24 research outputs found
Synthesis and evaluation of a desymmetrised synthetic lectin:An approach to carbohydrate receptors with improved versatility
A new design for carbohydrate receptors features unmatched apolar surfaces, and could lead to selectivities for a broader range of substrates.</p
Enantioselective carbohydrate recognition by synthetic lectins in water
These chiral āsynthetic lectinsā are the first to discriminate between carbohydrate enantiomers, and also show unprecedented affinities for monosaccharide substrates.</p
The bright future of unconventional s/p-hole interactions
[eng] Non-covalent interactions play a crucial role in (supramolecular) chemistry and much of biology. Supramolecular forces can indeed determine the structure and function of a host- guest system. Many sensors, for example, rely on reversible bonding with the analyte. Natural machineries also often have a significant non-covalent component (e.g. protein folding, recognition) and rational interference in such 'living' devices can have pharmacological implications. For the rational design/ tweaking of supramolecular systems it is helpful to know what supramolecular synthons are available and to understand the forces that make these synthons stick to one another. In this review we focus on s-hole and p-hole interactions. A s- or phole can be seen as positive electrostatic potential on unpopulated s* or p(*) orbitals, which are thus capable of interacting with some electron dense region. A s-hole is typically located along the vector of a covalent bond such as X H or X Hlg (X=any atom, Hlg=halogen), which are respectively known as hydrogen and halogen bond donors. Only recently it has become clear that s-holes can also be found along a covalent bond with chalcogen (X Ch), pnictogen (X Pn) and tetrel (X Tr) atoms. Interactions with these synthons are named chalcogen, pnigtogen and tetrel interactions. A p-hole is typically located perpendicular to the molecular framework of diatomic p-systems such as carbonyls, or conjugated p-systems such as hexafluorobenzene. Anion-p and lone-pair-p interactions are examples of named p-hole interactions between conjugated p-systems and anions or lone-pair electrons respectively. While the above nomenclature indicates the distinct chemical identity of the supramolecular synthon acting as Lewis acid, it is worth stressing that the underlying physics is very similar. This implies that interactions that are now not so well-established might turn out to be equally useful as conventional hydrogen and halogen bonds. In summary, we describe the physical nature of s- and p-hole interactions, present a selection of inquiries that utilise s- and p-holes, and give an overview of analyses of structural databases (CSD/PDB) that demonstrate how prevalent these interactions already are in solid-state structure
Influence of ring size on the strength of carbon bonding complexes between anions and perfluorocycloalkanes
[eng] In this paper we combine high level theoretical calculations (RI-MP2/def2-TZVP) with Cambridge Structural Database (CSD) analysis to demonstrate the importance of carbon bonding in cyclobutane rings. The higher ability of four-membered rings to interact with electron rich molecules is rationalized using several computational tools, including molecular electrostatic potential surfaces, energetic and geometric features of the complexes and ''atoms in molecules'' analysis. We have found that the solid state architectures of several X-ray structures, retrieved from the CSD searches, strongly support the theoretical calculations. In particular, carbon bonding interactions are quite common in nitro-substituted cubanes
Non-covalent sp3 carbon bonding with ArCF3 is analogous to CH-pi interactions.
[eng] A combined CSD and ab initio study reveals that the interaction between the sp3 C-atom in para-substituted ArCF3 and electron rich atoms is weak (o5 kcal mol 1), somewhat directional, and thus comparable to CH-p interactions
Small cycloalkane 1,1',2,2'-tetracyano structures are highly directional non-covalent carbon-bond donors
[eng] High-level calculations (RI-MP2/def2-TZVP) disclosed that the s-hole in between two C atoms of cycloalkane X2C CX2 structures (X=F, CN) is increasingly exposed with decreasing ring size. The interacting energy of complexes of F , HO , N C , and H2CO with cyclopropane and cyclobutane X2C CX2 derivatives was calculated. For X=F, these energies are small to positive, while for X=CN they are all negative, ranging from 6.8 to 42.3 kcal mol 1. These finding are corroborated by a thorough statistical survey of the Cambridge Structural Database (CSD). No clear evidence could be found in support of non-covalent carbon bonding between electron-rich atoms (El.R.) and F2C CF2 structures. In marked contrast, El.R.Ā·Ā·Ā·(CN)2C C(CN)2 interactions are abundant and highly directional. Based on these findings, the hydrophobic electrophilic bowl formed by 1,1',2,2'-tetracyano cyclopropane or cyclobutane derivatives is proposed as a new and synthetically accessible supramolecular synthon
Mechanistic Study of the L<sub>2</sub>Pd-Catalyzed Reduction of Nitrobenzene with CO in Methanol: Comparative Study between Diphosphane and 1,10-Phenanthroline Complexes
The catalytic activity of Pd<sup>II</sup> compounds supported
by
1,10-phenanthroline (phen) or the bidentate diarylphosphane ligand
L4X has been studied in the reaction of nitrobenzene with CO in methanol.
Both systems are ā¼70% selective for azoxybenzene and azobenzene
but also produce carbonylation products (methyl phenyl carbamate (MPC)
and <i>N,Nā</i>-diphenylurea (DPU)) and hydrogenation
products (aniline and DPU). The Pd<sup>II</sup>(L4X) system also produces
methanol oxidation products (dimethyl carbonate, dimethyl oxalate,
water). Upon the addition of a catalytic amount of acid, the coupling
reaction is suppressed in favor of either the carbonylation reaction
(for Pd<sup>II</sup>(phen)) or of both the carbonylation and hydrogenation
reaction (for Pd<sup>II</sup>(L4X)). The palladacycle L<sub>2</sub>PdCĀ(O)ĀNĀ(Ph)ĀOCĀ(O) (<b>C7</b>) and palladiumāimido species
L<sub>2</sub>Pdī»NPh (<b>C3</b>) were considered as possible
carbonylation product-releasing species, where L<sub>2</sub> is phen
or the diphosphane ligands L4X and L3X. A ligand exchange experiment
of phen-<b>C7</b> with L4X and L3X, ESI-MS analysis of L3X-<b>C7</b> and phen-<b>C7</b>, and a DFT study of nitrobenzene
deoxygenation intermediates to L<sub>2</sub>Pdī»NPh all suggest
that <b>C7</b> is not the major product-releasing intermediate;
all data suggest that the barrier for <b>C7</b> decarbonylation
(āCO) is lower than that for decarboxylation (āCO<sub>2</sub>). <b>C7</b> is thus thought to be part of an āNPh
reservoirā consisting of palladacycles that are mutually accessible
through carbonylation/decarbonylation. Under <i>acidic conditions</i> the decarboxylation barrier is lowered; for phen-<b>C7</b> apparently to the point where decarboxylative alcoholysis is favored
relative to decarbonylation, but for L4X-<b>C7</b> the decarbonylation
barrier still seems lowest due to the destabilizing effect that this
bulkier ligand has on such palladacycles. It is thus concluded that
the L<sub>2</sub>Pdī»NPh complex <b>C3</b> is the prime
āNPhā product-releasing intermediate and only under
acidic conditions and in an alcoholic environment may <b>C7</b>īøfor phenanthrolineīøbecome the predominant carbamate
product releasing intermediate
ĻāHole Interactions Involving Nitro Compounds: Directionality of Nitrate Esters
The
MEPs of a variety of nitro compounds (RāNO<sub>2</sub>) suggest
the existence of a Ļ-hole with a potential of up
to +54 kcal/mol in <b>10</b> (R = CF<sub>3</sub>). Several of
these nitro compounds were considered as partners for anions (F<sup>ā</sup>, Cl<sup>ā</sup>, NC<sup>ā</sup>) and
the electron rich molecules acetonitrile and dimethyl ether. In most
cases a Ļ-hole complex was obtained with calculated binding
energies of up to 20 kcal/mol with anions and 5 kcal/mol with the
neural molecules. A thorough analysis of the CSD revealed that nitrate
esters are highly directional Ļ-holes in the solid state, for
at least sp<sup>2</sup> O atoms. This was further illustrated by highlighting
several crystal structures where more than 0.2 Ć
van der Waals
overlap was observed between the N atom of the nitrate ester and an
electron rich atom like oxygen