; Livant, P.; Majors, A. W. Can. J.
Chem. 1984, 62, 1339-1343.
(16) Devadoss, C.; Fessenden, R. W. J. Phys. Chem. 1990, 94, 4540-
4549.
(7) What little kinetic data on aminium radical reactions is available is
summarized in the following: Ingold, K. U. In Radical Reaction Rates in
Liquids; Fischer, H., Ed.; Springer-Verlag: Berlin, 1983; Vol. 13, subvolume
c, and 1994, Vol. 18, subvolume c.
(17) Zhang, X.; Yeh, S.-R.; Hong, S.; Freccero, M.; Albini, A.; Falvey,
D. E.; Mariano, P. S. J. Am. Chem. Soc. 1994, 116, 4211-4220.
(18) Workentin, M. S.; Wayner, D. D. M.; Johnston, L. J.; Parker, V.
D. J. Am. Chem. Soc. 1994, 116, 8279-8287.
0002-7863/96/1518-0013$12.00/0 © 1996 American Chemical Society