The Curious Case of 2-Propyl-1<i>H</i>-benzimidazole in the Solid State: An Experimental and Theoretical Study

Spontaneous deracemizations is a challenging, multidisciplinary subject in current chirality research. In the absence of any chiral inductors, an achiral substance or a racemic composition is driven into an enantioenriched or even homochiral state through a selective energy input, e.g., chemical potential, photoirradiation, mechanical grinding, ultrasound waves, thermal gradients, etc. The most prominent examples of such transformations are the Soai reaction and the Viedma deracemization. In this review, we track the most recent developments in this topic and recall that many other deracemizations have been reported for solutions from mesophases to conglomerate crystallizations. A compiled set of simply available achiral organic, inorganic, organometallic, and MOF compounds, yielding conglomerate crystals, should give the impetus to realize new experiments on spontaneous deracemizations. Taking into account thermodynamic constraints, modeling efforts have shown that structural features alone are not sufficient to describe spontaneous deracemizations. As a guideline of this review, particular attention is paid to the physicochemical origin and symmetry requirements of such processes.

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Section: Organic Conglomeratesmentioning

confidence: 99%

Section: Organic Conglomeratesmentioning

confidence: 99%

Spontaneous Deracemizations

et al. 2021

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“…Second, it can form from non‐permanent chiral subunits that synchronize their chirality . This is observed for 2‐propyl‐1‐H‐benzimidazole in the solid state . The molecular subunit is transiently chiral, although free rotation around a single bond allows fast interconversion between the enantiomers in solution.…”

Section: Resultsmentioning

confidence: 99%

Solid‐state synthesis of cyclo LD‐diphenylalanine: A chiral phase built from achiral subunits

2020

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The solid-state structure of LL/DD or LD/DL diphenylalanine diluted in KBr pellets is studied by infrared (IR) absorption and vibrational circular dichroism (VCD) spectroscopy. The structure depends on the absolute configuration of the residues. The natural LL diphenylalanine exists as a mixture of neutral and zwitterionic structures, depending on the humidity of the sample, while mostly the zwitterion is observed for LD diphenylalanine whatever the experimental conditions. The system undergoes spontaneous cyclization upon heating at 125 C, resulting to the formation of a diketopiperazine (DKP) dipeptide as the only product. The reaction is faster for LD than for LL diphenylalanine. As expected, LL and DD diphenylalanine react to form the LL and DD enantiomers of cyclo diphenylalanine. Interestingly, the DKP dipeptides formed from the LD or DL diphenylalanine show unexpected optical activity, with opposite VCD spectra for the products formed from the LD and DL reagents. This is explained in terms of chirality synchronization between the monomers within the crystal, which retain the symmetry of the reagent, resulting to the formation of a new chiral phase made from transiently chiral molecules. K E Y W O R D Schirality synchronization, diketopiperazine (DKP), peptides, vibrational circular dichroism (VCD), vibrational spectroscopy

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“…The unit can be thought of as a monomer, like in solution . In other cases, one has to take into account the crystal unit cell itself or a cluster of definite size like a dimer or a tetramer . Still, the intensity enhancement observed in the solid state has to be fully understood …”

Section: Introductionmentioning

confidence: 99%

“…31,32 In other cases, one has to take into account the crystal unit cell itself or a cluster of definite size like a dimer or a tetramer. 24,33 Still, the intensity enhancement observed in the solid state has to be fully understood. 28,34,35 We present here a comparative Fourier-transform infrared spectroscopy (FTIR) and VCD study of the solid-state cis-and trans-ACBC derivatives, Boc−(cis-ACBC)−NH−Bn and Boc−(trans-ACBC)−NH−Bn, shown in Figure 1.…”

mentioning

confidence: 99%

Vibrational circular dichroism as a probe of solid‐state organisation of derivatives of cyclic β‐amino acids: Cis‐ and trans‐2‐aminocyclobutane‐1‐carboxylic acid

et al. 2019

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Peptide models built from cis‐ and trans‐2‐aminocyclobutane‐1‐carboxylic acids (ACBCs) are studied in the solid phase by combining Fourier‐transform infrared spectroscopy (FTIR) absorption spectroscopy, vibrational circular dichroism (VCD), and quantum chemical calculations using density functional theory (DFT). The studied systems are N‐tert‐butyloxycarbonyl (Boc) derivatives of 2‐aminocyclobutanecarboxylic acid (ACBC) benzylamides, namely Boc−(cis‐ACBC)−NH−Bn and Boc−(trans‐ACBC)−NH−Bn. These two diastereomers show very different VCD signatures and intensities, which of the trans‐ACBC derivative being one order of magnitude larger in the region of the ν (CO) stretch. The spectral signature of the cis‐ACBC derivative is satisfactorily reproduced by that of the monomer extracted from the solid‐state geometry of related ACBC derivatives, which shows that no long‐range effects are implicated for this system. In terms of hydrogen bonds, the geometry of this monomer is intermediate between the C6 and C8 structures (exhibiting a 6‐ or 8‐membered cyclic NH⋯O hydrogen bond) previously evidenced in the gas phase. The benzyl group must be in an extended geometry to reproduce satisfactorily the shape of the VCD spectrum in the ν (CO) range, which qualifies VCD as a potential probe of dispersion interaction. In contrast, reproducing the IR and VCD spectrum of the trans‐ACBC derivative requires clusters larger than four units, exhibiting strong intermolecular H‐bonding patterns. A qualitative agreement is obtained for a tetramer, although the intensity enhancement is not reproduced. These results underline the sensitivity of VCD to the long‐range organisation in the crystal.

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The Curious Case of 2-Propyl-1H-benzimidazole in the Solid State: An Experimental and Theoretical Study

Cited by 16 publications

References 41 publications

Spontaneous Deracemizations

Spontaneous Deracemizations

Solid‐state synthesis of cyclo LD‐diphenylalanine: A chiral phase built from achiral subunits

Vibrational circular dichroism as a probe of solid‐state organisation of derivatives of cyclic β‐amino acids: Cis‐ and trans‐2‐aminocyclobutane‐1‐carboxylic acid

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