Conformation of N-methyl-4-piperidyl 2,4-dinitrobenzoate

Article abstract of DOI:10.1107/S0108270102022254

The crystal structure of N-methyl-4-piperidyl 2,4-dinitro-benzoate. C₁₃H₁₅N₃O₆ (I), at 130 (2) K reveals that, in the solid state, the molecule exists in the equatorial conformation, (leq). Thus, the through-bond interaction present in the axial conformation, (lax), is not strong enough to overcome the syn-diaxial interactions between the axial methyl substituent and the axial H atoms on the two piperidyl ring C atoms either side of the ester-linked ring C atom. The carboxylate group in (I) is orthogonal to the aromatic ring, in contrast with other 2,4-dinitrobenzoates, which are coplanar. The piperidyl-ester C-O bond distance is 1.467 (3) A, which is actually shorter than other equatorial cyclohexyl-ester C-O distances. This shorter piperidyl-ester C-O bond distance is due to the reduced electron demand of the orthogonal ester group.

Full text of DOI:10.1107/S0108270102022254

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
OPNB distance (White & Robertson, 1993). The 2,4-dinitro-
benzoate ester is more strongly electron demanding than the
4-nitrobenzoate ester, and this is re¯ected in the relative pKa
values for the parent acids; 4-nitrobenzoic and 2,4-dinitro-
benzoic acids have pKa values of 3.4 and 1.4, respectively
(Dean, 1992). The stronger electron demand of the 2,4-di-
nitrobenzoate ester substituent would result in a stronger
through-bond interaction with the nitrogen lone pair. Thus, we
were interested in establishing whether (I) would adopt
conformation (Iax). If this axial conformation were observed,
then we would be interested in seeing the effects of the
through-bond interaction on the CÐODNB distance
compared with a typical cyclohexyl 2,4-dinitrobenzoate.
ISSN 0108-2701
Conformation of N-methyl-4-piperidyl
2,4-dinitrobenzoate
Laura Andrau and Jonathan White*
School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australia
Correspondence e-mail: whitejm@unimelb.edu.au
Received 22 November 2002
Accepted 2 December 2002
Online 11 January 2003
The crystal structure of N-methyl-4-piperidyl 2,4-dinitro-
benzoate, C13H15N3O6, (I), at 130 (2) K reveals that, in the
solid state, the molecule exists in the equatorial conformation,
(Ieq). Thus, the through-bond interaction present in the axial
conformation, (Iax), is not strong enough to overcome the
syn±diaxial interactions between the axial methyl substituent
and the axial H atoms on the two piperidyl ring C atoms either
side of the ester-linked ring C atom. The carboxylate group in
(I) is orthogonal to the aromatic ring, in contrast with other
2,4-dinitrobenzoates, which are coplanar. The piperidyl±ester
Ê
CÐO bond distance is 1.467 (3) A, which is actually shorter
than other equatorial cyclohexyl±ester CÐO distances. This
shorter piperidyl±ester CÐO bond distance is due to the
reduced electron demand of the orthogonal ester group.
Comment
As part of our studies of the factors in¯uencing CÐOR bond
distances, where R is hydroxy, ester or ether (White &
Robertson, 1992; White et al., 2000; Pool et al., 2000), we have
determined the structure of the title compound, (I), which was
prepared in two steps from 4-piperidone, (II), with an overall
yield of 60%.
Compound (I) can conceivably exist in solution in two
conformations, (Iax) and (Ieq), which interconvert by nitrogen
inversion. Although conformation (Ieq) is expected to be
favoured on steric grounds, the axial conformation, (Iax), is
stabilized by a through-bond interaction between the nitrogen
lone-pair electrons and the low-lying CÐODNB (ODNB is
dinitrobenzoate) antibonding orbital (Fig. 1).
Previous to this study, we determined the structure of
N-methyl-4-piperidinyl 4-nitrobenzoate, (IV) (Andrau &
White, 2003), which was shown to adopt a conformation
analogous to (Ieq) in the solid state, suggesting that the
through-bond interaction between the nitrogen lone-pair and
the CÐOPNB antibonding orbital was not strong enough to
overcome the steric repulsion associated with the axial methyl
group in (Iax). The CÐOPNB (OPNB is 4-nitrobenzoate)
The crystal structure of (I) at 130 (2) K reveals, disap-
pointingly, that ester (I) exists in the solid state in the equa-
torial conformation (Ieq), suggesting that the through-bond
interaction in conformation (Iax) is still not suf®ciently
stabilizing to overcome the repulsive syn±diaxial interactions.
However, there is an interesting aspect to this structure.
Examination of the dihedral angle between the carboxyl ring
and the aromatic ring [O2ÐC7ÐC8ÐC9 = 78.7 (3)ꢀ] reveals
that these two groups are close to orthogonal, whereas in
other 2,4-dinitrobenzoate esters whose structures we have
determined, these two groups are essentially coplanar (Green
et al., 1995). This conformation brings nitro atom O3 into a
close contact with carbonyl atom C7, with an O3Á Á ÁC7 distance
Ê
of 2.599 (3) A. Furthermore, the carbonyl C atom deviates
from the plane of the attached atoms (O1, O2 and C8) by
Ê
0.043 (2) A towards atom O3. These latter structural effects
are consistent with the early stages of nucleophilic addition of
the nitro O atom to the ester carbonyl group (Burgi et al.,
1973).
Ê
The orthogonal relationship between the aromatic ring and
the carboxyl group in (I) would result in poor ꢀ overlap
bond distance in (IV) is 1.4630 (16) A, which is not signi®-
cantly lengthened compared with the standard cyclohexyl CÐ
o60 # 2003 International Union of Crystallography
DOI: 10.1107/S0108270102022254
Acta Cryst. (2003). C59, o60±o61

organic compounds
Data collection
between the electron-de®cient aromatic ring and the carboxyl
group. This, in addition to the interaction with the nitro O
atom discussed above, would make the ester O atom less
electron-demanding. The result of decreased electron demand
at the ester O atom would be twofold. Firstly, the strength of
the through-bond interaction would be decreased, and
secondly, the C1ÐO1 distance, which is sensitive to electron
demand (Amos et al., 1992), would be shorter than expected
for a cyclohexyl 2,4-dinitrobenzoate ester. The C1ÐO1
Bruker APEX CCD area-detector
diffractometer
' and ! scans
7269 measured re¯ections
2486 independent re¯ections
1840 re¯ections with I > 2ꢅ(I)
Rint = 0.046
ꢃmax = 25ꢀ
h = 7 ! 7
k = 8 ! 15
l = 20 ! 20
Re®nement
Re®nement on F2
R[F2 > 2ꢅ(F2)] = 0.051
wR(F2) = 0.118
S = 1.06
2486 re¯ections
w = 1/[ꢅ2(Fo2) + (0.0423P)2
+ 0.2224P]
where P = (Fo2 + 2Fc2)/3
(Á/ꢅ)max < 0.001
Ê
distance in (I) is 1.468 (3) A, which is in fact shorter than that
observed for other equatorial cyclohexyl 2,4-dinitrobenzoate
3
Ê
Áꢆmax = 0.23 e A
3
Ê
esters [1.476 (2) A; Green et al., 1994]. A similar situation
arises in the structures of phenylselenyl cyclohexyl 2,4-di-
nitrobenzoates (V) and (VI) (White et al., 2002). For example,
Ê
0.24 e A
259 parameters
All H-atom parameters re®ned
Áꢆmin
=
Table 1
Selected geometric parameters (A, ).
ꢀ
Ê
C1ÐO1
C1ÐC5
C1ÐC2
C2ÐC3
1.467 (3)
1.507 (3)
1.511 (3)
1.526 (3)
C3ÐN1
C4ÐN1
C4ÐC5
C6ÐN1
1.468 (3)
1.465 (3)
1.525 (3)
1.466 (3)
Figure 1
The two possible conformations of (I), illustrating the through-bond
interaction present in (Iax). ODNB denotes dinitrobenzoate.
O1ÐC1ÐC5
O1ÐC1ÐC2
C5ÐC1ÐC2
C1ÐC2ÐC3
106.35 (18)
111.25 (19)
110.5 (2)
N1ÐC3ÐC2
N1ÐC4ÐC5
C1ÐC5ÐC4
111.6 (2)
111.16 (19)
109.4 (2)
108.8 (2)
O1ÐC1ÐC2ÐC3
C5ÐC1ÐC2ÐC3
C1ÐC2ÐC3ÐN1
O1ÐC1ÐC5ÐC4
C2ÐC1ÐC5ÐC4
174.9 (2)
57.0 (3)
57.7 (3)
178.15 (19)
57.3 (3)
N1ÐC4ÐC5ÐC1
O2ÐC7ÐC8ÐC13
O1ÐC7ÐC8ÐC13
O2ÐC7ÐC8ÐC9
57.8 (3)
98.2 (3)
75.8 (3)
78.7 (3)
Figure 2
A view of the molecule of (I), with displacement ellipsoids drawn at the
20% probability level. H atoms have been omitted for clarity.
Data collection: SMART (Bruker, 2000); cell re®nement: SMART;
data reduction: SAINT (Bruker, 1999); program(s) used to solve
structure: SHELXTL (Bruker, 1997); program(s) used to re®ne
structure: SHELXTL; molecular graphics: SHELXTL.
ester (V), which has the carboxylate group orthogonal to the
Ê
aromatic ring, has a C1ÐO1 distance of 1.474 (2) A, which is
Ê
signi®cantly shorter than the C1ÐO1 distance of 1.487 (2) A
The authors acknowledge ®nancial support from The
University of Melbourne.
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: TA1405). Services for accessing these data are
described at the back of the journal.
observed in ester (VI). Notably in ester (VI), the carboxyl
group is coplanar with the ring. This again demonstrates the
reduced electron demand of the orthogonal carboxylate
group.
References
Amos, R. D., Handy, N. C., Jones, P. G., Kirby, A. J., Parker, J. M., Percy, M. D.
& Su, J. (1992). J. Chem. Soc. Perkin Trans. 2, pp. 549±558.
Andrau, L. & White, J. M. (2003). Acta Cryst. E59, o77±o79.
Bruker (1997). SHELXTL. Version 5.10. Bruker AXS Inc., Madison,
Wisconsin, USA.
Bruker (1999). SAINT. Version 6.02. Bruker AXS Inc., Madison, Wisconsin,
USA.
Bruker (2000). SMART. Version 5.55. Bruker AXS Inc., Madison, Wisconsin,
USA.
Experimental
4-Piperidone, (II), was reduced to 4-piperidol, (III), using sodium
borohydride in ethanol. The secondary alcohol (III) was converted
into the 2,4-dinitrobenzoate ester, (I), by stirring with 2,4-nitro-
benzoyl chloride in dichloromethane in the presence of sodium
bicarbonate and dimethylaminopyridine, followed by aqueous work-
up. Crystals of (I) were grown by slow evaporation of an ether
solution.
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5067.
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Crystal data
3
C13H15N3O6
Mr = 309.28
Monoclinic, P21=c
Ê
a = 6.040 (3) A
b = 13.303 (5) A
Ê
c = 17.699 (7) A
ꢁ = 94.096 (8)ꢀ
V = 1418.4 (10) A
Z = 4
Dx = 1.448 Mg m
Mo Kꢂ radiation
Cell parameters from 1163
re¯ections
ꢃ = 2.3±22.5ꢀ
ꢄ = 0.12 mm
T = 130 (2) K
Ê
1
3
Ê
Rod, orange
0.30 Â 0.12 Â 0.08 mm
White, J. M. & Robertson, G. B. (1992). J. Org. Chem. 57, 4638±4644.
White, J. M. & Robertson, G. B. (1993). Acta Cryst. C49, 347±350.
ꢁ
Acta Cryst. (2003). C59, o60±o61
Andrau and White C13H15N3O6 o61