Merging Constitutional and Motional Covalent Dynamics in Reversible Imine Formation and Exchange Processes

Abstract

The formation and exchange processes of imines of salicylaldehyde, pyridine-2-carboxaldehyde, and benzaldehyde have been studied, showing that the former has features of particular interest for dynamic covalent chemistry, displaying high efficiency and fast rates. The monoimines formed with aliphatic α,ω-diamines display an internal exchange process of self-transimination type, inducing a local motion of either "stepping-in-place" or "single-step" type by bond interchange, whose rate decreases rapidly with the distance of the terminal amino groups. Control of the speed of the process over a wide range may be achieved by substituents, solvent composition, and temperature. These monoimines also undergo intermolecular exchange, thus merging motional and constitutional covalent behavior within the same molecule. With polyamines, the monoimines formed execute internal motions that have been characterized by extensive one-dimensional, two-dimensional, and EXSY proton NMR studies. In particular, with linear polyamines, nondirectional displacement occurs by shifting of the aldehyde residue along the polyamine chain serving as molecular track. Imines thus behave as simple prototypes of systems displaying relative motions of molecular moieties, a subject of high current interest in the investigation of synthetic and biological molecular motors. The motional processes described are of dynamic covalent nature and take place without change in molecular constitution. They thus represent a category of dynamic covalent motions, resulting from reversible covalent bond formation and dissociation. They extend dynamic covalent chemistry into the area of molecular motions. A major further step will be to achieve control of directionality. The results reported here for imines open wide perspectives, together with other chemical groups, for the implementation of such features in multifunctional molecules toward the design of molecular devices presenting a complex combination of motional and constitutional dynamic behaviors.

Artificial molecular motors

“…They are the first examples of translational synthetic small-molecule motors. Other dynamic covalent chemistry-based walkers have been reported [45][46][47][48] . However, in those cases directionality results from the spontaneous downhill migration of the walker towards a thermodynamic sink at one end of the track (i.e.…”

A walk along DNA using bipedal migration of a dynamic and covalent crosslinker

“…As described below, DNA is shown to provide a convenient surface for processive and bipedal diffusion of a dynamic crosslink in addition to its already well-characterized use in transport based on non-covalent exchange. DNA offers all of the benefits of easy assembly and creates defined surfaces containing nucleophilic footholds that are held more rigidly than those currently available with a linear aliphatic oligomer 11,22,30 .…”

Strain‐Induced Reactivity in the Dynamic Covalent Chemistry of Macrocyclic Imines

“…One may also note that in the strained [1+ +1]-macrocycles distortion/rotation of the CÀCH=NÀ group out of the pyridine plane should both deconjugate it from the pyridine group and change itso rientation with respect to the pyridyl nitrogen, possibly causing some destabilisation.S omewhat related is the fact that pyridine-2-aldehyde forms al arger amount of imine (61 %) with isopentylamine than benzaldehyde (20 %). [29] Both of the strain-induced structurale ffects evoked, that is, in-plane distorsion and out-of plane rotation, lead in the direction of al ess stable/more reactive imine group.…”