Search

A comprehensive mechanistic study on the title reaction by using DFT(B3LYP-D3) computational method is reported. Explicit consideration of mono- (m-xylylic) and dicarboxylic acid (phthalic) in the key transition states reveals active participation of the carboxylic acid, beginning with the generation of a monomeric Rh(I) active catalyst and in the ensuing catalytic steps. In the early catalytic event, uptake of alkyne is predicted to take place only after the oxidative addition of the Rh(I) active catalyst to the carboxylic acid. The hydrometalation of the alkyne bound to the Rh(III)-H intermediate then generates a Rh(III)-vinyl intermediate, which in turn converts to a Rh(III)-allyl species. The inclusion of m-xylylic acid results in a two-step pathway to Rh(III)-allyl species via Rh-allene intermediate. A number of weak noncovalent interactions (hydrogen bonding and C-H center dot center dot center dot pi) between the catalyst and the substrates and that involving m-xylylic acid are found to have a direct impact on the regiochemical preference toward the branched product and the enantiocontrolling hydroamination step involving C-N bond formation leading to the major enantiomer (S-allylic amine). The chiral induction is enabled by cumulative effect of noncovalent interactions, which is an insight that could aid future developments of chiral ligands for asymmetric hydroamination.

The origin of enantioselectivity in asymmetric catalysis is attributed to the energy difference between lower and higher energy diastereomeric transition states, which are respectively responsible for the formation of major and minor enantiomers. Although the increase in the number of transition state models emphasizes the role of weak non-covalent interactions in asymmetric induction, the strength of such interactions is seldom quantified. Through this article, we propose a simple and effective method of quantifying the total non-covalent interaction in stereocontrolling transition states belonging to a group of three representative asymmetric catalytic reactions involving chiral phosphoric acids. Our method relies on rational partitioning of a given transition state into two (or three) sub-units, such that the complex network of intramolecular interactions can be ameliorated to a set of intermolecular interactions between two sub-units. The computed strength of interaction obtained using the counterpoise (CP) method on suitably partitioned transition states provides improved estimates of non-covalent interactions, which are also devoid of basis set superposition error (BSSE). It has been noted that catalysts decorated with larger aromatic arms provide cumulative non-covalent interactions (C-H center dot center dot center dot pi, N-H center dot center dot center dot pi and pi center dot center dot center dot pi) to the tune of 10 to 15 kcal mol(-1). Fine-tuning of the magnitude and nature of these interactions can provide valuable avenues in the design of asymmetric catalysts.

In this paper we report the synthesis of 4,4'-bis(benzylideneamino)diphenyl ether (1), 4,4'-bis(p-methoxybenzylideneamino)diphenyl ether (2), 4,4'-bis(p-nitrobenzylideneamino)diphenyl ether (3), 4,4'-bis(benzylamino)diphenyl ether (4) and 4,4'-bis(p-methoxybenzylamino)diphenyl ether (5). The new compounds were characterized by using standard spectroscopic techniques. Supramolecular structures of 1-4 have been studied by using single-crystal X-ray diffraction to gauge the influence of substituents, present on para-positions of peripheral phenyl rings, on the association of molecules in the solid state. The study showed that introduction of p-OCH3 and p-NO2 substituents on peripheral phenyl rings, successfully switches off CH center dot center dot center dot pi synthons seen in compound 1 and 4 and that the new compounds adopt other packing strategies based on CH center dot center dot center dot O (2) and CH center dot center dot center dot N, CH center dot center dot center dot O and pi center dot center dot center dot pi (3) interactions. The optical band gaps (Eg), calculated by optical absorption method fall in the range of 2.16-3.0 eV and demonstrate a wide band gap semiconducting nature of these compounds. The thermogravimetric study of these compounds suggests the total destruction of the molecules up to 550 degrees C with a remaining mass of

The role of noncovalent interactions in carbohydrate recognition by aromatic amino acids has long been reported. To develop a molecular understanding of noncovalent interactions in the recognition process, we have examined a series of binary complexes between 3-methylindole (3-MeIn) and sugars. In particular, the geometries and binding affinities of 3-MeIn with alpha/beta-D-glucose, beta-D-galactose, alpha-D-mannose and alpha/beta-L-fucose are obtained using the MP2(full)/6-31G(d,p) and the M06/TZV2D//MP2/6-31G(d,p) level of theories. The conventional hydrogen bonding such as N-H center dot center dot center dot O and C-H center dot center dot center dot O as well as nonconventional O-H center dot center dot center dot pi and C-H center dot center dot center dot pi type of interactions is, in general, identified as responsible for the moderately strong interaction energies. Large variations in the position orientations of 3-MeIn with respect to saccharide are noticed, within the same sugar family, as well as across different sugar series. Furthermore, complexes with large differences in their geometries are recognized as capable of exhibiting very similar interaction energies, underscoring the significance of exhaustive conformation sampling, as carried out in the present study. These observations are readily attributed to the differences in the efficiency of the type of interactions enlisted above. The highest and lowest interaction energies, upon inclusion of 50% BSSE correction, are found to be -16.02 and -6.22 kcal mol(-1), respectively, for alpha-D-glucose (1a) and alpha-L-fucose (5j). While more number of prominent conventional hydrogen bonding contacts remains as a characteristic feature of the strongly bound complexes, the lower end of the interaction energy spectrum is dominated by multiple C-H center dot center dot center dot pi interactions. The complexes exhibiting as many as four C-H center dot center dot center dot pi contacts are identified in the case of alpha/beta-D-glucose, beta-D-galactose, and alpha/beta-L-fucose with an interaction energy hovering around 8 kcal mol(-1). The presence of effective C-H center dot center dot center dot pi interactions is found to be dependent on the saccharide configuration as well as the area of the apolar patch constituted by the C-H groups. The study offers a comprehensive set of binary complexes, across different saccharides, which serves as an illustration of the significance and ubiquitous nature of C-H center dot center dot center dot pi interactions in carbohydrate binding in saccharide-protein complexes.

Submitted by Fátima Piedade (fpiedade@sa.isel.pt) on 2013-01-31T17:58:18Z No. of bitstreams: 1 Evaluation of intermolecular interactions in thioxanthone derivatives substituent effect on crystal diversity.rep.pdf: 204126 bytes, checksum: 000f7f3c3235e6a32977e4b322a3424f (MD5) Made available in DSpace on 2013-01-31T17:58:18Z (GMT). No. of bitstreams: 1 Evaluation of intermolecular interactions in thioxanthone derivatives substituent effect on crystal diversity.rep.pdf: 204126 bytes, checksum: 000f7f3c3235e6a32977e4b322a3424f (MD5) Previous issue date: 2011

The structure of the binary complex between phenylacetylene and borane-trimethylamine has been elucidated using IR-UV double resonance spectroscopy in combination with high level ab initio calculations at the CCSD(T) level. Borane-trimethylamine interacts primarily through multiple C-H center dot center dot center dot pi interactions with the p electron density of the benzene ring in phenylacetylene. CCSD(T) level calculations provide reliable estimates for the interaction energy and free energy, which are in accord with the experimental observations. The DFT-SAPT calculations point out that the dispersion interaction plays a major role in the formation of the experimentally observed complex, along with a sizable contribution from electrostatics.

The calculations have been done for CH/pi interaction with pi-system of Ni(II)-acetylacetonato chelate ring. The results show that there is an attractive electrostatic interaction, while dispersion component is a major source of attractive interacting energies. The interaction was compared with CH/pi, interaction between two benzene rings. The comparison shows that two interactions are quite similar, enabling to estimate the energy of CH/pi interaction with pi-system of Ni(II)-acetylacetonato chelate ring to be about 10.5 kJ/mol. The results indicate that CH/pi interactions of chelate ring in various molecular systems can be as important as CH/pi interactions of phenyl ring. (c) 2006 Elsevier B.V. All rights reserved.

The structures of the binary complexes between phenylacetylene and several tertiary amines viz., triethylamine, 1-ethylpiperidine, 1-ethylpiperazine, 1-azabicyclo[2.2.2]octane, and 1,4-diazabicyclo[2.2.2]octane were inferred using infrared-optical double resonance spectroscopy. The IR spectra in the acetylenic C-H stretching region clearly rule out the formation of electrostatic dominated C-H center dot center dot center dot N hydrogen bonded complexes. The IR spectra also point to the fact that all the five tertiary amines interact with the extended pi electron density of the phenylacetylene moiety, leading to the formation of multidentate C-H center dot center dot center dot pi hydrogen bonded complexes. Additionally a very weak electrostatic C-H center dot center dot center dot N hydrogen bond enhances the stability of the complex marginally. The multidentate C-H center dot center dot center dot pi hydrogen bonded complexes are stabilized by a substantial contribution from the dispersion energy.