assess the viability of this conjecture, 2,2-dichloronor-
bornane was treated with SbF5 in SO2ClF at -100 °C.
On the basis of the 1H and 13C NMR and correlation
(COSY, HMQC, DEPT, NOESY) NMR spectra of the
resulting solution at -100 °C, as well as comparison to
Lew is Acid -Ca ta lyzed Rea r r a n gem en t of
2,2-Dich lor on or bor n a n e to
1-Ch lor on or bor n a n e
Kenneth Smith,† Nicie Conley,† George Hondrogiannis,†
Lyle Glover,† J ames F. Green,†
1
the H and 13C NMR spectra of the 2-phenyl-2-norbornyl
Andrew Mamantov,*,‡ and Richard M. Pagni*,†
cation (5),2,10,11 the observed species is identified as the
2-chloro-2-norbornyl cation (3). See Table 1 for NMR
assignments of 3 and 5.
Department of Chemistry, University of Tennessee,
Knoxville, Tennessee 37996-1600, and Environmental
Protection Agency, 1200 Pennsylvania Avenue,
Washington, D.C. 20460
rpagni@utk.edu
Received November 14, 2003
When the solution containing the 2-chloro-2-norbornyl
cation (3) was warmed to -20 °C, the ion was converted
into a new species which in turn decomposed. However,
the new species could be characterized as the 1-chloro-
2-norbornyl cation (4). The original seven resonances in
the proton-decoupled 13C NMR spectrum were replaced
by seven new ones [5 sharp (284.4, 91.1, 61.3, 42.7, 37.2)
and 2 broad (∼78, ∼41) due to chemical exchange]. The
C-2 resonance in the original ion at δ 284.5 was replaced
by a new one at δ 284.4, while C-1 in the original ion at
δ 76.9 was replaced by a peak at δ 91.1 which is assigned
to C1, the one bonded to chlorine. In the 1H NMR
spectrum of the new ion a resonance appeared at δ 5.6
which is consistent with H2, the one attached to the
charge-bearing carbon.
Abstr a ct: The mechanism for the unusual AlCl3-catalyzed
rearrangement of 2,2-dichloronorbornane to 1-chloronorbor-
nane in pentane has been elucidated; the reaction, which
also yields four isomeric dichloronorbornanes, occurs in three
steps: (1) ionization to form the 2-chloro-2-norbornyl cation,
which was fully characterized by two-dimensional 1H and
13C NMR in SbF5/SO2ClF; (2) Wagner-Meerwein shift to
yield the 1-chloro-2-norbornyl cation, which was partially
1
characterized by H NMR; and (3) hydride abstraction.
1-Chloronorbornane (1) is an interesting molecule
because its geometry precludes it undergoing the SN2
reaction and inhibits the SN1 reaction. It is also a source
of the 1-norbornyl cation,1-3 radical,4,5 and anion.6,7 It is
prepared by the unusual aluminum chloride-catalyzed
rearrangement of 2,2-dichloronorbornane (2) in pentane
(eq 1). Overall, one chlorine appears to change position
while the second is replaced by hydrogen. 2,2-Dibro-
monorbornane reacts similarly, yielding 1-bromonorbor-
nane in AlBr3/pentane.9 How do these remarkable rear-
rangements occur? What is the source of hydrogen in the
product?
A similar set of reactions must also occur in AlCl3/
pentane, which elicits the question: Where does the
hydride come from that quenches the 1-chloro-2-nor-
bornyl cation? There are only two possibilities: the
substrate or a product derived from the substrate and
the solvent. To answer this question, the reaction was
run in pentane and pentane-d12 and the product mixture
analyzed by GC/MS. In pentane two significant observa-
tions were made: (1) Four dichloronorbornanes (6-9) (21
are theoretically possible) were formed in yields similar
to that of 1-chloronorbornane (eq 3). This observation was
never mentioned in the literature syntheses of 1-chlo-
ronorbornane. Although the four new products were not
definitively identified, each of them clearly had a chlorine
at C-1 and likely did not have a chlorine at C-4 or C-7.
This suggestion is based on the very prominent peaks in
the mass spectrum of each compound at m/z 101 and 103,
which are due to the cyclopentadienyl cation 10 and m/z
129 and 131 due to the 1-chloronorbornyl cation. One can
reasonably assign the compounds as 1,2-exo- (6), 1,2-
endo- (7), 1,3-exo- (9), and 1,3-endo-dichloronorbornane
(10).12 (2) Numerous trace products with molecular
weights around 130/132 ((2), the molecular weight of
The initial steps in the rearrangement of 2,2-dichlo-
ronorbornane appear to be ionization to form the 2-chloro-
2-norbornyl cation (3) followed by a Wagner-Meerwein
shift to yield the 1-chloro-2-norbornyl cation (eq 2). To
† University of Tennessee.
‡ Environmental Protection Agency.
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10.1021/jo030350e CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/29/2004
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