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