Surfing π Clouds for Noncovalent Interactions: Arenes versus Alkenes

Abstract

A comparative study of molecular balances by NMR spectroscopy indicates that noncovalent functional‐group interactions with an arene dominate over those with an alkene, and that a π‐facial intramolecular hydrogen bond from a hydroxy group to an arene is favored by approximately 1.2 kJ mol−1. The strongest interaction observed in this study was with the cyano group. Analysis of the series of groups CH2CH3, CHCH2, CCH, and CN shows a correlation between conformational free‐energy differences and the calculated charge on the Cα atom of these substituents, which is indicative of the electrostatic nature of their π interactions. Changes in the free‐energy differences of conformers show a linear dependence on the solvent hydrogen bond acceptor parameter β.

Some Recent Advances in the Design and Use of Molecular Balances for the Experimental Quantification of Intramolecular Noncovalent Interactions of π Systems

“…Although some of these assumptions are justifiable, nonetheless some verification must be provided. For example, a replacement of an aromatic ring with a double bond in a control balance may not lead to a removal of intramolecular substituent⋅⋅⋅π interactions, as it was assumed by Shimizu et al., since the interactions of the π(aromatic) fragment in the main molecular balance are replaced by those of the π(olefinic) fragment, as shown by us previously …”

“…As only a few groups have studied alkene π‐interactions, we have used the inherent symmetry of the bridged bicyclic system to design a modified balance for comparison of the interactions of alkenes and arenes . The “alkene versus arene” measurements were carried out using the basic framework of the balances shown in Figure , which can be used to form double mutant cycles.…”

Noncovalent Lone Pair⋅⋅⋅(No‐π!)‐Heteroarene Interactions: The Janus‐Faced Hydroxy Group

“…As shown in Figure 1, we have previously introduced13 the dibenzobicyclo[3.2.2]nonane framework 1 as the pivotal element of a top‐pan molecular balance for quantifiable comparison of functional group–arene interactions in a range of solvents through variation of the two substituents (Y and Z) and determination of the conformational population [up (U) or down (D)] of the more electronegative substituent (Z) by NMR spectroscopy. We have used similar models to compare arene versus alkene interactions 13c. Accordingly, the present study required construction of the differentiated systems 2 and 3 (Figure 1).…”

“…This result is in startling contrast to the strong solvent dependence of p D observed for 1 , 2 b , and 3 b (Table 1), the hydroxy groups of which can form strong but instantaneous hydrogen bonds with the more polar solvents CD 3 CN, CD 3 OD, [D 5 ]pyridine, and [D 6 ]DMSO, thus leading to decreased p D values (for detailed discussion of solvent effects for OH‐to‐arene conformers, including dependence of p D on the hydrogen‐bond acceptor parameter β, see Ref. 13c). The relatively high strength of the lp⋅⋅⋅heteroarene interaction is also mirrored in the measured populations for the two diastereoisomeric cyanohydrin derivatives.…”

Preise 2015 der Société Chimique de France

“…Seine Forschungsinteressen umfassen die (physikalische) organische und die Organometallchemie, darunter Zinkcarbenoide, übergangsmetallkatalysierte Reaktionen und die Chemie freier Radikale. Zu seinen neuesten Veröffentlichungen in der Angewandten Chemie gehören eine über nichtkovalente Wechselwirkungen funktioneller Gruppen3a und eine über Wechselwirkungen zwischen freien Elektronenpaaren und Heteroarenen 3b…”