Studies On Dynamic Covalent Chemistry Modulated By Donor-Acceptor Orbital Interaction

Supramolecular chemistry has been found application in many aspects,such as chemical sensing,molecular catalysis,intelligent materials,as well as biological labeling.As the cornerstone of molecular recognition and self-assembly,noncovalent interactions play an indispensable role.Therefore,it is of great significance to explore the discovery,strength,modulation,and functionalization of noncovalent interactions.Dynamic covalent bonds have advantages of both the reversibility of supramolecular interactions and the stability of classical covalent bonds,through which the structural diversity and therefore systematic complexity would be efficiently accomplished.Herein we demonstrate the investigation of n??~* interaction,which is of critical importance in organic chemistry,such as conformational analysis,molecular recognition,catalysis,protein engineering,and photoelectric materials,by using the strategy of DCC,and further exhibit its application in the modulation of dynamic covalent reactions and stabilization of imine bond.In order to examine the relationship between n??~* interactions and dynamic covalent reactions(DCRs),a series of 2-X-2'-formylbiphenyl derivatives with different substituents X were designed and synthesized.The reaction of aldehydes and amines or thiols were conducted for revealing the role of n??~* interactions in modulating kinetics and thermodynamics of DCRs.The reaction between aldehyde and primary amine would be decelerated by n??~* interactions.The kinetics of imine exchange reaction was very slow,and it was catalyzed through the addition of water.The equilibrium of imine exchange prefers more electron-deficient aldehydes,which indicate the stabilizing effect of n??~* interactions on aldehydes.Hammett analysis was carried out on the exchange equilibrium of different substituted imines and further confirm the stabilized role and modulation mechanism of n??~* interactions.Moreover,the existence of n??~* interactions was verified by X-ray crystal analysis and DFT calculation,which also allowed the quantification of such interactions.Solvents have remarkable influence on chemical reactions and supramolecular interactions.Imine formation and exchange performed at varied solvents were monitored,by which the effect of solvents on n??~* interaction and DCRs could be illuminated.Kinetics of imine formation and exchange were accelerated to varying degrees in protic solvents,with the maximum effect observed in water.The equilibrium of imine exchange has opposite linear correlation with the cohesive energy density(ced)parameters of protic and aprotic solvents.We rationalize that in protic solvents and aprotic solvents the n??~* interaction has different mechanisms,which further impact the equilibrium positions of imine exchange in different solvents.The n??~* interaction between X and imine would be enhanced resulting from the difference in intermolecular hydrogen bond between aldehyde oxygen or imine nitrogen with protic solvents.Such solvents effect further enabled the creation and stabilization of imine in aqueous solutions at physiological pH condition.This strategy has potential applications in biomolecular labelling and modification.Finally,the potential application of n??~* interaction and dynamic covalent reaction in regulating the molecular rotors was preliminarily explored.Three kinds of biphenyl rotors were designed and synthesized.Their rotational energy barriers in different solvents were detected by VT-NMR,which laid a foundation for the future work.