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1. Taming the excited state reactivity of imines – from non-radiative decay to aza Paternò–Büchi reaction

   https://doi.org/10.1039/d0cs00717j  

   Kandappa, Sunil Kumar; Valloli, Lakshmy Kannadi; Ahuja, Sapna; Parthiban, Jayachandran; Sivaguru, J. (February 2021, Chemical Society Reviews)

   null (Ed.)

   This review highlights the excited state characteristics of imines and processes that govern their photochemical and photophysical properties. This review examines the pathways for deactivation and types of photochemical reactions that originate from excited imines. This review also features recent strategies that are developed to circumvent the fundamental issues that have plagued the development of the aza Paternò–Büchi reaction. 

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2. Visible light–mediated aza Paternò–Büchi reaction of acyclic oximes and alkenes to azetidines

   https://doi.org/10.1126/science.adj6771  

   Wearing, Emily R; Yeh, Yu-Cheng; Terrones, Gianmarco G; Parikh, Seren G; Kevlishvili, Ilia; Kulik, Heather J; Schindler, Corinna S (June 2024, Science)

   The aza Paternò–Büchi reaction is a [2+2]-cycloaddition reaction between imines and alkenes that produces azetidines, four-membered nitrogen-containing heterocycles. Currently, successful examples rely primarily on either intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò–Büchi reactions, it is essential to extend the reaction to acyclic imine equivalents. Here, we report that matching of the frontier molecular orbital energies of alkenes with those of acyclic oximes enables visible light–mediated aza Paternò–Büchi reactions through triplet energy transfer catalysis. The utility of this reaction is further showcased in the synthesis ofepi-penaresidin B. Density functional theory computations reveal that a competition between the desired [2+2]-cycloaddition and alkene dimerization determines the success of the reaction. Frontier orbital energy matching between the reactive components lowers transition-state energy (ΔG^ǂ) values and ultimately promotes reactivity. 

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3. Chiral Brønsted acid-controlled intermolecular asymmetric [2 + 2] photocycloadditions

   https://doi.org/10.1038/s41467-021-25878-9  

   Sherbrook, Evan M.; Genzink, Matthew J.; Park, Bohyun; Guzei, Ilia A.; Baik, Mu-Hyun; Yoon, Tehshik P. (September 2021, Nature Communications)

   Abstract Control over the stereochemistry of excited-state photoreactions remains a significant challenge in organic synthesis. Recently, it has become recognized that the photophysical properties of simple organic substrates can be altered upon coordination to Lewis acid catalysts, and that these changes can be exploited in the design of highly enantioselective catalytic photoreactions. Chromophore activation strategies, wherein simple organic substrates are activated towards photoexcitation upon binding to a Lewis acid catalyst, rank among the most successful asymmetric photoreactions. Herein, we show that chiral Brønsted acids can also catalyze asymmetric excited-state photoreactions by chromophore activation. This principle is demonstrated in the context of a highly enantio- and diastereoselective [2+2] photocycloaddition catalyzed by a chiral phosphoramide organocatalyst. Notably, the cyclobutane products arising from this method feature atrans-cisstereochemistry that is complementary to other enantioselective catalytic [2+2] photocycloadditions reported to date. 

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4. Modeling the Unimolecular Decay Dynamics of the Fluorinated Criegee Intermediate, CF3CHOO

   https://doi.org/10.3390/photochem3030020  

   Guidry, Lily M.; Poirier, Courtney A.; Ratliff, Jordyn M.; Antwi, Ernest; Marchetti, Barbara; Karsili, Tolga N. (September 2023, Photochem)

   When volatile alkenes are emitted into the atmosphere, they are rapidly removed by oxidizing agents such as hydroxyl radicals and ozone. The latter reaction is termed ozonolysis and is an important source of Criegee intermediates (CIs), i.e., carbonyl oxides, that are implicated in enhancing the oxidizing capacity of the troposphere. These CIs aid in the formation of lower volatility compounds that typically condense to form secondary organic aerosols. CIs have attracted vast attention over the past two decades. Despite this, the effect of their substitution on the ground and excited state chemistries of CIs is not well studied. Here, we extend our knowledge obtained from prior studies on CIs by CF3 substitution. The resulting CF3CHOO molecule is a CI that can be formed from the ozonolysis of hydrofluoroolefins (HFOs). Our results show that the ground state unimolecular decay should be less reactive and thus more persistent in the atmosphere than the non-fluorinated analog. The excited state dynamics, however, are predicted to occur on an ultrafast timescale. The results are discussed in the context of the ways in which our study could advance synthetic chemistry, as well as processes relevant to the atmosphere. 

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5. Asymmetric allylic substitution–isomerization to axially chiral enamides via hydrogen-bonding assisted central-to-axial chirality transfer

   https://doi.org/10.1039/D0SC02828B  

   Sun, Chao; Qi, Xiaotian; Min, Xiao-Long; Bai, Xue-Dan; Liu, Peng; He, Ying (September 2020, Chemical Science)

   null (Ed.)

   Axially chiral enamides bearing a N–C axis have been recently studied and were proposed to be valuable chiral building blocks, but a stereoselective synthesis has not been achieved. Here, we report the first enantioselective synthesis of axially chiral enamides via a highly efficient, catalytic approach. In this approach, C(sp 2 )–N bond formation is achieved through an iridium-catalyzed asymmetric allylation, and then in situ isomerization of the initial products through an organic base promoted 1,3-H transfer, leading to the enamide products with excellent central-to-axial transfer of chirality. Computational and experimental studies revealed that the 1,3-H transfer occurs via a stepwise deprotonation/re-protonation pathway with a chiral ion-pair intermediate. Hydrogen bonding interactions with the enamide carbonyl play a significant role in promoting both the reactivity and stereospecificity of the stepwise 1,3-H transfer. The mild and operationally simple formal N -vinylation reaction delivered a series of configurationally stable axially chiral enamides with good to excellent yields and enantioselectivities. 

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