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Mechanism of the PyryliumPyridinium Ring Interconversion. Mild Preparative Conditions for Conversion of Amines into Pyridinium Ions

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Mechanism of the Pyrylium/Pyridinium Ring
Interconversion. Mild Preparative Conditions for
Conversion of Amines into Pyridinium Ions
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By Alan R. Katritzky, Ruben H . Manzo, Jeremy M. Lloyd,
and Ranjan C. Patel"]
I3C-NMR studies have demonstrated that excess n-butylamine reacts with 2,4,6-triphenylpyrylium tetrafluoroborate
(1) by rapid ring-opening to the "vinylogous amide" (4) followed by slow reclosure to give the pyridinium salt"]. Extensive spectrophotometric kinetic studies with a variety of amines and pyryliums have now clarified the reaction mechanism: the ultraviolet spectra of species ( I ) , (4), and (5) are
readily distinguished.
Prof. Dr. A. R. Katrttzky, Dr. R H Manzo, J M.Lloyd, Dr. R. C. Patel
School of Chemical Sciences, University of East Anglia
Nonvich NR4 7TJ (England)
0 Verlag Chemie, GmbH, 6940 Weinheim, 1980
f [mini
Fig. 1. Plot of absorption at 418 nm of a CH2CI2solution of 2.4.6-triphenylpyry.
solid line shows
lium perchlorate (6.1 x ~ O - ' M ) and aniline (12.2 x 1 0 - 3 ~ )The
) added after 16
the variation when triethylamine (to bring to 6.1 x 1 0 - I ~ was
min and acetic acid (to bnng to 2.46 x
M) was added after 37 min. The dotted line shows the unchanged absorbance in the absence of NEt3 (acetic acid was
added as before after 37 min).
The sequence (1)+(4) is base catalyzed. For amines of
pK, > 8, excess of amine acts as base. For aniline, and for mand p-nitroaniline, the sequence (1)-+ (4) becomes fast in the
presence of NEt, as catalyst. For pyryliums possessing a 2- or
6-phenyl or 2- or 6-methyl group, the equilibrium of type
(1)<(4) is displaced extensively towards (4): however, if both
the 2- and 6-positions are occupied by tert-butyl or by fused
rings, appreciable quantities of the pyrylium salt remain in
The rate-determining step in the reaction of (1) with excess
n-butylamine is the ring closure (4), R = nBu+ (5), R = nBu
which is found kinetically to be first order in (1) and zeroth
order in nBuNH2. The rate depends on the solvent (relative
rates in dimethylformamide, acetonitrile, and dichloromethane were 1 :20: 270).
The rate of ring closure (4)-+(5) varies strongly with the
structure of the amine. Taking the rate for nBuNH, as unity,
rates for amines RCH2NH2 were 1.3-0.7 and for amines
RRCHNH, were 0.01-0.002, while those for aniline and mnitroaniline were respectively 0.02 and 0.0007 (obtained by
extrapolation). Ring closure is clearly hindered both sterically and electronically by electron withdrawal.
The ring closure (4)+ (5) is strongly acid catalyzed. Addition of two moles of AcOH to a mixture of (1) and iPrNH2
[after the formation of the intermediate (4)] increased the
rate 325 times; the rate for the nBuNH, reaction was too fast
to measure under these conditions. Catalysis by AcOH of the
reactions of (1) with iPrNH, and with PhNH2obey equations
of the Michaelis-Menten type indicating a pre-equilibrium of
vinylogous amide and catalyst to give protonated vinylogous
amide which then reacts further by a unimoIecular pathwayf21.
The influence of successive basic and acidic catalysis of
the reaction of (1) with aniline is shown in Figure 1. Such catalysis can dramatically improve preparative procedures for
converting pyrylium into pyridinium. The pyrylium salt, amine, and triethylamine (equimolar quantities) are stirred in
CH2C12for 5 min to form the vinylogous amide, 2 mol of
AcOH is added, the mixture kept 15 min and diluted with
diethyl ether to yield the pyridinium.
In this way the following improvements have been made:
(i) For amines RCH2NH2 reaction times have been reduced from ca. 12 h (e.g. ref. I31) to 20 min, while at least
maintaining the yield ( 280%).
(ii) For amines RR'CHNH2 reaction times have been reduced from 12-30 h used to prepare iodidesf4'to 20 min to
obtain the more generally useful tetrafluoroborates in excellent yield (? 73%)IS1.
(iii) Reaction of arylamines with 2,4,6-triphenylpyrylium
previously required much stronger conditions: 80 "C for aniline[*]and m-nitroanilinei6'and refluxing dimethylformamide
for p-nitroaniline[']. Provided reagents are rigorously dried,
the above procedure at 20 "C gives the I-phenyl-, I-(m-nitropheny1)-, and I-@-nitropheny1)pyridiniums in 77, 66, and
47% yield, respectively. The last two yields can be raised to
81 and 69% by slightly longer reaction times (at 20 "C).
(iv) Uncatalyzed reactions of even amines RCH2NH2with
more complex pyryliums can be very slow: thus at 20 "C the
reaction of n-BuNH, with 2-tert-butyl-5,6-dihydro-4-phenylnaphtho[l,2-b]pyrylium is incomplete after 7 days at 20 "C:
the catalyzed reaction leads to quantitative conversion at
20°C in 15 min. Amines RR'CHNH, do not react with $6dihydro-2,4-diphenylnaphthopyryliumin the absence of
AcOH; the catalyzed reaction proceeds quantitatively at
20°C in ca. 10 min.
Received: October 11, 1979 [Z 426 IE]
German version: Angew. Chem. 92, 315 (1980)
[I] A. R. Kafrirzky, R. T. C. Brownlee, G. Musumarm, Tetrahedron, in press.
121 See, e.g., k B. Wiberg- Physical Organic Chemistry. Wiley, New York 1964,
p. 329.
[3] A. R. Katrirzky, U.Grunrz. D. K Kenny. M. C. Rerende, H. Sheikh, J. Chem.
SOC.Perktn Trans. 1 1979. 430.
141 A. R. Karrifzky, N . F Eweiss. P.-L. Nie,J. Chem. SOC.Perkin Trans. 1 1979,
151 Satisfactory analyses were obtained for all new compounds reported.
[6] A. R. Katrirzky, A. KmroSikoua, C. A. Ramsden, J. Lewis, Coll. Czech. Chem.
Commun. 43, 2046 (1978).
I71 E. A. Zuezdina, M. P. Zhdanoua, V. A. Bren'. G. N . Dorofeyenko, Khim. Geterotsikl. Soedin. 1974, 1461.
S 02.50/0
Angew. Chem. Inr. Ed. Engl. 19 (1980) NO. 4
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preparation, pyryliumpyridinium, mild, mechanism, interconversion, ring, ions, conditions, amines, conversion, pyridinium
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