Relatively few catalytic systems are able to control the stereochemistry of electronically excited organic intermediates. Here we report the discovery that a chiral Lewis acid complex can catalyze ...triplet energy transfer from an electronically excited photosensitizer. We applied this strategy to asymmetric 2 + 2 photocycloadditions of 2'-hydroxychalcones, using tris(bipyridyl) ruthenium(ll) as a sensitizer. A variety of electrochemical, computational, and spectroscopic data rule out substrate activation by means of photoinduced electron transfer and instead support a mechanism in which Lewis acid coordination dramatically lowers the triplet energy of the chalcone substrate. We expect that this approach will enable chemists to more broadly apply their detailed understanding of chiral Lewis acid catalysis to stereocontrol in reactions involving electronically excited states.
Site selectivity represents a key challenge for non-directed C–H functionalization, even when the C–H bond is intrinsically reactive. Here, we report a copper-catalyzed method for benzylic C–H ...azidation of diverse molecules. Experimental and density functional theory studies suggest the benzyl radical reacts with a CuII-azide species via a radical-polar crossover pathway. Comparison of this method with other C–H azidation methods highlights its unique site selectivity, and conversions of the benzyl azide products into amine, triazole, tetrazole, and pyrrole functional groups highlight the broad utility of this method for target molecule synthesis and medicinal chemistry.
Aziridines-three-membered nitrogen-containing cyclic molecules-are important synthetic targets. Their substantial ring strain and resultant proclivity towards ring-opening reactions makes them ...versatile precursors of diverse amine products
, and, in some cases, the aziridine functional group itself imbues important biological (for example, anti-tumour) activity
. Transformation of ubiquitous alkenes into aziridines is an attractive synthetic strategy, but is typically accomplished using electrophilic nitrogen sources rather than widely available amine nucleophiles. Here we show that unactivated alkenes can be electrochemically transformed into a metastable, dicationic intermediate that undergoes aziridination with primary amines under basic conditions. This new approach expands the scope of readily accessible N-alkyl aziridine products relative to those obtained through existing state-of-the-art methods. A key strategic advantage of this approach is that oxidative alkene activation is decoupled from the aziridination step, enabling a wide range of commercially available but oxidatively sensitive
amines to act as coupling partners for this strain-inducing transformation. More broadly, our work lays the foundations for a diverse array of difunctionalization reactions using this dication pool approach.
A catalytic enantioselective syn-1,4-bromolactonization of conjugated (Z)-enynes was reported. Diastereomeric ratios >20:1 and up to 99% enantiomeric excesses were observed. Di-, tri-, and ...tetra-substituted bromoallenes were prepared together with lactone heterocycles efficiently and stereoselectively. Preliminary investigations suggest that the chiral catalyst may serve as a bifunctional reagent by interacting with both a carboxylic acid nucleophile and NBS electrophile.
Organic-inorganic hybrid perovskites attract considerable attention owing to their applications in high-efficiency solar cells and light emission. Compared with three-dimensional perovskites, ...two-dimensional （2D） layered hybrid perovskites have a higher exciton binding energy and potentially higher light- emission efficiency. The growth of high-quality crystalline 2D perovskites with a well-defined nanoscale morphology is desirable because they can be suitable building blocks for integrated optoelectronics and （nano）photonics. Herein, we report the facile solution growth of single-crystal microplates of 2D perovskites based on a 2-phenylethylammonium （C6HsCH2CH2NHG PEA） cation, （PEA）2PbX- （X = Br, I）, with a well-defined rectangular geometry and nanoscale thickness through a dissolution-recrystallization process. The crystal structures of （PEA）2PbX4 are first confirmed using single-crystal X-ray diffraction. A solution-phase transport-growth process is developed to grow microplates with a typical size of tens of micrometers and thickness of hundreds of nanometers on another clean substrate different from the substrate coated with lead-acetate precursor film. Surface-topography analysis suggests that the formation of the 2D microplates is likely driven by the wedding-cake growth mechanism. Through halide alloying, the photoluminescence emission of （PEA）2Pb（Br, I）4 perovskites with a narrow peak bandwidth is readily tuned from violet （-410 nm） to green （-530 nm）.
Asymmetric nitrene‐transfer reactions are a powerful tool for the preparation of enantioenriched amine building blocks. Reported herein are chemo‐ and enantioselective silver‐catalyzed aminations ...which transform di‐ and trisubstituted homoallylic carbamates into 4.1.0‐carbamate‐tethered aziridines in good yields and with ee values of up to 92 %. The effects of the substrate, silver counteranion, ligand, solvent, and temperature on both the chemoselectivity and ee value were explored. Stereochemical models were proposed to rationalize the observed absolute stereochemistry of the aziridines, which undergo nucleophilic ring opening to yield enantioenriched amines with no erosion in stereochemical integrity.
Silver cat.: Asymmetric nitrene‐transfer reactions are a powerful tool for preparing enantioenriched amines. The reported chemo‐ and enantioselective silver‐catalyzed aminations transform di‐ and trisubstituted homoallylic carbamates into aziridines in good yields and with up to 92 % ee. Stereochemical models are proposed to rationalize the absolute stereochemistry of the products.
Carbonyl–carbonyl interactions between adjacent backbone amides have been implicated in the conformational stability of proteins. By combining experimental and computational approaches, we show that ...relevant amidic carbonyl groups associate through an n→π* donor–acceptor interaction with an energy of at least 0.27 kcal/mol. The n→π* interaction between two thioamides is 3-fold stronger than between two oxoamides due to increased overlap and reduced energy difference between the donor and acceptor orbitals. This result suggests that backbone thioamide incorporation could stabilize protein structures. Finally, we demonstrate that intimate carbonyl interactions are described more completely as donor–acceptor orbital interactions rather than dipole–dipole interactions.
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 a
trans
-
cis
stereochemistry that is complementary to other enantioselective catalytic 2+2 photocycloadditions reported to date.
The reaction of rhodium‐bound carbenes with strained bicyclic methylene aziridines results in a formal 3+1 ring expansion to yield highly substituted methylene azetidines with excellent regio‐ and ...stereoselectivity. The reaction appears to proceed through an ylide‐type mechanism, where the unique strain and structure of the methylene aziridine promotes a ring‐opening/ring‐closing cascade that efficiently transfers chirality from substrate to product. The resultant products can be elaborated into new azetidine scaffolds containing vicinal tertiary‐quaternary and even quaternary‐quaternary stereocenters.
A tale of two rings: A one carbon ring expansion of methylene aziridines to methylene azetidines occurs upon reaction with a rhodium‐bound carbene. This 3+1 ring expansion efficiently delivers substituted azetidines in high yields and stereoselectivities.
Stereochemical control of electronically excited states is a long-standing challenge in photochemical synthesis, and few catalytic systems that produce high enantioselectivities in triplet-state ...photoreactions are known. We report herein an exceptionally effective chiral photocatalyst that recruits prochiral quinolones using a series of hydrogen-bonding and π–π interactions. The organization of these substrates within the chiral environment of the transition-metal photosensitizer leads to efficient Dexter energy transfer and effective stereoinduction. The relative insensitivity of these organometallic chromophores toward ligand modification enables the optimization of this catalyst structure for high enantiomeric excess at catalyst loadings as much as 100-fold lower than the optimal conditions reported for analogous chiral organic photosensitizers.
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