Mechanistic Details of Asymmetric Bromocyclization with BINAP Monoxide: Identification of Chiral Proton-Bridged Bisphosphine Oxide Complex and Its Application to Parallel Kinetic Resolution

Switchable organocatalytic enantioselective sulfenocyclization of cyclohexadienes enabling chemodivergent access to chiral bicyclo[m.n.1] ring systems

The precise control over regio- and stereoselectivity from the same substrate represents a significant challenge in organic chemistry. Herein, a switchable organocatalytic enantioselective carbosulfenylation/sulfenolactonization of cyclohexa-1,4-dienes to access the chiral bicyclo[m.n.1] ring systems, which are the critical core skeleton of many important natural products and biologically active compounds, is achieved. By simply tuning the substituent of the sulfenylating agent, a series of synthetically challenging chiral bridged bicyclo[3.3.1]nonanes and 2-oxabicyclo[3.2.1]octanes bearing three consecutive stereocenters are obtained with good yields and excellent enantioselectivities (up to 94% yield and 97% ee). Furthermore, the initial investigation of the bicyclic derivative as a chiral ligand in metal catalysis is also conducted. Our findings offer a version of switchable divergent asymmetric synthesis in which different products can be controllably generated from an identical set of substrates by simply adjusting reaction parameters.

Functionalized Terthiophene as an Ambipolar Redox System: Structure, Spectroscopy, and Switchable Proton-Coupled Electron Transfer

Organic redox systems that can undergo oxidative and reductive (ambipolar) electron transfer are elusive yet attractive for applications across synthetic chemistry and energy science. Specifically, the use of ambipolar redox systems in proton-coupled electron transfer (PCET) reactions is largely unexplored but could enable "switchable" reactivity wherein the uptake and release of hydrogen atoms are controlled using a redox stimulus. Here, we describe the synthesis and characterization of an ambipolar functionalized terthiophene (TTH) bearing methyl thioether and phosphine oxide groups that exhibits switchable PCET reactivity. Electrochemical studies established that the functionalized TTH can be reversibly oxidized and reduced, prompting the synthesis and characterization of cationic and anionic radicals on a single TTH platform. Combined structural, spectroscopic, and computational investigations revealed the influence of the methyl thioether and phosphine oxide moieties on the TTH electronic structure that results in the stabilization of both cationic and anionic radicals. Upon single-electron oxidation, the functionalized TTH serves as a hydrogen atom acceptor and undergoes PCET with 1,4-dihydroquinone to generate a TTH hydroxyphosphonium species. The process was found to be reversible upon single-electron reduction, with functionalized TTH acting as a hydrogen atom donor in a PCET reaction with 2,3-dimethylanthraquinone. The thermochemistry of the O-H bond formed and cleaved in functionalized TTH during the reaction sequence was investigated, revealing that a bond weakening of 30 kcal/mol underpins the switchable PCET reactivity. Overall, these studies provide an electrochemical, structural, spectroscopic, and thermochemical foundation for the use of ambipolarity to control PCET reactions in organic redox systems.

Triphenylphosphine Oxide: A Versatile Covalent Functionality for Carbon Nanotubes

Broadening the scope of functionalities that can be covalently bound to single-walled carbon nanotubes (SWCNTs) is crucial for enhancing the versatility of this promising nanomaterial class in applied settings. Here we report the covalent linkage of triphenylphosphine oxide [Ph3P(O)] to SWCNTs, a hitherto overlooked surface functionality. We detail the synthesis and structural characterization of a new family of phosphine oxide-functionalized diaryliodonium salts that can facilitate direct Ph3P(O) transfer and afford novel SWCNTs with tunable Ph3P(O) content (SWCNT-P). The molecularly-distributed and robust nature of the covalent Ph3P(O) attachment in SWCNT-P was supported by a combination of characterization methods including Raman, infrared, UV/Vis-NIR and X-ray photoelectron spectroscopies coupled with thermogravimetric analysis. Electron microscopy further revealed the effectiveness of the Ph3P(O) moiety for de-bundling SWCNTs to yield SWCNT-P with superior dispersibility and processability. Finally, electrochemical studies established that SWCNT-P is sensitive to the presence of Li+, Na+ and K+ wherein the Gutmann-Beckett Lewis acidity parameters of the ions were quantitatively transduced by Ph3P(O) to electrochemical responses. This work hence presents a synthetic, structural, spectroscopic and electrochemical foundation for a new phosphorus-enriched responsive nanomaterial platform featuring the Ph3P(O) functionality.

Chemodivergent Parallel Kinetic Resolution of Paracyclophanes: Enantiomer Fishing with Different Substrates

An unprecedented chemodivergent strategy for parallel kinetic resolution (PKR) is disclosed through which two planar chiral products bearing different structures were simultaneously afforded with opposite stereoselectivities. Two achiral esters are activated by one single chiral N-heterocyclic carbene (NHC) catalyst to react with the different enantiomers of the racemic imine substrate in a parallel fashion. Two products bearing distinct structures and opposite stereoselectivities are respectively afforded from the same reaction system in good to excellent yields, enantio- and diastereoselectivities. Control experiments and kinetic studies are carried out to probe the kinetic and dynamic properties during the reaction progress. The planar chiral pyridine and lactam products show interesting applications in both asymmetric synthesis and pesticide development.

Cooperative Photoredox and Cobalt-Catalyzed Acceptorless Dehydrogenative Functionalization of Cyclopropylamides towards Allylic N,O-Acyl-acetal Derivatives

We disclose herein a novel photoredox and cobalt co-catalyzed ring-opening/acceptorless dehydrogenative functionalization of mono-donor cyclopropanes. This sustainable and atom-economic approach allows the rapid assembly of a wide range of allylic N,O-acyl-acetal derivatives. The starting materials are readily available and the reaction features mild conditions, broad substrate scope, and excellent functional group compatibility. The optimized conditions accommodate assorted cycloalkylamides and primary, secondary, and tertiary alcohols, with applications in late-stage functionalization of pharmaceutically relevant compounds, stimulating further utility in medicinal chemistry. Moreover, selective nucleophilic substitutions with various carbon nucleophiles were achieved in a one-pot fashion, offering a reliable avenue to access some cyclic and acyclic derivatives.

Intramolecular chaperone-assisted dual-anchoring activation (ICDA): a suitable preorganization for electrophilic halocyclization

The halocyclization reaction represents one of the most common methodologies for the synthesis of heterocyclic molecules. Many efforts have been made to balance the relationship between structure, reactivity and selectivity, including the design of new electrophilic halogenation reagents and the utilization of activating strategies. However, discovering universal reagents or activating strategies for electrophilic halocyclization remains challenging due to the case-by-case practice for different substrates or different cyclization models. Here we report an intramolecular chaperone-assisted dual-anchoring activation (ICDA) model for electrophilic halocyclization, taking advantage of the non-covalent dual-anchoring orientation as the driving force. This protocol allows a practical, catalyst-free and rapid approach to access seven types of small-sized, medium-sized, and large-sized heterocyclic units and to realize polyene-like domino halocyclizations, as exemplified by nearly 90 examples, including a risk-reducing flow protocol for gram-scale synthesis. DFT studies verify the crucial role of ICDA in affording a suitable preorganization for transition state stabilization and X+ transfer acceleration. The utilization of the ICDA model allows a spatiotemporal adjustment to straightforwardly obtain fast, selective and high-yielding synthetic transformations.

Phosphonium-Catalyzed Monoreduction of Bisphosphine Dioxides: Origin of Selectivity and Synthetic Applications

Controlling product selectivity in successive reactions of the same type is challenging owing to the comparable thermodynamic and kinetic properties of the reactions involved. Here, the synergistic interaction of the two phosphoryl groups in bisphosphine dioxides (BPDOs) with a bromo-phosphonium cation was studied experimentally to provide a practical tool for substrate-catalyst recognition. As the eventual result, we have developed a phosphonium-catalyzed monoreduction of chiral BPDOs to access an array of synthetically useful bisphosphine monoxides (BPMOs) with axial, spiro, and planar chirality, which are otherwise challenging to synthesize before. The reaction features excellent selectivity and impressive reactivity. It proceeds under mild conditions, avoiding the use of superstoichiometric amounts of additives and metal catalysts to simplify the synthetic procedure. The accessibility and scalability of the reaction allowed for the rapid construction of a ligand library for optimization of asymmetric Heck-type cyclization, laying the foundation for a broad range of applications of chiral BPMOs in catalysis.

trans-Cyclooctenes as Scavengers of Bromine Involved in Catalytic Bromination

Scavengers that capture reactive chemical substances are used to prevent the decomposition of materials. However, in the field of catalysis, the development of scavengers that inhibit background pathways has attracted little attention, although the concept will open up an otherwise inaccessible reaction space. In catalytic bromination, fast non-catalyzed background reactions disturb the catalytic control of the selectivity, even when using N-bromoamide reagents, which have a milder reactivity than bromine (Br2 ). Here, we developed a trans-cyclooctene (TCO) bearing a 2-pyridylethyl group to efficiently retard background reactions by capturing Br2 in bromocyclization using N-bromosuccinimide. The use of less than a stoichiometric amount of the TCO was sufficient to inhibit non-catalyzed reactions, and mechanistic studies using the TCO revealed that in situ-generated Br2 provides non-catalyzed reaction pathways based on a chain mechanism. The TCO is useful as an additive for improving enantioselectivity and regioselectivity in catalytic reactions. Cooperative systems using the TCO with selective catalysts offer an alternative strategy for optimizing catalyst-controlled selectivity during bromination. Moreover, it also served as an indicator of Br2 involved in catalytic reaction pathways; thus, the TCO was useful as a probe for mechanistic investigations into the involvement of Br2 in bromination reactions of interest.

Parallel kinetic resolution of aziridines via chiral phosphoric acid-catalyzed apparent hydrolytic ring-opening

We report a chiral phosphoric acid catalyzed apparent hydrolytic ring-opening reaction of racemic aziridines in a regiodivergent parallel kinetic resolution manner. Harnessing the acyloxy-assisted strategy, the highly stereocontrolled nucleophilic ring-opening of aziridines with water is achieved. Different kinds of aziridines are applicable in the process, giving a variety of enantioenriched aromatic or aliphatic amino alcohols with up to 99% yields and up to >99.5 : 0.5 enantiomeric ratio. Preliminary mechanistic study as well as product elaborations were inducted as well.