Light-controlled Intramolecular Cooperative Catalysis Based On Azobenzene-bridged Chiral Salen Transition-metal Complexes

Cooperative bimetallic catalysis is a fundamental phenomenon in biosphere,where two or more metal centers in the active site work synergistically to promote a catalytic transformation.In most case,the cooperative action is controlled by molecular units that undergo changes in response to external stimuli.The obtained chiral binuclear metallosalen catalysts enforced an intramolecular.If the linker in bimetallic catalyst undergoes conformational changes in response to external stimuli,the relative positioning of two active sites can be regulated in a stimuli-controllable way.In this paper,serial chiral salen transition metal complex catalysts in light-controlling molecules are designed by using azobenzene with photoisomerization as the linking group.The catalyst can be transformed from a planar trans-structure to a curved cis-isomer by ultraviolet radiation,and the center positions of two metals in the molecule are changed,thereby regulating the synergistic effect of the two transition metal salen complexes in the molecule in asymmetric catalytic reactions.?1?The asymmetric oxidation of sulfide is of great significance in organic synthesis,and its synthetic chiral sulfoxide is a valuable chiral intermediate.Chiral salen Ti?IV?complex was able to catalyze this transformation through a bimetallic,cooperative mechanism.In the system,a di-?-oxo Ti?salen?complex of[{?salen?Ti??-O?}₂]was the real catalyst species for the sulfoxidation.Based on the cis/trans photoisomerization of azobenzene,we employed 4,4'-dihydroxyazobenzene as a bridge to covalently bond two chiral salen Ti?IV?complexes.The obtained azobenzene-bridged chiral salen Ti?IV?catalyst?ALGC?underwent reversible photo-responsive structural changes.In the dark,the trans-type azobenzene linker placed two salen Ti?IV?units separately,it was difficult to establish the intramolecular cooperative function for efficient sulfoxidation.Moderate conversion?69%?and selectivity?in terms of chemo-and enantioselectivity?were observed over the dark-adapted ALGC.Upon UV irradiation,the azobenzene linker was in cis-state.It located the two salen Ti?IV?units in proximity to each other,enforcing their intramolecular,cooperative function which enhanced catalytic efficiency.Indeed,only 0.1 mol%of ALGC was sufficient to give remarkable activity?87%of conversion?and selectivity?89%of chemoselectivity and 80%of ee values?to chiral phenyl methyl sulfoxide.?2?Although displayed a similar UV-enforced cooperation mode,the catalytic efficiency of azobenzene-bridged chiral salen transition-metal complexes strongly depended on the flexibility of spacer.In addition,the[{?salen?Ti??-O?}₂]compound bridged by?-oxo can more effectively catalyze the oxidation of asymmetric thioethers through a bimetallic synergy mechanism.Herein,we introduced various flexible alkyl chains between azobenzene and salen Ti?IV?moieties,giving azobenzene-bridged bimetallic salen Ti?IV?catalysts of ATGC-n?where n represented the corresponding numbers of methylene unit in aliphatic linkers,n=2,3 and 4?.the cis/trans photoisomerization of azobenzene in the linking group allow the relative position of the two chiral salen Ti?IV?units in the catalyst molecule can be regulated,Thereby regulating its cooperative catalytic performance in the asymmetric oxidation of thioether.Furthermore,the catalytic efficiency improved with lengthening the azobenzene spacer with aliphatic chain.When 4,4'-dipropoxy azobenzene was used as a linker,only 0.1 mol%of ATGC-3demonstrated the highest catalytic efficiency in the reaction.Almost quantitative yield?98%?of?R?-sulfoxide with up to 99%ee value was obtained over the UV-treated ATGC-3.?3?Asymmetric epoxidation of unfunctionalized olefins with chiral salen Mn?III?complexes is the most effective method for the synthesis of chiral epoxides.Herein,two chiral salen Mn?III?complexes were bridged through a hydroxyl-terminated azobenzene via direct axial coordination of the metal center with terminated hydroxyl group in azobenzen.Benefiting from the photosensitive of azobenzene bridge,the obtained serial bimetalic chiral salen Mn?III?catalyst?AXGC-n,where n represented the corresponding numbers of methylene unit in aliphatic linkers,n=0,2 and 3?exhibited reversibly cis/trans photoisomerization by alternating dark and UV irradiation.Under dark condition,the hydroxyl-terminated azobenzene has a trans-configuration,and the chiral salen Mn?III?complex in AXGC-n was relatively far away,which is unfavorable for intra-molecular synergistic catalysis.Moderate conversion and selectivity?in terms of chemo-and enantioselectivity?were observed over the dark-adapted AXGC-n.Under UV irradiation,the two catalytic active centers in the catalyst structure are close to each other,which was conducive to the intra-molecular synergistic catalysis.Only 1.5 mol%of AXGC-3 was sufficient to give 98%conversion of?-methylstyrene epoxide with 82%selectivity.