More Like this

1. Enantioselective Paternò–Büchi Reactions: Strategic Application of a Triplet Rebound Mechanism for Asymmetric Photocatalysis

   https://doi.org/10.1021/jacs.4c02975  

   Kidd, Jesse B; Fiala, Tahoe A; Swords, Wesley B; Park, Yerin; Meyer, Kent A; Sanders, Kyana M; Guzei, Ilia A; Wright, John C; Yoon, Tehshik P (June 2024, Journal of the American Chemical Society)

   The Paternò–Büchi reaction is the [2+2] photocycloaddition of a carbonyl with an alkene to afford oxetane products. Enantioselective catalysis of this classical photoreaction, however, has proven to be a long-standing challenge. Many of the best-developed strategies for asymmetric photochemistry are not suitable to address this problem because the interaction of carbonyls with Brønsted or Lewis acidic catalysts can alter the electronic structure of their excited state and divert their reactivity towards alternate photoproducts. We show herein that an alternative triplet rebound strategy enables the stereocontrolled reaction of an excited-state carbonyl compound in its native, unbound state. These studies have resulted in the development of the first highly enantioselective catalytic Paternò–Büchi reaction, cata-lyzed by a novel hydrogen-bonding chiral Ir photocatalyst. 

   more » « less
2. 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. 

   more » « less
3. 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. 

   more » « less
4. 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. 

   more » « less
5. Ultrafast Processes in Upper Excited Singlet States of Free and Caged 7-Diethylaminothiocoumarin

   https://doi.org/10.1021/acs.jpca.4c03139  

   Dutta, Abhijit; Ghosh, Sujit Kumar; Mandal, Satyajit; Srinivasan, Varadharajan; Ramamurthy, Vaidhyanathan; Sen, Pratik (August 2024, The Journal of Physical Chemistry A)

   The photochemistry and photophysics of thiocarbonyl compounds, analogues of carbonyl compounds with sulfur, have long been overshadowed by their counterparts. However, recent interest in visible light reactions has reignited attention toward these compounds due to their unique excited-state properties. This study delves into the ultrafast dynamics of 7- diethylaminothiocoumarin (TC1), a close analogue of the wellknown probe molecule coumarin 1 (C1), to estimate intersystem crossing rates, understand the mechanisms of fluorescence and phosphorescence, and evaluate TC1’s potential as a solvation dynamics probe. Enclosing TC1 within an organic capsule indicates its potential applications, even in aqueous environments. Ultrafast studies reveal a dominant subpicosecond intersystem crossing process, indicating the importance of upper excited singlet and triplet states in the molecule’s photochemistry. The distinct fluorescence and phosphorescence origins, along with the presence of closely spaced singlet excited states, support the observed efficient intersystem crossing. The sulfur atom alters the excited-state behavior, shedding light on reactive triplet states and paving the way for further investigations. 

   more » « less