Tandem Asymmetric Michael/Transfer Hydrogenation by Using a Dual Rh-Incorporated Micelle Reactor

Key words

1,4-addition - transfer hydrogenation - micelle reactor - photoswitching - rhodium catalysis - tandem catalysis

Significance

Homochiral Rh–diene and Rh–TsDPEN units were incorporated stepwise into the hydrophilic outer shell and the hydrophobic core ­regions, respectively, of micelles of a polyoxazoline-based amphiphilic triblock copolymer bearing spiropyran crosslinks (d-Rh-mcl). The d-Rh-mcl micelles catalyzed an asymmetric 1,4-addition of arylboronic acids to α,β-unsaturated ketones in water (15 h) to give (S)-3-arylketone intermediates; successive asymmetric transfer hydrogenation of these (S)-intermediates under UV irradiation (15 h) gave the corresponding (1R,3S)-3-arylbutanols in high yields and with excellent enantioselectivity.

Comment

The spiropyran cross-linker is a photoresponsive unit that isomerizes to a hydrophilic zwitterionic merocyanine under UV irradiation. Thus, the initial 1,4-addition took place in the hydrophilic corona region of the anchored Rh-diene complex in the absence of UV irradiation. Upon UV irradiation, the zwitterionic merocyanine increased the permeability of the Rh–TsDPEN core region toward water-soluble HCOONa, to promote the transfer hydrogenation in the second reaction step. The 1,4-addition and transfer hydrogenation compete under homogeneous conditions. The photoinduced gating between the compartmentalized catalytic regions was essential for regulating the incompatible multiple catalytic steps.

Publication History

Article published online:
17 June 2021