Deprotonation of 1-(carbethoxyalkyl)pyridinium halides with strong N-bases

Article abstract of DOI:10.1002/(SICI)1099-1395(199901)12:1<39::AID-POC84>3.0.CO;2-4

The rate constants were measured for deprotonation of 1-(carbethoxymethyl)pyridinium chloride and 1-(2-carbethoxyethyl)pyridinium bromide with strong bases (DBU, MTBD, TBD and P2-Et) in acetonitrile. The UV spectra and semiempirical calculations are consistent with an ylide structure of the deprotonated species. The ylides obtained slowly decompose, and the reaction products were identified by ¹H NMR spectroscopy; 1-(carbethoxymethyl)pyridinium chloride gives N-methylpyridinium cation and ethanol and 1-(2-carbethoxyethyl)pyridinium bromide converts to pyridine and ethyl acrylate. Copyright

Full text of DOI:10.1002/(SICI)1099-1395(199901)12:1<39::AID-POC84>3.0.CO;2-4

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 12, 39–46 (1999)
Deprotonation of 1-(carbethoxyalkyl)pyridinium halides
with strong N-bases
Zo®a Dega-Szafran,* Grzegorz Schroeder and Mirostaw Szafran
Faculty of Chemistry, A. Mickiewicz University, 60780 Poznan, Poland
Received 1 November 1997; revised 20 February 1998; accepted 16 March 1998
ABSTRACT: The rate constants were measured for deprotonation of 1-(carbethoxymethyl)pyridinium chloride and
1-(2-carbethoxyethyl)pyridinium bromide with strong bases (DBU, MTBD, TBD and P2-Et) in acetonitrile. The UV
spectra and semiempirical calculations are consistent with an ylide structure of the deprotonated species. The ylides
obtained slowly decompose, and the reaction products were identified by 1H NMR spectroscopy; 1-
(carbethoxymethyl)pyridinium chloride gives N-methylpyridinium cation and ethanol and 1-(2-carbethoxyethyl)pyr-
idinium bromide converts to pyridine and ethyl acrylate. Copyright 1999 John Wiley & Sons, Ltd.
KEYWORDS: carbethoxyalkylpyridinium halides; deprotonation; strong bases; ylide
INTRODUCTION
group in carbethoxymethylpyridinium salts becomes
comparable to that of the methylene groups in b-keto
esters.12 In these compounds, the action of base rapidly
generates ylides and further reaction results in cleavage
to an acid and an N-methylpyridinium salt,13 by a
mechanism probably similar to that involved in the
fission of b-dicarbonyl compounds. In the case of ylides
with a b-hydrogen, there is a possibility of Hofmann
elimination to form an alkene and pyridine or a less
substituted amine.1
Most alkene-forming elimination reactions proceed
through a one-step concerted E2 mechanism or a
stepwise E1cB mechanism with a carbanionic intermedi-
ate.14 Bunting et al.15 reported a detailed kinetic study of
the hydroxide ion-catalysed elimination of pyridines
from N-(2-cyanoethyl)pyridinium cations in aqueous
solution for a pyridine group having a pKBH in the range
1.5–9.7. Reaction rates are both pH and X-substituent
dependent, with a total range of 4 Â 105-fold in pseudo-
first-order rate constants. Brønsted plots as a function of
the basicity of the pyridine leaving group are concave-
down, which is consistent with a change in rate-
determining step within an E1cB mechanism. These
plots are characterized by blg = 0.30 for the rate-
determining deprotonation for pKBH < 5.8 and
blg = 0.93 for the rate-determining expulsion of the
pyridine nucleofuge from the carbanionic intermediate
for pKBH > 5.8. These reactions are further perturbed by
hydrogen–deuterium exchange and by the Michael-type
addition of pyridines (pKBH > 6) to acrylonitrile to
produce N-(2-cyanoethyl)pyridinum cations.
Strong bases can abstract either a- or b-protons from
quaternary ammonium halides. Abstraction of an a-
proton gives ylides as the intermediates. When a b-proton
is removed, an elimination can occur that is similar to a
Hofmann elimination.1
Most N-ylides are capable of only fleeting existence.2
On the other hand, in pyridinium-type ylides a deloca-
lization of charge into the heterocyclic ring may occur
and the stability of such compounds is enhanced.
Pyridine ylides with C = O or CN groups are so stable
that their structures could be determined by x-ray
diffraction.3–6
Ammonium ylides are important precursors in organic
synthesis, owing to their rearrangement under mild
conditions to form highly substituted organic compounds
and the ability to prepare compounds stereospecifically
from ammonium ylides.7 The major drawback in the use
of ammonium ylides in synthesis is the competition
between the two primary rearrangement pathways of
these compounds, the Stevens [1, 2] and the Sommelet–
Hauser [2, 3] rearrangements.7,8
Pyridinium ylides without a b-hydrogen react with
olefinic dipolarophiles.9–11 The reactivity of the N-CH2
*Correspondence to: Z. Dega-Szafran, Faculty of Chemistry, A.
Mickiewicz University, ul. Grunwaldzka 6, 60780 Poznan, Poland.
Email: szafran@main.amu.edu.pl
Contract/grant sponsor: Polish Committee for Scientific Research
(KBN); Contract/grant number: 2P 303 06907.
Copyright 1999 John Wiley & Sons, Ltd.
CCC 0894–3230/99/010039–08 $17.50

40
Z. DEGA-SZAFRAN, G. SCHROEDER AND M. SZAFRAN
In this work we investigated the deprotonation
slope gives k values and the intercept k- values (Tables 1
and Table 2). This indicates that Eqn (1) describes the
investigated reaction properly. Note however that in the
case of Eqn (2) the term [B H] is not constant; the
process for the forward reaction is first order but that for
the backward reaction is second order.
reactions of 1-(carbethoxymethyl)pyridinium chloride
(1a) and 1-(2-carbethoxyethyl)pyridinium bromide (2a)
by strong bases in aprotic solvents to obtain information
on the relative acidity of hydrogens in methylene groups
in the a- and b-positions.
‡
Although Eqn (1) correctly fits the experimental data
model, Eqn (3) cannot be completely neglected. The
dissociation of the hydrogen-bonded ion pairs into ions
increases with increasing dilution and temperature. It
seems that the contribution of solvated ions can be
neglected in the range of concentration and temperature
applied in this work.
In ylides the carbon is basic and acts as a hydrogen
bond acceptor. According to ab initio calculations (MP2/
6–311 ‡ ‡ G**), ylides form hydrogen bonds even with
such weak proton donors as methanol.19
RESULTS AND DISCUSSION
The strong N-bases 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU)
(pKa = 23.9),16
7-methyl-1,5,7-triazabicy-
clo[4.4.0]dec-5-ene (MTBD) (pKa = 24.70),17 1,5,7-tria-
zabicyclo[4.4.0]dec-5-ene (TBD) (pKa = 24.97)17 and
Schwesinger base, phosphazene (P2-Et) (pKa = 32.80)18
were used for deprotonation reactions; pKa values were
determined in acetonitrile solution (Scheme 1).
More reactive 1a undergoes deprotonation with all
bases in acetonitrile and DMSO solutions to give ylide
1b. However, P2-Et, as the strongest base, reacts too fast
When a 1-(carbethoxyalkyl)pyridinium halide (AH) is
treated with an excess of
a strong base (B)
([B0] 4 [AH0]), the following processes can occur:
k
‡
„
AH ‡ B
ꢀA ; B H†; kobs ˆ k‰B0Š ‡ k
ꢀ1†
ꢀ2†
ꢀ3†
k
k
‡
‡
„
AH ‡ B
A ‡ B H; kobs ˆ k‰B0Š ‡ k ‰B HŠ
k
KD
k
„
k
‡
‡
‡
AH ‡ B
ꢀA ; B H† „ A ‡ B H; kobs ˆ k‰B0Š ‡ k ‰B HŠ=KD
where k is the rate constant for the deprotonation
(forward) reaction, k- is the rate constant for the
protonation (backward) reaction, [B0] is the initial base
to be investigated quantitatively (Table 1).
The activation parameters were calculated by the
linear least-squares fit of lnk vs 1/T (Table 3). The values
of DG≠ are independent of the N-base used, within
experimental error. The large negative values for the
entropy of activation (DS≠) reveal that solvation in the
transition state is highly ordered.
‡
concentration, [B H] is the concentration of the cation
and KD is the dissociation constant of the ion pair (A ,
‡
B H) (Scheme 2).
The kobs values were calculated from the traces of
absorbance (1a, 420 nm; 2a, 368 nm) vs time and are
listed in Tables 1 and 2. When kobs is plotted vs [B0] an
excellent straight line is obtained (r > 0.99), where the
The deprotonation rate constants of 1a are comparable
to those for deprotonation of dimethyl (4-nitrophenyl)-
Scheme 1
Scheme 2
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 39–46 (1999)

DEPROTONATION OF CARBETHOXYALKYLPYRIDINIUM HALIDES
41
Table 1. Kinetic data (standard deviation in parentheses) for the deprotonation reaction of 1-(carbethoxymetyl)pyridinium
5
chloride (1a) (3 Â 10 M) with bases (0.0005±0.0075 M) in acetonitrile
1
k
obs (s
)
1
1
1
Base
DBU
Temperature (°C)
0.0005 M
0.0025 M
0.0050 M
0.0075 M
10 4 k (l mol
s
)
Int (s )
5
200 (3)
285 (4)
365 (5)
500 (6)
615 (6)
230 (4)
254 (3)
260 (2)
263 (5)
285 (4)
263 (4)
302 (5)
342 (3)
389 (5)
430 (4)
285 (4)
402 (5)
485 (3)
640 (3)
774 (8)
336 (3)
377 (6)
413 (5)
443 (5)
480 (6)
421 (4)
500 (5)
557 (4)
621 (8)
675 (8)
410 (5)
535 (6)
655 (8)
835 (5)
986 (7)
480 (4)
555 (8)
608 (8)
658 (11)
726 (11)
642 (8)
760 (10)
827 (9)
897 (9)
1004 (11)
530 (7)
673 (9)
830 (12)
1027 (11)
1210 (12)
630 (8)
729 (9)
810 (9)
871 (10)
986 (11)
856 (8)
4.75 (9)
5.51 (7)
172 (5)
260 (3)
325 (7)
457 (5)
566 (7)
197 (3)
207 (3)
218 (3)
221 (4)
216 (6)
215 (6)
260 (13)
294 (8)
325 (6)
366 (5)
10
15
20
25
5
10
15
20
25
5
6.67 (15)
7.57 (13)
8.51 (15)
5.76 (10)
6.94 (7)
MTBD
TBD
7.86 (6)
8.71 (9)
10.27 (13)
8.52 (12)
9.51 (28)
10.46 (17)
11.56 (13)
12.77 (11)
10
15
20
25
974 (9)
1072 (13)
1194 (11)
1320 (13)
a
P2-Et
—
a
The reaction is too fast.
Table 2. Kinetic data (standard deviation in parentheses) for the deprotonation reaction of 1-(carbethoxyethyl)pyridinium
5
bromide (2a) (3 Â 10 M) with P2-Et (0.0005±0.0125 M) in acetonitrile
1
k
obs (s
)
1
1
1
Temperature (°C)
0.0005 M
0.0075 M
0.0100 M
0.0125 M
k (l mol
s
)
Int (s )
5
0.035 (2)
0.053 (2)
0.112 (4)
0.173 (4)
0.250 (5)
0.075 (2)
0.100 (3)
0.175 (4)
0.254 (4)
0.362 (4)
0.103 (5)
0.135 (3)
0.235 (4)
0.341 (3)
0.485 (5)
0.143 (3)
0.186 (4)
0.294 (4)
0.425 (5)
0.611 (4)
141 (7)
174 (8)
242 (8)
337 (3)
474 (9)
0.030 (1)
0.030 (1)
0.030 (1)
0.003 (1)
0.010 (1)
10
15
20
25
Table 3. Thermodynamic data
1
1
1
Compound
Base
DH≠ (kJ mol
)
DS≠ (J mol 1 K
)
DG≠ (kJ mol
)
1a
DBU
MTBD
TBD
18.11 Æ 0.90
16.71 Æ 1.02
11.24 Æ 0.42
40.12 Æ 2.46
90 Æ 3
93 Æ 4
109 Æ 2
60 Æ 8
44.87 Æ 0.90
44.47 Æ 1.02
43.91 Æ 0.42
57.89 Æ 2.46
2a
P2-Et
malonate.20 This result conforms to that of compounds
containing a CH2 group flanked by two activating
carbonyl substituents.
Figure 1 illustrates the UV spectrum of 1a in
acetonitrile and its time-dependent changes on addition
of P2-Et. The absorption at 420 nm is a result of the
resonance interactions of the ylide chromophore with the
COOEt substituent (1b . 1c) (Scheme 3). The signifi-
cant decrease in intensity of the 420 nm absorption
reflects conversion of the ylide to products as shown in
Scheme 3. These reaction products were identified by 1H
NMR spectroscopy. For 1a the reaction products have
spectroscopic properties identical with those of a mixture
of N-methylpyridinium iodide21 and ethanol. The 1H
NMR chemical shifts of 1a, its reaction mixture with P2-
Et and the decomposition products of the ylide are listed
in Table 4. When P2-Et is added to 1a, the signals due to
5
Figure 1. UV±visible spectra of (a) 1a (5 Â 10 M) and a
5
2
mixture of 1a (4 Â 10 M) with P2-Et (1 Â 10 M) in
acetonitrile after (b) 0.5, (c) 6, (d) 12 and (e) 20 min
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 39–46 (1999)

42
Z. DEGA-SZAFRAN, G. SCHROEDER AND M. SZAFRAN
Scheme 3
a
Table 4. 1H NMR data for products resulting from the reaction between 1a and P2-Et in DMSO-d6
Assignment
P2-Et
CH3CH2OH
Ha
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
r
9.33(broad)
8.98(d)
8.28(broad)
8.11(t)
8.75(broad)
8.57(t)
5.96(broad)
2.80(m)
2.64(s)
2.63(s)
2.61(s)
9.29(d)
8.30(t)
8.75(t)
5.93(s)
—
—
—
—
—
—
—
—
—
9.00(d)b
8.12(t)b
Hb
Hg
8.57(t)b
—
N-CH2
N-CH3
2.80(m)
2.61(s)
2.58(s)
2.54(s)
2.50(s)
2.60(s)
2.52(s)
5.02(s)
4.37(s)b
—
OCH2
CH3
4.21(d, broad)
3.44(q)
4.24(q)
1.29(t)
—
3.44(q)
1.05(t)
1.24(t)
1.09(t)
1.05(t)
—
s
t
0.95
a
d = Doublet; m = multiplet; t = triplet; q = quartet.
b Data from Ref. 21.
the protons of the pyridine ring lost their multiplicity and
become broad (Fig. 2). This suggests that 1a is in a
dynamic equilibrium with 1b and 1c.
hydrogen atoms are slightly more acidic than CH2COO.
a-Deprotonation of the second ring carbon was also
observed when N-ethylpiperidine was treated with a
super-base (s-BuLi–t-BuOK).22
Deprotonation of the less reactive species, 2a, was
investigated only with P2-Et (Table 2), because the
reaction with other bases was too slow to study. In 2a
there are two CH2 groups and their relative acidities are
not known. One may expect that the formation of the
ylide 2b would produce a considerable red shift and
hyperchromic effect in the UV spectra because of
lengthening of the conjugated system. On the other hand,
the formation of 2c would cause little change in the
absorption. As Fig. 3 shows, addition of P2-Et to 2a shifts
the absorption to ca 368 nm and proves that 2b is formed.
Semiempirical calculations support the above conclu-
sion. From Table 5, it is seen that 2b is more stable than
A typical time-dependent UV spectrum showing the
disappearance of 2b is shown in Fig. 3. The ylides are not
very stable compounds and slowly rearrange to more
stable products. The 1H NMR spectrum shows that cation
2a on addition of P2-Et slowly undergoes elimination to
give pyridine and ethyl acrylate (Fig. 4 Table 6), but the
reaction does not go to completion, because these com-
pounds remain in equilibrium. The elimination reaction
of acrylates is reversible.15 The formation of ethyl
acrylate established that the elimination occurs via 2c,
which can be generated directly from 2a and additionally
via 2b (Scheme 4). Thus P2-Et abstracts both a- and b-
protons from 2a but abstraction of the a-hydrogen atom is
‡
2c by 65 kJ mol 1. Hence it seems that the N CH2
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 39–46 (1999)

DEPROTONATION OF CARBETHOXYALKYLPYRIDINIUM HALIDES
43
1
Figure 2. H NMR spectrum of a mixture of 1a and P2-Et in DMSO-d6
the rate-determining step. The elimination step is much
slower than the deprotonation reaction.
The time-dependent UV spectra of substituted 1-(2-
cyanoethyl)pyridinium cations in aqueous alkaline solu-
tions show that the reaction mixture is transparent above
300 nm.15 The absorptions of the pyridinium cations are
more intense than those of neutral pyridines but their
wavelengths are comparable. The only exceptions are
cations with amino and methoxy substituents, which
show a more significant spectral change as a result of the
resonance interactions of the substituents with the
pyridine ring. These data show that N-(2-cyanoethyl)pyr-
idinium cations in alkaline aqueous solution do not form
ylides. Hence, the deprotonation mechanism of pyridine
cations by strong bases in aprotic solvents is different
from that in aqueous alkaline solution.
Table 5. Calculated heats of formation (DHf) and dipole
moments (m)
1
DHf (kJ mol
)
m (D)
SAM1
Compound
PM3
SAM1
PM3
1b
2b
2c
94.98
116.98
44.56
106.02
111.54
45.77
2.55
2.10
10.31
2.78
2.30
10.45
5
Figure 3. UV±visible spectra of (a) 2a (5 Â 10 M) and a
5
2
mixture of 2a (4 Â 10 M) with P2-Et (1 Â 10 M) in
acetonitrile after (b) 0.5, (c) 6, (d) 12, (e) 18 and (f) 30 min
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 39–46 (1999)

44
Z. DEGA-SZAFRAN, G. SCHROEDER AND M. SZAFRAN
Figure 4. 1H NMR spectrum of a mixture of 2a with P2-Et in DMSO-d6
spectroscopy and PM3 semiempirical calculations. The
CONCLUSIONS
kobs data fits Eqn (1) well. The UV spectra show that both
compounds form ylides, which undergo decomposition.
These decomposition reactions were followed by 1H
NMR spectroscopy. The N-methylpyridinium cation and
ethanol were detected in the mixture containing 1a,
whereas pyridine and ethyl acrylate were detected in the
mixture containing 2a.
The deprotonation of la with DBU, MTBD, TBD and P2-
Et and 2a with P2-Et in aprotic solvents has been
investigated by kinetic measurements, UV and 1H NMR
It is interesting that deprotonation of the 1-(2-
cyanoethyl)pyridinium cation in aqueous alkaline solu-
tion does not proceed via ylides.15 Thus deprotonation
with strong bases in aprotic solvents differs from that in
alkaline aqueous solution.
EXPERIMENTAL
An equimolar mixture of pyridine and the appropiate !-
haloalkanecarboxylic ethyl ester was allowed to stand at
room temperature until a solid was precipitated. This
product was filtered off and washed with diethyl ether.
Scheme 4
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 39–46 (1999)

DEPROTONATION OF CARBETHOXYALKYLPYRIDINIUM HALIDES
45
a
Table 6. 1H NMR data for products resulting from the reaction between 2a and P2-Et in DMSO-d6
Assignment
P2-Et
Ha
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
r
9.18(d)
8.58(d)
8.19(t)
7.39(t)
8.64(t)
7.80(t)
4.88(t)
2.80(m)
2.64(s)
2.63(s)
2.61(s)
2.60(s)
2.52(s)
3.21(t)
6.17(dd)
6.32(dd)
5.94(dd)
4.15(q)
4.06(q)
1.23(t)
1.16(t)
1.09(t)
9.22(d)
8.20(t)
8.66(t)
4.90(t)
—
—
—
—
—
—
—
—
—
8.59(d)
7.41(t)
Hb
Hg
7.81(t)
—
N-CH2
N-CH3
2.89(m)
2.61(s)
2.58(s)
2.54(s)
2.50(s)
—
CH2COO
Ha
Hb
Hc
OCH2
3.19(t)
—
—
—
—
—
—
—
—
—
—
—
—
—
6.17(dd)
6.33(dd)
5.95(dd)
4.15(q)
—
s
t
u
w
z
4.06(q)
1.15(t)
CH3
—
1.23(t)
0.95(t)
a
d = Doublet; m = multiplet; t = triplet; q = quartet.
The 1-(carbethoxyalkyl)pyridinium halides were very
hygroscopic and could be hydrolysed to the correspond-
ing 1-(carboxyalkyl)pyridinium halides by standing at
room temperature for several weeks; 1-(carbethoxy-
methyl)pyridinium chlorides (1a), m.p. 62–65°C (lit.,23
m.p. 70°C), 1-(2-carbethoxyethyl)pyridinium bromides
(2a), m.p. 67–69 (lit.,24 m.p. 70–72°C).
optimization was carried out without any symmetry
constraints.
Acknowledgement
This work was supported by the Polish Committee for
Scientific Research (KBN), grant 2P 303 06907.
The
bases,
1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU),
7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene
(MTBD), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)
and Schwesinger base, phosphazene base (P2-Et), were
purchased from Fluka and were used as received.
Acetonitrile was purified by the method of O’Donnell
et al.25 with a final distillation from P2O5 (b.p. 81.5–
82.0°C).
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