Dynamic Control of Chiral Space in a Catalytic Asymmetric Reaction Using a Molecular Motor (5 pages)
Enzymes and synthetic chiral catalysts have found widespread application to produce single enantiomers, but in situ switching of the chiral preference of a catalytic system is very difficult to achieve. Here, we report on a light-driven molecular motor with integrated catalytic functions in which the stepwise change in configuration during a 360° unidirectional rotary cycle governs the catalyst performance both with respect to activity and absolute stereo control in an asymmetric transformation. During one full rotary cycle, catalysts are formed that provide either racemic (R,S) or preferentially the R or the S enantiomer of the chiral product of a conjugate addition reaction. This catalytic system demonstrates how different molecular tasks can be performed in a sequential manner, with the sequence controlled by the directionality of a rotary cycle
(H/T Foresight nanodot)
Schematic illustration of an integrated unidirectional light-driven molecular motor and bifunctional organocatalyst (top) and the molecular structure of (2R,2′R)-(P,P)- trans-1 (bottom). The motor comprises a rotor and stator connected by an alkene moiety that functions as the axle. A and B are DMAP and thiourea catalytic groups, respectively, that can cooperate as Brønsted base and hydrogen bond donor in an organocatalytic conjugate addition. Clockwise rotation (seen from the stator side) of the rotor around the axle, by photochemically and thermally induced steps, controls the position and helical orientation of the catalytic groups A and B, providing sequentially catalysts I, II, and III with different activities and stereoselectivities. The last two isomerization steps [steps 3 and 4 of the 360°C rotary cycle (see Fig. 2)] reset the catalyst to its initial stage I.
Coupling of unidirectional switching to catalytic function, as demonstrated here, may prove to be a key design tool in the construction of future catalysts that can perform multiple tasks in a sequential manner.
Foresight notes -
The molecular specificity of this initial proof-of-principle demonstration is only partial. The differences in catalytic activity and the differences in chiral ratios of the reaction products are only of the order of three- or four-fold. We can hope that continued work in this direction will lead to cleaner reaction specificities resulting from programmable control of artificial molecular machines. Eventually we hope to see arrays of programmable molecular catalysts executing complex reaction sequences, leading to productive nanosysems and atomically precise manufacturing.
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