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.
Azidoformates are interesting potential nitrene precursors, but their direct photochemical activation can result in competitive formation of aziridination and allylic amination products. Herein, we ...show that visible‐light‐activated transition‐metal complexes can be triplet sensitizers that selectively produce aziridines through the spin‐selective photogeneration of triplet nitrenes from azidoformates. This approach enables the aziridination of a wide range of alkenes and the formal oxyamination of enol ethers using the alkene as the limiting reagent. Preparative‐scale aziridinations can be easily achieved under continuous‐flow conditions.
A fresh spin on heterocycles: Synthetically useful photochemical aziridinations can be achieved by using a visible‐light‐activated transition‐metal photosensitizer to produce nitrenes exclusively in their triplet spin state. A wide range of aliphatic and aromatic alkenes can be readily aziridinated without competitive allylic insertion reactions. TrocN3=2,2,2‐trichloroethyl azidoformate.
MP2 and CCSD(T) complete basis set (CBS) limit relative electronic energies (ΔE e) have been determined for eight low-lying structures of the water hexamer by combining explicitly correlated MP2-R12 ...computations with higher-order correlation corrections from CCSD(T) calculations. Higher-order correlation effects are quite substantial and increase ΔE e by at least +0.19 kcal mol−1 and as much as +0.59 kcal mol−1. The effects from zero-point vibrational energy (ZPVE) have been assessed from unscaled harmonic vibrational frequencies computed at the MP2 level with a correlation consistent triple-ζ basis set (cc-pVTZ for H and aug-cc-pVTZ for O). ZPVE effects are even more significant than higher-order correlation effects and are uniformly negative, decreasing the relative energies by −0.16 kcal mol−1 to −1.61 kcal mol−1. Although it has been widely accepted that the cage becomes the lowest-energy structure after ZPVE effects are included Nature 1996, 381, 501−503, the prism is consistently the most stable structure in this work, lying 0.06 kcal mol−1 below the nearly isoenergetic cage isomer at the electronic MP2 CBS limit, 0.25 kcal mol−1 below at the electronic CCSD(T) CBS limit, and 0.09 kcal mol−1 below at the harmonic ZPVE corrected CCSD(T) CBS limit. Moreover, application of any uniform scaling factor less than unity to correct for anharmonicity further stabilizes the prism and increases the relative energies of the other structures.
Hydrazine is an important industrial chemical and fuel that has attracted considerable attention for use in liquid fuel cells. Ideally, hydrazine could be prepared via direct oxidative coupling of ...ammonia, but thermodynamic and kinetic factors limit the viability of this approach. The present study evaluates three different electrochemical strategies for the oxidative homocoupling of benzophenone imine, a readily accessible ammonia surrogate. Hydrolysis of the resulting benzophenone azine affords hydrazine and benzophenone, with the latter amenable to recycling. The three different electrochemical N–N coupling methods are (1) a proton-coupled electron-transfer process promoted by a phosphate base, (2) an iodine-mediated reaction involving intermediate N–I bond formation, and (3) a copper-catalyzed N–N coupling process. Analysis of the thermodynamic efficiencies for these electrochemical imine-to-azine oxidation reactions reveals low overpotentials (η) for the copper- and iodine-mediated processes (390 and 470 mV, respectively), but a much higher value for the proton-coupled pathway (η ≈ 1.6 V). A similar approach is used to assess molecular electrocatalytic methods for electrochemical oxidation of ammonia to dinitrogen.
Site-selective radical reactions of benzylic C–H bonds are now highly effective methods for C(sp3–H) functionalization and cross-coupling. The existing methods, however, are often ineffective with ...heterobenzylic C–H bonds in alkyl-substituted pyridines and related aromatic heterocycles that are prominently featured in pharmaceuticals and agrochemicals. Here, we report new synthetic methods that leverage polar, rather than radical, reaction pathways to enable the selective heterobenzylic C–H chlorination of 2- and 4-alkyl-substituted pyridines and other heterocycles. Catalytic activation of the substrate with trifluoromethanesulfonyl chloride promotes the formation of enamine tautomers that react readily with electrophilic chlorination reagents. The resulting heterobenzyl chlorides can be used without isolation or purification in nucleophilic coupling reactions. This chlorination–diversification sequence provides an efficient strategy to achieve heterobenzylic C–H cross-coupling with aliphatic amines and a diverse collection of azoles, among other coupling partners.
Herein, we describe a practical protocol for the removal of alcohol functional groups through reductive cleavage of their benzoate ester analogs. This transformation requires a strong single electron ...transfer (SET) reductant and a means to accelerate slow fragmentation following substrate reduction. To accomplish this, we developed a photocatalytic system that generates a potent reductant from formate salts alongside Brønsted or Lewis acids that promote fragmentation of the reduced intermediate. This deoxygenation procedure is effective across structurally and electronically diverse alcohols and enables a variety of difficult net transformations. This protocol requires no precautions to exclude air or moisture and remains efficient on multigram scale. Finally, the system can be adapted to a one‐pot benzoylation‐deoxygenation sequence to enable direct alcohol deletion. Mechanistic studies validate that the role of acidic additives is to promote the key C(sp3)−O bond fragmentation step.
A new protocol for alcohol deoxygenation that functions across a wide range of alcohol structures is reported. This approach combines the reductive potency of carbon dioxide radical anion with acidic additives to promote the kinetically slow C(sp3)−O bond cleavage.
The action of molecular catalysts comprises multiple microscopic kinetic steps whose nature is of central importance in determining catalyst activity and selectivity. Single-molecule microscopy ...enables the direct examination of these steps, including elucidation of molecule-to-molecule variability. Such molecular diversity is particularly important for the behavior of molecular catalysts supported at surfaces. We present the first combined investigation of the initiation dynamics of an operational palladium cross-coupling catalyst at the bulk and single-molecule levels, including under turnover conditions. Base-initiated kinetics reveal highly heterogeneous behavior indicative of diverse catalyst population. Unexpectedly, this distribution becomes more heterogeneous at increasing base concentration. We model this behavior with a two-step saturation mechanism and identify specific microscopic steps where chemical variability must exist in order to yield observed behavior. Critically, we reveal how structural diversity at a surface translates into heterogeneity in catalyst behavior, while demonstrating how single-molecule experiments can contribute to understanding of molecular catalysts.
An ongoing challenge in modern catalysis is to identify and understand new modes of reactivity promoted by earth-abundant and inexpensive first-row transition metals. Herein, we report a mechanistic ...study of an unusual copper(I)-catalyzed 1,3-migration of 2-bromostyrenes that reincorporates the bromine activating group into the final product with concomitant borylation of the aryl halide bond. A combination of experimental and computational studies indicated this reaction does not involve any oxidation state changes at copper; rather, migration occurs through a series of formal sigmatropic shifts. Insight provided from these studies will be used to expand the utility of aryl copper species in synthesis and develop new ligands for enantioselective copper-catalyzed halogenation.
The structures of more than 70 low-lying water clusters ranging in size from (H2O)3 to (H2O)10 have been fully optimized with several different quantum mechanical electronic structure methods, ...including second-order Møller–Plesset perturbation theory (MP2) in conjunction with correlation consistent triple-ζ basis sets (aug-cc-pVTZ for O and cc-pVTZ for H, abbreviated haTZ). Optimized structures obtained with less demanding computational procedures were compared to the MP2/haTZ ones using both MP2/haTZ single point energies and the root-mean-square (RMS) deviations of unweighted Cartesian coordinates. Based on these criteria, B3LYP/6-31+G(d,2p) substantially outperforms both HF/haTZ and MP2/6-31G*. B3LYP/6-31+G(d,2p) structures never deviate from the MP2/haTZ geometries by more than 0.44 kcal mol–1 on the MP2/haTZ potential energy surface, whereas the errors associated with the HF/haTZ and MP2/6-31G* structures grow as large as 12.20 and 2.98 kcal mol–1, respectively. The most accurate results, however, were obtained with the two-body:many-body QM:QM fragmentation method for weakly bound clusters, in which all one- and two-body interactions are calculated at the high-level, while a low-level calculation is performed on the entire cluster to capture the cooperative effects (nonadditivity). With the haTZ basis set, the MP2:HF two-body:many-body fragmentation method generates structures that deviate from the MP2/haTZ ones by 0.01 kcal mol–1 on average and not by more than 0.03 kcal mol–1.
We describe programs developed and implemented at the University of Wisconsin–Madison Department of Chemistry aimed at increasing representation and improving outcomes for graduate students from ...underrepresented minority groups. We briefly describe our Research Experience for Undergraduate (REU), Chemistry Opportunities (CHOPS), Catalyst, and Bridge to the Chemistry Doctorate Programs. Our experiences have yielded both successes and failures that provide direction for future improvements and continuing efforts.
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