Development of the Squaramide Scaffold for High Potential and Multielectron Catholytes for Use in Redox Flow Batteries.

Nonaqueous organic redox flow batteries (N-ORFBs) are a promising technology for grid-scale storage of energy generated from intermittent renewable sources. Their primary benefit over traditional aqueous RFBs is the wide electrochemical stability window of organic solvents, but the design of catholyte materials, which can exploit the upper range of this window, has proven challenging. We report herein a new class of N-ORFB catholytes in the form of squaric acid quinoxaline (SQX) and squaric acid amide (SQA) materials.

Indolo[2,3-]quinoxaline as a Low Reduction Potential and High Stability Anolyte Scaffold for Nonaqueous Redox Flow Batteries.

Redox flow batteries (RFBs) are a promising stationary energy storage technology for leveling power supply from intermittent renewable energy sources with demand. A central objective for the development of practical, scalable RFBs is to identify affordable and high-performance redox-active molecules as storage materials. Herein, we report the design, synthesis, and evaluation of a new organic scaffold, indolo[2,3-]quinoxaline, for highly stable, low-reduction potential, and high-solubility anolytes for nonaqueous redox flow batteries (NARFBs).

Development of high-voltage bipolar redox-active organic molecules through the electronic coupling of catholyte and anolyte structures.

All-organic non-aqueous redox flow batteries (O-NRFBs) are a promising technology for grid-scale energy storage. However, most examples of high-voltage (>2 V) O-NRFBs rely upon the use of distinct anolytes and catholytes separated by a membrane or porous separator which can result in crossover of redox active material from one side of the battery to the other. The resulting electrolyte mixing leads to irreversible reductions in energy density and capacity.

Catalytic Asymmetric Synthesis of the Pentacyclic Core of (+)-Citrinadin A.

The synthesis of the pentacylic core of (+)-citrinadin A is described. Our strategy harnesses the power of palladium-catalyzed trimethylenemethane chemistry (Pd-TMM) to form the key spirooxindole motif in a catalytic, asymmetric fashion. Upon the conversion of this spirooxindole to a vinyl epoxide electrophile, the piperidine ring is directly added via a diastereoselective metalation followed by an S2' addition.

Catalytically Generated Vanadium Enolates Formed via Interruption of the Meyer-Schuster Rearrangement as Useful Reactive Intermediates.

Enolate chemistry is one of the most fundamental strategies for the formation of carbon-carbon and carbon-heteroatom bonds. Classically, this has been accomplished through the use of stoichiometric quantities of strong base and cryogenic reaction temperatures. However, these techniques present issues related to enolate regioselectivity and functional group tolerance.

Ruthenium-catalysed multicomponent synthesis of the 1,3-dienyl-6-oxy polyketide motif.

Polyketide natural products are an important class of biologically active compounds. Although substantial progress has been made on the synthesis of repetitive polyketide motifs through the iterative application of a single reaction type, synthetic access to more diverse motifs that require more than one type of carbon-carbon bond connection remains a challenge. Here we describe a catalytic, multicomponent method for the synthesis of the privileged polyketide 1,3-dienyl-6-oxy motif.

Acyclic Branched α-Fluoro Ketones for the Direct Asymmetric Mannich Reaction Leading to the Synthesis of β-Tetrasubstituted β-Fluoro Amines.

The preparation of acyclic β-fluoro amines bearing tetrasubstituted fluorine stereocenters is described via a direct Zn/ProPhenol-catalyzed Mannich reaction. The reaction utilizes branched vinyl or alkynyl α-fluoro ketones that can be coupled with a range of aryl, heteroaryl, vinyl, or cyclopropyl aldimines in high yield and with excellent diastereo- (up to >20:1) and enantioselectivity (up to 99 %). The use of readily cleaved tert-butoxycarbonyl (Boc) or carboxybenzyl (Cbz) imine protecting groups adds utility to the reaction by allowing for easy access to the free amine products under mild and chemoselective reaction conditions.

Vanadium-Catalyzed Coupling of Allenols with Electrophilic Halide Sources for the Formation of α-Halo-α',β'-unsaturated Ketones.

A vanadium-catalyzed coupling of allenylic alcohols with electrophilic halide sources to form α-halo-α',β'-unsaturated ketones is described. The process proceeds through a metal enolate formed from the 1,3-transposition of an allenol that is initiated by a cheap and earth-abundant vanadium oxo catalyst. Fluorine, chlorine, and bromine electrophiles can be utilized, and the resulting products can give rise to the introduction of nitrogen, oxygen, sulfur, and iodine nucleophiles α to the ketone through substitution chemistry.

Direct Enantio- and Diastereoselective Vinylogous Addition of Butenolides to Chromones Catalyzed by Zn-ProPhenol.

We report the first enantio- and diastereoselective 1,4-addition of butenolides to chromones. Both α,β- and β,γ-butenolide nucleophiles are compatible with the Zn-ProPhenol catalyst, and preactivation as the siloxyfurans is not required. The scope of electrophiles includes a variety of substituted chromones, as well as a thiochromone and a quinolone, and the resulting vinylogous addition products are generated in good yield (31 to 98%), diastereo- (3:1 to >30:1), and enantioselectivity (90:10 to 99:1 er).

Highly Chemoselective Deprotection of the 2,2,2-Trichloroethoxycarbonyl (Troc) Protecting Group.

Nonreducing, pH-neutral conditions for the selective cleavage of the 2,2,2-trichloroethoxycarbonyl (Troc) protecting group are reported. Using trimethyltin hydroxide in 1,2-dichloroethane, Troc-protected alcohols, thiols, and amines can be selectively unmasked in the presence of various functionalities that are incompatible with the reducing conditions traditionally used to remove the Troc group. This mild deprotection protocol tolerates a variety of other hydrolytically sensitive and acid/base-sensitive moieties as well.

Zn-ProPhenol Catalyzed Enantio- and Diastereoselective Direct Vinylogous Mannich Reactions between α,β- and β,γ-Butenolides and Aldimines.

We report a Zn-ProPhenol catalyzed reaction between butenolides and imines to obtain tetrasubstituted vinylogous Mannich products in good yield and diastereoselectivity with excellent enantioselectivity (97 to >99.5% ee). Notably, both α,β- and β,γ-butenolides can be utilized as nucleophiles in this transformation.

Carbon-Nitrogen Bond Formation via the Vanadium Oxo Catalyzed Sigmatropic Functionalization of Allenols.

A vanadium catalyzed 1,3-rearrangement of allenols to form transient vanadium enolates that selectively couple with electrophilic nitrogen sources is reported even in the presence of competing simple protonation and Alder-ene pathways. Hydrazine products can be cyclized in a 6-endo-trig fashion which, upon reductive cleavage of the N-N bond, yield 1,4-diamines. Additionally, cleavage of the N-N bond before cyclization can be achieved to form β-hydroxy amines, a common structural motif of biologically active compounds.

Contemporaneous Dual Catalysis: Aldol Products from Non-Carbonyl Substrates.

The aldol reaction represents an important class of atom-economic carbon-carbon bond-forming reactions vital to modern organic synthesis. Despite the attention this reaction has received, issues related to chemo- and regioselectivity as well as reactivity of readily enolizable electrophiles remain. To help overcome these limitations, a new direct approach toward aldol products that does not rely upon carbonyl substrates is described.