Catalytic Enantioselective 6π Photocyclization of Acrylanilides

Abstract

Controlling absolute stereochemistry in catalytic photochemical reactions is generally challenging owing to high rates of background reactivity. Successful strategies broadly rely on selective excitation of the reaction substrate when associated with a chiral catalyst. Recent studies have demonstrated that chiral Lewis acid complexes can enable selective energy transfer from a photosensitizer to facilitate enantioselective triplet state reactions. Here, we apply this approach to the enantioselective catalysis of a 6π photocyclization through the design of an iridium photosensitizer optimized to undergo energy transfer to a reaction substrate only in the presence of a chiral Lewis acid complex. Among a group of iridium(III) sensitizers, enantioselectivity and yield closely correlate with photocatalyst triplet energy within a narrow window enabled by a modest reduction in substrate triplet energy upon binding a scandium/ligand complex. These results demonstrate that photocatalyst tuning offers a means to suppress background reactivity and improve enantioselectivity in photochemical reactions.

Feng chiralN,N′-dioxide ligands: uniqueness and impacts

“…With its ever-increasing popularity and impacts in synthetic organic chemistry, recent examples have opened avenues for controlling the stereoselectivity of radical-related bond-forming processes by virtue of Feng ligand-metal Lewis acid catalysis. [44][45][46]60,61 In the near future, we expect a greatly expanded chemical space of Feng ligands in manipulating such transient intermediates with greener processes, as well as applications in material science.…”

“…56,57,58,59 Huang and colleagues 60 achieved convergent synthesis of chiral vicinal amino alcohols via an enantioselective reductive radical-type coupling reaction of nitrones with aromatic ketyl radicals generated from photocatalyzed single-electron transfer reduction of carbonyls (Scheme 3e). Paton, Smith, and colleagues 61 recently demonstrated that the use of L 3 -PrPr 2 -Sc (OTf) 3 could lower the triplet energy of acrylanilides by forming complexes with the acrylanilides, to which selective energy transfer from a photosensitizer proceeded, thereby enabling an asymmetric 6π photocyclization reaction (Scheme 3f).…”

Investigating the Effect of Lewis Acid Co-catalysts on Photosensitized Visible-Light De Mayo Reactions

“…Lewis acids can have multiple effects in sensitized photoreactions. They can increase the efficiency of both electron- and energy-transfer activation processes. − They can also influence the dynamics of bond-forming processes that occur after the initial photocatalytic activation step . The effect of Lewis acids on De Mayo reactions was previously investigated by Chow and Cheng, who noted that, although dibenzoylmethane ( 9 ) does not react productively with aliphatic olefins upon direct irradiation, the corresponding preformed BF 2 adduct 10 reacts smoothly to afford De Mayo product 11 (Figure C) .…”

Endo/Exo-Controllable Photocyclization by EnT-SET-Switch

“…Intramolecular hydroarylation has been proven to be a straight and versatile method for construction of heteroaromatics. Photoinduced intramolecular hydroarylation of N -arylacrylamide normally undergoes a 6- endo -trig cyclization, whereas a 5- exo -trig cyclization could be achieved via a transition-metal-catalyzed oxidative transformation or a redox-neutral reaction or by using stereometric FeCl 3 and NaBH 4 . The site-selectivity control for visible-light-induced hydroarylation of N -arylacrylamide (6- endo -trig vs 5- exo -trig cyclization) requires different catalytic systems.…”

“…Nevertheless, the site-selectivity control at the same catalyst by switching the nitrogen substituents has not been well-documented. − Besides, cyclization of unprotected N -H- N -arylacrylamides via visible light photocatalysis has not been reported.…”