NMR spectral study of some 2r,6c-diarylpiperidin-4-one (3′-hydroxy- 2′-naphthoyl)hydrazones with special reference to γ-syn effect

Article abstract of DOI:10.1016/j.saa.2010.09.015

¹H and ¹³C NMR spectra have been recorded for 2r,6c-diarylpiperidin-4-one (3′-hydroxy-2′-naphthoyl)hydrazones 10-17 and 3,3-dimethyl-2r,6c-bis(p-methoxyphenyl)piperidin-4-one (5). For selected compounds 2D NMR spectra have been recor

Full text of DOI:10.1016/j.saa.2010.09.015

Spectrochimica Acta Part A 78 (2011) 153–159
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
NMR spectral study of some 2r,6c-diarylpiperidin-4-one
(3ꢀ-hydroxy-2ꢀ-naphthoyl)hydrazones with special reference to ꢀ-syn effect
S. Sylvestre, K. Pandiarajan∗
Department of Chemistry, Annamalai University, Annamalainagar, Chidambaram, Tamilnadu 608 002, India
a r t i c l e i n f o
a b s t r a c t
1H and 13C NMR spectra have been recorded for 2r,6c-diarylpiperidin-4-one (3ꢀ-hydroxy-2ꢀ-
naphthoyl)hydrazones 10–17 and 3,3-dimethyl-2r,6c-bis(p-methoxyphenyl)piperidin-4-one (5). For
selected compounds 2D NMR spectra have been recorded. The spectral data along with those reported for
related compounds are used to study the effect of a heteroatom X on the 13C chemical shift of a ꢀ-carbon
with X C˛ Cˇ Cꢀ torsional angle close to 0◦, termed as ꢀ-syn effect. Also ꢀ-gauche and ı-effects of the
alkyl groups at C-3 on the carbons of the aryl group at C-2 have been studied. The chemical shifts for the
naphthalene ring are in accord with the mesomeric and steric effects of the carbonyl and hydroxy groups.
Article history:
Received 3 July 2010
Received in revised form 18 August 2010
Accepted 8 September 2010
Keywords:
Piperidin-4-ones
Hydrazones
1H NMR
13C NMR
© 2010 Elsevier B.V. All rights reserved.
Conformation
ꢀ-Syn effect
1. Introduction
reported [27,28] the signals of the naphthalene ring have not been
assigned using 2D spectra.
Nuclear magnetic resonance spectroscopy has been widely
used in the study of organic compounds [1–18] and reviews
[19,20] also have appeared on this subject. 1H chemical shifts
are influenced by electronic, steric and magnetic anisotropic
effects whereas 13C chemical shifts are largely influenced by elec-
tronic and steric effects. The ˛, ˇ, ꢀ-gauche, ꢀ-anti and ı effects
of substituents on the chemical shifts of ring carbons in satu-
rated six-membered ring compounds have been well discussed
[21–23].
In the present study, the above-mentioned aspects are investi-
gated using the NMR spectral data of piperidin-4-one 5, hydrazones
10–17 and related compounds reported in the literature.
2. Results and discussion
2.1. Assignments of NMR signals
The numbering of carbon atoms in 10–17 is shown in Scheme 1.
Protons are numbered accordingly. Thus, the proton at C-2 is
denoted as H-2 and that at C-5ꢀ is denoted as H-5ꢀ. The atoms in
5 also are numbered accordingly.
In cyclohexanones, adopting chair conformations, the carbon of
an equatorial alkyl group attached to the ring carbon adjacent to the
carbonyl group, will be at the ꢀ-position with respect to the car-
bonyl oxygen. Also this carbon should be coplanar with the CC
O
For 5, 13, 16 and 17 the signals were assigned based on the
correlations in the 2D spectra. For 10, 11, 12, 14 and 15 the
signals were assigned by comparison with 13 and using known
effects [29,30] of the CH3, OMe and Cl substituents in the aryl
rings. The observed 1H and 13C chemical shifts are given in
Tables 1 and 2, respectively. The chemical shifts of naphthalene
also are given for comparison. The 1H–1H coupling constants are
given in Table 3.
plane and its chemical shift should be influenced by the carbonyl
oxygen. A similar effect can be expected in oximes and hydra-
zones with a nitrogen atom instead of the oxygen atom. Though
13C NMR spectral data are available for large number of such com-
pounds, in no study this effect has been addressed. Also the effect
of substituent on a ı-carbon in close proximity, has not been stud-
ied.
The derivatives of 3-hydroxy-2-naphthoic acid hydrazide (1)
have been found to possess valuable biological activities [24–27].
Though NMR spectral data of the derivatives of 1 have been
2.2. Conformation of the piperidine ring
Based on a previous study [5], piperidin-4-ones 2–9 should
exist in chair conformations 2C–9C. In these conformations the aryl
groups are equatorial and the alkyl group at C-3 is equatorial in
the 3t-alkyl compounds. The observed vicinal coupling constants
∗
Corresponding author. Tel.: +91 9994469980.
E-mail address: profkprajan@yahoo.co.in (K. Pandiarajan).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.09.015

154
S. Sylvestre, K. Pandiarajan / Spectrochimica Acta Part A 78 (2011) 153–159
Table 1
1H chemical shifts (ppm) of 5 and 10–17.
Proton
Compound
5
10
11
12
13
14
15
16
17
Naphthalene
NH, OH
H-2a
H-3a
H-5e
H-5a
–
3.76
–
2.26
2.84
3.94
–
–
–
–
–
11.46
3.57
2.64
3.07
2.33
3.93
8.54
7.31
7.73
7.52
7.34
7.93
7.51
7.52
7.36
7.38
7.28
0.88
11.45
3.57
2.64
3.06
2.32
3.93
8.53
7.31
7.73
7.52
7.34
7.93
7.50
7.52
7.36
7.38
–
11.63
3.51
2.57
3.02
2.27
3.86
8.56
7.29
7.70
7.47
7.31
7.93
7.38
7.39
6.87
6.91
–
11.42
3.70
–
11.44
3.78
–
11.48
3.73
–
11.48
3.69
2.51
3.03
2.30
3.93
8.54
7.29
7.71
7.49
7.32
7.92
7.49
7.49
7.32
7.37
11.46
3.43
2.55
2.99
2.39
4.34
8.52
7.28
7.71
7.51
7.36
7.91
7.48
7.50
7.34
7.35
2.87
2.41
3.82
8.53
7.28
7.72
7.47
7.34
7.93
7.36
7.48
6.90
6.92
–
2.90
2.38
3.89
8.52
7.29
7.73
7.48
7.40
7.93
7.49
7.60
7.40
7.43
–
2.87
2.41
3.84
8.53
7.28
7.72
7.48
7.33
7.93
7.34
7.44
7.14
7.17
–
H-6a
H-1ꢀ
(7.84)
(7.84)
(7.84)
(7.48)
(7.48)
(7.84)
H-4ꢀ
H-5ꢀ
H-6ꢀ
H-7ꢀ
H-8ꢀ
–
H-2ꢀꢀ and H-6ꢀꢀ
H-2ꢀꢀꢀ and H-6ꢀꢀꢀ
H-3ꢀꢀ and H-5ꢀꢀ
H-3ꢀꢀꢀ and H-5ꢀꢀꢀ
H-4ꢀꢀ and H-4ꢀꢀꢀ
3-Alkyl protons
7.35
7.44
6.91
6.91
–
0.80
1.06
3.77
7.24, 7.29
1.23 (CH2)
0.83 (CH3)
–
7.25, 7.27
1.89 (CH)
1.11, 1.05 (CH3)
–
0.88
0.93
1.04
1.13
3.75
3.72
–
1.05
1.12
–
1.05
1.13
–
p-OCH3
p-CH3
–
–
–
–
3.75
3.72
–
–
–
2.30
–
–
Chemical shifts of naphthalene [33] are given in the parentheses.
suggest that hydrazones 10–17 also must adopt such chair confor-
mations.
CO–C(2ꢀ) bonds. In related hydrazones, among the two possible
rotamers about the N–N bond, the anti form (C N and NCO bonds
are anti) has been found to be stable [6,9]. The two rotamers about
the CO–NH bond have been found to give rise to distinct NMR sig-
nals [9,32]. Also, in the E-forms of N-isonicotinylhydrazones [31]
the equatorial methyl protons have been found to be shielded
markedly by the magnetic anisotropic effect of the aromatic group
attached to carbonyl group. In 10–17 such a shielding is not
observed. Hence, it can be concluded that hydrazones 10–17 exist in
the Z-form about the CO–NH bond in solution. Thus, two rotamers
R1 and R2 may be considered for the hydrazones. In R1 there can be
N–H···O intramolecular hydrogen bonding whereas in R2 there can
be C O···H–O intramolecular hydrogen bonding. In salicylamide [7]
and related compounds [6,7,9] separate signals have been observed
for OH and NH protons. However, only one signal is observed for NH
2.3. Configuration about the C N bond
Compared to 5, in 13 H-5e is deshielded by 0.51 ppm and H-5a
is shielded by 0.43 ppm. Also, C-5 is shielded by 13.7 ppm whereas
C-3 is shielded only by 6.7 ppm. These observations are similar to
those made in 3t-alkyl-2r,6c-diarylpiperidin-4-one oximes [3] with
E configuration about the C N bond. Obviously, in 13 the configu-
ration about C N bond is E. In the other hydrazones also H–5e has a
higher chemical shift than H–5a and C–5 has a lower chemical shift
than C–3. Hence, in all the hydrazones the configuration about the
C
N bond should be E. Therefore, hydrazones 10–17 must exist in
chair conformations 10C–17C.
2.4. Conformation of the NHCO (3ꢀ-hydroxynaphthyl) moiety
and OH protons in 10–17. This shows that there is a fast exchange
between OH and NH protons. Hence, we suggest that, compounds
10–17 may exist as rotamer R1 rather than as rotamer R2. In R1
The NHCO (3ꢀ-hydroxynaphthyl) moiety in 10–17 can adopt
eight conformations due to rotation about N–N, CO–NH and

S. Sylvestre, K. Pandiarajan / Spectrochimica Acta Part A 78 (2011) 153–159
155
Scheme 1. Synthesis of 2r,6c-diarylpiperidin-4-one (3ꢀ-hydroxy-2ꢀ-naphthoyl)hydrazones.
2.5. 13C chemical shifts of the methyl groups at C-3
the exchange between OH and NH protons should be faster than in
R2.
3t-Alkyl-2r,6c-diphenylpiperidines (18–20) have been shown
to adopt chair conformations with equatorial orientations of
the substituents, based on the observed vicinal coupling con-
stants [32]. The chemical shift of the methyl carbon in 18
has been found as 18.7 ppm [32]. However, the methyl car-
bon appears at 13.0 ppm in 10 and 10.1 ppm in
2 [1].
Also, the chemical shift of the methyl carbon is 14.8 ppm in
2-methylcyclohexanone (21) [33] but 23.0 ppm in methylcy-
clohexane (22) [33]. These observations can be explained as
follows.

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S. Sylvestre, K. Pandiarajan / Spectrochimica Acta Part A 78 (2011) 153–159
rial methyl group the chemical shift of the methyl carbon in 21
is much less than that in 22. It is also interesting to note that in
4-methylcyclohexanone the methyl carbon appears at 21.0 ppm
[33].
In 3t,5c-dimethyl-2r,6c-diphenylpiperidine (24) the chemical
shifts of the axial and equatorial methyl carbons have been found
as 12.8 and 18.6 ppm, respectively [32]. This is because the axial
methyl carbon is gauche to the nitrogen atom and the methylene
carbon C-5. In ketone 5 and hydrazones 13–15 the axial methyl
carbon is subjected to similar gauche interactions. In addition the
equatorial methyl group exerts a ˇ-effect on this methyl carbon.
The axial methyl group exerts a similar ˇ-effect on the equato-
rial methyl carbon. Based on this, one must expect that in the
3,3-dimethyl compounds 5, 13, 14 and 15 the axial and equa-
torial methyl carbons should differ in their chemical shifts by
Fig. 1. Conformations of the methyl group in 2, 10 and 18.
about 6.0 ppm. However, the observed difference is quite small.
The CH bonds of the axial methyl group are far away from the car-
bonyl oxygen in the ketone or nitrogen of the C N group in the
hydrazones. Hence, the axial methyl carbon is not shielded by the
interaction of the carbonyl oxygen in the ketone or the nitrogen
of the C N group in the hydrazones. The shielding experienced by
the equatorial methyl carbon is almost the same as the shielding
experienced by the axial methyl carbon due to the ꢀ-gauche effects.
The conformation of the methyl group in 10 is shown in Fig. 1.
It is seen that the nitrogen atom of the C N group can polarize
C–HA and C–HB bonds so that HA and HB get partial positive charges
and the methyl carbon gets a partial negative charge. The negative
charge on the carbon shields it. In 2 this shielding is larger due to the
greaterelectronegativity ofoxygen than nitrogen. Hence, the chem-
ical shift of the methyl carbon increases in the order 2 < 10 < 18.
Since 21 and 22 should adopt chair conformations with equato-
Table 2
13C chemical shifts (ppm) of 5 and 10–17.
Compound
Carbon
5
10
11
12
13
14
15
16
17
Naphthalene
–C
O
–
68.4
49.6
212.6
47.1
60.4
–
–
–
–
–
162.3
69.0
45.1
162.5
37.4
60.4
162.5
69.0
45.1
162.5
37.4
60.4
162.1
68.3
45.1
162.5
37.3
59.8
162.1
69.1
42.9
165.7
33.4
59.6
162.6
69.3
43.2
165.3
33.7
59.8
162.5
70.0
43.4
165.9
33.9
60.5
162.6
67.2
51.8
161.0
37.7
60.4
161.9
63.7
54.8
160.0
36.9
58.1
C-2
C-3
C-4
C-5
C-6
C-1ꢀ
C-2ꢀ
C-3ꢀ
C-4ꢀ
C-5ꢀ
C-6ꢀ
C-7ꢀ
C-8ꢀ
C-9ꢀ
131.9
121.4
153.7
111.0
126.1
127.9
124.2
128.4
127.1
136.1
143.4
143.5
128.8
127.8
128.7
128.6
127.3
127.5
13.0
131.9
121.4
153.7
111.0
126.1
127.9
124.2
128.7
127.5
136.1
143.5
143.6
128.8
127.3
127.8
128.4
129.3
131.9
13.0
131.9
121.4
154.1
111.0
126.1
128.4
124.0
128.4
127.4
135.3
136.2
136.2
129.3
128.5
114.0
114.1
159.0
131.3
120.7
153.3
110.5
125.6
128.0
123.6
128.7
126.9
135.6
132.4
136.2
129.9
127.9
112.8
113.7
158.4
158.5
23.4
131.8
121.3
153.6
111.0
126.2
127.9
124.2
128.8
127.5
136.1
138.8
143.6
131.3
128.6
129.3
128.6
132.2
132.3
23.3
131.9
121.3
153.7
111.0
126.1
127.5
124.2
128.6
127.2
136.1
137.9
141.6
129.4
128.5
128.5
129.3
136.7
136.9
21.7
131.9
121.4
153.9
111.0
126.2
127.5
124.2
128.7
127.8
136.2
143.5
144.3
128.8
127.3
128.6
128.5
127.9
127.8
19.5 (CH2)
12.5 (CH3)
–
131.5
120.7
153.2
110.5
126.7
127.7
123.7
128.8
127.2
135.7
144.4
144.4
128.3
127.3
128.2
128.0
127.0
125.7
28.5 (CH)
21.0, 18.8 (CH3)
–
(127.8)
(125.8)
(125.8)
(127.8)
(127.8)
(125.8)
(125.8)
(127.8)
(133.5)
(133.5)
–
–
–
–
C-10ꢀ
–
C-1ꢀꢀ
131.0
136.1
130.2
128.3
113.4
114.1
149.0
C-1ꢀꢀꢀ
C-2ꢀꢀ, C-6ꢀꢀ
C-2ꢀꢀꢀ, C-6ꢀꢀꢀ
C-3ꢀꢀ, C-5ꢀꢀ
C-3ꢀꢀꢀ, C-5ꢀꢀꢀ
C-4ꢀꢀ, C-4ꢀꢀꢀ
20.2
21.1
55.6
55.5
–
13.0
3-Alkyl carbons
p-OCH3
21.7
55.6
55.4
–
21.6
–
21.2
–
–
–
–
–
55.5
55.5
–
p-CH3
–
23.4
–
–
Chemical shifts of naphthalene [33] are given in the parentheses.

S. Sylvestre, K. Pandiarajan / Spectrochimica Acta Part A 78 (2011) 153–159
157
Table 3
19 the chemical shift of the methylene carbon is 25.1 ppm [32]. It is
interesting to note that the chemical shift of the methylene carbon
increases in the order 8 < 16 < 19. However, the chemical shifts of
the methyl carbon of the ethyl group in 8, 16 and 19 are 12.2, 12.5
and 11.1 ppm, respectively.
1H–1H coupling constants (Hz) of 10–17.
Compound
3J6a,5a
3J2a,3a
2J5e,5a
3J6a,5e
3J3a,alkyl
10
11
12
13
14
15
16b
17c
11.5
11.5
11.0
11.0
11.5
11.0
11.0
11.7
10.0
9.5
10.0
–
–
–
14.0
12.0
12.0
12.0
10.0
12.0
11.5
14.8
2.5
6.5
6.0
6.5
–
–
–
a
a
a
a
a
a
2.7. 13C chemical shifts of isopropyl group at C-3
From X-ray crystallographic study [35], it has been found that
9 adopts chair conformation with equatorial orientations of all the
substituents and the isopropyl group adopts a conformation so that
the isopropyl methine proton is anti to C(3)–C(4) bond. From the
observed NOEs and vicinal coupling constants it has been found
that 9 adopts such a conformation in solution also [8]. The vicinal
coupling constant between the isopropyl methine proton and H-3
10.0
13.0
3.0, 7.0
2.0
3.2
a
b
c
H-6 and H-5a were observed only as doublets due to poor resolution.
Methyl protons appeared as a triplet with a J value of 5.0 Hz.
The methyl protons appeared as a doublet with a J value of 5.0 Hz.
Hence, axial and equatorial methyl carbons do not differ much in
their chemical shifts in 5, 13, 14 and 15.
2.6. 13C chemical shifts of the ethyl group at C-3
is 2.0 Hz in 17 suggesting that in 17 also the isopropyl group adopts
a similar conformation. Hence, the CH bond of the isopropyl group
cannot interact with the oxygen in 9 or with the nitrogen of the
From X-ray crystallographic study [34], it has been found that
3t-ethyl-2r,6c-bis(p-methoxyphenyl)piperidin-4-one (23) adopts
chair conformation with equatorial orientations of all the sub-
stituents. Interestingly, the torsional angle for the segment
C
N group in 17. In 9 and 17 there may be a shielding due to the
eclipsing of C X bond with the C(3)–CHMe2 bond. In 9, 17 and
20 the chemical shifts of the isopropyl methine carbon are 25.5
[32], 28.5 and 27.1 ppm [32], respectively. It is seen that relative to
the isopropyl methine carbon in 20, in 9 it is shielded by 1.6 ppm
whereas in 17 it is deshielded by 1.4 ppm. Thus, the effect due to
bond eclipsing should be only small and may be +ve or −ve. The
chemical shifts of one methyl carbon in 9, 17 and 20 are 21.1, 21.0
and 21.7 ppm, respectively. The corresponding chemical shifts of
the other methyl carbon are 17.8, 18.8 and 16.2 ppm, respectively.
O
CC(3)CH2 has been found as 1◦. The methyl group is gauche
to C(3)–C(4) bond but anti to C(2)–C(3) bond in the solid state.
However, in solution the ethyl group can adopt three different con-
formations A, B and C shown in Fig. 2. In conformation C there will
be severe nonbonded interaction between the methyl and 2-phenyl
groups, and its contribution can be ignored.
In conformation A one methylene proton is anti to H-3 and
the vicinal coupling constant between this proton and H-3 will be
around 10–12 Hz. The other methylene proton is gauche to H-3 and
the coupling constant between this proton and H-3 will be around
2–4 Hz. However, in conformation B both the methylene protons
are gauche to H-3 and both the vicinal coupling constants will be
around 2–4 Hz.
In 16 the vicinal coupling constants of H-3a with the methy-
lene protons of the ethyl group are found as 7.0 and 3.0 Hz. The
observed vicinal coupling constants suggest that 16 exists as an
equilibrium mixture of conformations A and B. Hence, in 16 one
C–H bond will be polarized by the nitrogen atom of the C N group.
A similar conformational equilibrium has been proposed for 8 [8].
Thus, the methylene carbon should be shielded in 8 and 16 com-
pared to 19. Indeed, the chemical shifts of the methylene carbon in
8 and 16 are 17.8 [32] and 19.5 ppm [1], respectively, whereas in
2.8. ꢀ-Syn effect
The effect of a heteroatom X on the ꢀ-carbon with the tor-
sional angle X C˛ Cˇ Cꢀ close to 0◦ may be termed as ꢀ-syn effect.
Indeed the observed shielding on C-5 in compounds 9–17 may be
attributed to this effect. This is because C-5, C-4, nitrogen atom of
the C N bond and the amide nitrogen should be coplanar.
By comparing the chemical shifts of 10 with those [1] in 2 the
shielding on C-3 and C-5 due to hydrazone formation are found
as 6.5 and 13.5 ppm, respectively. The corresponding shieldings
in 3t-methyl-2r,6c-diphenylpiperidin-4-one oxime (25) [3] due to
oximation have been found as 8.4 and 17.1 ppm, respectively. The
shielding on C-3 has been attributed to lower polarity of the C
N
bond than the C O bond [3]. The observed shielding on C-3 in 13
Fig. 2. Conformations of ethyl group.

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S. Sylvestre, K. Pandiarajan / Spectrochimica Acta Part A 78 (2011) 153–159
suggests that the C N bond in the hydrazone is somewhat more
polar than the C N bond in oxime. Assuming the polar effect to be
same on C-3 and C-5 the shielding on C-5 due to the ꢀ-syn effect
may be taken as 8.7 ppm (17.1–8.4) in 25. In 10 the shielding on C-5
due to ꢀ-syn effect comes as 7.0 ppm (13.5–6.5). The ꢀ-syn effect
is larger in oxime than in the hydrazone because in oximes the ꢀ-
syn effect is due to oxygen whereas in the hydrazone it is due to
nitrogen.
Though H-4ꢀ is shielded by the mesomeric effect of the OH group
it is deshielded by the polarization effect. The mesomeric effect out-
weighs the polarization effect causing a shielding on H-4ꢀ. However,
both these effects shield C-4ꢀ and the shielding on C-4ꢀ is abnormally
high compared to that on H-4ꢀ.
3. Conclusion
The observed vicinal proton–proton coupling constants sug-
gest that in hydrazones 10–17 the piperidine ring adopts chair
conformation with equatorial orientations of the aryl groups. The
observed chemical shifts of H-5e, H-5a and C-5 are in accord with E
configuration about the C N bond. In 10–16 the carbon of the equa-
torial alkyl carbon attached to C–3 is shielded by the nitrogen atoms
of the C N bond due to the interaction of the nitrogen atom with
the C–H bonds of this carbon. This effect is termed as ꢀ-syn effect.
This effect is somewhat greater in the corresponding ketones. The
chemical shifts for the naphthalene ring could be accounted by
the mesomeric effects of the carbonyl and hydroxy groups. How-
ever, the chemical shifts of carbons adjacent to the substituents and
protons on them are influenced by polarization effects also. How-
ever, the chemical shifts of carbons adjacent to the substituents and
protons on them are influenced by polarization effects also.
2.9. ꢀ-Gauche effect
The ꢀ-gauche effect by an axial substituent in cyclohexanes has
been well discussed [22]. Polarization of the axial C–H bond has
been advocated as the major contribution to ꢀ-gauche effect. How-
ever, this effect has been termed as syn-ꢀ-effect in that study since
the axial C–H bond, which is polarized by the substituent (or atom
in the case of halogens), is parallel to the C–X bond. However, this
term is not appropriate because the affected carbon is only gauche
to the substituent X (or atom in the case of halogens).
By comparing 12 and 13 it is seen that the axial methyl group
at C-3 shields C-5 by 3.9 ppm. This ꢀ-gauche effect due to an axial
methyl group has been found as 6.9 ppm in cyclohexanes [22]. Prob-
ably the ꢀ-gauche effect is less on a carbon already compressed by
ꢀ-syn effect.
In 5 and 10–17 C-1ꢀꢀ is gauche to the alkyl groups at C-3. Thus,
in all those compounds C-1ꢀꢀ should have a lower chemical shift
than C-1ꢀꢀꢀ. However, only in the 3,3-dimethyl compounds 5, 13,
14 and 15 the chemical shifts of C-1ꢀꢀand C-1ꢀꢀꢀ differ significantly.
The observed shieldings on C-1ꢀꢀ in 5, 13, 14 and 15 suggest that
ꢀ-gauche effect may have another origin apart from polarization of
a C–H bond on the ꢀ-carbon atom.
4. Experimental
4.1. Materials
3-Hydroxy-2-naphthoic acid hydrazide was purchased from
Sigma–Aldrich and was used as such. All the reagents and solvents
were of laboratory grade.
4.2. Methods
2.10. ı-Effect
The melting points were determined in open capillaries and are
uncorrected. IR spectra were recorded on an AVATAR 330 FT-IR
Thermo Nicolet spectrometer in KBr pellets. Elemental analysis was
performed on an Elementar Vario EL III CHNS analyser.
It has been found that [22] in cyclohexanes a substituent (equa-
torial or axial) shields the ı-carbon and this shielding has been
attributed to linear electric field effect. In these cases the ı-carbons
are not involved in a steric interaction with the substituents. In
10–17 the ortho carbons C-2ꢀꢀ and C-6ꢀꢀ are at ı-positions relative
to the alkyl groups at C-3. In these cases there is steric interaction
between the ı-carbons and the substituents. From Table 2 it is seen
that the ortho carbons C-2ꢀꢀ and C-6ꢀꢀ are deshielded by the alkyl
groups at C-3. The reason for this deshielding is not clear.
1H NMR spectra were recorded on
a Bruker DRX-500
NMR spectrometer operating at 500.03 MHz for 1H with the
following spectral parameters; acquisition time = around 3.0 s,
number of scans = 100, number of data points = 32 K and spectral
width = 10,330 Hz.
Proton decoupled 13C NMR spectra were recorded on a Bruker
DRX-500 NMR spectrometer operating at 125.77 MHz for 13C with
the following spectral parameters; acquisition time = around 0.5 s;
number of scans = 1000; number of data points = 32 K; spectral
width = 30,000 Hz.
2.11. Chemical shifts of the naphthyl protons and carbons
Based on the mesomeric effects of the OH and CO groups one
should expect that in 10–17 C-4ꢀ, C-5ꢀ, C-7ꢀ, C-9ꢀ, H-4ꢀ, H-5ꢀ and H-
7ꢀ should be shielded and C-1ꢀ, C-6ꢀ, C-8ꢀ, C-10ꢀ, H-1ꢀ, H-6ꢀ and H-8ꢀ
should be deshielded relative to those in naphthalene. Indeed the
observed chemical sifts suggest so. However, the effects on C-5ꢀ,
C-6ꢀ and H-6ꢀ are not significant.
1H-1H COSY spectrum of 13, HSQC spectra of 5, 13, 16 and 17,
HMBC spectra of 13, 16 and 17 and phase-sensitive NOESY spec-
trum of 13 were recorded on a Bruker DRX-500 NMR spectrometer
using standard parameters. The number of data points was 1 K. For
NOESY spectrum the mixing time was 800 ms.
Moreover, compared to the deshielding on C-1ꢀ that on H-1ꢀ
seems to be abnormally high. Also, the shielding on C-4ꢀ seems to
be abnormally high compared to that on H-4ꢀ. These observations
suggest that the C(1ꢀ)–H(1ꢀ) and C(4ꢀ)–H(4ꢀ) bonds are polarized by
interaction with C O and OH bonds, respectively. Due to this, par-
tial positive charges are placed on H-1ꢀ and H-4ꢀ whereas partial
negative charges are placed on C-1ꢀ and C-4ꢀ. Thus, C-1ꢀ is sub-
jected to a shielding by this polarization and to deshielding by the
mesomeric effect of the carbonyl group. The mesomeric effect out-
weighs the polarization effect causing a net deshielding on C-1ꢀ.
However, H-1ꢀ is deshielded by both these effects and the mag-
netic anisotropic effect of the carbonyl group. Thus, the observed
deshielding on H-1ꢀ is abnormally higher compared to the deshield-
ing on C-1ꢀ.
All NMR measurements were made in 5 mm NMR tubes using
solutions made by dissolving about 10 mg of the material in 0.5 mL
of DMSO-d6.
4.3. Preparation of compounds
Piperidin-4-ones 2–9 were prepared following the procedure of
Noller and Baliah [36].
4.3.1. 3t-Methyl-2r,6c-diarylpiperidin-4-ones (2–4)
Dry ammonium acetate (100 mmol) was dissolved in ethanol
and the solution was mixed with 2-butanone (100 mmol) and
appropriate substituted benzaldehyde (200 mmol). The mixture
was just heated to boil and allowed to stand at room temperature

S. Sylvestre, K. Pandiarajan / Spectrochimica Acta Part A 78 (2011) 153–159
159
Table 4
Yields, melting points, elemental analysis and IR stretching frequency of 10–17.
m.p.
(◦C)
Elemental analysis
Calculated (%)
IR stretching frequencies (cm−1
)
Compd.
Yield
(%)
Found (%)
C
N
C
O
NH (pip)
NH (amide)
OH group
C
H
N
C
H
N
10
11
12
13
14
15
16
17
177–179 80
222–224 90
190–192 85
202–204 85
231–234 90
221–224 78
220–221 80
223–224 80
–
–
–
–
6.05
6.34
5.08
–
–
–
8.13
8.02
7.86
–
–
–
–
–
6.09
6.50
5.27
–
–
–
8.35
8.03
7.89
–
1627
1600
1616
1608
1600
1594
1596
1597
1655
1639
1644
1649
1633
1633
1654
1652
3309
3315
3315
3309
3320
3281
3306
3273
3309
3271
3282
3266
3278
3281
3306
3273
3419
3435
3424
3419
3402
3446
3408
3397
72.43
73.60
67.59
–
–
–
73.08
73.42
67.68
–
–
–
–
–
–
–
–
–
–
–
overnight. The reaction mixture was diluted with ether (100 mL)
and treated with conc. HCl (20 mL). The precipitated hydrochloride
was washed with ethanol–ether. The hydrochloride was suspended
in acetone and neutralized with aqueous ammonia. Dilution with
water gave the free base which was recrystallized from ethanol.
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Acknowledgements
The authors are thankful to SIF, Indian Institute of Science, Ban-
galore and to SAIF IIT Chennai for recording NMR spectra. Thanks
are due to CECRI, Karaikudi for elemental analysis. One of the
authors (S. Sylvestre) is thankful to UGC for the award of a fellow-
ship.
Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2010.09.015.