Lewis acid-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane

Article abstract of DOI:10.1021/jo030350e

The mechanism for the unusual AlCl₃-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane in pentane has been elucidated; the reaction, which also yields four isomeric dichloronorbornanes, occurs in three steps: (1) ionization

Full text of DOI:10.1021/jo030350e

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.
(1) Schleyer, P. v. R.; Watts, W. E.; Fort, R. C., J r.; Comisaror, M.
B.; Olah, G. A. J . Am. Chem. Soc. 1964, 86, 5679.
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M. S. C. J . Am. Chem. Soc. 1993, 115, 10728.
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J .; Mueller, P.; Rossier, J .-C. J . Org. Chem. 1999, 64, 6401.
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10.1021/jo030350e CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/29/2004
J . Org. Chem. 2004, 69, 4843-4844
4843

TABLE 1. 1H a n d 13C Ch em ica l Sh ifts of 3 a n d 5
52,10,11
presence of AlCl3.13 The isomerization of pentane is
initiated by hydride abstraction, most likely from a
secondary carbon, to form AlCl3H- (or AlCl3D-) which,
in this instance, quenches the 1-chloro-2-norbornyl cat-
ion.
3
position
1H
13C
1H
13C
1
2
3
4
5
6
7
4.93
76.9
284.5
61.0
42.5
22.2
4.83
-
3.49
3.20
1.41
2.00, 2.81
2.04
59.3
260.1
50.4
39.4
25.3
34.1
41.4
-
3.48, ∼3.6a
3.40
2.32, ∼3.6a
1.66, 2.07
2.47
Exp er im en ta l Section
40.5
36.0b
In a 25-mL, three-necked, round-bottomed flask containing
0.175 g (1.07 mmol) of 2,2-dichloronorbornane in 5.0 g (6.8 mL)
of pentane-d12 was added, under nitrogen, 0.070 g (0.53 mmol)
of AlCl3 via a sidearm attachment over 3 h. After the reaction
mixture was stirred for an additional 40 h at room temperature,
the pentane was decanted from the reaction sludge which in turn
was washed with additional pentane. The combined pentane
extracts were washed with brine and water, dried, and removed
in vacuo. The residue was analyzed by GC/MS. The 1-chloronor-
bornane peak was identified by comparison to that of the known
compound. The deuterium content of 1-chloronorbornane was
determined from the peak areas of the molecular ions.
a
b
C-3 and C-5 have hydrogens that overlap at ∼3.6. C-7 is
severely broadened, even at -100 °C, suggesting that an unknown
dynamic reaction is occurring
1-chloronorbornane, are also produced.
The NMR samples were prepared by standard techniques11,14
When the reaction was repeated in pentane-d12, the
resulting 1-chloronorbornane consisted of 47% d0 and
53% d1 (eq 3). Interestingly, none of the dichloronorbor-
nanes was deuterated (eq 3).
1
and the H and 13C spectra recorded on a 400 MHz instrument.
Ack n ow led gm en t. This work was supported by the
Environmental Protection Agency, Office of Pollution
Prevention and Toxics, Exposure Evaluation Division.
J O030350E
(13) Olah, G. A.; Molna´r, AÄ . Hydrocarbon Chemistry, 2nd ed.; Wiley-
Interscience: New York, 2003.
(14) Olah, G. A.; Prakash, G. K. S.; Sommer, J . Superacids; Wiley:
New York, 1985.
One may conclude from the labeling experiment that
the hydride arises from both sources, substrate/product
and solvent. Alkanes, including pentane, isomerize in the
4844 J . Org. Chem., Vol. 69, No. 14, 2004