Enantioselective [2+2] Cycloadditions of Cinnamate Esters: Generalizing Lewis Acid Catalysis of Triplet Energy Transfer

Abstract

We report the enantioselective [2+2] cycloaddition of simple cinnamate esters, the products of which are useful synthons for the controlled assembly of cyclobutane natural products. This method utilizes a co-catalytic system in which a chiral Lewis acid accelerates the transfer of triplet energy from an excited-state Ir(III) photocatalyst to the cinnamate ester. Computational evidence indicates that the principal role of the Lewis acid co-catalyst is to lower the absolute energies of the substrate frontier molecular orbitals, leading to greater electronic coupling between the sensitizer and substrate and increasing the rate of the energy transfer event. These results suggest Lewis acids can have multiple beneficial effects on triplet sensitization reactions, impacting both the thermodynamic driving force and kinetics of Dexter energy transfer.

Reactivity control of a photocatalytic system by changing the light intensity

“…The ndings presented herein on controlled substrate activation through light-power-density regulation might well stimulate a broader application of this concept in the continuouslygrowing eld of photo(redox) catalysis. Possible further directions include the incorporation of our approach into cascadetype reactions, in which triplets 28,38,[60][61][62][63][64][65][66] or intermediate radicals 43,67-69 are trapped by further additives resulting in more Fig. 3 3 Irsppy (left) and e aq c À (right) induced reductive activation of 4-(chloromethyl)benzoic acid resulting in the hydrogen abstraction and the dimerization main product, respectively, together with rate constant determinations of the two substrate activation reactions.…”

Let there be light

“…These reactions include ring formations through [3+2] cycloadditions leading to derivatives of pyrrolidine (an important ring structure containing four carbon atoms and one nitrogen) 2 , and [2+2] cycloadditions. 3 In a recent analysis using a chiral iridium complex as a sensitiser for chiral catalysis of an intermolecular cycloaddition, the group could separate the functions of energy transfer and structural information transfer, showing that, contrary to previously held beliefs, a preassociation of molecules is not required for such reactions. 4 Anamitra Chatterjee and Burkhard König from the University of Regensburg, Germany, recently demonstrated another new reaction triggered by light.…”

Metal-supported and -assisted stereoselective cooperative photoredox catalysis

“…(2.0 mol%), Sc-cat. as a LA catalyst, 38 successfully used in previous reports for cyclobutane synthesis by Bach 39,40 and Brown. 41…”