Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536807048003/ya2056sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536807048003/ya2056Isup2.hkl |
CCDC reference: 669136
The title compound was synthesized by treating of 2-methylpyridine (1.10 g, 11 mmol) with (S)-(+)-2-bromomethylpyrrolidine hydrobromide (2.50 g, 10 mmol; see Xu et al., 2006 for the synthesis of the latter) in MeCN (30 ml) under stirring at 353 K for 24 h (yield 95%). The reaction is accompanied by racemization of the L-proline derivative. Suitable crystals of the title compound were obtained by slow evaporation of ethanol solution at room temperature (m.p. 421 K).
The H211 and H212 atoms were located in a difference Fourier map; all carbon-bound H atoms were placed in calculated position with C—H = 0.98 Å (sp), C—H = 0.97 Å (sp2), C—H = 0.96 Å (sp3), C—H = 0.93 Å (aromatic). All H atoms included in the final cycles of refinement in the riding motion approximation, with Uiso(H) = 1.2Ueq of the carrier atoms.
There has been growing interest (Ishii et al., 2004; Andrey et al., 2004) in the studies of catalytic properties of proline and its derivatives. The title compound (I), a quaternary ammonium salt obtained from L-proline, can act as organocatalyst in several important processes such as the Michael reaction, the Mannich reaction and others. When this compound was used as a catalyst of the Michael addition of cyclohexanone to nitroolefins, the products were obtained in good yields, however the reactions showed poor enantioselectivities. In order to establish the structure of (I) unambiguously, as well as to investigate the reason for the above mentioned lack of enantioselectivity, the X-ray analysis of (I) was undertaken. The structure of (I) is shown in Fig. 1.
It came as a surprise, that the crystal of (I) turned out to be racemic and crystallize in centrosymmetric space group, even though it was prepared from the chiral L-proline derivative. It is known however that in basic conditions under heating L-proline and its derivatives may undergo racemization (Ebbers et al., 1997).
The crystal of (I) is built of pyrrolidinium dications and bromide anions. The N21 atom of the pyrrolidinium ring and the C5 atom are displaced from the mean plane of C1, C2, C3, C4 in the same direction by 0.533 (6) and 0.625 (8) Å. Thanks to two N—H···Br bonds (Table 2) the dications and anions form "ion-pairs" in crystal of (I).
See Ishii et al. (2004) and Andrey et al. (2004) for the catalytic properties of L-proline. The synthesis of (S)-(+)-2-bromomethylpyrrolidine hydrobromide is described by Xu et al. (2006). Details of the racemization of amino acids under basic conditions are given by Ebbers et al. (1997). For the method used for the extinction correction, see: Larson (1970).
Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO (Rigaku, 1998); data reduction: CrystalStructure (Rigaku/MSC & Rigaku, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: CRYSTALS (Watkin et al., 1996); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: CrystalStructure (Rigaku/MSC & Rigaku, 2004).
C11H18N22+·2Br− | F(000) = 672.00 |
Mr = 338.08 | Dx = 1.612 Mg m−3 |
Monoclinic, P21/c | Melting point: 148 K |
Hall symbol: -P 2ybc | Mo Kα radiation, λ = 0.71075 Å |
a = 12.588 (3) Å | Cell parameters from 9608 reflections |
b = 10.023 (4) Å | θ = 3.1–27.5° |
c = 12.461 (4) Å | µ = 5.81 mm−1 |
β = 117.617 (13)° | T = 298 K |
V = 1393.1 (8) Å3 | Block, yellow |
Z = 4 | 0.26 × 0.19 × 0.17 mm |
Rigaku R-AXIS RAPID diffractometer | 2192 reflections with I > 2σ(I) |
Detector resolution: 10.00 pixels mm-1 | Rint = 0.032 |
ω scans | θmax = 27.5° |
Absorption correction: multi-scan (ABSCOR; Higashi, 1995) | h = −16→16 |
Tmin = 0.225, Tmax = 0.372 | k = −12→13 |
13165 measured reflections | l = −14→16 |
3178 independent reflections |
Refinement on F2 | H-atom parameters constrained |
R[F2 > 2σ(F2)] = 0.045 | w = 1/[0.0017Fo2 + 1.0σ(Fo2)]/(4Fo2) |
wR(F2) = 0.139 | (Δ/σ)max < 0.001 |
S = 1.01 | Δρmax = 0.88 e Å−3 |
3178 reflections | Δρmin = −0.69 e Å−3 |
137 parameters | Extinction correction: Larson (1970) |
0 restraints | Extinction coefficient: 28 (2) |
C11H18N22+·2Br− | V = 1393.1 (8) Å3 |
Mr = 338.08 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 12.588 (3) Å | µ = 5.81 mm−1 |
b = 10.023 (4) Å | T = 298 K |
c = 12.461 (4) Å | 0.26 × 0.19 × 0.17 mm |
β = 117.617 (13)° |
Rigaku R-AXIS RAPID diffractometer | 3178 independent reflections |
Absorption correction: multi-scan (ABSCOR; Higashi, 1995) | 2192 reflections with I > 2σ(I) |
Tmin = 0.225, Tmax = 0.372 | Rint = 0.032 |
13165 measured reflections |
R[F2 > 2σ(F2)] = 0.045 | 0 restraints |
wR(F2) = 0.139 | H-atom parameters constrained |
S = 1.01 | Δρmax = 0.88 e Å−3 |
3178 reflections | Δρmin = −0.69 e Å−3 |
137 parameters |
Refinement. Refinement using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F2. R-factor (gt) are based on F. The threshold expression of F2 > 2.0 σ(F2) is used only for calculating R-factor (gt). |
x | y | z | Uiso*/Ueq | ||
Br1 | 0.41438 (4) | 0.56892 (5) | 0.64583 (4) | 0.06054 (16) | |
Br2 | 0.08574 (4) | 0.16025 (5) | 0.45070 (4) | 0.05775 (16) | |
N21 | 0.3228 (3) | 0.3124 (3) | 0.4761 (2) | 0.0487 (9) | |
N22 | 0.1717 (2) | 0.5515 (3) | 0.3133 (2) | 0.0516 (10) | |
C1 | 0.4223 (4) | 0.2119 (6) | 0.5167 (5) | 0.0741 (17) | |
C2 | 0.4018 (6) | 0.1369 (6) | 0.4052 (5) | 0.094 (2) | |
C3 | 0.3048 (5) | 0.2097 (5) | 0.2990 (4) | 0.0762 (18) | |
C4 | 0.2946 (3) | 0.3451 (4) | 0.3492 (3) | 0.0481 (12) | |
C5 | 0.1739 (4) | 0.4108 (4) | 0.2777 (3) | 0.0511 (12) | |
C6 | 0.1146 (4) | 0.5837 (6) | 0.3801 (4) | 0.087 (2) | |
C7 | 0.1090 (7) | 0.7146 (7) | 0.4121 (7) | 0.119 (3) | |
C8 | 0.1624 (7) | 0.8095 (9) | 0.3721 (7) | 0.106 (3) | |
C9 | 0.2193 (5) | 0.7798 (4) | 0.3032 (5) | 0.106 (2) | |
C10 | 0.2226 (3) | 0.6472 (3) | 0.2744 (4) | 0.0577 (13) | |
C11 | 0.2778 (6) | 0.6100 (7) | 0.1949 (6) | 0.119 (2) | |
H4 | 0.3570 | 0.4043 | 0.3502 | 0.058* | |
H6 | 0.0792 | 0.5168 | 0.4044 | 0.104* | |
H7 | 0.0712 | 0.7378 | 0.4582 | 0.142* | |
H8 | 0.1598 | 0.8982 | 0.3927 | 0.127* | |
H9 | 0.2539 | 0.8461 | 0.2773 | 0.128* | |
H101 | 0.4190 | 0.1520 | 0.5763 | 0.089* | |
H102 | 0.4995 | 0.2563 | 0.5514 | 0.089* | |
H111 | 0.3484 | 0.5576 | 0.2404 | 0.143* | |
H112 | 0.2213 | 0.5592 | 0.1271 | 0.143* | |
H113 | 0.2989 | 0.6896 | 0.1663 | 0.143* | |
H201 | 0.3764 | 0.0464 | 0.4090 | 0.112* | |
H202 | 0.4750 | 0.1345 | 0.3974 | 0.112* | |
H211 | 0.3489 | 0.3871 | 0.5289 | 0.057* | |
H212 | 0.2604 | 0.2740 | 0.4796 | 0.057* | |
H301 | 0.3271 | 0.2205 | 0.2347 | 0.091* | |
H302 | 0.2295 | 0.1617 | 0.2683 | 0.091* | |
H501 | 0.1534 | 0.4084 | 0.1925 | 0.061* | |
H502 | 0.1146 | 0.3610 | 0.2905 | 0.061* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Br1 | 0.0632 (3) | 0.0577 (3) | 0.0598 (3) | −0.0061 (2) | 0.0278 (2) | −0.0152 (2) |
Br2 | 0.0576 (3) | 0.0641 (3) | 0.0493 (3) | −0.0091 (2) | 0.0228 (2) | 0.0049 (2) |
N21 | 0.0508 (19) | 0.050 (2) | 0.0420 (18) | −0.0043 (16) | 0.0184 (15) | −0.0055 (15) |
N22 | 0.0438 (18) | 0.064 (2) | 0.0402 (18) | 0.0065 (17) | 0.0138 (14) | −0.0026 (17) |
C1 | 0.071 (3) | 0.074 (3) | 0.068 (3) | 0.024 (2) | 0.024 (2) | 0.005 (2) |
C2 | 0.125 (5) | 0.070 (4) | 0.074 (3) | 0.039 (3) | 0.036 (3) | 0.005 (3) |
C3 | 0.112 (4) | 0.060 (3) | 0.062 (3) | 0.015 (3) | 0.045 (3) | −0.008 (2) |
C4 | 0.057 (2) | 0.043 (2) | 0.048 (2) | −0.0016 (19) | 0.0284 (19) | −0.0019 (18) |
C5 | 0.056 (2) | 0.050 (2) | 0.041 (2) | −0.012 (2) | 0.0174 (18) | −0.0013 (19) |
C6 | 0.053 (2) | 0.147 (6) | 0.051 (2) | 0.025 (3) | 0.017 (2) | −0.004 (3) |
C7 | 0.087 (5) | 0.153 (12) | 0.079 (5) | 0.023 (6) | 0.008 (4) | −0.019 (7) |
C8 | 0.081 (7) | 0.110 (8) | 0.075 (6) | 0.067 (6) | −0.007 (4) | −0.012 (5) |
C9 | 0.087 (4) | 0.058 (3) | 0.107 (5) | 0.011 (3) | −0.012 (4) | −0.007 (3) |
C10 | 0.051 (2) | 0.044 (2) | 0.061 (2) | 0.002 (2) | 0.011 (2) | 0.006 (2) |
C11 | 0.148 (6) | 0.089 (5) | 0.110 (5) | −0.021 (5) | 0.052 (5) | 0.030 (4) |
N21—C1 | 1.500 (6) | C1—H101 | 0.970 |
N21—C4 | 1.488 (6) | C1—H102 | 0.970 |
N22—C5 | 1.483 (5) | C2—H201 | 0.970 |
N22—C6 | 1.367 (7) | C2—H202 | 0.970 |
N22—C10 | 1.361 (6) | C3—H301 | 0.970 |
C1—C2 | 1.494 (9) | C3—H302 | 0.970 |
C2—C3 | 1.509 (7) | C4—H4 | 0.980 |
C3—C4 | 1.524 (7) | C5—H501 | 0.970 |
C4—C5 | 1.511 (5) | C5—H502 | 0.970 |
C6—C7 | 1.383 (10) | C6—H6 | 0.930 |
C7—C8 | 1.384 (13) | C7—H7 | 0.930 |
C8—C9 | 1.382 (13) | C8—H8 | 0.930 |
C9—C10 | 1.382 (6) | C9—H9 | 0.930 |
C10—C11 | 1.496 (11) | C11—H111 | 0.960 |
N21—H211 | 0.949 | C11—H112 | 0.960 |
N21—H212 | 0.895 | C11—H113 | 0.960 |
Br1···N21 | 3.186 (3) | H101···H201ix | 3.192 |
Br2···N21 | 3.234 (4) | H101···H202ix | 3.119 |
Br2···N22i | 3.554 (2) | H101···H301vi | 2.998 |
Br2···C10i | 3.586 (3) | H102···Br1iii | 3.571 |
N21···Br1 | 3.186 (3) | H102···C9iii | 3.161 |
N21···Br2 | 3.234 (4) | H102···C10iii | 3.293 |
N22···Br2ii | 3.554 (2) | H102···C11iii | 3.381 |
C10···Br2ii | 3.586 (3) | H102···H9iii | 3.017 |
Br1···H4iii | 2.868 | H102···H111iii | 3.045 |
Br1···H9iv | 3.250 | H102···H113iii | 3.292 |
Br1···H101v | 3.217 | H102···H201ix | 3.343 |
Br1···H102iii | 3.571 | H111···Br1iii | 2.934 |
Br1···H111iii | 2.934 | H111···H101xiii | 3.326 |
Br1···H113iv | 2.895 | H111···H102iii | 3.045 |
Br1···H202iii | 3.429 | H111···H202xiv | 3.467 |
Br1···H202vi | 3.514 | H111···H211iii | 3.596 |
Br1···H211 | 2.239 | H112···Br2xiii | 3.018 |
Br1···H301iii | 3.574 | H112···C7xi | 3.288 |
Br1···H301vi | 3.461 | H112···C8xi | 3.193 |
Br2···H7vii | 2.883 | H112···H7xi | 2.908 |
Br2···H8viii | 2.986 | H112···H8xi | 2.686 |
Br2···H112vi | 3.018 | H112···H101xiii | 3.540 |
Br2···H212 | 2.351 | H113···Br1xi | 2.895 |
Br2···H302 | 3.501 | H113···C7xi | 3.106 |
Br2···H501vi | 2.810 | H113···C8xi | 3.248 |
Br2···H502 | 2.973 | H113···H7xi | 2.929 |
N21···H301vi | 3.215 | H113···H8xi | 3.150 |
C1···H201ix | 3.437 | H113···H102iii | 3.292 |
C1···H301vi | 3.505 | H113···H202xiv | 3.336 |
C2···H9viii | 3.428 | H201···C1ix | 3.437 |
C2···H201ix | 3.246 | H201···C2ix | 3.246 |
C2···H202ix | 3.510 | H201···C8viii | 3.458 |
C5···H7i | 3.572 | H201···C9viii | 3.221 |
C7···H112iv | 3.288 | H201···H8viii | 3.027 |
C7···H113iv | 3.106 | H201···H9viii | 2.600 |
C7···H501ii | 3.520 | H201···H101ix | 3.192 |
C7···H502ii | 3.141 | H201···H102ix | 3.343 |
C8···H112iv | 3.193 | H201···H201ix | 3.026 |
C8···H113iv | 3.248 | H201···H202ix | 2.903 |
C8···H201x | 3.458 | H202···Br1iii | 3.429 |
C8···H502ii | 3.160 | H202···Br1xiii | 3.514 |
C9···H102iii | 3.161 | H202···C2ix | 3.510 |
C9···H201x | 3.221 | H202···H101ix | 3.119 |
C10···H102iii | 3.293 | H202···H111xv | 3.467 |
C11···H7xi | 3.268 | H202···H113xv | 3.336 |
C11···H8xi | 3.348 | H202···H201ix | 2.903 |
C11···H102iii | 3.381 | H202···H202ix | 3.566 |
H4···Br1iii | 2.868 | H211···Br1 | 2.239 |
H7···Br2vii | 2.883 | H211···H111iii | 3.596 |
H7···C5ii | 3.572 | H211···H301vi | 2.910 |
H7···C11iv | 3.268 | H212···Br2 | 2.351 |
H7···H112iv | 2.908 | H212···H301vi | 2.888 |
H7···H113iv | 2.929 | H301···Br1iii | 3.574 |
H7···H501ii | 3.086 | H301···Br1xiii | 3.461 |
H7···H502ii | 3.144 | H301···N21xiii | 3.215 |
H8···Br2x | 2.986 | H301···C1xiii | 3.505 |
H8···C11iv | 3.348 | H301···H101xiii | 2.998 |
H8···H112iv | 2.686 | H301···H211xiii | 2.910 |
H8···H113iv | 3.150 | H301···H212xiii | 2.888 |
H8···H201x | 3.027 | H302···Br2 | 3.501 |
H8···H302x | 3.378 | H302···H8viii | 3.378 |
H8···H501ii | 3.578 | H302···H9viii | 3.175 |
H8···H502ii | 3.159 | H501···Br2xiii | 2.810 |
H9···Br1xi | 3.250 | H501···C7i | 3.520 |
H9···C2x | 3.428 | H501···H7i | 3.086 |
H9···H102iii | 3.017 | H501···H8i | 3.578 |
H9···H201x | 2.600 | H502···Br2 | 2.973 |
H9···H302x | 3.175 | H502···C7i | 3.141 |
H101···Br1xii | 3.217 | H502···C8i | 3.160 |
H101···H111vi | 3.326 | H502···H7i | 3.144 |
H101···H112vi | 3.540 | H502···H8i | 3.159 |
C1—N21—C4 | 104.9 (4) | C3—C2—H201 | 110.1 |
C5—N22—C6 | 119.6 (4) | C3—C2—H202 | 110.1 |
C5—N22—C10 | 119.1 (4) | H201—C2—H202 | 109.5 |
C6—N22—C10 | 121.2 (4) | C2—C3—H301 | 110.6 |
N21—C1—C2 | 105.5 (4) | C2—C3—H302 | 110.6 |
C1—C2—C3 | 107.0 (5) | C4—C3—H301 | 110.6 |
C2—C3—C4 | 105.0 (4) | C4—C3—H302 | 110.6 |
N21—C4—C3 | 102.5 (3) | H301—C3—H302 | 109.5 |
N21—C4—C5 | 113.7 (4) | N21—C4—H4 | 109.0 |
C3—C4—C5 | 113.5 (3) | C3—C4—H4 | 109.0 |
N22—C5—C4 | 113.0 (3) | C5—C4—H4 | 109.0 |
N22—C6—C7 | 120.7 (6) | N22—C5—H501 | 108.6 |
C6—C7—C8 | 116.8 (9) | N22—C5—H502 | 108.6 |
C7—C8—C9 | 123.6 (8) | C4—C5—H501 | 108.6 |
C8—C9—C10 | 117.1 (6) | C4—C5—H502 | 108.6 |
N22—C10—C9 | 120.6 (5) | H501—C5—H502 | 109.5 |
N22—C10—C11 | 120.0 (4) | N22—C6—H6 | 119.7 |
C9—C10—C11 | 119.3 (5) | C7—C6—H6 | 119.7 |
C1—N21—H211 | 109.2 | C6—C7—H7 | 121.6 |
C1—N21—H212 | 108.0 | C8—C7—H7 | 121.6 |
C4—N21—H211 | 113.5 | C7—C8—H8 | 118.2 |
C4—N21—H212 | 111.3 | C9—C8—H8 | 118.2 |
H211—N21—H212 | 109.6 | C8—C9—H9 | 121.5 |
N21—C1—H101 | 110.5 | C10—C9—H9 | 121.5 |
N21—C1—H102 | 110.5 | C10—C11—H111 | 109.5 |
C2—C1—H101 | 110.5 | C10—C11—H112 | 109.5 |
C2—C1—H102 | 110.5 | C10—C11—H113 | 109.5 |
H101—C1—H102 | 109.5 | H111—C11—H112 | 109.5 |
C1—C2—H201 | 110.1 | H111—C11—H113 | 109.5 |
C1—C2—H202 | 110.1 | H112—C11—H113 | 109.5 |
C1—N21—C4—C3 | 39.2 (4) | N21—C1—C2—C3 | 9.6 (7) |
C1—N21—C4—C5 | 162.2 (3) | C1—C2—C3—C4 | 14.4 (7) |
C4—N21—C1—C2 | −30.8 (5) | C2—C3—C4—N21 | −32.9 (6) |
C5—N22—C6—C7 | −178.5 (4) | C2—C3—C4—C5 | −155.9 (5) |
C6—N22—C5—C4 | −106.0 (4) | N21—C4—C5—N22 | 73.6 (5) |
C5—N22—C10—C9 | 178.2 (3) | C3—C4—C5—N22 | −169.7 (4) |
C5—N22—C10—C11 | 1.1 (5) | N22—C6—C7—C8 | 0.7 (7) |
C10—N22—C5—C4 | 76.7 (4) | C6—C7—C8—C9 | 0.2 (7) |
C6—N22—C10—C9 | 1.0 (5) | C7—C8—C9—C10 | −0.4 (9) |
C6—N22—C10—C11 | −176.2 (3) | C8—C9—C10—N22 | −0.1 (5) |
C10—N22—C6—C7 | −1.3 (6) | C8—C9—C10—C11 | 177.0 (5) |
Symmetry codes: (i) −x, y−1/2, −z+1/2; (ii) −x, y+1/2, −z+1/2; (iii) −x+1, −y+1, −z+1; (iv) x, −y+3/2, z+1/2; (v) −x+1, y+1/2, −z+3/2; (vi) x, −y+1/2, z+1/2; (vii) −x, −y+1, −z+1; (viii) x, y−1, z; (ix) −x+1, −y, −z+1; (x) x, y+1, z; (xi) x, −y+3/2, z−1/2; (xii) −x+1, y−1/2, −z+3/2; (xiii) x, −y+1/2, z−1/2; (xiv) −x+1, y+1/2, −z+1/2; (xv) −x+1, y−1/2, −z+1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
N21—H211···Br1 | 0.95 | 2.24 | 3.186 (3) | 176 |
N21—H212···Br2 | 0.90 | 2.35 | 3.233 (3) | 169 |
Experimental details
Crystal data | |
Chemical formula | C11H18N22+·2Br− |
Mr | 338.08 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 298 |
a, b, c (Å) | 12.588 (3), 10.023 (4), 12.461 (4) |
β (°) | 117.617 (13) |
V (Å3) | 1393.1 (8) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 5.81 |
Crystal size (mm) | 0.26 × 0.19 × 0.17 |
Data collection | |
Diffractometer | Rigaku R-AXIS RAPID |
Absorption correction | Multi-scan (ABSCOR; Higashi, 1995) |
Tmin, Tmax | 0.225, 0.372 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 13165, 3178, 2192 |
Rint | 0.032 |
(sin θ/λ)max (Å−1) | 0.650 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.045, 0.139, 1.01 |
No. of reflections | 3178 |
No. of parameters | 137 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.88, −0.69 |
Computer programs: PROCESS-AUTO (Rigaku, 1998), CrystalStructure (Rigaku/MSC & Rigaku, 2004), SHELXS97 (Sheldrick, 1997), CRYSTALS (Watkin et al., 1996), ORTEP-3 for Windows (Farrugia, 1997).
D—H···A | D—H | H···A | D···A | D—H···A |
N21—H211···Br1 | 0.949 | 2.238 | 3.186 (3) | 176 |
N21—H212···Br2 | 0.895 | 2.351 | 3.233 (3) | 169 |
There has been growing interest (Ishii et al., 2004; Andrey et al., 2004) in the studies of catalytic properties of proline and its derivatives. The title compound (I), a quaternary ammonium salt obtained from L-proline, can act as organocatalyst in several important processes such as the Michael reaction, the Mannich reaction and others. When this compound was used as a catalyst of the Michael addition of cyclohexanone to nitroolefins, the products were obtained in good yields, however the reactions showed poor enantioselectivities. In order to establish the structure of (I) unambiguously, as well as to investigate the reason for the above mentioned lack of enantioselectivity, the X-ray analysis of (I) was undertaken. The structure of (I) is shown in Fig. 1.
It came as a surprise, that the crystal of (I) turned out to be racemic and crystallize in centrosymmetric space group, even though it was prepared from the chiral L-proline derivative. It is known however that in basic conditions under heating L-proline and its derivatives may undergo racemization (Ebbers et al., 1997).
The crystal of (I) is built of pyrrolidinium dications and bromide anions. The N21 atom of the pyrrolidinium ring and the C5 atom are displaced from the mean plane of C1, C2, C3, C4 in the same direction by 0.533 (6) and 0.625 (8) Å. Thanks to two N—H···Br bonds (Table 2) the dications and anions form "ion-pairs" in crystal of (I).