Deprotonation of N-phenacyl- and N-acetonyl-4-cyanopyridinium halides with 1,4-diazabicyclo[2,2,2]octane

Article abstract of DOI:10.1016/S0022-2860(02)00394-0

In N-phenacyl- and N-acetonyl-4-cyanopyridinium halides (1a·Br and 1b·Cl) the N⁺CH₂ hydrogens are replaced by deuterium in D₂0 or CH₃OD solutions, similarly as in β-diketones, proving a prototropic equilibrium 1 = 2. The rate constants have been measured for deprotonation of 1a·Br and 1b·Cl with 1,4-diazabicyclo[2,2,2]octane (DABCO) in solutions. UV, FTIR, ¹H and ¹³C NMR spectra are consistent with the ylide structure of the deprotonated species. N-phenacyl-4- cyanopyridinium ylide (3a) is relatively stable in the solid state and unstable in solutions. The observed parallel changes in ΔS ≠ and the time of disappearance of the ylide absorption band suggest that solvation is responsible for the ylide stability. The UV-Vis spectra of 1a·Br and 1b·Cl with DABCO show solvent dependent isosbestic points, which suggest that the decomposition reactions of ylides are very probably uniform. The PM3 and B3LYP calculations were carried out to investigate the structures of the cations, ylides and their complexes with ammonia and DABCO. Ylides are formed by abstraction of hydroxy proton from enol form (2) via complex 8.

Full text of DOI:10.1016/S0022-2860(02)00394-0

Journal of Molecular Structure 643 (2002) 55–68
www.elsevier.com/locate/molstruc
Deprotonation of N-phenacyl- and N-acetonyl-4-cyanopyridinium
halides with 1,4-diazabicyclo[2,2,2]octane
*
M. Szafran , A. Szwajca, B. L⁄ e˛ska, G. Schroeder, Z. Dega-Szafran
´
Faculty of Chemistry, Adam Mickiewicz University, ul Grunwaldzka 6, 60780 Poznan, Poland
Received 12 March 2002; revised 27 June 2002; accepted 12 July 2002
Abstract
In N-phenacyl- and N-acetonyl-4-cyanopyridinium halides (1a·Br and 1b·Cl) the NþCH2 hydrogens are replaced by
deuterium in D2O or CH3OD solutions, similarly as in b-diketones, proving a prototropic equilibrium 1 O 2. The rate constants
have been measured for deprotonation of 1a·Br and 1b·Cl with 1,4-diazabicyclo[2,2,2]octane (DABCO) in solutions. UV,
FTIR, 1H and 13C NMR spectra are consistent with the ylide structure of the deprotonated species. N-phenacyl-4-
cyanopyridinium ylide (3a) is relatively stable in the solid state and unstable in solutions. The observed parallel changes in DS –
and the time of disappearance of the ylide absorption band suggest that solvation is responsible for the ylide stability. The UV–
Vis spectra of 1a·Br and 1b·Cl with DABCO show solvent dependent isosbestic points, which suggest that the decomposition
reactions of ylides are very probably uniform. The PM3 and B3LYP calculations were carried out to investigate the structures of
the cations, ylides and their complexes with ammonia and DABCO. Ylides are formed by abstraction of hydroxy proton from
enol form (2) via complex 8.
q 2002 Elsevier Science B.V. All rights reserved.
Keywords: N-phenacyl-4-cyanopyridinium bromide; N-acetonyl-4-cyanopyridinium chloride; Deprotonation reaction; Ylides; Spectroscopy;
DFT and PM3 calculations
1. Introductions
enolate form, connected by intramolecular or inter-
molecular O–H· · ·O hydrogen bonds with O· · ·O
In N-alkyl-pyridinium salts of the type $Nþ–
CH2–Y, where Y is a strong electron-withdrawing
substituent, the methylene protons are acidic and their
reactivity becomes comparable with that of methylene
group of 1,3-dicarbonyl compounds. The pKa values
of a number of N-phenacyl-pyridinium salts are in the
7–10.9 range [1,2], acetylacetone 8.16 and benzoy-
lacetone 8.23 [3]. In crystals, 1,3-diketones exist in
distance in the 2.465–2.629 A range [4]. 1,3-
˚
Dicarbonyl compounds in solutions may exist in
three forms: the diketo, cis-enolic and trans-enolic
[5]. The equilibrium constants of acetyloacetone,
KT ¼ [cis-enolic]/[diketo] are solvent dependent and
change from 19 in hexane to 1.6 in DMSO [6]. N-
phenacyl-pyridinium ion in aqueous solution exists
mainly in the keto form (1) with some amount of the
enol form (2) (Schemes 1 and 2); KT ¼ [enol (2)]/
[keto(1)] ¼ 7.7 £ 1027 [2].
*
Corresponding author. Tel.: þ48-61-829-13209; fax: þ48-61-
In basic solutions the N-phenacyl-pyridinium ion
rapidly generates an ylide, which also can exists in an
865-8008.
E-mail address: szafran@main.amu.edu.pl (M. Szafran).
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(02)00394-0

56
M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
N-phenacyl-4-cyanopyridinium ylide (3a) was
obtained by mixing an aqueous solution of 1a·Br
and an aqueous solution of 20% sodium carbonate.
The ylide was then filtered off and dried over P2O5,
mp 126 8C dec., Ref. [1] 114–6 8C dec. N-acetonyl-4-
cyanopyridinium ylide (3b) is soluble in water.
The NMR spectra were recorded on a Varian
Gemini 300VT spectrometer, operating at 300.07 and
1
75.46 MHz for H and 13C, respectively. The FTIR
spectra were recorded in Nujol and Fluorolube mulls
on a Nicolet Magna spectrometer at 2 cm21 resol-
ution. Each spectrum consists of 200 scans. The UV
Scheme 1.
spectra were recorded on
spectrometer.
a JASCO V-550
equilibrium between zwitterion (3) and betaine (4).
Although nitrogen ylides are widely used as inter-
mediate in organic synthesis [7–10], no systematic
study of their formation rate has been reported. We
have recently presented [11] a detailed kinetic study
of the deprotonation reaction of several quaternary
pyridinium halides, (4-R–C5H4NþCH2Y)X2;
R ¼ Me and NMe2, Y ¼ CONH2, COOEt, COMe,
COC6H5 and CN, X ¼ Cl or Br, with very strong
bases in DMSO solution. Now, we report on the
deprotonation of more acidic N-phenacyl-4-cyano-
pyridinium bromide (1a·Br) and N-acetonyl-4-cyano-
pyridinium chloride (1b·Cl), which occurs with
involvement of a much weaker base, such as 1,4-
diazabicyclo[2,2,2]octane (DABCO) (Scheme 1).
The rate of proton transfer was measured spectro-
photometrically on an Applied Photophysics stopped-
flow apparatus with the cell block thermostated to
^1 8C. The reactions were monitored at the maxi-
mum absorption of ylide (3a, 468–496 nm and 3b,
496 nm) and were conducted by mixing a solution of
1a·Br (1 £ 1025 M) with 1,4-diazabicyclo[2,2,2]oc-
tane (1–5 £ 1024 M) in the following solvents: H2O,
MeOH, MeCN þ 5% MeOH and DMSO, and 1b·Cl
(1 £ 1025 M) in H2O.
The PM3 [12] semiempirical method was used, as
implemented in the AMPAC program package [13]. In
all cases, the PRECISE and GRAD key words were
used. The DFT calculations have been carried out
using the GAUSSIAN 98 package [14]. The calcu-
lations employed the BLYP and B3LYP functionals
(Becke three-parameter Lee–Yang–Parr) exchange-
correlation functional [15,16], which combines
the hybrid exchange functional of Becke [15] with
the gradient-corrected correlation functional of Lee,
Yang and Parr [16].
2. Experimental
N-phenacyl-4-cyanopyridinium bromide (1a·Br)
was prepared by refluxing of 4-cyanopyridine with
the equimolar amount of BrCH2COPh in acetone for
10 h. The excess of the solvent was removed under
reduced pressure, the crude product was washed with
diethyl ether and recrystallized from methanol contain-
ing 2% HBr, mp 223–5 8C dec., 85% yield, Ref. [1]
236–7 8C dec.
N-acetonyl-4-cyanopyridinium chloride (1b·Cl)
was prepared by refluxing of 4-cyanopyridine with
the 10% excess of ClCH2COCH3 in acetonitrile for
10 h. The solvent was removed under reduced
pressure, the crude product was washed with diethyl
ether and recrystallized from ethanol containing 2%
HCl, mp 205 8C dec., 43% yield.
3. Results and discussion
TheNþCH2 hydrogensin1a·Br and1b·Clare easily
replaced bydeuterium inD2OorCH3OD solutions. Ina
similar way the CH2 hydrogens in acetylacetone were
exchanged by deuterium in D2O solution [17]. On the
other hand, in betaines, $ NþCH2COO2and their
hydrogen halides, $NþCH2COOH·X2, the CH2
hydrogens are not replaced by deuterium in D2O
solutions [18-20]. This confirms the presence of 2
in solutions of 1a·Br and 1b·Cl, which undergo

M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
57
and 400 nm in CHCl3, 242 nm in MeOH and 240 nm
in H2O. This suggests two different mechanisms of the
decomposition reaction. In CHCl3 ylide, 3a decom-
poses to 4-cyanopyridine and phenacylo carbene, with
a probable rearrangement to ketene or dimerized to
1,2-dibenzoylethylene. Formation of 4-cyanopyridine
(a-H, 8.82, b-H, 7.54 ppm) was confirmed by 1H
NMR spectroscopy (Fig. 3). From aqueous solution
benzoic acid was separated. This indicates that in
aqueous solution of ylides, the bond between Nþ–CH
and carbonyl groups is broken.
1
The H and 13C NMR chemical shifts of 1a·Br,
1b·Cl, 3a and 3b in DMSO-d6 are listed in Table 1.
The proton and carbon-13 signals have been easily
assigned by comparison with the spectra of N-
phenacyl-pyridinium halides [1,11,25], N-methyl-
pyridinium iodide [26] and acetophenone [27]. The
signal of a-protons of the pyridine ring in the ylide is
broad and shifted downfield as compared to that of the
a-protons of the pyridine ring in the starting salts.
This broadening and deshielding can be explained by
the interaction of these protons with the oxygen atom
in the betaine form (4). Henrick et al. [28] have noted
a similar NMR effect but attributed it to a deshielding
of the a-protons resulting from the resonance
structure.
Scheme 2.
deprotonation on addition of 1,4-diazabicy-
clo[2,2,2]octane, DABCO (pKBH ¼ 8:82 [21], PA ¼
963:4 kJ mol21 [22]) to give 3a and 3b ylides,
respectively (Scheme 2). The characteristic ylide
absorption is solvent dependent (see below).
3.2. Kinetic study of proton transfer to amine
When quaternary pyridinium halides (AH) are
treated by an excess of a strong base (B)
{[Bo] q [AHo]} the deprotonation reaction is
described by Eq. (1) [29]:
3.1. FTIR, UV–Vis and NMR spectra
In Fig. 1 the IR spectra of 1b·Cl and 1a·Br and
their deuterated derivatives are compared. The nCD2
vibrations are at 2216 and 2243 cm21, respectively.
The nCyO band in 1a·Br is at 1688 cm21, while in 3a
is at a fairly low frequency, 1522 cm21. Matsubayashi
[23] explains this low frequency of the nCyO band by
elongation of the C–O bond in ylides. The C9–O1
distances confirm this explanation (see below). In
1b·Cl the nCyO band, as expected, is at a slightly
higher frequency (1731 cm21).
KD
k
AþH þ B O ðA^· · ·H–BþÞ O A^ þ BþH;
k2
ð1Þ
kobs ¼ k½B0ꢀ þ k2½BþHꢀ=KD
where k is the rate constant for the deprotonation
(forward) reaction, k2 is the rate constant for the
protonation (backward) reaction, [B0] is the initial
base concentration, [BþH] is the concentration of the
protonated base, and KD is the dissociation constant of
the ion pair (A^· · ·H–Bþ).
The UV–Vis spectra of 1a·Br (322 and 395 nm)
and 1b·Cl (269 and ,331 nm) with DABCO show
two isosbestic points (Fig. 2), which suggest that the
decomposition reactions of ylides are very probably
uniform [24]. The isosbestic points are solvent
dependent and in the case of 1a·Br they are: 325
The kobs values were calculated from the traces
of absorbance (3a, H2O, 468 nm, MeOH, 481 nm,
MeCN þ 5% MeOH, 487 nm, DMSO, 496 nm, and
3b, H2O, 496 nm) vs. time and are listed in Table 2.

58
M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
Fig. 1. FTIR spectra of N-acetonyl-4-cyano-pyridinium chloride, 1b·Cl, (a), N-phenacyl-4-cyano-pyridinium bromide, 1a·Br (b) and N-
phenacyl-4-cyano-pyridinium ylide, 3a (c) in the solid state; dotted line after deuteration).
When kobs was plotted vs. [B0], excellent straight
lines were obtained (r . 0.97), where the slope
gives k values and the intercept, the values of
k2[BþH]/KD (Table 2), which means that the
reactions studied are correctly described by Eq. (1).
As Table 2 shows, the rate of deprotonation is
solvent dependent. The fastest reaction occurs in
DMSO and the slowest in methanol. The replacement
of Ph group in 1a by Me (1b) slows down the
deprotonation ca. twice. A similar slow down process
occurs in deprotonation of 4-methylpyridinium
(7-methyl-1,5,7-triazabicyclo[4,4,0]dec-5-ene) in
DMSO [11]. However, when CN is replaced by
N(Me)2 the reverse effect was found [11].
The substituent in the pyridine ring has a greater
effect than that at the carbonyl group. The deproto-
nation of N-phenacyl-4-methylpyridinium bromide
by a stronger base, MTBD (pKa (CH3CN) ¼ 24.17
[30], PA ¼ 1015.5 kJ/mol [31]) is ca. 18 times
slower than that of 1a by DABCO. These data
confirmed that electron-withdrawing substituents
accelerate deprotonation, while electron-donating
ones slow it down. A similar effect of the ring
quaternary salts by
a stronger base MTBD

M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
59
Fig. 2. Time-dependent (min) UV–Vis spectra of (a and b) 1a·Br (1 mM) with DABCO (10 mM) in DMSO and (c and d) 1b·Cl (1 mM) with
DABCO (10 mM) in CH3CN þ 5% MeOH, cell 0.5 cm.
substituents on the elimination reaction of b-proton
from N-(2-cyanoethyl)-R-pyridinium cation in aqu-
eous solution was observed by Bunting et al. [32].
The activation parameters (Table 3) were calcu-
lated from the Eyring equation in the form Eq. (2)
[33,34]:
is the gas constant. The DS – values (Table 3)
demonstrate that ylide (3a) in DMSO is much more
solvated than in water.
3.3. PM3 and DFT calculations
The conformers of N-phenacyl-4-cyanopyridinium
cation, ylide and their complexes with ammonia found
by the PM3 calculations are collected in Fig. 4(a) and
(b), the geometrical parameters are listed in Table 4
and the energies in Table 5. The atom numbering is
given in Scheme 1. The PM3 geometries were chosen
as starting structures of the DFT calculations and the
obtained energies are collected in Table 6. As shown
by the data of Table 5, 1a is by 13.8 kJ/mol less stable
k
kB
h
DS–
DH–
log ¼ log
þ
2
T
2:303R
2:303RT
DS–
1000 DH–
19:148T
¼ 10:319 þ
þ
ð2Þ
19:148
where kB ¼ 1:380658ð12Þ £ 10223 J K21 is the Boltz-
mann constant, h ¼ 6:6260755ð40Þ £ 10234 J s is the
Planck’s constant and R ¼ 8:314510ð70Þ J mol21 K21

60
M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
than 2a(I) but more stable than conformer 2a(II) (Fig.
4(a)). The reverse stability relation is predicted by the
B3LYP computation (Table 6). An analogous reverse
stability order predicted by the PM3 and B3LYP/6-
31G(d,p) calculations has been reported by Zhu and
Lifshitz [35] for complexation of protonated betaine
with ammonia.
The PM3 calculations reproduce five complexes
withammonia(5–9)(Fig. 4(b)). Theirrelativeenergies
decrease in the order 7ac . 6ac . 5ac . 8ac . 9ac
and 7bc . 6bc . 5bc . 8bc . 9bc (Table 5). In the
gas phase the molecular complexes are usually more
stable than their ion pairs. Thus, the lower energies of
9acand9bcincomparisonwiththosein8acand8bcare
surprising. Ammonia forms molecular complex in the
gas phase with a much stronger acid like HCl [36]. The
O1· · ·N3distancein9acisslightlyshorterincomparison
to that in 8ac (Table 7) and this probably caused the
lowering of the observed Erel. in the former complex
(Table 5).
The structures of 5ac and 7ac are the first order
transition states with one imaginary frequency.
Complex 6ac has two imaginary frequencies and
is a second order saddle point. For the remaining
complexes (8ac and 9ac) the calculated frequencies
are positive. The binding energy of the NþH4/ylide
(9ac) is 104.2 kJ/mol, which is considerably higher
Fig. 3. The 1H NMR spectra in CDCl3 of 3a (a) immediately after
p
solution and (b) after 3.5 h; denote chemical shifts of 4-CN-
pyridine.
Table 1
1H and 13C NMR chemical shift (ppm) and multiplicity in DMSO-d6 of 1a·Br, 1b·Cl, 3a and 3b
1a·Br
1b·Cl
3a
3b
1H
13C
1H
13C
1H
1Ha
13C
1H
Pyridine ring
a
b
g
9.35, d
8.84, d
147.65
130.74
127.85
9.39, d
8.80, d
147.05
130.35
127.34
9.58, b
9.32, b
7.37, d
140.89
126.25
105.64
9.27, b
7.72, d
7.83, m
Phenyl ring
Ipso
o
133.35
128.38
129.23
134.91
67.13
128.94
128.75
128.19
129.93
102.78
173.45
117.31
8.08, d
7.67, t
7.81, t
6.67, s
7.83, m
7.45, m
7.45, m
7.45, m
7.80, d
7.40, m
7.40, m
6.85, b
m
p
CH2/CH
CO
CN
CH3
6.13, s
2.35, s
68.82
198.37
114.67
27.22
6.67, s
1.91, s
189.59
114.89
s ¼ singlet, d ¼ doublet, t ¼ triplet, m ¼ multiplet, b ¼ broad.
a
In CDCl3.

M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
61
Table 2
Kinetic data and activation parameters for the deprotonation of 4-CN–C5H4Nþ–CH2–COPh·Br2 (1a·Br) and 4-CN–C5H4Nþ–CH2–
COMe·Cl2 (1b·Cl)
Solvent
T (8C)
kobs (s21
)
k (M21 s21)1024
k2 [BþH]/KD (s21
)
4-CN–C5H4Nþ–CH2–COPh·Br2
B0 (M) £ 1024
MeOH
1
2
3
4
5
25
30
35
40
7.2(1)
13.4(1)
19.6(8)
27.6(2)
12.5(1)
18.1(1)
29.4(5)
38.8(1)
18.0(2)
24.0(2)
35.4(7)
52.0(3)
22.5(2)
32.0(2)
45.4(9)
61.8(4)
25.1(3)
37.3(9)
52.5(9)
72.4(5)
4.5(3)
6.1(2)
3.4(1.0)
6.5(8)
8.1(3)
12.0(9)
16.9(1.2)
11.1(4)
MeCN þ 5%MeOH
25
30
35
40
13.0(1)
21.4(1)
32.2(1)
47.2(2)
19.9(1)
30.4(1)
40.2(1)
55.1(2)
23.8(1)
37.6(1)
50.4(2)
66.6(2)
30.2(1)
45.4(1)
61.4(2)
83.4(3)
36.6(1)
54.0(2)
73.3(2)
98.3(4)
5.7(3)
8.0(2)
7.4(9)
13.7(6)
10.3(4)
13.0(10)
20.5(1.4)
30.9(3.3)
DMSO
25
30
35
40
207.4(3)
273.2(3)
323.4(4)
407.3(3)
300.6(1.1)
375.4(1.4)
454.8(1.3)
533.6(2.4)
381.7(2.1)
469.5(2.3)
566.2(3.1)
652.4(2.5)
428.7(2.2)
525.6(3.3)
622.5(2.5)
745.7(4.5)
74.4(73)
84.8(77)
68.7(27.0)
113.7(28.4)
138.4(44.2)
187.6(19.9)
100.8(120)
113.3(54)
H2O
25
30
35
40
24.4(1)
32.8(1)
42.0(3)
56.8(3)
34.5(1)
43.3(1)
58.7(3)
84.5(4)
42.3(1)
57.0(2)
76.2(4)
107.8(6)
53.4(1)
69.0(2)
94.0(5)
127.0(7)
58.0(2)
80.7(3)
8.6(5)
12.1(2)
17.3(1)
24.1(9)
16.7(1.8)
20.1(8)
111.0(6)
156.0(8)
24.3(4)
34.1(2.9)
4-CN–C5H4Nþ–CH2–COMe·Cl2
B0 (M) £ 1024
2
3
4
5
6
H2O
25
30
35
40
21.0(1)
30.0(3)
44.0(3)
56.5(6)
25.3(1)
34.1(2)
50.8(3)
64.2(4)
30.6(1)
39.1(2)
56.2(3)
71.3(5)
33.5(2)
44.1(2)
62.3(4)
80.6(7)
38.5(2)
50.3(3)
69.7(4)
89.2(6)
4.3(1)
5.3(2)
6.6(2)
8.4(2)
12.1(0.4)
18.3(1.0)
29.2(0.8)
38.3(0.7)
than the binding energy of 49.4 kJ/mol for NH3/
enol (8ac). The difference in the binding energies
between 5ac and 6ac is 5.9 kJ/mol, while between
7ac and 5ac is 9.2 kJ/mol (Table 5).
N-phenacyl-4-cyanopyridinium cation and ylide is
negligible.
According to the B3LYP/6-31G(d,p) calculations
only two complexes, 5ac and 8ac are formed, whose
When DABCO is used for deprotonation, the
PM3 method reproduces only three complexes with
a reasonably increasing relative energy, 8ad , 5
ad , 9ad and 8bd , 9bd < 5bd (Table 5). The
decrease of the number of complexes may be
explained by a larger volume of the DABCO
molecule.
Table 3
Activation parameters for the deprotonations of 4-CN–C5H4Nþ–
CH2–COPh·Br2 (1a·Br) and 4-CN–C5H4Nþ–CH2–COMe·Cl2
(1b·Cl) with 1,4-diazabicyclo[2,2,2]octane (DABCO)
Solvent
DH –
(kJ/mol)
DS –
(J mol21 deg21
DG –
(kJ mol21
)
)
Although DABCO is a weaker base in water and
slightly stronger in the gas phase, than ammonia
ðpKa ¼ 9:36 ð208Þ; PA ¼ 853:5 kJ=mol [21]), the
calculated complexation energies (ECOMPL) do not
reflect this difference (Table 5). A replacement of
Ph group by Me has little effect on the complexa-
tion energy. This suggests that the coupling
between the phenyl ring and the oxygen atom in
4-CN–C5H4Nþ–CH2–COPh·Br2
MeOH 43.7(1.0)
29(3)
MeCN þ 5%MeOH 39.6(2.1) 221(7)
46.5(1.0)
45.8(2.1)
39.5(1.1)
44.9(0.4)
DMSO
H2O
19.8(1.1) 266(4)
50.9(0.4) 20(2)
4-CN–C5H4Nþ–CH2–COMe·Cl2
H2O 32.5(1.0) 247(3)
46.6(1.0)

62
M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
Fig. 4. The optimized PM3 structures of (a) 1a–3a and (b) 5ac–9ac.
energies are at the minima on the potential energy
surface, as all their frequencies are positive.
The BLYP calculations reproduce also complex 6ac
(Tables 4 and 6, Fig. 4(b)).
As shown by the data in Table 4, the majority
of bond lengths calculated by the PM3 method for
1a, 2a and 3a are slightly longer in comparison to
the corresponding bonds reproduced by the B3LYP

Table 4
˚
Bond lengths (A), bond and torsion angles (8) of 1-phenacyl-4-cyanopyridinium cations (1a, 2a(I)) and 1-phenacyl-4-cyanopyridinium ylide (3a) calculated by B3LYP and PM3
level of theory
Parameter
Bond
1a
B3LYP
1a
PM3
2a(I)
B3LYP
2a(I)
PM3
3a 3a
B3LYP BLYP
3a
PM3
Parameter
Angle
1a
1a
2a(I)
2a(I) 3a
3a
B3LYP PM3 B3LYP PM3 B3LYP BLYP PM3
3a
N1· · ·O1
N1–C2
N1–C6
N1–C8
C2–C3
C3–C4
C4–C5
C5–C6
C8–C9
C9–O1
C4–C7
C7–N2
C9–C10
C10–C11
C11–C12
C12–C13
2.666
2.762
1.376
1.377
1.487
1.391
1.401
1.402
1.390
1.541
1.214
1.420
1.159
1.475
1.398
1.392
1.390
1.395
1.386
1.401
0.011
2.936
2.872
2.925
1.388
1.387
1.372
1.376
1.406
1.409
1.370
1.429
1.249
1.424
1.165
1.506
1.403
1.392
1.397
1.396
1.394
1.403
2.952
2.881
1.356
1.357
1.473
1.383
1.402
1.407
1.379
1.564
1.220
1.431
1.162
1.471
1.409
1.389
1.399
1.398
1.391
1.408
1.369
1.372
1.417
1.381
1.402
1.406
1.375
1.362
1.450
1.429
1.163
1.472
1.408
1.392
1.397
1.397
1.392
1.406
1.376
1.381
1.426
1.392
1.399
1.402
1.389
1.369
1.352
1.421
1.159
1.468
1.401
1.389
1.393
1.392
1.390
1.399
0.024
1.404
1.402
1.383
1.383
1.420
1.422
1.378
1.439
1.264
1.427
1.179
1.516
1.414
1.402
1.407
1.406
1.404
1.415
0.08
1.392 N1–C2–C3
1.395 N1–C6–C5
1.357 N1–C8–C9
1.381 C2–N1–C6
1.402 N2–C7–C4
1.398 C2–C3–C4
1.385 C3–C4–C5
1.455 C4–C5–C6
1.233 C8–C9–O1
1.419 C7–C4–C3
1.160 C8–C9–C10
1.493 O1–C8–C1’
120.8
121.0
109.9
120.7
179.9
119.3
119.0
119.1
117.7
120.6
117.8
124.5
120.5 121.2
120.5 121.9
113.4 128.4
120.4 118.8
180.0 180.0
119.4 120.4
119.7 118.0
119.5 119.7
119.5 120.8
120.2 121.1
115.3 121.5
125.2 120.8
119.5 119.2
120.9 121.3
119.9 120.1
120.2 120.0
120.3 120.1
120.0 120.1
120.0 120.2
119.6 119.5
110.0 113.3
110.6
121.1 121.2
120.9 122.4
126.6 125.1
119.5 117.5
180.0 179.8
119.4 121.6
119.5 116.8
119.5 120.6
118.8 124.4
120.3 121.8
118.8 116.7
119.6 118.9
120.0 117.2
120.0 123.9
119.8 120.7
119.8 120.5
120.2 120.1
120.0 119.7
120.2 120.1
119.9 118.8
115.8 113.9
121.2 121.7
122.3 121.4
125.3 124.5
117.4 118.0
179.9 179.9
121.8 119.7
116.6 119.3
116.6 119.8
124.2 122.1
121.9 120.4
116.8 116.8
119.0 121.0
117.2 119.6
117.2 121.3
120.7 120.3
120.6 120.4
120.1 120.2
119.7 119.8
120.2 120.2
118.7 119.1
113.6 118.4
1.398 C9–C10–C11 117.5
1.389 C8–C10–C15 122.9
1.391 C10–C11–C12 120.1
1.390 C10–C15–C14 120.0
1.391 C11–C12–C13 120.0
1.398 C12–C13–C14 120.4
0.014 C13–C14–C15 119.9
C
13–C14
C14–C15
15–C10
C
r.m.s
Torsion
C11–C10–C15 119.6
N1–C8–C9–O1
C2–N1–C8–C9
C6–N1–C8–C9
C10–C9–C8–N1
C2–N1–C8–O1
214.2
14.2
25.1
20.5
22.4
22.3
1.2
H–C8–N1
H–C8–C9
r.m.s
108.3
108.8
111.9
109.6
63.9
2110.4
167.3
289.4
89.0
3.8
2177.2
176.9
20.5
179.8
179.7
20.9
178.8
179.6
20.7
21.4
178.6
179.2
23.2
175.8
21.5
178.5
179.3
23.1
179.7
20.3
179.7
108.3 118.3
108.7
117.8 121.0
1.2
121.3 117.0
2166.5
259.5
83.9
2178.5
0.8
40.3
21.8
143.4
1.5
0.9
1.9
C15–C10–C9–O1 2177.2
C8–C9–O1H
2179.5
169.7
C10–C9–O1H
212.2

64
M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
Table 5
Total energies (kJ/mol), enthalpies offormation (kJ/mol), relative energies (kJ/mol) and dipole moments (Debye) calculated by the PM3 method
of investigated compounds
Compound
DHf
Erel.
ECOMPL
m
Compound
DHf
Erel
ECOMPL
m
N-phenacyl
1a
N-acetonyl
1b
925.5
13.8
0
4.94
788.7
777.4
805.0
152.3
727.2
731.8
745.2
711.7
696.6
760.2
737.2
759.4
11.7
11.3
0
5.89
2a(I)
2a(II)
3a
911.7
943.9
297.9
864.3
870.3
882.4
849.3
835.5
885.7
877.8
898.7
213.0
641.8
1.20
1.66
4.12
4.87
5.95
10.55
2.82
13.30
5.45
6.06
6.52
1.55
0.00
2b(I)
2b(II)
3b
6.79
3.74
3.94
6.68
6.76
32.2
27.6
0
5ac
6ac
28.9
34.7
46.9
13.8
0
248.1
242.3
257.3
249.4
2104.2
251.5
245.6
288.3
5bc
6bc
30.5
35.1
48.5
15.1
0
248.5
243.9
248.9
277.4
297.5
240.2
251.9
282.0
7ac
8ac
7bc
8bc
12.4
7.62
17.4
9ac
5ad
9bc
5bd
11.7
0
23.0
0
8.59
6.56
8.35
0.00
5.36
8ad
9ad
8bd
9bd
18.0
22.2
NH3
NH4
DABCO
DABCOH
689.1
calculations. For 3a the bond lengths calculated by
BLYP are slightly longer in comparison to the
values obtained by B3LYP and PM3 methods. No
systematic differences have been observed for the
bond angles. The formation of ylide 3a from cation
1a causes the following geometrical changes: (i)
the N1–C8 and C8–C9 bonds become shorter ca.
N-acetonyl-4-cyano-pyridinium cations and ylides
have similar conformation, respectively.
The DABCO complexes 8ad and 8bd have the
lowest energies (Table 5). This suggests that ylides
are formed by abstraction of hydrogen atom from the
enolic form (2a, 2b) via complex 8 (Scheme 3).
˚
0.1 A; (ii) the C9–O1 bond becomes up to 0.03 A
˚
Table 6
longer; (iii) the intramolecular N1· · ·O1 distance
Total energies (in hartrees), relative energies (kJ/mol) and dipole
moments (Debye) of investigated compounds calculated by DFT
methods
˚
˚
elongates ca. 0.1 A (B3LYP) and 0.12 A (PM3).
The N1–C8–C9–O1 moiety in 1a is perpendicular,
while in 2a and 3a it is coplanar with the pyridine
ring plane. The C8–C9 in 1a is a typical single
bond, in 2a it is a typical double bond, while in 3a
it is between a single and a double bond. All the
above data indicate that in 3a there is a strong
electron coupling between the pyridine ring and the
oxygen atom through bonds, while in 1a there is an
electrostatic interaction between the oppositely
charged nitrogen and oxygen atoms via space.
Interesting differences between the B3LYP and
PM3 data are observed for the torsion angles
(Table 4), which indicate dissimilarity of their
conformations.
Compound
Method
Energy (a.u.)
Erel.
EHB
m
1a
B3LYP
BLYP
2724.599633
2724.339333
2724.590666
2724.203651
2723.947487
2781.179102
2780.886634
2781.184868
2780.896082
2532.850504
2532.844801
2532.461362
2532.281287
256.557768
256.905864
0
4.38
4.23
2.62
4.48
4.55
5.48
5.53
5.26
6.06
6.03
7.40
4.39
4.53
1.85
0.00
2a(I)
3a
B3LYP
B3LYP
BLYP
23.4
5ac
6ac
8ac
B3LYP
BLYP
15.1
0
257.0
295.6
B3LYP
BLYP
1b
B3LYP
B3LYP
B3LYP
BLYP
0
2b(I)
3b
15.1
The geometrical parameters for 1b, 2b and 3b
listed in Table 8 are very similar to these for 1a, 2a
and 3a collected in Table 4. Thus N-phenacyl- and
NH3
NH4
B3LYP
B3LYP

Table 7
Selected distances (A), angles and torsion angles (deg) derived by the PM3, B3LYP and BLYP calculations for 1-phenacyl-4-cyanopyridine derivatives from Scheme 2
˚
Parameter
5ac
B3LYP
5ac
PM3
6ac
BLYP
6ac
PM3
7ac
PM3
8ac
B3LYP
8ac
PM3
9ac
PM3
5ad
PM3
8ad
PM3
9ad
PM3
C9–O1
1.130
1.213
1.235
1.216
1.229
1.335
1.334
1.265
1.213
1.333
1.261
N1· · ·O1
2.673
1.474
1.558
1.475
2.781
1.487
1.537
1.478
2.711
1.479
1.575
1.480
3.224
2.111
2.763
1.479
1.529
1.479
2.912
1.742
2.874
1.420
1.479
1.487
2.725
1.650
2.963
1.415
1.388
1.490
2.874
1.420
1.375
1.473
2.906
1.389
1.410
1.490
2.778
1.487
1.537
1.478
2.865
1.421
1.377
1.473
2.904
1.386
1.414
1.491
N1–C8
C8–C9
C9–C10
C8· · ·N3
(C8)H· · ·N3
C8H· · ·N3
C6· · ·N3
172.9
177.1
172.9
3.244
2.158
2.892
1.728
2.903
1.740
(C6)H· · ·N3
C6·H· · ·N3
O1· · ·N3
171.3
177.9
177.9
2.711
2.963
1.537
2.696
1.722
2.682
1.708
2.699
1.740
2.750
1.713
O1· · ·HN3/O1H· · ·N3
O1H·N3
172.1
167.4
155.7
162.7
172.7
C8–C9–O1
118.3
124.0
120.3
124.8
118.1
119.9
124.3
217.8
80.7
121.5
121.7
121.8
119.5
24.3
2.7
119.6
120.2
20.3
123.9
119.0
1.0
120.4
124.8
210.0
76.0
119.0
121.3
21.5
3.4
123.9
119.4
1.5
C10–C8–O1
123.7
221.7
64.2
N1–C8–C9–O1
C2–N1–C8–C9
C6–N1–C7–C8
11.7
13.4
224.3
22.2
263.7
111.6
282.8
96.7
20.8
20.4
179.6
2179.0
0.9
2109.9
160.8
298.3
163.2
50.8
2160.6
159.3
2177.7
178.6
21.5
140.2
161.6
240.6
240.6
179.1
2179.8
20.9
2103.3
170.9
53.5
2175.7
178.5
1.6
2179.1
2178.6
2.0
C10–C9–C8–N1
2169.3
242.2
176.5
2167.4
255.7
2148.7
C2–N1· · ·C9–O1
C15–C10–C9–O1
C8–C9–O1–H
C10 C9–O1–H
34.4
2179.6
153.3
158.6
128.5
176.6
23.9
122.2
147.9
132.0
177.5
22.5
248.3
114.2
C11–C10–C9–O1
22.8
30.2
2.8
225.1
218.9
252.3
258.6
231.1
266.5

Table 8
Selected distances (A), angles and torsion angles (deg) calculated by the PM3 method of 1-acetonyl-4-cyanopyridine derivatives from Scheme 2
˚
Parameters
1b
2b(I)
3b
5bc
6bc
7bc
8bc
9bc
5bd
8bd
1.335
9bd
1.260
C9–O1
1.211
1.351
1.231
1.211
2.779
1.489
1.536
1.497
1.213
1.226
1.333
1.264
1.211
2.775
1.488
1.536
1.497
N1· · ·O1
2.765
1.489
1.539
1.496
2.874
1.429
1.366
1.489
2.874
1.357
1.455
1.503
2.770
1.480
1.527
1.497
2.909
1.741
2.886
1.422
1.478
1.502
2.726
1.648
2.871
1.422
1.374
1.491
2.903
1.387
1.412
1.502
2.864
1.423
1.373
1.491
2.899
1.385
1.416
1.503
C8–N1
C8–C9
C9–C10
C8· · ·N3
(C8)H· · ·N3/C8· · ·HN3þ
C8·H·N3
174.5
174.1
C6· · ·N3
2.889
1.725
2.859
1.748
(C6)H· · ·N3
C6·H·N3
174.9
157.2
O–H
0.953
0.989
2.698
1.722
0.988
2.719
1.734
O1· · ·N3
4.658
2.658
1.712
4.953
2.745
1.712
O1· · ·HN3/O1H· · ·N3
O1HN3
168.4
150.6
174.7
171.0
C8–C9–O1
120.4
118.9
123.1
120.8
120.9
124.7
8.6
122.1
122.3
28.0
119.3
121.1
20.1
0.3
123.3
120.3
20.1
0.1
120.8
119.2
120.5
20.3
6.0
123.3
120.6
0.6
C10–C9–O1
125.4
2.3
120.1
20.0
20.4
121.9
20.0
20.2
125.0
23.8
284.3
95.3
125.0
0.6
N1–C7–C8–O1
C2–N1–C8–C9
C6–N1–C8–C9
C10–C9–C8–N1
C2–N1· · ·C9–O1
C8–C9–O1–H
290.3
88.6
101.8
277.6
2171.8
88.6
224.0
158.7
2154.2
3.2
285.7
93.4
2.5
2179.8
180.0
20.4
2180.0
180.0
2179.7
179.8
20.2
179.6
20.3
2179.8
179.9
0.0
2174.2
179.1
4.9
2177.3
2179.6
2.7
2177.6
270.7
176.3
271.7
179.3
268.7
20.2
2179.9
0.1
177.6
21.8
C10–C9–O1–H

M. Szafran et al. / Journal of Molecular Structure 643 (2002) 55–68
67
5bd. The replacement of Ph group by Me one
was found to cause an insignificant effect. This
suggests that electron coupling occurred mainly
between the pyridine ring and the oxygen.
Ylides are formed by abstraction of the hydroxyl
proton from enolic form (2a,2b) via complex 8.
Acknowledgments
´
We acknowledge the Poznan Supercomputing and
Network Centre for access to their computers.
Scheme 3.
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