Urea-Based [2]Rotaxanes as Effective Phase-Transfer Organocatalysts: Hydrogen-Bonding Cooperative Activation Enabled by the Mechanical Bond.

We finely designed a set of [2]rotaxanes with urea threads and tested them as hydrogen-bonding phase-transfer catalysts in two different nucleophilic substitutions requiring the activation of the reactant fluoride anion. The [2]rotaxane bearing a fluorinated macrocycle and a fluorine-containing urea thread displayed significantly enhanced catalytic activity in comparison with the combination of both noninterlocked components. This fact highlights the notably beneficial role of the mechanical bond, cooperatively activating the processes through hydrogen-bonding interactions.

Mechanically Planar-to-Point Chirality Transmission in [2]Rotaxanes.

Herein we describe an effective transmission of chirality, from mechanically planar chirality to point chirality, in hydrogen-bonded [2]rotaxanes. A highly selective mono-N-methylation of one (out of four) amide N atom at the macrocyclic counterpart of starting achiral rotaxanes generates mechanically planar chirality. Followed by chiral resolution, both enantiomers were subjected to a base-promoted intramolecular cyclization, where their interlocked threads were transformed into new lactam moieties. As a matter of fact, the mechanically planar chiral information was effectively transferred to the resulting stereocenters (covalent chirality) of the newly formed heterocycles. Upon removing the entwined macrocycle, the final lactams were obtained with high enantiopurity.

Intramolecular Cyclization of Azido-Isocyanides Triggered by the Azide Anion: An Experimental and Computational Study.

This work describes the unprecedented intramolecular cyclization occurring in a set of α-azido-ω-isocyanides in the presence of catalytic amounts of sodium azide. These species yield the tricyclic cyanamides [1,2,3]triazolo[1,5-a]quinoxaline-5(4H)-carbonitriles, whereas in the presence of an excess of the same reagent, the azido-isocyanides convert into the respective C-substituted tetrazoles through a [3 + 2] cycloaddition between the cyano group of the intermediate cyanamides and the azide anion. The formation of tricyclic cyanamides has been examined by experimental and computational means. The computational study discloses the intermediacy of a long-lived N-cyanoamide anion, detected by NMR monitoring of the experiments, subsequently converting into the final cyanamide in the rate-determining step. The chemical behavior of these azido-isocyanides endowed with an aryl-triazolyl linker has been compared with that of a structurally identical azido-cyanide isomer, experiencing a conventional intramolecular [3 + 2] cycloaddition between its azido and cyanide functionalities. The synthetic procedures described herein constitute metal-free approaches to novel complex heterocyclic systems, such as [1,2,3]triazolo[1,5-a]quinoxalines and 9H-benzo[f]tetrazolo[1,5-d][1,2,3]triazolo[1,5-a][1,4]diazepines.

Violations to the principle of least motion: the shortest path is not always the fastest.

The reaction between two molecules is usually envisioned as following a least-motion path with both molecules travelling minimum distances to meet each other. However, the reaction path of lowest activation energy is not only determined by practicality but mainly by the orbital symmetry of the involved reactants and the efficiency of their mutual interaction. The term non-least-motion was born to design those reactions in which reactants follow, in their route to products, pathways longer than those intuitively expected. In this review we summarize the theoretical and experimental studies that describe and rationalize reactions following non-least-motion paths, starting with the dimerizations of carbenes and followed by additional processes of these and other reactive species (silylenes, carbynes) such as insertions into single bonds and additions to π-bonds. Other examples involving less reactive partners are also included.

Ring-to-Thread Chirality Transfer in [2]Rotaxanes for the Synthesis of Enantioenriched Lactams.

The synthesis of chiral mechanically interlocked molecules has attracted a lot of attention in the last few years, with applications in different fields, such as asymmetric catalysis or sensing. Herein we describe the synthesis of orientational mechanostereoisomers, which include a benzylic amide macrocycle with a stereogenic center, and nonsymmetric N-(arylmethyl)fumaramides as the axis. The base-promoted cyclization of the initial fumaramide thread allows enantioenriched value-added compounds, such as lactams of different ring sizes and amino acids, to be obtained. The chiral information is effectively transmitted across the mechanical bond from the encircling ring to the interlocked lactam. High levels of enantioselectivity and full control of the regioselectivity of the final cyclic compounds are attained.

Size-Driven Inversion of Selectivity in Esterification Reactions: Secondary Beat Primary Alcohols.

Relative rates for the Lewis base-mediated acylation of secondary and primary alcohols carrying large aromatic side chains with anhydrides differing in size and electronic structure have been measured. While primary alcohols react faster than secondary ones in transformations with monosubstituted benzoic anhydride derivatives, relative reactivities are inverted in reactions with sterically biased 1-naphthyl anhydrides. Further analysis of reaction rates shows that increasing substrate size leads to an actual acceleration of the acylation process, the effect being larger for secondary as compared to primary alcohols. Computational results indicate that acylation rates are guided by noncovalent interactions (NCIs) between the catalyst ring system and the DED substituents in the alcohol and anhydride reactants. Thereby stronger NCIs are formed for secondary alcohols than for primary alcohols.

Unraveling the computed non-least motion pathway for the homodimerization of superchameleonic isocyanides: the peculiar nonsymmetrical (F-NC)2 reactant complex.

Isocyanides are commonly qualified as chameleonic compounds because of their reactions with both nucleophiles and electrophiles. In some instances, their chameleonic behavior changes to superchameleonic when they are involved in homodimerization processes, with the two initially identical isocyanide units adopting different roles along the reaction coordinate. We present here a detailed analysis of the computed non-least motion pathway that two isocyanides, the superchameleonic F-NC and, for the sake of comparison, the standard Me-NC, follow when reacting with themselves by comparing the evolution of a series of representative geometrical and electronic parameters along the respective reaction coordinates. This study shows that the two F-NC units are notoriously distinguishable from each other in all the parameters under scrutiny. Furthermore, we envisage that the superchameleonic character of F-NC seems to be most likely due to a minimal electrostatic interaction between the two entities at the earliest stage of the reaction. We also show that MeO-NC, MeS-NC and Me3P[double bond, length as m-dash]N-NC might be postulated as new examples of superchameleonic isocyanides.

Unmasking the elusive 1,4-diazabutatrienes: the stabilizing role of the N-substituents.

The geometrical and electronic properties of a representative set of diversely-substituted 1,4-diazabutatrienes are analyzed by theoretical and statistical methods. The influence of the substituents on the stabilization of these exotic azacumulenes has been estimated through a homodesmotic reaction and compared with related heterocumulenes. The 1,4-diazabutatrienes are stabilized by π-donor or σ-acceptor substituents and, in some cases, by the combination of one donor with one acceptor substituents at both N termini, a fact that might be associated with the ideally linear geometry of the heterocumulenic core for keeping the optimal orbital overlapping between its atoms.

A structural analysis of 2,5-diaryl-4H-2,4-dihydro-3H-1,2,4-triazol-3-ones: NMR in the solid state, X-ray crystallography, and GIPAW calculations.

The 1 H, 13 C, 15 N, and 19 F nuclear magnetic resonance (NMR) spectra of 11 2,5-diaryl-2,4-dihydro-3H-1,2,4-triazol-3-ones have been acquired in DMSO-d6 solution and the 13 C, 15 N, and 19 F NMR spectra have also been acquired in the solid state (solid-state nuclear magnetic resonance [SSNMR] and magic angle spinning [MAS]). The X-ray structures of Compounds 3, 5, and 6 have been determined by X-ray diffraction. Theoretical calculations at the gauge-independent atomic orbital (GIAO)/B3LYP/6-311++G(d,p) level have provided a set of 321 chemical shifts that were compared with 310 experimental values in DMSO-d6 . To obtain good agreements, some effects need to be included. The SSNMR chemical shifts have been compared with gauge-including projector-augmented wave (GIPAW) calculations and with the heavy atom-light atom (HALA) effects.

Effective Encapsulation of C60 by Metal-Organic Frameworks with Polyamide Macrocyclic Linkers.

A flexible benzylic amide macrocycle, functionalized with two carboxylic acid groups, was employed as the organic ligand for the preparation of robust copper(II)- and zinc(II)-based metal-organic frameworks. These polymers crystallized in the C2/m space group of the monoclinic crystal system, creating non-interpenetrated channels in one direction with an extraordinary solvent-accessible volume of 46 %. Unlike metal-organic rotaxane frameworks having benzylic amide macrocycles as linkers, the absence of the thread in these novel reticular materials causes a decrease of dimensionality and an improvement of pore size and dynamic guest adaptability. We studied the incorporation of fullerene C60 inside the adjustable pocket generated between two macrocycles connected to the same dinuclear clusters, occupying a remarkable 98 % of the cavities inside the network. The use of these materials as hosts for the selective recognition of different fullerenes was evaluated, mainly encapsulating the smaller size fullerene derivative in several mixtures of C60 and C70 .