Scheme 2. Dechlorinative Surzur-Tanner Rearrangement of
Table 1. CuCl/bpy-Promoted Dechlorinative Rearrangement of
2,2,2-Trichloroethyl Carboxylates (1) to
2,2,2-Trichloroethyl Carboxylates
1-Acyloxy-1-chloroethenes (2)
time
(h)
yield
(%)
entry
1
estera
1
solvent
product 2
1a
DCE
benzene
THF
DCE
DCE
DCE
DCE
DCE
DCE
3
3
6
2
3
2
4
2.5
2.5
2a
92
1a , 2a
1a :2a
2b
2c
2d
2e
2f
2g
39, 45b
10:90c
93
2
3
4
5
6
7
1b
1c
1d
1e
1f
90
98
84
97
1g
91
1
a Concentration ) 0.125 M. b Yields calculated from H NMR spectra
of the mixture. c Molar ratio, calculated from 1H NMR spectra of the
mixture.
sphere to realize the above expectations. The reaction
proceeded smoothly to give 1-chloroalkenyl carboxylates 2
in high yields without any significant side products (Table
1), apparently by migration of the acyloxy group followed
by elimination of a chlorine atom, and required 2 equiv of
the CuCl/bpy reagent for completion. The structures of the
products were established by their spectral analysis and
hydrolysis. Though the expected formation of 5 by transfer
of a chlorine atom to the rearranged radical 4 from CuCl2
/bpy was not observed, it was not disappointing in view of
the fact that a new carbon-carbon double bond functionality
was created in the product, providing greater latitude for
further manipulation. The observed elimination of a chlorine
atom after the rearrangement suggested that the elimination
in the rearranged radical is faster than chlorine atom
abstraction from CuCl2/bpy probably due, among other
factors, to the relief in steric strain. It is possible that 5 is a
true stable intermediate that is converted into 2 by a
subsequent radical reaction. The reaction proceeds faster than
many reported â-acyloxyalkyl radical rearrangements as
revealed by a much shorter reaction time (2-4 h, Table 1)
that compares well with that reported for some of the
synthetically useful faster category of reactions (∼6 h).2-5
Interestingly, an apparently more stable radical (3, R′ ) H)
rearranges to a less stable radical (4, R′ ) H). Probably, the
availability of an inherent mechanism for stabilization of the
rearranged radical 4 by elimination of a chlorine atom or
the lower reactivity of the transferred chlorine atom in
intermediate 5 toword further abstraction drives the equi-
librium in the forward direction and forces it to occur in a
direction opposite to what has generally been observed so
far. Phenyl migration, which has been reported to compete
with this rearrangement,13 was not observed in the case of
substrates 1e-g. Crossover experiments on a mixture of 1d
and 1e gave only 2d and 2e without any detectable crossover
products. The reaction of 1a was not inhibited by 4-tert-
butylcatechol in accordance with other Cu(I)-catalyzed
group.7 However, small proportions of the products of direct
reduction still persisted.
Transition metal complexes are known to abstract a
halogen atom homolytically from polyhalo and other active
halogen compounds to produce radicals by a redox reaction.
The metal halide in the higher oxidation state thus formed
is a good halogen transfer agent and controls the reactivity
of the radical by transferring the halogen atom strategically
during the subsequent radical reactions. These radicals have
been exploited in what are known as halogen atom transfer
radical (HATR) addition/cyclization8-10 and living polym-
erization11 reactions, which are at the focus of great current
interest. Our experience in HATR cyclization12 led us to use,
for the first time, a suitable transition metal complex to
generate the initial radical in the Surzur-Tanner rearrange-
ment under nonreducing conditions. This strategy was
expected not only to obviate the problem of direct reduction
and loss of halogen functionality but also to allow the
reaction to be carried out at higher concentrations. Lewis
acid catalysis6 by the metal complex was also envisioned.
Thus, 2,2,2-trichloroethyl carboxylates 1 (Scheme 2) were
treated with a mixture of CuCl and bpy (1:1 molar ratio) in
DCE (1,2-dichloroethane) at reflux under a nitrogen atmo-
(7) Lacote, E.; Renaud, P. Angew. Chem., Int. Ed. 1998, 37, 2259.
(8) Byers, J. In Radicals in Organic Synthesis; Renaud, P., Sibi, M. P.,
Eds.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 1, Chapter 1.5.
(9) For reviews, see: (a) Jasperse, C. P.; Curran, D. P.; Fevig, T. L.
Chem. ReV. 1991, 91, 1237. (b) Iqbal, J.; Bhatia, B.; Nayyar, N. K. Chem.
ReV. 1994, 94, 519. (c) Giese, B.; Kopping, B.; Gobel, T.; Dickhaut, J.;
Thoma, G.; Kulicke, K. J.; Trach, F. Org. React. 1996, 48, 301.
(10) For some recent references, see: (a) Clark, A. J.; Campo, F. D.;
Deeth, R. J.; Filik, R. P.; Gatard, S.; Hunt, N. A.; Lastecoueres, D.; Thomas,
G. H.; Verlhac, J.-B.; Wongtap, H. J. Chem. Soc., Perkin Trans. 1 2000,
671. (b) Bryans, J. S.; Chessum, N. E. A.; Parsons, A. F.; Ghelfi, F.
Tetrahedron Lett. 2001, 42, 2901. (c) Nagashima, H.; Isono, Y.; Iwamatsu,
S.-i. J. Org. Chem. 2001, 66, 315.
(11) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921.
(12) Ram, R. N.; Charles, I. Chem. Commun. 1999, 2267.
(13) Beckwith, A. L. J.; Thomas, C. B. J. Chem. Soc., Perkin Trans. 2
1973, 861.
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Org. Lett., Vol. 5, No. 2, 2003