J . Org. Chem. 1998, 63, 1327-1329
1327
Sch em e 1
On e-Ca r bon Rin g Exp a n sion in
Cyclop en ta n on es a s a F r ee-Ra d ica l Clock
C. Chatgilialoglu,* V. I. Timokhin,1 and M. Ballestri
I.Co.C.E.A., Consiglio Nazionale delle Ricerche,
Via P. Gobetti 101, 40129 Bologna, Italy
Received September 3, 1997
In tr od u ction
Free radicals are of considerable importance in the
development of organic chemistry, and many radical-
based strategies employ group-14 hydrides.2 One of the
propagation steps of these radical chain reactions is the
hydrogen transfer from the reducing agent to a radical
(eq 1). To modulate the hydrogen donating abilities of
the hydrides, a significant number of substrates has been
introduced.3,4
An indirect procedure for measuring the rate constant
of eq 1 involves a competition between this process and
a unimolecular path of the radical (free-radical clocks).5
In his review Newcomb summarized competition methods
as well as calibrated unimolecular rearrangements of
radicals for this purpose.3 Kinetic scales for alkyl radical
reactions span literally the entire range of radical kinetics
in solution. The most popular example of primary alkyl
radicals is the 5-exo cyclization of the 5-hexenyl radical
(1) for which the rate constant of 2.3 × 105 s-1 at 298 K6
Ta ble 1. Kin etic Da ta for th e Rea ction of Br om id e 4
w ith (TMS)3SiH in Tolu en e or Ben zen e a t Va r iou s
Tem p er a tu r esa
T, K
kH/kre,b M-1
T, K
kH/kre,b M-1
333
343
353
363
4.11
3.19
2.75
2.77
383
393
403
413
1.96
2.11
1.43
1.37
a
b
Range of silane concentration, 0.5-0.9 M. Average of several
different experiments (at least four).
low regions of the kinetic scale. Therefore, it was
necessary to identify a free-radical clock (i) that does not
involve carbon-carbon double bonds, (ii) for which the
unimolecular rearrangement is ca. 105 s-1 at room
temperature, and (iii) for which the starting bromide is
easily accessible.
A general method of one-carbon ring expansion of
cycloketones was introduced in 1987 concurrently by
Beckwith9 and Dowd10 and later showed to be a powerful
synthetic tool.11 We have recently studied these reactions
from the mechanistic point of view, showing that the ring
expansion occurs via three-membered cyclic intermediate
radicals (or transition states), excluding the intermediacy
of acyl radicals.12 Furthermore, by using free-radical
clock methodology we have obtained the Arrhenius
parameters for the ring expansion of the secondary alkyl
is useful for middle regions of the kinetic scale. In a few
cases, the presence of an external double bond has caused
some inconvenience due to the similar reactivities of
bromine abstraction from 5-hexenyl bromide and addition
to the double bond.7 In principle, this inconvenience can
be overcome by using the neophyl radical (2) rearrange-
ment.8 However, this process occurs with a rate constant
of 9.0 × 102 s-1 at 298 K, which is generally useful for
(1) Visiting scientist. Permanent address: Department of Physico-
Chemistry, Institute of Physical Chemistry, National Academy of
Sciences of Ukraine, 3A Naukova Street, 290053 Lviv, Ukraine.
(2) For example, see: (a) Curran, D. P.; Porter, N.; Giese, B.
Stereochemistry of Radical Reactions; VCH: Weinheim, 1995. (b)
Motherwell, W. B.; Crich, D. Free Radical Chain Reactions in Organic
Synthesis; Academic Press: London, 1992.
(3) Newcomb, M. Tetrahedron 1993, 49, 1151-1176.
(4) Chatgilialoglu, C. In Chemical Synthesis: Gnosis to Prognosis;
Chatgilialoglu, C., Snieckus, V., Eds.; Kluwer: Dordrecht, 1996; pp
263-276.
(5) Griller, D.; Ingold, K. U. Acc. Chem. Res 1980, 13, 317-323.
(6) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J . C. J . Am. Chem.
Soc. 1981, 103, 7739-7742.
(9) (a) Beckwith, A. L. J .; O’Shea, D. M.; Gerba, S.; Westwood, S.
W. J . Chem. Soc., Chem. Commun. 1987, 666-667. (b) Beckwith, A.
L. J .; O’Shea, D. M.; Westwood, S. W. J . Am. Chem. Soc. 1988, 110,
2565-2575.
(7) For example, see: (a) Chatgilialoglu, C.; Guerra, M.; Guerrini,
A.; Seconi, G.; Clark, K. B.; Griller, D.; Kanabus-Kaminska, J .;
Martino-Simo˜es, J . A. J . Org. Chem. 1992, 57, 2427-2433. (b)
Chatgilialoglu, C.; Ferreri, C.; Vecchi, D.; Lucarini, M.; Pedulli, G. F.
J . Organomet. Chem. 1997, in press.
(8) Franz, J . A.; Barrows, R. D.; Camaioni, D. M. J . Am. Chem. Soc.
1984, 106, 3964-3967.
(10) (a) Dowd, P.; Choi, S.-C. J . Am. Chem. Soc. 1987, 109, 3493-
3494. (b) Dowd, P.; Choi, S.-C. Tetrahedron 1989, 45, 77-90.
(11) For review, see: Dowd, P.; Zhang, W. Chem. Rev. (Washington,
D.C.) 1993, 93, 2091-2115.
(12) (a) Chatgilialoglu, C.; Ferreri, C.; Sommazzi, A. J . Am. Chem.
Soc. 1996, 118, 7223-7224. (b) Chatgilialoglu, C.; Ferreri, C.; Lucarini,
M.; Venturini, A.; Zavitsas, A. A. Chem. Eur. J . 1997, 3, 376-387.
S0022-3263(97)01633-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/28/1998