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

The crystal structure of N-methyl-4-piperidyl 2,4-di­nitro­benzoate, 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 C-O bond distance is 1.467 (3) Å, 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.

S1. Comment

As part of our studies of the factors influencing C—OR bond distances, where R is hydroxy, ester or ether (White & Robertson, 1992; Green 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%. \sch

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 C-OPNB Please define 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 C-OPNB antibonding orbital was not strong enough to overcome the steric repulsion associated with the axial methyl in (Iax). The C-OPNB bond distance in (IV) was 1.4630 (16) Å, which was not significantly lengthened compared with the standard cyclohexyl C-OPNB distance (White & Robertson, 1993). The 2,4-dinitrobenzoate ester is more strongly electron-demanding than a 4-nitrobenzoate, and this is reflected in the relative pK_a values for the parent acids; 4-nitrobenzoic and 2,4-dinitrobenzoic acids have pK_a values of 3.4 and 1.4, respectively (Dean, 1992). The stronger electron demand of the 2,4-dinitrobenzoate 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 was observed, then we were interested in seeing the effects of the through-bond interaction on the C-ODNB distance compared with a typical cyclohexyl 2,4-dinitrobenzoate.

The crystal structure of (I) at 130 (2) K reveals, disappointingly, that ester (I) exists in the solid state in the equatorial conformation (Ieq), suggesting that the through-bond interaction in conformation (Iax) is still not sufficiently 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 [O2—C7—C8—C9 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 the nitro atom O3 into a close contact with the carbonyl atom C7, with O3···C7 2.599 (3) Å. Furthermore, the carbonyl C atom deviates from the plane of the attached atoms (O1, O2 and C8) by 0.043 (2) Å 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 (Burgi et al., 1973).

The orthogonal relationship between the aromatic ring and the carboxyl group in (I) would result in poor π overlap between the electron-deficient 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 C1—O1 distance, which is sensitive to electron demand (Amos et al., 1987), would be shorter than expected for a cyclohexyl 2,4-dinitrobenzoate ester. The C1—O1 distance in (I) is 1.468 (3) Å, which is in fact shorter than that observed for other equatorial cyclohexyl 2,4-dinitrobenzoate esters [1.476 (2) Å; Green et al., 1994]. A similar situation arises in the structures of the phenylselenyl cyclohexyl 2,4-dinitrobenzoates, (V) and (VI) (White et al., 2002). For example, ester (V), which has the carboxylate group orthogonal to the aromatic ring, has a C1—O1 distance of 1.474 (2) Å, which is significantly shorter than the C1—O1 distance of 1.487 (2) Å 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.

S2. Experimental

The title compound was prepared as follows. 4-Piperidone 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-nitrobenzoyl chloride in dichloromethane in the presence of sodium bicarbonate and dimethylaminopyridine, followed by aqueous workup. Crystals of (I) were grown by slow evaporation of an ether solution.

S3. Refinement

All H atoms were located by difference methods.

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SMART; data reduction: SAINT (Bruker, 1999); program(s) used to solve structure: SHELXTL (Bruker, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The two possible conformations of (I), illustrating the through-bond interaction present in (Iax).
	Fig. 2. A view of the molecule of (I), with displacement ellipsoids drawn at the 20% probability level. H atoms have been omitted for clarity.

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

Crystal data

C13H15N3O6	Dx = 1.448 Mg m^−3
Mr = 309.28	Melting point = 341–342 K
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 6.040 (3) Å	Cell parameters from 1163 reflections
b = 13.303 (5) Å	θ = 2.3–22.5°
c = 17.699 (7) Å	µ = 0.12 mm^−1
β = 94.096 (8)°	T = 130 K
V = 1418.4 (10) Å^3	Rod, orange
Z = 4	0.30 × 0.12 × 0.08 mm
F(000) = 648

Data collection

Make Model CCD area-detector diffractometer	1840 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.046
Graphite monochromator	θmax = 25.0°, θmin = 1.9°
ϕ and ω scans	h = −7→7
7269 measured reflections	k = −8→15
2486 independent reflections	l = −20→20

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.051	Hydrogen site location: difference Fourier map
wR(F^2) = 0.118	All H-atom parameters refined
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0423P)^2 + 0.2224P] where P = (Fo^2 + 2Fc^2)/3
2486 reflections	(Δ/σ)max < 0.001
259 parameters	Δρmax = 0.23 e Å^−3
0 restraints	Δρmin = −0.24 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.5166 (4)	0.58042 (16)	0.37429 (14)	0.0246 (6)
C2	0.7151 (4)	0.59858 (18)	0.42946 (16)	0.0288 (6)
C3	0.7719 (4)	0.71038 (18)	0.42932 (16)	0.0281 (6)
C4	0.3909 (4)	0.75462 (18)	0.39534 (15)	0.0266 (6)
C5	0.3232 (4)	0.64419 (18)	0.39445 (15)	0.0257 (6)
C6	0.6440 (5)	0.87898 (19)	0.44762 (17)	0.0328 (6)
C7	0.5287 (4)	0.41043 (17)	0.33026 (13)	0.0259 (6)
C8	0.4029 (4)	0.31227 (17)	0.33187 (13)	0.0241 (5)
C9	0.4847 (4)	0.22388 (17)	0.36624 (13)	0.0238 (5)
C10	0.3691 (4)	0.13469 (18)	0.36076 (13)	0.0251 (6)
C11	0.1634 (4)	0.13496 (16)	0.32174 (12)	0.0234 (5)
C12	0.0744 (4)	0.22087 (18)	0.28761 (13)	0.0267 (6)
C13	0.1948 (4)	0.30890 (18)	0.29322 (14)	0.0274 (6)
N1	0.5832 (3)	0.77231 (14)	0.44892 (11)	0.0253 (5)
N2	0.6986 (3)	0.22487 (16)	0.41149 (11)	0.0290 (5)
N3	0.0317 (3)	0.04132 (15)	0.31820 (11)	0.0291 (5)
O1	0.4419 (2)	0.47566 (11)	0.37704 (9)	0.0272 (4)
O2	0.6703 (3)	0.42739 (12)	0.28798 (9)	0.0351 (5)
O3	0.7879 (3)	0.30659 (13)	0.42425 (10)	0.0367 (5)
O4	0.7777 (3)	0.14396 (13)	0.43376 (9)	0.0366 (5)
O5	0.1025 (3)	−0.03040 (12)	0.35554 (10)	0.0362 (5)
O6	−0.1423 (3)	0.04116 (13)	0.27776 (10)	0.0391 (5)
H1	0.558 (4)	0.5948 (17)	0.3217 (13)	0.028 (6)*
H2A	0.842 (4)	0.5577 (17)	0.4169 (12)	0.024 (6)*
H2B	0.682 (4)	0.5762 (19)	0.4790 (16)	0.041 (8)*
H3A	0.889 (4)	0.7261 (16)	0.4664 (13)	0.025 (6)*
H3B	0.818 (4)	0.7298 (19)	0.3766 (16)	0.045 (8)*
H4A	0.420 (4)	0.7760 (18)	0.3414 (16)	0.039 (7)*
H4B	0.269 (4)	0.7947 (17)	0.4102 (13)	0.026 (6)*
H5A	0.286 (3)	0.6235 (15)	0.4466 (12)	0.017 (6)*
H5B	0.196 (4)	0.6338 (16)	0.3589 (13)	0.024 (6)*
H6A	0.678 (4)	0.8990 (18)	0.3949 (15)	0.039 (7)*
H6B	0.771 (4)	0.8896 (18)	0.4832 (14)	0.035 (7)*
H6C	0.516 (4)	0.9206 (19)	0.4643 (14)	0.042 (7)*
H10	0.423 (3)	0.0770 (17)	0.3838 (12)	0.019 (6)*
H12	−0.069 (4)	0.2177 (18)	0.2611 (14)	0.038 (7)*
H13	0.135 (4)	0.3702 (17)	0.2696 (12)	0.024 (6)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0265 (13)	0.0169 (12)	0.0307 (15)	−0.0050 (10)	0.0032 (11)	−0.0006 (10)
C2	0.0235 (13)	0.0218 (14)	0.0407 (17)	0.0035 (11)	−0.0008 (12)	0.0010 (12)
C3	0.0201 (13)	0.0279 (14)	0.0355 (16)	−0.0019 (10)	−0.0041 (12)	−0.0038 (12)
C4	0.0230 (13)	0.0239 (14)	0.0324 (15)	0.0032 (11)	−0.0011 (11)	−0.0003 (12)
C5	0.0199 (13)	0.0282 (14)	0.0285 (14)	−0.0013 (10)	−0.0026 (11)	−0.0023 (11)
C6	0.0356 (16)	0.0254 (14)	0.0374 (17)	−0.0058 (12)	0.0033 (14)	−0.0028 (13)
C7	0.0250 (13)	0.0255 (14)	0.0268 (14)	0.0016 (10)	−0.0006 (11)	−0.0030 (11)
C8	0.0270 (13)	0.0227 (13)	0.0231 (13)	0.0000 (10)	0.0038 (10)	−0.0024 (10)
C9	0.0223 (12)	0.0264 (13)	0.0226 (12)	0.0038 (10)	0.0004 (10)	−0.0045 (10)
C10	0.0299 (14)	0.0212 (14)	0.0243 (13)	0.0050 (10)	0.0032 (11)	0.0007 (11)
C11	0.0268 (13)	0.0211 (13)	0.0225 (13)	0.0000 (10)	0.0020 (10)	−0.0041 (10)
C12	0.0254 (13)	0.0293 (14)	0.0245 (13)	−0.0014 (11)	−0.0036 (11)	−0.0017 (11)
C13	0.0289 (14)	0.0203 (13)	0.0325 (14)	0.0029 (11)	−0.0023 (11)	0.0026 (11)
N1	0.0239 (10)	0.0200 (11)	0.0315 (12)	−0.0025 (8)	−0.0012 (9)	−0.0019 (9)
N2	0.0245 (11)	0.0342 (13)	0.0283 (12)	0.0025 (10)	0.0005 (9)	−0.0030 (10)
N3	0.0341 (12)	0.0236 (12)	0.0296 (12)	−0.0024 (9)	0.0026 (10)	−0.0040 (9)
O1	0.0293 (9)	0.0206 (9)	0.0322 (10)	−0.0033 (7)	0.0049 (7)	−0.0034 (7)
O2	0.0377 (10)	0.0305 (10)	0.0388 (11)	−0.0033 (8)	0.0143 (9)	−0.0037 (8)
O3	0.0314 (10)	0.0386 (11)	0.0391 (11)	−0.0082 (8)	−0.0044 (8)	−0.0042 (8)
O4	0.0315 (10)	0.0387 (11)	0.0389 (11)	0.0115 (8)	−0.0035 (8)	0.0037 (9)
O5	0.0388 (10)	0.0223 (10)	0.0474 (11)	0.0019 (8)	0.0031 (9)	0.0070 (8)
O6	0.0382 (11)	0.0341 (11)	0.0432 (11)	−0.0103 (8)	−0.0103 (9)	−0.0029 (8)

Geometric parameters (Å, º)

C1—O1	1.467 (3)	C6—H6C	1.01 (3)
C1—C5	1.507 (3)	C7—O2	1.197 (3)
C1—C2	1.511 (3)	C7—O1	1.333 (3)
C1—H1	1.00 (2)	C7—C8	1.512 (3)
C2—C3	1.526 (3)	C8—C13	1.389 (3)
C2—H2A	0.98 (2)	C8—C9	1.398 (3)
C2—H2B	0.96 (3)	C9—C10	1.376 (3)
C3—N1	1.468 (3)	C9—N2	1.471 (3)
C3—H3A	0.95 (2)	C10—C11	1.378 (3)
C3—H3B	1.03 (3)	C10—H10	0.92 (2)
C4—N1	1.465 (3)	C11—C12	1.383 (3)
C4—C5	1.525 (3)	C11—N3	1.477 (3)
C4—H4A	1.02 (3)	C12—C13	1.378 (3)
C4—H4B	0.96 (2)	C12—H12	0.96 (2)
C5—H5A	1.01 (2)	C13—H13	0.97 (2)
C5—H5B	0.96 (2)	N2—O3	1.227 (2)
C6—N1	1.466 (3)	N2—O4	1.231 (2)
C6—H6A	1.01 (3)	N3—O5	1.221 (2)
C6—H6B	0.97 (3)	N3—O6	1.228 (2)
O1—C1—C5	106.35 (18)	H6A—C6—H6C	110 (2)
O1—C1—C2	111.25 (19)	H6B—C6—H6C	108 (2)
C5—C1—C2	110.5 (2)	O2—C7—O1	126.6 (2)
O1—C1—H1	108.1 (13)	O2—C7—C8	123.8 (2)
C5—C1—H1	111.2 (13)	O1—C7—C8	109.26 (19)
C2—C1—H1	109.4 (13)	C13—C8—C9	117.8 (2)
C1—C2—C3	108.8 (2)	C13—C8—C7	117.2 (2)
C1—C2—H2A	111.3 (13)	C9—C8—C7	124.9 (2)
C3—C2—H2A	111.1 (13)	C10—C9—C8	122.2 (2)
C1—C2—H2B	109.5 (15)	C10—C9—N2	117.9 (2)
C3—C2—H2B	111.3 (15)	C8—C9—N2	119.9 (2)
H2A—C2—H2B	105 (2)	C9—C10—C11	117.9 (2)
N1—C3—C2	111.6 (2)	C9—C10—H10	121.8 (13)
N1—C3—H3A	105.3 (14)	C11—C10—H10	120.3 (14)
C2—C3—H3A	111.6 (13)	C10—C11—C12	122.0 (2)
N1—C3—H3B	109.9 (14)	C10—C11—N3	118.9 (2)
C2—C3—H3B	108.8 (14)	C12—C11—N3	119.1 (2)
H3A—C3—H3B	110 (2)	C13—C12—C11	118.9 (2)
N1—C4—C5	111.16 (19)	C13—C12—H12	121.9 (15)
N1—C4—H4A	112.3 (14)	C11—C12—H12	119.2 (15)
C5—C4—H4A	108.8 (14)	C12—C13—C8	121.2 (2)
N1—C4—H4B	108.7 (13)	C12—C13—H13	120.3 (13)
C5—C4—H4B	109.1 (14)	C8—C13—H13	118.6 (13)
H4A—C4—H4B	106.7 (19)	C3—N1—C4	110.49 (19)
C1—C5—C4	109.4 (2)	C3—N1—C6	109.9 (2)
C1—C5—H5A	107.1 (12)	C4—N1—C6	109.45 (19)
C4—C5—H5A	109.4 (12)	O3—N2—O4	124.1 (2)
C1—C5—H5B	110.9 (13)	O3—N2—C9	117.73 (19)
C4—C5—H5B	110.1 (13)	O4—N2—C9	118.2 (2)
H5A—C5—H5B	109.8 (18)	O5—N3—O6	124.5 (2)
N1—C6—H6A	109.8 (14)	O5—N3—C11	118.0 (2)
N1—C6—H6B	108.6 (14)	O6—N3—C11	117.51 (19)
H6A—C6—H6B	111 (2)	C7—O1—C1	117.54 (18)
N1—C6—H6C	109.2 (14)
O1—C1—C2—C3	−174.9 (2)	C11—C12—C13—C8	0.4 (4)
C5—C1—C2—C3	−57.0 (3)	C9—C8—C13—C12	−1.3 (3)
C1—C2—C3—N1	57.7 (3)	C7—C8—C13—C12	175.8 (2)
O1—C1—C5—C4	178.15 (19)	C2—C3—N1—C4	−58.7 (3)
C2—C1—C5—C4	57.3 (3)	C2—C3—N1—C6	−179.6 (2)
N1—C4—C5—C1	−57.8 (3)	C5—C4—N1—C3	58.4 (3)
O2—C7—C8—C13	−98.2 (3)	C5—C4—N1—C6	179.5 (2)
O1—C7—C8—C13	75.8 (3)	C10—C9—N2—O3	−171.0 (2)
O2—C7—C8—C9	78.7 (3)	C8—C9—N2—O3	7.6 (3)
O1—C7—C8—C9	−107.4 (2)	C10—C9—N2—O4	9.4 (3)
C13—C8—C9—C10	2.1 (3)	C8—C9—N2—O4	−171.9 (2)
C7—C8—C9—C10	−174.8 (2)	C10—C11—N3—O5	6.2 (3)
C13—C8—C9—N2	−176.5 (2)	C12—C11—N3—O5	−172.1 (2)
C7—C8—C9—N2	6.6 (3)	C10—C11—N3—O6	−173.9 (2)
C8—C9—C10—C11	−2.0 (3)	C12—C11—N3—O6	7.8 (3)
N2—C9—C10—C11	176.65 (19)	O2—C7—O1—C1	3.6 (3)
C9—C10—C11—C12	1.1 (3)	C8—C7—O1—C1	−170.20 (17)
C9—C10—C11—N3	−177.1 (2)	C5—C1—O1—C7	149.1 (2)
C10—C11—C12—C13	−0.4 (4)	C2—C1—O1—C7	−90.6 (2)
N3—C11—C12—C13	177.8 (2)