Hiroko Suezawa

CH/π hydrogen bonds in organic and organometallic chemistry

This treatise is an update to a preceding highlight (CH/π hydrogen bonds in crystals) published in this journal 5 years ago (M. Nishio, CrystEngComm, 2004, 6, 130–156). After the introductory part (sections 1 and 2), we survey recent results (mostly since 2004) relevant to the CH/π hydrogen bond: crystal conformation, packing and host/guest chemistry (section 3). Section 4 summarizes the results obtained by crystallographic database (CSD and PDB) analyses. In section 5, several topics in related fields (selectivity in organic reactions, surface chemistry, structural biology, drug design and high-level ab initio calculations of protein/substrate complexes and natural organic compounds) are introduced, and in the final part we comment on the prospects of this emerging field of chemistry.

CH/π Interactions Implicated in the Crystal Structure of Transition Metal Compounds − A Database Study

A statistical study was carried out to investigate the role of the CH/π interaction in the crystal structure of transition metal compounds. Thus, short CH/π distances were surveyed in crystal structures deposited in the Cambridge Structural Database. Among organometallic entries bearing C6 or C5 aromatic rings, a substantial part of the structures has been found to have intermolecular CH/π contacts shorter than the van der Waals distance. Further, in many structures short intramolecular CH/π contacts have been found. Interligand and intraligand CH/π interactions were also surveyed in coordination compounds bearing the typical ligands 1,10-phenanthroline, 2,2′-bipyridine, and 2,2′:6′,2′′-terpyridine, as well as triphenylphosphane complexes of ruthenium, rhenium, and rhodium. The results were discussed in the context of the CH/π interaction in controlling the crystal packing and the molecular structure of transition metal compounds. The compact structure with CH/π interactions is a general aspect in coordination and organometallic chemistry. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Intramolecular CH-? interaction. Substituent effect as a probe for hydrogen bond-like character

The contribution of a charge-transfer term to the CH–π interaction is supported by ab initio calculations and experiments on the substituent effect. NOE is an effective tool to determine the CH–π interacted folded conformers separately from the stretched conformer. Copyright

CH/π interactions as disclosed on the fullerene convex surface. A database study

A systematic search in the Cambridge Structural Database has disclosed that many CH groups are involved in the interaction with the fullerene convex surface; aliphatic as well as aromatic CHs are concerned. CH/π interactions in C60 fullerene and C60 fulleride inclusion compounds were surveyed and compared. The mean CH/C60 intermolecular distance has been shown to be shorter in the fulleride complexes than in the fullerene complexes. The face-to-face type interaction was also surveyed, programmatically. The present study has demonstrated that the CH/π interaction plays a considerable part in fullerene supramolecular chemistry, together with the van der Waals interaction. The concept of the CH/π interaction will be of help in designing fullerene- and nanotube-based materials and in exploring the electronic structure of nonclassical π-systems.

The origin of the generalized anomeric effect: possibility of CH/n and CH/pi hydrogen bonds

Ab initio MO calculations were carried out at the MP4/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level to investigate the conformational Gibbs energy of a series of methyl ethers CH(3)O-CH(2)-X (X=OH, OCH(3), F, Cl, Br, CN, C triple bond CH, C(6)H(5), CHO). It was found that the Gibbs energy of the gauche conformers is lower in every case than that of the corresponding anti conformers. In the more stable gauche conformers, the interatomic distance between X and the hydrogen atom was shorter than the sum of the van der Waals radii. The natural bonding orbital (NBO) charges of group X were more negative in the gauche conformers than in the anti conformers. We suggest that the CH/n and CH/pi hydrogen bonds play an important role in stabilizing the gauche conformation of these compounds.

Preferred conformations of 1-alkyl-2-arylethane and 1,2-diaryl-ethane derivatives in relation to the attractive interaction between hydrocarbon groups

Abstract The preferred conformations of a series of substituted 1-alkyl-2-arylethanes (ArCHR-CHXR′) and 1,2-diarylethanes (ArCHR-CHXAr′) were determined by NMR and other spectroscopic methods and compared with those estimated by molecular force field calculations. These substituted ethanes often take Ar/R′ gauche and Ar/Ar′ gauche conformations preferably in accord with the prediction from the force field calculations. Nature of the attractive interaction between the hydrocarbon moieties (R and Ar) is discussed briefly.

The molecular structure of acyclic aralkyl compounds studied by a crystallographic database survey. Relevance of the intramolecular CH/π hydrogen bond to conformation

A database study was carried out, by the use of the Cambridge Structural Database, to investigate the conformation of aralkyl compounds ArCH2XCH2Y 1 and ArCHCH3XCH 2 in crystals. The structure bearing R (R: any group) and Ar in the syn relationship has often been found in these compounds. The proportion of crystal structures bearing R and Ar in the syn relationship relative to the anti conformation (rsyn/anti) varied from 0.55 for 1 to 3.68 for 2. The logarithm of rsyn/anti was plotted against the difference in Gibbs energy ΔGsyn − anti obtained by MO calculations of model compounds at the MP2/6-311G(d,p)//MP2/6-31G(d) level: C6H5CH2XCH3 and C6H5CHCH3XCH3. A linear correlation has been shown between lnrsyn/anti and ΔGsyn − anti. The CH/π interaction is suggested to operate in controlling the R/Ar-folded crystal structure of these aralkyl compounds.

Electronic substituent effect on intramolecular CH/π interaction as evidenced by NOE experiments

In order to demonstrate the hydrogen-bond-like character of the CH/π interaction, electronic substituent effects on the equilibria between the stretched and the folded conformers of series of compounds capable of forming CH/π interactions were examined by measurements of NOE enhancements of 1H NMR signals. Nuclear Overhauser enhancement is shown to be useful to determine the abundance of the CH/π proximate folded conformer. The Hammett plots of all series of the compounds capable of having CH/π interaction gave negative ρ values. Together with other substituent effects (effects of electronegative substituents, on the CH donor, of ring size, and of α-alkyl substituent), the involvement of delocalization interaction and the hydrogen-bond-like character of the CH/π interaction were established.

Origin of the diastereofacial selectivity in the nucleophilic addition to chiral acyclic ketones. An ab initio MO study

Ab initio MO calculations were carried out, at the MP2/6-311G(d,p)//MP2/6-31G(d) level, to investigate the conformational Gibbs energy of alkyl 1-phenylethyl ketones, C6H5CHCH3COR, 1 (R = CH3, C2H5, i-C3H7, t-C4H9). Rotamers a and a′ whereby R is synclinal to C6H5 and the benzylic methyl group is nearly eclipsed to CO have been shown to be the most stable in every case. Rotamer a is stabilized by a 5-member CH/π hydrogen bond and is more abundant than a′, which is stabilized by a less effective 6-member CH/π bond. The diastereomeric ratio of the product secondary alcohols in the nucleophilic addition to 1 was estimated on the basis of the ground-state rotamer distribution. The Gibbs energy of the diastereomeric transition states was also calculated for a model reaction (C6H5CHCH3COR + LiH) at the same level of approximation. The transition-state geometries leading to the predominant product are similar to those of the ground-state conformation. In geometries leading to the minor product, the relevant torsion angles are twisted to avoid unfavourable steric interactions. The short CH/π and CH/O distances suggest that these weak hydrogen bonds are operating in stabilizing the transition structures. The above two methods gave results consistent with each other. The mechanism of 1,2-asymmetric induction can thus be explained on the basis of the simple premise that the geometry of the transition state resembles the ground-state conformation of the substrate and the nucleophilic reagent approaches from the less hindered side of the carbonyl π-face.

Rotational Barriers and 15N Chemical Shifts of N‐Acyl‐N‐alkyl‐substituted Amino Acids

Rotational barriers about the C—N bonds of several N‐acyl‐N‐methyl‐α‐amino acids and their esters R3CO—N(R4)—CR1R2—COOR5, and also N‐Boc‐protected dipeptides, were determined and the steric effects caused by the substituents R1–R5 are discussed by comparing them with the results of MM3 calculations on these amides. Bulky substituents on both the acyl group and the nitrogen atom were shown to have lower ΔG‡. In the series of N‐acylglycines with variable R3, the 15N chemical shifts were correlated with ΔG‡.