A novel access to 3-aryl-2-norbornyl cation

Article abstract of DOI:10.1039/b300839h

A novel access to a 2-norbornyl cation under mild, non acidic conditions is found in the addition of photochemically generated 4-dimethylaminophenyl cation to 2-norbornene. Deprotonation to nortricyclene or nucleophile addition ensue depending on the solvent characteristics.

Full text of DOI:10.1039/b300839h

A novel access to 3-aryl-2-norbornyl cation
Mariella Mella, Silvia Esposti, Maurizio Fagnoni and Angelo Albini
Department of Organic Chemistry, University of Pavia, via Taramelli 10, 27100 Pavia, Italy.
E-mail: albini@chifis. unipv.it; Fax: 39 0382 507323; Tel: 39 0382 507316
Received (in Cambridge, UK) 21st January 2003, Accepted 12th February 2003
First published as an Advance Article on the web 25th February 2003
A novel access to a 2-norbornyl cation under mild, non acidic
conditions is found in the addition of photochemically
generated 4-dimethylaminophenyl cation to 2-norbornene.
Deprotonation to nortricyclene or nucleophile addition
ensue depending on the solvent characteristics.
nanes, present as three main isomers with two further
components in traces, in a proportion that depended on the
alcohol chosen. Thus, with methanol the yield of product 5 was
reduced to 17% and the ethers became the main products. These
were recognized as the following methyl ethers: 3-endo-aryl-
2-exo-norbornyl (6a, 11%), 7-syn-aryl-2-exo-norboryl (7a, the
main component, 30%) and 7-anti-aryl-2-exo-norbornyl (8a,
10%).7 Likewise, in trifluoroethanol the corresponding ethers
were by far the main products with structures 6d–8d (5 was
reduced to 4%).7 On the other hand, irradiation in iso-propanol
and in tert-butanol gave a progressively increasing proportion
of the tricyclene 5 and a lower yield of arylnorbornyl ethers.
These were again three main isomers as shown by GC/MS,
presumably of structure 6b–8b and 6c–8c, although in these
cases the small amount hindered a detailed identification. A
minor product in methanol (7%) and in iso-propanol (12%) was
2-Norbornyl cation (1+, Scheme 1) is the prototype ‘non
classical’ (alkyl bridged) carbocation and it has been the subject
of extensive investigation, with regard both to its role in
solvolytic reaction (Scheme 1, path a) and to its generation and
spectroscopic characterization under ‘superacid’ conditions
(paths b–d).1 The impressive amount of work that has been
devoted to this topic over more than four decades has been
based upon a small number of reaction types and it appears
desirable to explore different paths to such an intermediate.2
In this connection, we considered the recent finding that
photolysis of 4-chloro (and fluoro) anilines in a polar solvent
generates the corresponding phenyl cation (2+, Scheme 2),3 an
intermediate otherwise difficult to access in solution,4 and that
this adds to alkenes, not to s nucleophiles such as alcohols. As
an example, irradiation in the presence of alkenes in acetonitrile
gives 4-(b-chloroethyl)anilines and in methanol 4-(b-methox-
yethyl)anilines (4). These were rationalized through the inter-
mediacy of a phenonium cation (3+).
Table 1 Aminophenylnorbornane derivatives from the photolysis of 4-N,N-
dimethylchloroaniline in the presence of 1 M norbornane
Products (% Yield)
Solvent
MeCN
MeOH
i-PrOH
t-BuOH
CF3CH2OH
5
6
7
8
PhNMe2
33
17
20
39
4
14
27
10
7
16
2
The latter ion pertains to another family of extensively
investigated intermediates and, contrary to norbornyl cation,
has been characterized as a classical carbocation under
superacid conditions.5 The photolysis of chloroanilines in the
presence of norbornene was thus expected to lead to adduct
cation(s) under mild, non acidic conditions and we were curious
to explore which chemical paths would be followed.
a
a
a
b
b
b
18
24
17
a Products 6–8, cumulative yield 22%. b Cumulative yield 12%.
Irradiation of N,N-dimethyl-4-chloroaniline in acetonitrile
containing 1 M norbornene gave a single major product, which
was isolated in 33% yield and recognized as 2-arylnortricyclene
5 (see Table 1 and Scheme 3).6,7
The result changed when the irradiation was carried out in
nucleophilic, protic solvents such as alcohols. Under these
conditions product 5 was accompanied by arylalkoxynorbor-
Scheme 1
Scheme 2
CHEM. COMMUN., 2003, 738–739
Scheme 3
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This journal is © The Royal Society of Chemistry 2003

N,N-dimethylaniline. By comparison, this was the main product
from the irradiation of the chloroani1ine in neat acetonitrile
(47%), methanol or isopropanol (90%).
Thus, the phenyl cation is generated and effectively trapped
by the alkene (only in hydrogen-donating media such as MeOH
and i-PrOH reduction to aniline competes to a limited extent).
This yields the 3-aryl-2-norbornyl cation 9+, which gives the
reactions expected from a ‘nonclassical’ cation. In acetonitrile
loss of a proton leads to nortricylene 5. The reaction is the
reverse of the generation of the 2-norbornyl cation from
nortricyclene under superacidic conditions (path d in Scheme
1). The alternative product of deprotonation, 2-arynorbornene,
has not been detected, though the unsatisfactory material
balance leaves room for different paths.6
the 2-norbornyl cation, also when formed by phenyl cation
addition, different from the phenonium ion path followed with
simple olefins under the same conditions.
Support of this work by MURST, Rome and fruitful
discussions with Professor R. Gandolfi are gratefully ac-
knowledged.
The result in alcohols depends on the acidity and nucleophi-
licity of these solvents. Thus, bulky and poorly acidic tert-
butanol does not slow down deprotonation of the norbornyl
cation and gives only a small amount of ethers (ratio 5/ethers
3.25). With iso-propanol and methanol this ratio drops to 1 and
0.4. With acidic trifluoroethanol, deprotonation is effectively
suppressed (ratio 5/ethers 0.1).
The products obtained from the reaction in alcohols are those
expected from addition to cation 9+, with the nucleophile
entering exo as generally observed in these reactions.1,8 Further
isomers such as 5-aryl-2-norbornyl ethers which would arise
from different H-bonded isomers of 9+ are not among the main
products, but may correspond to further isomers present in
minute amounts.
Notes and references
1 G. A. Olah, J. Org. Chem., 2001, 66, 5943; H. C. Brown, The Non-
classical Ion Problem, Plenum, New York, 1977; P. D. Bartlett,
Nonclassical Ions, Benjamin, New York, 1965; S. Winstein, Quart. Rev.
(London), 1969, 23, 1411; W. Kirmse, Top. Curr. Chem., 1979, 80, 125;
P. R. Schreiner, D. L. Severance, W. L. Jorgesen, P. v. R. Schleyer and
H. F. Schaefer III, J. Am. Chem. Soc., 1995, 117, 2663; S. A. Perera and
R. J. Bartlett, J. Am. Chem. Soc., 1996, 118, 7849; N. H. Werstiuk and H.
M. Muchall, J. Phys. Chem. A, 2000, 104, 2054.
2 A particular case is the generation of the norbornyl cation through an
essentially non activated fragmentation, e.g. from norbornyldiazonium
salts or from norbornyloxychlorocarbenes. J. A. Berson and A.
Remanick, J. Am. Chem. Soc., 1964, 86, 1749; E. J. Corey, J. Casanova,
P. A. Vatakencherry and R. Winter, J. Am. Chem. Soc., 1963, 85, 169; R.
A. Moss, F. Zheng, R. R. Sauers and J. P. Toscano, J. Am. Chem. Soc.,
2001, 123, 8109.
3 B. Guizzardi, M. Mella, M. Fagnoni, M. Freccero and A. Albini, J. Org.
Chem., 2001, 66, 6353; B. Guizzardi, M. Mella, M. Fagnoni and A.
Albini, J. Org. Chem., 2003, 68, 1067.
4 P. J. Stang, in Dicordinate Carbocations, ed. Z. Rappoport and P. J.
Stang, Wiley, Ney York, 1997, p. 451.
5 D. J. Cram, J. Am. Chem. Soc., 1949, 71, 3863, 3871; G. A. Olah, N. J.
Head, G. Rasul and G. K. S. Prakash, J. Am. Chem. Soc., 1995, 117,
875.
6 Irradiation for 2 h by external 15 W lamps, 310 nm;3 samples were
routinely Ar-flushed; omitting this slowed the reaction to 3 h, but did not
change product distribution. No other volatile products are formed.
Addition to the solvent and polymerization are presumed to account for
the remaining material.
The product distribution obtained is closely reminiscent of
that resulting from solvolysis of 2-norbornyl derivatives, For
example, the acetolysis (AcOH/AcONa) of 2-(endo)phenyl-
3-(exo)tosyloxynorbornane yielded the norbornanols (R = H) 6
(23) and 7 (44) as the main products, accompanied by a minor
amount of 8 (5) and some phenyltricylene (14%).9 Neither in
such thermal solvolysis nor in the present photoinduced
addition were any 2-phenyl-2-norbornyl derivatives found.
Thus, 2, 3-hydrogen shift leading to (classical) 2-aryl-2-norbor-
nyl cation (10+) has no significant role. In contrast, we
previously found3 that addition of phenyl cation to simple
alkenes leads in part to rearranged benzyl derivatives through
hydrogen shift from a phenonium cation (here hypothetical 11+
? 10+).
In conclusion, the above reaction offers a novel entry to the
long studied 2-norbornyl cation, which occurs in solution under
unprecedented mild, non acidic conditions. The product
distribution obtained confirms the ‘non classical’ chemistry of
7 New products were analytically and spectroscopically characterized.
8 Except for the 2-exo-3-exo-aryl ethers, possibly due to steric hindering.
9 D. C. Kleinfelder, M. B. Watsky and W. E. Wilde, J. Org. Chem., 1973,
38, 4134.
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