Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins.

Bicarbonate-based electrolytes are ubiquitous in aqueous electrochemical CO2 reduction, particularly in heterogenous catalysis, where they demonstrate improved catalytic performance relative to other buffers. In contrast, the presence of bicarbonate in organic electrolytes and its roles in homogeneous electrocatalysis remain underexplored. Here, we investigate the influence of bicarbonate on iron porphyrin-catalyzed electrochemical CO2 reduction. We show that bicarbonate is a viable proton donor in organic electrolyte (pKa = 20.8 in dimethyl sulfoxide) and that urea pendants in the second coordination sphere can be used to template bicarbonate in the vicinity of a molecular iron porphyrin catalyst. The templated binding of bicarbonate increases its acidity, resulting in a 1500-fold enhancement in catalytic rates relative to unmodified parent iron porphyrin. This work emphasizes the importance of bicarbonate speciation in wet organic electrolytes and establishes second-sphere bicarbonate templating as a design strategy to harness this adventitious acid and enhance CO2 reduction catalysis.

A Concise Total Synthesis of (±)-Vibralactone.

Disclosed is a five-step synthesis of (±)-vibralactone, a biologically active terpenoid natural product. A key photochemical valence isomerization of 3-prenyl-pyran-2-one produces both the all-carbon quaternary stereocenter and the ß-lactone at an early stage. Cyclopropanation of the resulting bicyclic ß-lactone produces a strained housane structure that is converted to the natural product through a sequential ring expansion and reduction strategy. This concise and modular route to the natural product provides the shortest total synthesis of (±)-vibralactone reported to date.

Main Group-Catalyzed Cationic Claisen Rearrangements via Vinyl Carbocations.

We report a catalytic C-O coupling/Claisen cascade reaction enabled by interception of vinyl carbocations with allyl ethers. The reaction utilizes commercially available borate salts as catalysts and is effective at constructing sterically hindered C-C bonds. The reaction mechanism is studied experimentally and computationally to support a charge-accelerated [3,3] rearrangement of a silyloxonium cation. Our reaction is also applied to the highly stereoselective synthesis of fully substituted vinyl ethers.

α-Vinylation of Ester Equivalents via Main Group Catalysis for the Construction of Quaternary Centers.

A methodology for the construction of sterically congested quaternary centers via the trapping of vinyl carbocations with silyl ketene acetals is disclosed. This main group-catalyzed α-vinylation reaction is advantageous as methods to access these congested motifs are limited. Moreover, ß,γ-unsaturated carbonyl moieties and tetrasubstituted alkenes are present in various bioactive natural products and pharmaceuticals, and this catalytic platform offers a means of accessing them using simple and inexpensive materials.

Catalytic asymmetric C-H insertion reactions of vinyl carbocations.

From the preparation of pharmaceuticals to enzymatic construction of natural products, carbocations are central to molecular synthesis. Although these reactive intermediates are engaged in stereoselective processes in nature, exerting enantiocontrol over carbocations with synthetic catalysts remains challenging. Many resonance-stabilized tricoordinated carbocations, such as iminium and oxocarbenium ions, have been applied in catalytic enantioselective reactions. However, their dicoordinated counterparts (aryl and vinyl carbocations) have not, despite their emerging utility in chemical synthesis. We report the discovery of a highly enantioselective vinyl carbocation carbon-hydrogen (C-H) insertion reaction enabled by imidodiphosphorimidate organocatalysts. Active site confinement featured in this catalyst class not only enables effective enantiocontrol but also expands the scope of vinyl cation C-H insertion chemistry, which broadens the utility of this transition metal-free C(sp3)-H functionalization platform.

Dewar Heterocycles as Versatile Monomers for Ring-Opening Metathesis Polymerization.

We report the utility of readily available heterocycles as precursors to unique ring-opening metathesis polymerization (ROMP) monomers. Photochemical valence isomerization reactions of pyridones, dihydropyridines, and pyrones dearomatize the parent heterocycles to their highly strained Dewar isomers, which readily engage in controlled ROMP reactions using Grubbs catalysts. This strategy is used to access polymer backbones that contain strained ß-lactam and azetidine cores, which can be further derivatized using post-polymerization chemistries. We demonstrate this through the synthesis of water-soluble ß-amino acid polymers that have potential applications as biomedical materials, along with the synthesis of highly-soluble poly(acetylene) derivatives, which have potential applications as organic conductive materials derived from bio-feedstock chemicals.

Positional effects of second-sphere amide pendants on electrochemical CO2 reduction catalyzed by iron porphyrins.

The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach to transforming this greenhouse gas into value-added carbon products with sustainable energy input. Inspired by natural bioinorganic systems that feature precisely positioned hydrogen-bond donors in the secondary coordination sphere to direct chemical transformations occurring at redox-active metal centers, we now report the design, synthesis, and characterization of a series of iron tetraphenylporphyrin (Fe-TPP) derivatives bearing amide pendants at various positions at the periphery of the metal core. Proper positioning of the amide pendants greatly affects the electrocatalytic activity for carbon dioxide reduction to carbon monoxide. In particular, derivatives bearing proximal and distal amide pendants on the ortho position of the phenyl ring exhibit significantly larger turnover frequencies (TOF) compared to the analogous para-functionalized amide isomers or unfunctionalized Fe-TPP. Analysis of TOF as a function of catalyst standard reduction potential enables first-sphere electronic effects to be disentangled from second-sphere through-space interactions, suggesting that the ortho-functionalized porphyrins can utilize the latter second-sphere property to promote CO2 reduction. Indeed, the distally-functionalized ortho-amide isomer shows a significantly larger through-space interaction than its proximal ortho-amide analogue. These data establish that proper positioning of secondary coordination sphere groups is an effective design element for breaking electronic scaling relationships that are often observed in electrochemical CO2 reduction.