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G. A. Sereda / Tetrahedron Letters 45 (2004) 7265–7267
CH3
CH3
CH3
+
CH3
Bu-t
t-BuBr, reflux
CH3
t-Bu
9b
CH3
9a
Graphite
t-Cyclohexyl-Br, reflux
9a + 9b,
+
8c
8a
+
Graphite
1 : 1.4, 78%
BnCl, reflux
CH3
CH3
Graphite
Bn
7
8a + 8b + 8c,
+
8b
1.4 : 2.4 : 1, 90%
Bn
10b
10a
10a + 10b,
1:1.5, 100%
Scheme 2.
decoupling technique. The influence of the difference in
relaxation times was compensated by arraying the relax-
ation delays until the consistent integral intensities. The
integral intensities were measured at the relaxation delay
of 5s.
As opposed to the common alkylation on anhydrous
metal salts, graphite is less likely to cause carbon skele-
ton rearrangements, which would lead to undesired side-
products. Lack of reactivity of primary alkyl halides
brings a feature of selectivity to the reaction and widens
the scope of its application. The reaction proceeds selec-
tively for slightly activated aromatic nuclei. Benzyl chlo-
ride was not reactive toward benzene. The mechanism of
the reaction is a subject of a further study. As for now,
we believe that graphite acts as a Lewis base that coor-
dinates withcarbocation-like intermediates as opposed
to the traditional activation of alkyl halides by the coor-
dination of the Lewis acid catalyst with the halogen atom.
The ratio between the regioisomers was consistent with
the known procedure of cyclohexylation of toluene with
cyclohexyltosylate.10
tert-Butylation of toluene with tert-butyl bromide led to
a mixture of two isomers 9a,b.11 The identity of isomers
9a and 8b, as well as their relative ratio were determined
on a GC/MS instrument (column temperature 120°C).
The retention times were compared with those known
from the literature.11 The exact 9a:9b ratio was con-
Acknowledgements
1
firmed on the basis of the integral intensities of the H
NMR signals, corresponding to the methylene groups.
Lack of the ortho-isomer in the reaction product was
consistent withthe known procedure of tert-butylation
of toluene with tert-butyl bromide withthe use of Lewis
acids as catalysts.11 However, tert-butyl bromide (total
yield 78%) was a little less effective alkylating agent than
benzyl chloride (total yield 100%).
We thank the Department of Chemistry of the Univer-
sity of SouthDakota and the NSF (grant #0082978)
for financial support of this work.
Supplementary data
Supplementary data associated withthis article can be
13C-DEPT NMR spectra for all synthesized
compounds.
Benzylation of toluene led to a mixture of ortho- and
para-derivatives 10a,b. The identity of isomers 10a and
10b, as well as their relative ratio was determined by
1
comparison of the H NMR spectrum of the reaction
1
product withknown H-spectra of individual isomers.12
References and notes
We believe that the steric hindrance prevents tert-butyl-
ation at the ortho-position. Reaction of ortho-xylene
with tert-butyl bromide produced 4-tert-butyl-1,2-
dimethylbenzene 1113 as a single product withihgh
yield. Unlike o-xylene, p-xylene has only ortho-positions
available. In this case tert-butylation led to a compli-
cated mixture of unidentified products.
1. Kodomari, M.; Suzuli, Y.; Yoshida, K. Chem. Commun.
1997, 1567.
2. Suzuki, Y.; Matsushima, M.; Kodomari, M. Chem. Lett.
1998, 319.
3. Nagai, M.; Yoda, T.; Omi, S.; Kodomari, M. J. Catal.
2001, 1, 105.