Christo S. Sevov

Iridium-Catalyzed Intermolecular Asymmetric Hydroheteroarylation of Bicycloalkenes

Catalytic hydroarylation of alkenes is a desirable process because it can occur under neutral conditions with regioselectivity complementary to that of acid-catalyzed reactions and stereoselectivity derived from the catalyst. We report an intermolecular asymmetric addition of the C-H bonds of indoles, thiophenes, pyrroles, and furans to bicycloalkenes in high yield with high enantiomeric excess. These heteroarene alkylations occur ortho to the heteroatom. This selectivity is observed even with unprotected indoles, which typically undergo alkylation at the C3 position. Initial mechanistic studies revealed that oxidative addition of a heteroarene C-H bond to a neutral Ir(I) species occurs within minutes at room temperature and occurs in the catalytic cycle prior to the turnover-limiting step. Products from syn addition of the C-H bond across the olefin were observed.

Iridium-Catalyzed Intermolecular Hydroamination of Unactivated Aliphatic Alkenes with Amides and Sulfonamides

The intermolecular addition of N-H bonds to unactivated alkenes remains a challenging, but desirable, strategy for the synthesis of N-alkylamines. We report the intermolecular amination of unactivated α-olefins and bicycloalkenes with arylamides and sulfonamides to generate synthetically useful protected amine products in high yield. Mechanistic studies on this rare catalytic reaction revealed a resting state that is the product of N-H bond oxidative addition and coordination of the amide. Rapid, reversible dissociation of the amide precedes reaction with the alkene, but an intramolecular, kinetically significant rearrangement of the species occurs before this reaction with alkene.

Evolutionary Design of Low Molecular Weight Organic Anolyte Materials for Applications in Nonaqueous Redox Flow Batteries.

The integration of renewable energy sources into the electric grid requires low-cost energy storage systems that mediate the variable and intermittent flux of energy associated with most renewables. Nonaqueous redox-flow batteries have emerged as a promising technology for grid-scale energy storage applications. Because the cost of the system scales with mass, the electroactive materials must have a low equivalent weight (ideally 150 g/(mol·e(-)) or less), and must function with low molecular weight supporting electrolytes such as LiBF4. However, soluble anolyte materials that undergo reversible redox processes in the presence of Li-ion supports are rare. We report the evolutionary design of a series of pyridine-based anolyte materials that exhibit up to two reversible redox couples at low potentials in the presence of Li-ion supporting electrolytes. A combination of cyclic voltammetry of anolyte candidates and independent synthesis of their corresponding charged-states was performed to rapidly screen for the most promising candidates. Results of this workflow provided evidence for possible decomposition pathways of first-generation materials and guided synthetic modifications to improve the stability of anolyte materials under the targeted conditions. This iterative process led to the identification of a promising anolyte material, N-methyl 4-acetylpyridinium tetrafluoroborate. This compound is soluble in nonaqueous solvents, is prepared in a single synthetic step, has a low equivalent weight of 111 g/(mol·e(-)), and undergoes two reversible 1e(-) reductions in the presence of LiBF4 to form reduced products that are stable over days in solution.

Iridium-Catalyzed, Intermolecular Hydroamination of Unactivated Alkenes with Indoles

The addition of an N-H bond to an olefin is the most direct route for the synthesis of alkylamines. Currently, intermolecular hydroamination is limited to reactions of a narrow range of reagents containing N-H bonds or activated alkenes, and all the examples of additions to unactivated alkenes require large excesses of alkene. We report intermolecular hydroamination reactions of indoles with unactivated olefins. The reactions occur with as few as 1.5 equiv of olefin to form N-alkylindoles exclusively and in good yield. Characterizations of the catalyst resting state, kinetic data, labeling studies, and computational data imply that the addition occurs by olefin insertion into the Ir-N bond of an N-indolyl complex and that this insertion reaction is faster than insertion of olefin into the Ir-C bond of the isomeric C-2-indolyl complex.

Iridium-catalyzed oxidative olefination of furans with unactivated alkenes.

The oxidative coupling of arenes and alkenes is an attractive strategy for the synthesis of vinylarenes, but reactions with unactivated alkenes have typically occurred in low yield. We report an Ir-catalyzed oxidative coupling of furans with unactivated olefins to generate branched vinylfuran products in high yields and with high selectivities with a second alkene as the hydrogen acceptor. Detailed mechanistic experiments revealed catalyst decomposition pathways that were alleviated by the judicious selection of reaction conditions and application of new ligands.

Physical Organic Approach to Persistent, Cyclable, Low-Potential Electrolytes for Flow Battery Applications

The deployment of nonaqueous redox flow batteries for grid-scale energy storage has been impeded by a lack of electrolytes that undergo redox events at as low (anolyte) or high (catholyte) potentials as possible while exhibiting the stability and cycling lifetimes necessary for a battery device. Herein, we report a new approach to electrolyte design that uses physical organic tools for the predictive targeting of electrolytes that possess this combination of properties. We apply this approach to the identification of a new pyridinium-based anolyte that undergoes 1e- electrochemical charge-discharge cycling at low potential (-1.21 V vs Fc/Fc+) to a 95% state-of-charge without detectable capacity loss after 200 cycles.

Iridium-Catalyzed, Intermolecular Hydroetherification of Unactivated Aliphatic Alkenes with Phenols

Metal-catalyzed addition of an O-H bond to an alkene is a desirable process because it allows for rapid access to ethers from abundant starting materials without the formation of waste, without rearrangements, and with the possibility to control the stereoselectivity. We report the intermolecular, metal-catalyzed addition of phenols to unactivated α-olefins. Mechanistic studies of this rare catalytic reaction revealed a dynamic mixture of resting states that undergo O-H bond oxidative addition and subsequent olefin insertion to form ether products.

Macromolecular Design Strategies for Preventing Active-Material Crossover in Non-Aqueous All-Organic Redox-Flow Batteries

Intermittent energy sources, including solar and wind, require scalable, low-cost, multi-hour energy storage solutions in order to be effectively incorporated into the grid. All-Organic non-aqueous redox-flow batteries offer a solution, but suffer from rapid capacity fade and low Coulombic efficiency due to the high permeability of redox-active species across the batterys membrane. Here we show that active-species crossover is arrested by scaling the membranes pore size to molecular dimensions and in turn increasing the size of the active material above the membranes pore-size exclusion limit. When oligomeric redox-active organics (RAOs) were paired with microporous polymer membranes, the rate of active-material crossover was reduced more than 9000-fold compared to traditional separators at minimal cost to ionic conductivity. This corresponds to an absolute rate of RAO crossover of less than 3 μmol cm-2  day-1 (for a 1.0 m concentration gradient), which exceeds performance targets recently set forth by the battery industry. This strategy was generalizable to both high and low-potential RAOs in a variety of non-aqueous electrolytes, highlighting the versatility of macromolecular design in implementing next-generation redox-flow batteries.

Selectivity in the Electron Transfer Catalyzed Diels−Alder Reaction of (R)-α-Phellandrene and 4-Methoxystyrene

Electron transfer catalysis is an effective method for the acceleration of Diels-Alder reactions between two substrates of similar electron density. The dependence of the selectivity of the Diels-Alder reaction between (R)-alpha-phellandrene and 4-methoxystyrene catalyzed by photoinduced electron transfer with tris(4-methoxyphenyl) pyrylium tetrafluoroborate is studied. Despite the fact that the radical ions involved are highly reactive species, complete regioselectivity favoring attack on the more highly substituted double bond is observed. The endo/exo selectivity and the periselectivity between [4 + 2] and [2 + 2] cycloaddition is found to be solvent-dependent. Stereochemical analysis showed that the periselectivity is correlated with the facial selectivity, with attack trans to the isopropyl group leading to the [4 + 2] product and cis attack leading to the formation of the [2 + 2] product. A good correlation between the dielectric constant of the solvent and the endo/ exo ratio is found, but more polar solvents lead to lower periselectivity. The effect of reactant and catalyst concentrations is found to be smaller. These results are rationalized in the context of the relative stability of the ion-molecule complexes and the singly linked intermediate of the reaction.

High-Performance Oligomeric Catholytes for Effective Macromolecular Separation in Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries (NRFBs) represent an attractive technology for energy storage from intermittent renewable sources. In these batteries, electrical energy is stored in and extracted from electrolyte solutions of redox-active molecules (termed catholytes and anolytes) that are passed through an electrochemical flow cell. To avoid battery self-discharge, the anolyte and catholyte solutions must be separated by a membrane in the flow cell. This membrane prevents crossover of the redox active molecules, while simultaneously allowing facile transport of charge-balancing ions. A key unmet challenge for the field is the design of redox-active molecule/membrane pairs that enable effective electrolyte separation while maintaining optimal battery properties. Herein, we demonstrate the development of oligomeric catholytes based on tris(dialkylamino)cyclopropenium (CP) salts that are specifically tailored for pairing with size-exclusion membranes composed of polymers of intrinsic microporosity (PIMs). Systematic studies were conducted to evaluate the impact of oligomer size/structure on properties that are crucial for flow battery performance, including cycling stability, charge capacity, solubility, electron transfer kinetics, and crossover rates. These studies have led to the identification of a CP-derived tetramer in which these properties are all comparable, or significantly improved, relative to the monomeric counterpart. Finally, a proof-of-concept flow battery is demonstrated by pairing this tetrameric catholyte with a PIM membrane. After 6 days of cycling, no crossover is detected, demonstrating the promise of this approach. These studies provide a template for the future design of other redox-active oligomers for this application.