Synthetic Molecular Motors: Thermal N Inversion and Directional Photoinduced CN Bond Rotation of Camphorquinone Imines

Abstract

The thermal and photochemical E/Z isomerization of camphorquinone-derived imines was studied by acombination of kinetic, structural, and computational methods.T he thermal isomerization proceeds by linear Ninversion, whereas the photoinduced process occurs through C=Nbond rotation with preferred directionality as aresult of diastereoisomerism. Thereby, these imines are arguably the simplest example of synthetic molecular motors.T he generality of the orthogonal trajectories of the thermal and photochemical pathways allows for the postulation that every suitable chiral imine qualifies,in principle,a samolecular motor driven by light or heat.

Molecular rotary motors: Unidirectional motion around double bonds

“…That is, two photochemical isomerization steps and two thermally activated ring inversions give rise to 360°rotation. To show that imines can be used as two-step molecular motors in a more general sense and to provide more experimental and theoretical proof for the rotational behavior, camphorquinone imines were synthesized in a follow-up study (93).…”

An autonomous chemically fuelled small-molecule motor

“…22 Their designs culminated in a system that employed chemical reactions to bias a 120 degree rotation of a triptycene residue in one direction, 23 but attempts to extend this approach to repetitive 360 degree directional rotation proved unsuccessful. 24 Light-driven rotary molecular motors based on overcrowded alkenes 12,13 and imines 14,16 have been developed by the groups of Feringa and Lehn, while our group 25,26 and others [27][28][29] have made molecules in which the components can be rotated directionally step-wise by repetitively carrying out several chemical reactions in sequence. The latter systems all operate through Brownian ratchet mechanisms, differentiating the rates of random thermal motion of the components in each direction by the manipulation of kinetic (mainly steric) barriers.…”

A new fundamental type of conformational isomerism

“…In doubly bonded molecules such as substituted imines C(R 1 R 2 )=N(R 3 ), rotation about a bond and BAI provide alternate concerted mechanisms for (E/Z) stereoisomerisation. 5,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] The bond-rotation transition state in non-planar and naively involves sp 2 to sp 3 re-hybridisation at the N atom, whereas the planar BAI transition state involves sp 2 to sp re-hybridisation at the N. However, as the reaction products are the same, there has been no need for unique product or process names, with all isolated compounds amenable to standard nomenclature. As in the example of water discussed above, BAI in singly bonded systems has not been shown to lead to interesting chemistry.…”