Copper-catalyzed imination of pyridines using PhI=NTs as nitrene precursor

Article abstract of DOI:10.1016/S0040-4039(03)00939-0

The reaction of PhI=NTs with pyridines in the presence of a catalytic amount of Cu(II) triflate afforded the corresponding p-tolylsulfonyliminopyridinium ylides in excellent yields.

Full text of DOI:10.1016/S0040-4039(03)00939-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4385–4387
Copper-catalyzed imination of pyridines using PhIꢀNTs as
nitrene precursor
Suman L. Jain, Vishal B. Sharma and Bir Sain*
Chemical and Biosciences Division, Indian Institute of Petroleum, Dehradun 248005, India
Received 11 February 2003; revised 3 April 2003; accepted 10 April 2003
Abstract—The reaction of PhIꢀNTs with pyridines in the presence of a catalytic amount of Cu(II) triflate afforded the
corresponding p-tolylsulfonyliminopyridinium ylides in excellent yields. © 2003 Elsevier Science Ltd. All rights reserved.
1-Aminopyridinium ylides are important synthetic
intermediates as they undergo 1,3 dipolar cycloaddition
reactions with a variety of dipolarophiles to yield
indolizine derivatives, which have been used in the
synthesis of alkaloids,1 and undergo photoisomeriza-
tion to yield 1(1H),2-diazepine derivatives.2 The chem-
istry of 1-aminopyridinium ylides is underdeveloped
due to the lack of availability of methods for their
synthesis. Although these compounds can be prepared
either by acylation of 1-aminopyridinium salts followed
by deprotonation3 or by thermal decomposition of
azides in the presence of pyridines,4 both the methods
suffer from drawbacks such as very low yields of 1-
aminopyridinium ylides, formation of side products due
to competing hydrogen abstraction and insertion reac-
tions, and the hazardous nature of azides such as
sulfonyl azides which are extremely shock sensitive
reagents.5 It is therefore important to develop a new
methodology for the preparation of 1-aminopyridinium
ylides.
To evaluate the catalytic efficiency of various copper
catalysts, the reaction of pyridine with PhIꢀNTs was
studied using different copper-based catalysts with ace-
tonitrile as the solvent (substrate/PhIꢀNTs molar
,
ratio=2:1) in the presence of 5 A molecular sieves. The
results are summarized in Table 1 and show that Cu(II)
triflate is the most active catalyst for this reaction. To
examine the versatility of Cu(II) triflate as a catalyst, a
variety of pyridines and fused pyridine derivatives were
reacted with the nitrene generated from PhIꢀNTs.7
These results are presented in Table 2 and clearly
indicate that pyridines containing electron-donating
Scheme 1.
Recently Evans et al. reported that [N-(p-tolylsul-
fonyl)imino]phenyliodinane (PhIꢀNTs)6 is an efficient
nitrene precursor for aziridination of alkenes in the
presence of copper-based catalysts.7 Hence, we thought
it would be worthwhile to use this reagent as a nitrene
precursor and accordingly we now report a first cata-
lytic method for the preparation of p-tolylsul-
fonyliminopyridinium ylides by the reactions of
pyridines with the nitrene from PhIꢀNTs generated in
situ in the presence of Cu(II) triflate as catalyst (Scheme
1).
Table 1. Imination of pyridine with different copper
catalystsa
Entry
Catalyst
Additive
–
Yieldb (%)
1
2
3
4
5
Cu(OTf)2
Cu(OTf)2
Cu(acac)2
CuCl
60
75
70
60
50
,
MS-5 A
,
MS-5 A
,
MS-5 A
,
CuCl2
MS-5 A
a Reaction conditions: Substrate/PhIꢀNTs (2:1), catalyst (5 mol%),
Keywords: pyridines; pyridinium ylides; copper; PhIꢀNTs.
* Corresponding author. Tel.: +91-135-2660071; fax: +91-135-
2660202; e-mail: birsain@iip.res.in
acetonitrile (5 ml), 8 h, under a nitrogen atmosphere in the presence
,
of 5 A molecular sieves.
b Isolated yields.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00939-0

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S. L. Jain et al. / Tetrahedron Letters 44 (2003) 4385–4387
Table 2. Imination of pyridines with PhIꢀNTs using Cu(II) triflate as catalysta
,
groups (Table 2, entries 2–4) are more reactive and give
better yields of the products. Similarly pyridines (Table
2, entries 1–6) were found to be more reactive as
compared to their fused ring derivatives (Table 2,
entries 7 and 8). Although, in all the reactions described
in Tables 1 and 2, we used PhIꢀNTs as the nitrene
precursor, to evaluate the efficiency of other nitrene
precursors, the reaction of pyridine was also studied
with Chloramine-T, and Bromamine-T in the presence
of Cu(II) triflate as catalyst. The use of PhIꢀNTs as the
nitrene precursor gave the highest yield of the p-tolyl-
sulfonylpyridinium ylide with Cu(II) triflate as catalyst.
All these reactions were associated with the formation
of p-toluenesulfonamide as the by-product along with
desired products. Reactions conducted without the use
of 5 A molecular sieves showed a significant increase in
p-toluenesulfonamide formation. To investigate the
effect of solvents the imination of pyridine was also
performed in different solvents such as acetonitrile,
nitromethane, dichloroethane and toluene under similar
reaction conditions with PhIꢀNTs using Cu(II) triflate
as catalyst. Among the solvents studied, acetonitrile
and nitromethane were found to be the best solvents for
this reaction.
In summary, the present method represents the first
example of a catalytic method for the imination of
pyridines and fused pyridine derivatives to form the
corresponding p-tolylsulfonyliminopyridinium ylides
using PhIꢀNTs, Chloramine-T and Bromamine-T as

S. L. Jain et al. / Tetrahedron Letters 44 (2003) 4385–4387
4387
nitrene precursors. The better yields of the products,
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Acknowledgements
We are thankful to the Director, IIP for his kind
permission to publish these results. Suman L. Jain and
Vishal B. Sharma thank the CSIR, New Delhi for the
award of research fellowships.