Isomorphism of chiral ammonium salts Ph(All)N+Et(Me)X-·CHCl3

Article abstract of DOI:10.1016/j.mencom.2009.01.008

The title ammonium salts (X = Br, I) were synthesized, the identity of crystal structures for them (space groups P2₁2₁2₁, Z = 4) was found, spontaneous resolution of Ph(All)N⁺Et(Me)Br^-·CHCl₃ was performed, and the impossibility of replacement of the solvate molecule by CHBr₃ or CH₂Hal₂ (Hal = Cl, Br, I) was demonstrated by 1H NMR and X-ray diffraction studies.

Full text of DOI:10.1016/j.mencom.2009.01.008

Mendeleev
Communications
Mendeleev Commun., 2009, 19, 19–20
†
Isomorphism of chiral ammonium salts Ph(All)N+Et(Me)X–·CHCl3
Remir G. Kostyanovsky,*a Konstantin A. Lyssenko,b Oleg N. Krutiusa and Vasily R. Kostyanovskya
a N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 495 651 2191; e-mail: kost@chph.ras.ru
b A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5085; e-mail: kostya@xrlab.ineos.ac.ru
DOI: 10.1016/j.mencom.2009.01.008
The title ammonium salts (X = Br, I) were synthesized, the identity of crystal structures for them (space groups P212121, Z = 4)
was found, spontaneous resolution of Ph(All)N+Et(Me)Br–·CHCl3 was performed, and the impossibility of replacement of the
solvate molecule by CHBr3 or CH2Hal2 (Hal = Cl, Br, I) was demonstrated by 1H NMR and X-ray diffraction studies.
The crystal structure of Ph(All)N+Et(Me)I–·CHCl3 was studied
Ph
N
HA
previously.1 Chiral ammonium salts are widely used as catalysts
in asymmetric reactions,2 as ionic liquids,3 and in crystal
engineering.4 It seems reasonable to examine the usability of
optically active salts Ph(All)N+Et(Me)X–·CHCl3 1a–c (a X = Br,
b X = I, c X = Cl) in asymmetric allylation reactions. The salts
of this type are known to racemize in solutions, and the rate of
the process varies in the order: I > Br > Cl.5 That is why salts
1a,c and 1a' were prepared and studied in terms of conglo-
merate formation (Scheme 1).‡
HB
CH2=CHCH2Br
CHBr3 CHCl3
Ph(All)N+Et(Me)Br–
Me
Et
HC
1a'
Br–·CHCl3
LiAlH4
dioxane
PhNH2
PhN(Et)Me
1a
3
HC(OEt)3
PhNCH=O
CHCl3
CH2=CHCH2Cl
Ph(All)N+Et(Me)Cl–·CHCl3
Et
1c
2
An X-ray diffraction (XRD) study§ revealed that 1a is iso-
structural to 1b, it crystallizes in the same chiral space group
P212121. Furthermore, the refinement of Flack parameter [its
value was 0.009(3)] indicated that the crystal is not a racemic
Scheme 1
twin and formally contains only one enantiomer. At the same
time, the analysis of Fourier density synthesis demonstrated that
§
†
Crystals of 1a (C12H18N, CHCl3, Br, M = 375.55) are orthorhombic,
Asymmetric nitrogen, part 103.
‡
1H NMR spectra were measured on a Bruker WM-400 spectrometer
space group P212121, at 100 K: a = 9.730(2), b = 12.372(3) and c =
= 13.365(3) Å, V = 1608.9(6) Å3, Z = 4 (Z' = 1), dcalc = 1.550 g cm–3,
m(MoKα) = 30.37 cm–1, F(000) = 760. Intensities of 101238 reflections
were measured with a Bruker SMART APEX2 CCD diffractometer
[l(MoKα) = 0.71072 Å, w-scans, 2q < 90°] and 12963 independent reflec-
tions [Rint = 0.0396] were used in further refinement.
Crystals of 1a' (C12H18BrN, M = 256.18) are orthorhombic, space group
Pbcn, at 100 K: a = 10.8510(14), b = 12.4726(13) and c = 18.9661(17) Å,
V
F(000) = 1056. Intensities of 19464 reflections were measured with a
Bruker SMART APEX2 CCD diffractometer [l(MoKα) = 0.71072 Å,
w-scans, 2q < 66°] and 5063 independent reflections [Rint = 0.0616] were
used in further refinement.
The substantial redundancy in data allows empirical absorption correc-
tion to be applied using multiple measurements of equivalent reflections
with SADABS Bruker program. The structures were solved by direct
method and refined by the full-matrix least-squares technique against F2
in the anisotropic–isotropic approximation. Upon the refinement of
occupancies for two enantiomers the number of restrains, i.e. the DFIX
instructions for C–C bond lengths of allyl group and ADP for carbon
atoms were applied. Hydrogen atoms for main component in 1a were
located from the Fourier synthesis of the electron density and refined in
the isotropic approximation. For 1a the refinement converged to wR2 =
= 0.0595 and GOF = 1.017 for all independent reflections [R1 = 0.0310
was calculated against F for 10636 observed reflections with I > 2s(I)].
For 1a' the refinement converged to wR2 = 0.1065 and GOF = 0.975 for
all independent reflections [R1 = 0.0404 was calculated against F for 2749
observed reflections with I > 2s(I)]. All calculations were performed
using SHELXTL PLUS 5.0.
(400.13 MHz).
2: bp 112 °C (5 Torr). 1H NMR (400 MHz, CDCl3) d: 1.14 (t, 3H, Me,
3J 7.1 Hz), 3.84 (q, 2H, CH2N, 3J 7.1 Hz), 7.15, 7.28, 7.38 (m, 5H, Ph),
8.34 (s, 1H, HCO).
3: bp 52 °C (5 Torr). 1H NMR (CDCl3) d: 1.17 (t, 3H, MeCH2, 3J 7.1 Hz),
2.95 (s, 3H, MeN), 3.44 (q, 2H, CH2Me), 6.74, 6.77, 7.28 (m, Ph).
1
1a: yield 71%, mp 137.7 °C (in sealed capillary). H NMR (CD3CN)
= 2566.9(5) Å3, = 8 (Z' = 1), dcalc = 1.326 g cm–3, ) = 31.69 cm–1
Z m(MoKα ,
3
d: 1.05 (t, 3H, MeCH2N, J 7.1 Hz), 3.48 (s, 3H, MeN), 4.03 (m, 2H,
CH2Me, ABX3 spectrum, ΔnAB 79 Hz, 2JAB –14.0 Hz, 3JAX = 3JBX = 7.1 Hz),
2
4.58 (m, 2H, CH2CH, ABX spectrum, ΔnAB 52 Hz, JAB –13.0 Hz,
3JAX 4.8 Hz, 3JBX 6.4 Hz), 5.50 (d, 1H, HC, 3JCA 16.5 Hz), 5.57 (m, 1H,
HA, 3JAC 16.5 Hz, 3JAB 10.1 Hz, 3JH CCH N 4.8 and 6.4 Hz), 5.62 (d, 1H,
A
2
3
HB, JBC 10.1 Hz), 7.58, 7.64, 7.77 (5H, Ph), 7.71 [s, 1H, CHCl3 (for
CHCl3 in pure CD3CN, 7.58 ppm)].
Suitable for XRD study crystals were grown from the mixture
CHCl3–isooctane at –21 °C. For crystals 10 and 6 mg there was found
[a]D +31.0 and +26.2 (c 0.15, CHCl3), for next crystals 11 mg [a]D +16.2
(c 0.27, MeOH) and 6 mg [a]D +26.2 (c 0.15, CHCl3) and 6 mg [a]D –12.7
(c 0.15, MeOH). The absolute configuration (S)-(+) and (R)-(–) 1a is
obviously the same as for 1b, due to identity of the chiral cations.1
1a' was prepared by crystallization from a mixture of THF/MeCN
(2:1) and twofold excess of CHBr3, mp in sealed capillary 132–135 °C
(decomp.). 1H NMR (CDCl3) d: 1.06 (t, 3H, MeCH2N, 3J 7.1 Hz), 3.72,
(s, 3H, MeN), 4.56 (2H, CH2Me, ABX3 spectrum, 2J –14.1 Hz, 3J 7.1 Hz),
5.18 (2H, NCH2CH, ABX spectrum, 2J –12.7 Hz, 3J 5.8 and 8.1 Hz), 5.37
(dd, 1H, HA, 3JAC 16.1 Hz, 3JAB 10.1 Hz), 5.44 (dd, 1H, HB, 3JBA 10.1 Hz,
3JBC 1.3 Hz), 5.78 (dd, 1H, HC, 3J 16.1 Hz, 2J 1.3 Hz), 7.48 (t, 1H), 7.59
(t, 2H), 7.88 (d, 2H, Ph). For CHBr3 in CDCl3 d: 6.83 (s, HC).
1
CCDC 685726 and 713421 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
For details, see ‘Notice to Authors’, Mendeleev Commun., Issue 1, 2009.
1c: yield 21%, strongly hygroscopic solid, the H NMR spectrum in
CD3CN is the same as for 1a. By crystallization from mixtures CHCl3
with hydrocarbons (hexane, pentane, isooctane) and with ethyl acetate
crystals suitable for XRD study were not prepared.
– 19 –
© 2009 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2009, 19, 19–20
C(12)
C(2)
C(3)
C(3)
C(2)
C(12)
C(10')
C(11')
C(8')
C(7')
C(4)
C(9)
C(7')
N(1)
C(4)
C(10')
C(11)
C(11)
C(8')
C(7)
C(9')
C(8)
C(9')
C(10)
C(5)
C(6)
C(12')
N(1)
C(11')
C(10)
C(5)
C(7)
C(6)
C(12')
C(8)
C(9)
Figure 1 Superposition of two enantiomers in the crystal of 1a. The minor
enantiomer with occupancy equal to 0.121(2) is shown by dashed lines.
some residual density is located around N(1) that can be
interpreted as the presence of the opposite enantiomer. Taking
into account that the residual density does not exceed 1.0 eÅ–3,
we additionally collected the dataset with 2q £ 90° to increase
the resolution. The refinement of occupancies led to the value
for minor enantiomer as low as 0.121(2) (Figure 1).
This makes us to reinvestigate the previously published data
for iodide 1b. Unfortunately, the resolution of available dataset
was restricted to 2q £ 60° and, although the maxima of residual
density (0.56 eÅ–3), which can be interpreted as a methyl group,
was located in the vicinity of N(1) atom, it was impossible to
refine correctly the occupancy of the opposite enantiomer. Thus,
we can conclude that, unlike in the bromide salt, the occupancy
for minor enantiomer in the iodide is much lower or even close
to zero.
The crystal packing in 1a is similar to that in the previously
studied iodide salt. However, there are some distinct differences,
which are mainly reflected in the variation of interactions between
the halide anion and the CHCl3 solvate molecule. Indeed, in both
crystals X···Cl contacts assembles the anion and CHCl3 molecule
and the X···H–C interactions lead to the formation of a 3D frame-
work with the chiral ammonium located in the resulted pores.
Although these associates are similar, only Br(1)···H and
Br(1)···Cl(1) interactions are shortened [2.53 and 3.447(1) Å]
in comparison with iodide [2.81 and 3.599(2) Å] for account of
the van der Waals radii difference7 (0.17 Å), while Br(1)···Cl(2)
interaction becomes even longer [from 3.899(1) to 3.961(2) Å]
(Figure 2). Apparently, such a variation of X–···CHCl3 interac-
tions distance plays a crucial role in cation–anion binding and,
consequently, is reflected in the degree of enantiomer resolution.
Despite the minor enantiomer geometry was determined less
accurately, we can conclude that it participates in the formation
of C–H···Cl interactions between the allyl group and the solvate
molecule, while for the major enantiomer such interactions are
absent.
Figure 3 Superposition of two enantiomers in the crystal of 1a'. The minor
enantiomer with occupancy equal to 0.239(4) is shown by dashed lines.
tion in the presence of CHBr3 leads to bromide 1a' crystals,
where the absence of CHBr3 was confirmed by NMR and XRD
studies. In contrast to 1a and 1b, bromide 1a' crystallizes as a
racemate (space group Pbcn, Z = 8). Nevertheless, the same type
of disorder was found in 1a' (Figure 3). The occupancies of
two enantiomers are 0.76 and 0.24. The only type of interionic
interactions in the crystal of 1a' is C–H···Br contacts. The shortest
contacts are observed for the ordered phenyl ring (H···Br
ca. 2.8 Å), while the interactions for disordered part are slightly
weaker.
Thus, a new example of isomorphism in chiral ammonium
salts 1a,b was found and their spontaneous resolution was
carried out. Earlier, the same phenomenon was described for
salts Ph(Me)N+Bn(All)X– at X = Br, I.8 The available informa-
tion clearly shows that CHCl3···X (X = Br, I) interactions play
the major role in the formation of chiral crystal packing. At the
same time, on the basis of energy of cation–anion X···H–C
contacts, as well as the shape of the molecule, one cannot
adequately distinguish between the two enantiomers upon crys-
tallization.
This work was supported by the Russian Academy of
Sciences and the Russian Foundation for Basic research (grant
no. 06-03-32840).
References
1 R. G. Kostyanovsky, V. R. Kostyanovsky, G. K. Kadorkina and K. A.
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A possibility to replace the solvate molecule CHCl3 by CHBr3
was examined with bromide 1a'. Its synthesis and crystalliza-
Cl(1B)
C(1SB)
Cl(3A)
Cl(3B)
7 R. S. Rowland and R. Taylor, J. Phys. Chem., 1996, 100, 7384.
8 V. Yu. Torbeev, K. A. Lyssenko, O. N. Kharybin, M. Yu. Antipin and
R. G. Kostyanovsky, J. Phys. Chem. B, 2003, 107, 13523.
Cl(2B)
Cl(2A)
Br(1)
C(1SA)
Cl(1A)
Cl(1)
C(1S)
Cl(2)
Cl(3)
Figure 2 The scheme illustrating the bromide···CHCl3 interactions in the
crystal of 1a.
Received: 24th July 2008; Com. 08/3189
– 20 –