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The structure of 4-iso­butyl-1,3-oxazolidine-2,5-dione, C7H11NO3, has been determined to explain the polymerization of a series of amino acid N-carboxy anhydrides in the solid state. A dimer structure is formed between the L- and D-enantiomers around a crystallographic centre of symmetry via N—H...O hydrogen bonds. The five-membered rings are arranged in a layer, sandwiched between two layers of iso­propyl groups. This structure should be conducive to the polymerization of the compound in the solid state.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536803017185/ci6257sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S1600536803017185/ci6257Isup2.hkl
Contains datablock I

CCDC reference: 222866

Key indicators

  • Single-crystal X-ray study
  • T = 288 K
  • Mean [sigma](C-C) = 0.003 Å
  • R factor = 0.048
  • wR factor = 0.159
  • Data-to-parameter ratio = 12.0

checkCIF/PLATON results

No syntax errors found



Alert level A PLAT029_ALERT_3_A _diffrn_measured_fraction_theta_full Low ..... 0.92
Alert level C REFLT03_ALERT_3_C Reflection count < 95% complete From the CIF: _diffrn_reflns_theta_max 25.00 From the CIF: _diffrn_reflns_theta_full 0.00 From the CIF: _reflns_number_total 1347 TEST2: Reflns within _diffrn_reflns_theta_max Count of symmetry unique reflns 1462 Completeness (_total/calc) 92.13% PLAT022_ALERT_3_C Ratio Unique / Expected Reflections too Low .. 0.92 PLAT128_ALERT_4_C Non-standard setting of Space group P21/c .. P21/a PLAT242_ALERT_2_C Check Low U(eq) as Compared to Neighbors .. C5 PLAT432_ALERT_2_C Short Inter X...Y Contact:O1 .. C2 = 2.99 Ang.
1 ALERT level A = In general: serious problem 0 ALERT level B = Potentially serious problem 5 ALERT level C = Check and explain 0 ALERT level G = General alerts; check 0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data 2 ALERT type 2 Indicator that the structure model may be wrong or deficient 3 ALERT type 3 Indicator that the structure quality may be low 1 ALERT type 4 Improvement, methodology, query or suggestion

Comment top

N-Carboxy anhydrides (NCAs) of amino acids are crystalline compounds and are usually polymerized in solution to prepare polypeptides (Bamford et al., 1956). Purified amino acid NCA crystals are generally sensitive to moisture and are polymerized or decomposed by water. When butylamine is added to amino acid NCA crystals suspended in an inert solvent such as hexane or decane, polymerization takes place in the solid state. We have studied this solid-state polymerization and found that the polymerizability is quite different in each amino acid NCA.

The crystal structures of amino acid NCAs were not studied for many years after the very early initial report by Leuchs (1906). We have reported the crystal structures of glycine NCA (Kanazawa et al., 1976a) and L-alanine NCA (Kanazawa et al., 1976b), and found that the polymerizability depended on the crystal structure (Kanazawa & Kawai, 1980). In addition, the crystal structures of γ-benzyl-L-glutamate NCA (Kanazawa et al., 1978a), L-leucine NCA (Kanazawa et al., 1978b), L-valine NCA (Kanazawa et al., 1984), DL-Valine NCA (Takenaka et al., 1994), DL-phenylalanine NCA (Kanazawa et al., 1997), L-phenylalanine NCA (Kanazawa, 2000) and β-benzyl-L-aspartate NCA (Kanazawa & Magoshi, 2003) have also been determined.

We found that the polymerization of L-leucine NCA was the most reactive in the solid state among the amino acid NCAs examined, and the solution polymerization of L-alanine NCA in acetonitrile seemed to be more reactive than its solid state polymerization (Kanazawa et al., 1982; Kanazawa, 1992). However, we recently found that a partial polymerization initiated by moisture is not avoidable for highly purified amino acid NCA crystals under normal experimental conditions. Thus, a normal solution polymerization is affected by the partially polymerized amino acid NCAs dissolved in the solution, and the reactivity may be reported artificially high. In fact, many amino acid NCAs have been observed to be more reactive in the solid state than in the solution state when the experiments are carried out under cool conditions to avoid moisture.

As the title compound (DL-leucine NCA), (I), is a racemate, it polymerizes slowly in solution. However, we found that the solid state polymerization of compound (I) was much more reactive than its solution polymerization; polymer conversion was 100% in the solid state but only 10% in acetonitrile solution at 313 K under the same conditions (Kanazawa & Hayakawa, 2000). Therefore, it is important to determine its crystal structure. Here, we present the crystal and molecular structure of (I). \sch

The molecular structure of (I) and the atom-numbering scheme are shown in Fig. 1. In the crystal structure of (I), L– and D-enantiomers form dimeric pairs around a centre of symmetry. They are linked by N1—H1···O1i hydrogen bonds [N1···O1i 2.903 (2) Å, H1···O1i 2.08 Å, and N1—H1···O1i 173°; symmetry code: (i) 1 − x, 2 − y, 1 − z].

As seen in Fig. 2, the five-membered NCA rings in (I) are packed in a layer, with the isobutyl groups packed in another layer which alternates with the first. This sandwich structure is one of the important requirements for high reactivity in the solid state (Kanazawa, 1992, 1998), because the five-membered rings can react with each other within the layer. This structure seems to give a higher reactivity in the solid state than with the solution reaction. Although the hydrogen-bonded dimer structure is not formed in the crystal of DL-phenylalanine NCA (Kanazawa et al., 1997), the sandwich structure composed of D and L molecules is found and the crystal is also reactive in the solid state. Therefore, the DL dimer structure in the crystal of (I) is not considered to be an important requirement for high reactivity in the solid state.

Experimental top

Compound (I) was obtained by the reaction of DL-leucine with trichloromethyl chroloformate or triphosgen in tetrahydrofuran, as reported previously for other NCAs (Kanazawa, 1992). The reaction product was recrystallized in a mixture of ethyl acetate and hexane (Ratio?), avoiding contamination by moisture.

Refinement top

As compound (I) crystallizes as thin plates and is very unstable, many crystallization and data-collection attempts were carried out. As the crystal degraded rapidly, only a limited number of ocillation photographs were taken, which cover 120° rotation of the crystal. Thus, the completeness of the reflections up to 2θmax = 50° is relatively low (92%). H atoms were located from a difference Fourier map and their positional parameters were allowed to ride on the parent atoms, with N—H distances of 0.83 Å and C—H distances in the range 0.94–0.98 Å Is this added text OK?; their isotropic displacement parameters were freely refined.

Computing details top

Data collection: PROCESS (Rigaku, 1996); cell refinement: PROCESS; data reduction: PROCESS; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003).

Figures top
[Figure 1] Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids with the atom-numbering scheme.
[Figure 2] Fig. 2. Packing diagram for (I) viewed along the b axis. Hydrogen bonds are shown by dashed lines.
4-isobutyl-1,3-oxazolidine-2,5-dione top
Crystal data top
C7H11NO3F(000) = 336
Mr = 157.17Dx = 1.261 Mg m3
Monoclinic, P21/aMo Kα radiation, λ = 0.71070 Å
Hall symbol: -P 2yabCell parameters from 3 reflections
a = 9.8780 (9) Åθ = 2.7–25.0°
b = 5.640 (2) ŵ = 0.10 mm1
c = 15.448 (2) ÅT = 288 K
β = 105.844 (3)°Plate, colourless
V = 827.9 (3) Å30.55 × 0.40 × 0.15 mm
Z = 4
Data collection top
Rigaku R-AXIS IV
diffractometer
1347 independent reflections
Radiation source: fine-focus sealed tube1162 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.048
Detector resolution: 10 pixels mm-1θmax = 25.0°, θmin = 2.7°
ω scansh = 1111
Absorption correction: multi-scan
(ABSCOR; Higashi, 1995)
k = 66
Tmin = 0.938, Tmax = 0.99l = 1817
3732 measured reflections
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.048Only H-atom displacement parameters refined
wR(F2) = 0.159 w = 1/[σ2(Fo2) + (0.1091P)2]
where P = (Fo2 + 2Fc2)/3
S = 1.17(Δ/σ)max < 0.001
1347 reflectionsΔρmax = 0.15 e Å3
112 parametersΔρmin = 0.26 e Å3
0 restraintsExtinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.099 (17)
Crystal data top
C7H11NO3V = 827.9 (3) Å3
Mr = 157.17Z = 4
Monoclinic, P21/aMo Kα radiation
a = 9.8780 (9) ŵ = 0.10 mm1
b = 5.640 (2) ÅT = 288 K
c = 15.448 (2) Å0.55 × 0.40 × 0.15 mm
β = 105.844 (3)°
Data collection top
Rigaku R-AXIS IV
diffractometer
1347 independent reflections
Absorption correction: multi-scan
(ABSCOR; Higashi, 1995)
1162 reflections with I > 2σ(I)
Tmin = 0.938, Tmax = 0.99Rint = 0.048
3732 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0480 restraints
wR(F2) = 0.159Only H-atom displacement parameters refined
S = 1.17Δρmax = 0.15 e Å3
1347 reflectionsΔρmin = 0.26 e Å3
112 parameters
Special details top

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 (Å2) top
xyzUiso*/Ueq
O10.60790 (14)1.2638 (2)0.49473 (9)0.0683 (5)
O20.81780 (13)1.2334 (2)0.59985 (8)0.0592 (4)
O31.00525 (14)1.0851 (3)0.70271 (10)0.0743 (5)
N10.65918 (15)0.9542 (3)0.59640 (9)0.0552 (5)
H10.58400.88300.57400.064 (6)*
C10.68312 (19)1.1532 (3)0.55696 (11)0.0536 (5)
C20.88426 (18)1.0672 (3)0.66228 (11)0.0560 (5)
C30.77737 (17)0.8814 (3)0.66966 (10)0.0526 (5)
H20.80930.72660.65950.054 (4)*
C40.74324 (18)0.8905 (3)0.76091 (11)0.0566 (5)
H30.71811.05060.77030.054 (4)*
H40.66210.79250.75660.065 (5)*
C50.8605 (2)0.8073 (4)0.84246 (12)0.0655 (6)
H50.94660.89510.84420.076 (6)*
C60.8906 (3)0.5446 (5)0.83784 (18)0.0978 (9)
H90.96350.49480.88780.108 (8)*
H100.80740.45670.83470.115 (10)*
H110.91890.51140.78360.137 (11)*
C70.8178 (3)0.8673 (6)0.92822 (14)0.1005 (9)
H60.79661.03210.93110.145 (13)*
H70.88730.82360.98120.136 (11)*
H80.73180.78060.92790.108 (9)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0673 (9)0.0715 (9)0.0619 (8)0.0029 (6)0.0104 (6)0.0200 (7)
O20.0606 (8)0.0532 (8)0.0617 (8)0.0035 (5)0.0129 (6)0.0099 (5)
O30.0574 (9)0.0798 (10)0.0779 (10)0.0092 (6)0.0051 (7)0.0120 (7)
N10.0532 (8)0.0575 (9)0.0519 (8)0.0066 (6)0.0089 (6)0.0054 (7)
C10.0567 (10)0.0551 (10)0.0485 (10)0.0004 (7)0.0134 (8)0.0029 (7)
C20.0596 (11)0.0536 (10)0.0530 (10)0.0009 (7)0.0124 (8)0.0023 (8)
C30.0592 (10)0.0457 (9)0.0497 (10)0.0000 (7)0.0097 (8)0.0012 (7)
C40.0654 (11)0.0505 (10)0.0534 (10)0.0017 (7)0.0152 (8)0.0028 (7)
C50.0705 (12)0.0670 (12)0.0547 (11)0.0089 (9)0.0096 (9)0.0087 (8)
C60.111 (2)0.0754 (15)0.0915 (19)0.0146 (15)0.0005 (15)0.0211 (13)
C70.128 (2)0.118 (2)0.0532 (13)0.0048 (17)0.0210 (14)0.0073 (13)
Geometric parameters (Å, º) top
O1—C11.215 (2)C4—H30.96
O2—C21.376 (2)C4—H40.96
O2—C11.390 (2)C5—C61.517 (4)
O3—C21.192 (2)C5—C71.534 (3)
N1—C11.329 (2)C5—H50.98
N1—C31.446 (2)C6—H90.94
N1—H10.83C6—H100.95
C2—C31.514 (2)C6—H110.97
C3—C41.537 (2)C7—H60.96
C3—H20.96C7—H70.95
C4—C51.534 (2)C7—H80.98
C2—O2—C1108.88 (13)C3—C4—H4107.8
C1—N1—C3113.08 (14)H3—C4—H4107.9
C1—N1—H1117.9C6—C5—C4111.97 (17)
C3—N1—H1128.8C6—C5—C7111.11 (19)
O1—C1—N1130.53 (18)C4—C5—C7108.54 (18)
O1—C1—O2120.59 (16)C6—C5—H5108.4
N1—C1—O2108.87 (14)C4—C5—H5108.8
O3—C2—O2121.86 (16)C7—C5—H5107.9
O3—C2—C3129.73 (16)C5—C6—H9111.6
O2—C2—C3108.37 (14)C5—C6—H10109.5
N1—C3—C2100.09 (12)H9—C6—H10110.0
N1—C3—C4112.08 (14)C5—C6—H11109.6
C2—C3—C4112.20 (13)H9—C6—H11108.3
N1—C3—H2111.3H10—C6—H11107.8
C2—C3—H2110.6C5—C7—H6111.7
C4—C3—H2110.3C5—C7—H7112.6
C5—C4—C3115.96 (15)H6—C7—H7109.2
C5—C4—H3109.4C5—C7—H8109.2
C3—C4—H3107.5H6—C7—H8106.5
C5—C4—H4108.0H7—C7—H8107.3
C3—N1—C1—O1179.47 (17)O3—C2—C3—N1174.62 (19)
C3—N1—C1—O20.43 (19)O2—C2—C3—N17.75 (16)
C2—O2—C1—O1175.06 (14)O3—C2—C3—C466.4 (2)
C2—O2—C1—N15.79 (18)O2—C2—C3—C4111.27 (15)
C1—O2—C2—O3173.56 (17)N1—C3—C4—C5177.77 (14)
C1—O2—C2—C38.58 (17)C2—C3—C4—C570.54 (19)
C1—N1—C3—C24.43 (17)C3—C4—C5—C666.3 (2)
C1—N1—C3—C4114.67 (16)C3—C4—C5—C7170.64 (17)

Experimental details

Crystal data
Chemical formulaC7H11NO3
Mr157.17
Crystal system, space groupMonoclinic, P21/a
Temperature (K)288
a, b, c (Å)9.8780 (9), 5.640 (2), 15.448 (2)
β (°) 105.844 (3)
V3)827.9 (3)
Z4
Radiation typeMo Kα
µ (mm1)0.10
Crystal size (mm)0.55 × 0.40 × 0.15
Data collection
DiffractometerRigaku R-AXIS IV
diffractometer
Absorption correctionMulti-scan
(ABSCOR; Higashi, 1995)
Tmin, Tmax0.938, 0.99
No. of measured, independent and
observed [I > 2σ(I)] reflections
3732, 1347, 1162
Rint0.048
(sin θ/λ)max1)0.595
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.048, 0.159, 1.17
No. of reflections1347
No. of parameters112
H-atom treatmentOnly H-atom displacement parameters refined
Δρmax, Δρmin (e Å3)0.15, 0.26

Computer programs: PROCESS (Rigaku, 1996), PROCESS, SIR92 (Altomare et al., 1994), SHELXL97 (Sheldrick, 1997), PLATON (Spek, 2003).

Selected geometric parameters (Å, º) top
O1—C11.215 (2)C2—C31.514 (2)
O2—C21.376 (2)C3—C41.537 (2)
O2—C11.390 (2)C4—C51.534 (2)
O3—C21.192 (2)C5—C61.517 (4)
N1—C11.329 (2)C5—C71.534 (3)
N1—C31.446 (2)
C2—O2—C1108.88 (13)N1—C3—C2100.09 (12)
C1—N1—C3113.08 (14)N1—C3—C4112.08 (14)
O1—C1—N1130.53 (18)C2—C3—C4112.20 (13)
O1—C1—O2120.59 (16)C5—C4—C3115.96 (15)
N1—C1—O2108.87 (14)C6—C5—C4111.97 (17)
O3—C2—O2121.86 (16)C6—C5—C7111.11 (19)
O3—C2—C3129.73 (16)C4—C5—C7108.54 (18)
O2—C2—C3108.37 (14)
 

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