Rapid and Modular Access to Multifunctionalized 1,2-Azaborines via Palladium/Norbornene Cooperative Catalysis

Expedient access to bora-butenolide bioisosteres by counteranion-mediated trans-hydroboration of alkynes

The hydroboration of alkynes is a textbook example of a syn-selective concerted addition reaction, while trans-selective additions of borane to alkynes remain to be developed. We herein report a transition metal-free anti-addition of pinacolborane to alkynes, facilitated by the counteranion effect. This work further develops Chan alkyne reduction by utilizing the borane instead of aluminohydride reagents, enabling the facile synthesis of valuable five-membered boracycles that constitute isosteric alternatives to bioactive butenolides and a versatile platform for abundant downstream transformations. The practical method is distinguished by excellent regioselectivity, a broad substrate scope, and high compatibility with a variety of functional groups. The exploration of trans-selective patterns affords not only a stereo-complementary approach to traditional organic synthesis, but also mandates a new perspective on the noncanonical trans-hydroboration mechanism. A combination of control experiments and computational studies at the DFT level of theory reveal the previously unrecognized role of the HMDS counteranion in a stepwise intermolecular hydrogen transfer process.

BN-embedded aromatic hydrocarbons: synthesis, functionalization and applications

Substituting CC double bonds with B-N bonds in polycyclic aromatic hydrocarbons (PAHs) has emerged as a promising approach to advance and diversify organic functional materials. This structural modification not only imparts unique electronic and optical properties, but also enhances chemical stability, thereby opening new avenues for material design and applications. However, the widespread adoption of BN-fused aromatic hydrocarbons in practical applications is still in its nascent phase. This constraint stems primarily from the challenges in precisely tailoring molecular structures to optimize photophysical and electronic properties, thereby influencing their efficacy in targeted applications. Consequently, a comprehensive evaluation of historical, current, and prospective developments in BN-fused aromatic hydrocarbons is deemed essential. This review offers an in-depth overview of recent advancements in BN-fused aromatic hydrocarbons, focusing on synthetic strategies, fundamental properties, and emerging applications. Additionally, we elucidate the pivotal role of computational chemistry in directing the design, discovery, and optimization of these materials. Our objective is to foster interdisciplinary collaboration and stimulate innovative approaches to fully harness the potential of azaborinine chemistry across various fields, including organic optoelectronics, biomedicine, and related disciplines.

Synthesis, Structure, and Properties of Two Planar BN-Benzofluorenes

Two planar BN doped benzofluorenes (BN-BkF and BN-BjF) were synthesized separately through palladium-catalyzed intramolecular C-C or C-N coupling reactions. The structures of both BN-BkF and BN-BjF are unambiguously confirmed by X-ray crystallographic analysis. Moreover, the BN unit doping leads to a much lower HOMO level and higher LUMO level as compared to their carbon analogues BkF and BjF. In comparison to BkF and BjF, both BN-BkF and BN-BjF exhibit blue shifts in their ultraviolet-visible (UV-vis) absorption and fluorescence emission spectra. Furthermore, halogenation of BN-BkF and BN-BjF afforded monohalogenated BN-benzofluorenes in good yields. These monohalogenated BN-benzofluorenes can serve as convenient intermediates for palladium-catalyzed coupling reactions to yield a series of functionalized BN-benzofluorene derivatives. The UV-vis absorption and emission spectroscopies in dichloromethane of these BN-benzofluorene derivatives were studied, and the photophysics of these compounds exhibited a high degree of substituent dependency.

Palladium/Norbornene Cooperative Catalysis 2-Fold C-H/C-X Coupling: Construction of Polycyclic Aromatic Hydrocarbons from Brominated Benzimidazoles

A palladium/norbornene (NBE)-catalyzed 2-fold C-H/C-X coupling reaction of aryl iodides is reported. Bromine-substituted benzimidazoles were chosen as electrophilic and termination reagents, and the versatile polycyclic aromatic hydrocarbon products were successfully obtained. The strategy overcomes the challenge of catalyst poisoning by heterocyclic substrates. In addition, the imidazole moiety in the product is endowed with a localization role, thus enabling the compounds to be applied in a wider synthetic scenario, and the fluorescence persisted. Furthermore, the bioactivity evaluation has identified three promising leading compounds 3b, 4e, and 4i.

Synthesis of Polysubstituted Aromatics via the Pd(II)-Initiated Borono-Catellani Reaction

Herein, we report a novel palladium(II)-initiated borono-Catellani reaction that utilizes widely accessible aryl boronic acids that serve as both the reaction-initiating and -terminating substrates for the first time. The borono-Catellani reaction was facilitated by cooperative catalysis between Pd(OAc)2 and NBE, with air serving as the oxidizing agent, opening new venues for developing novel Catellani-type reactions. Importantly, this method is compatible with a wide range of substrates, including naphthaleneboronic acid, phenylboric acid derivatives, alkyl iodides, and aryl iodides, under these environmentally friendly and mild reaction conditions, thereby demonstrating versatile functional group compatibility for the synthesis of valuable polysubstituted aromatics.

Palladium-Catalyzed Enantioselective Synthesis of P(V)-Stereogenic Compounds via Desymmetric Annulation of Prochiral Phosphinamides and Aryl Iodides

The enantioselective synthesis of P(V)-stereogenic compounds has emerged as an interesting research topic primarily due to their significant biological activity and broad application prospects. Herein, we disclose a method for the construction of P(V)-stereogenic compounds from prochiral phosphinamides and aryl iodides via palladium- and chiral norbornene-catalyzed desymmetric annulation. The P(V)-stereogenic compounds were formed with a broad scope with excellent enantiomeric excesses. It is noteworthy that the synthetic value of this procedure was proven by a variety of transition metal-catalyzed cross-coupling reactions using the C-Br bond on the product as a versatile linchpin electrophile.

Palladium-Catalyzed Dual C-H Arylation/Cyclization Reaction of Iodoferrocenes with ortho-Bromobenzamides for the Construction of Arylated Isoquinolone-Fused Ferrocenes

We reported a palladium/norbornene-catalyzed dual intermolecular C-H arylation/intramolecular cyclization reaction of iodoferrocenes with ortho-bromobenzamides, enabling the formation of arylated isoquinolone-fused ferrocenes in a straightforward and effective manner. This method has a broad substrate scope and good functional group compatibility, while the gram-scale reaction demonstrates the practicality of this method.

Palladium/norbornene-catalyzed C-H/N-H cycloaddition of carbazoles with 2-halobenzoic acid derivatives

An example of palladium/norbornene-catalyzed C-H/N-H cycloaddition of carbazoles with 2-bromobenzoic acids is presented, in which a collection of important carbazoles is expeditiously obtained. Derivatives, including acyl halides, α-oxocarboxylic acids, anhydrides, and even amides, are all allowed. Preliminary mechanistic studies reveal that a rare six-membered spiropalladacycle is involved.

Solvent-Controlled Enantiodivergent Construction of P(V)-Stereogenic Molecules via Palladium-Catalyzed Annulation of Prochiral N-Aryl Phosphonamides with Aromatic Iodides

In this work, we describe an efficient and modular method for enantiodivergent accessing P(V)-stereogenic molecules by utilizing the catalytic atroposelective Catellani-type C-H arylation/desymmetric intramolecular N-arylation cascade reaction. The enantioselectivity of this protocol was proved to be tuned by the polarity of the solvent, thus providing a wide range of both chiral P(V)-stereogenic enantiomers in moderate to good yields with good to excellent enantiomeric excesses. Noteworthy is that the strategy developed herein represents an unprecedented example of solvent-dictated inversion of the enantioselectivity of P(V)-stereogenic compounds.

Removing Neighboring Ring Influence in Monocyclic B-OH Diazaborines: Properties and Reactivity as Phenolic Bioisosteres with Dynamic Hydroxy Exchange

The design of small molecules with unique geometric profiles or molecular connectivity represents an intriguing yet neglected challenge in modern organic synthesis. This challenge is compounded when emphasis is placed on the preparation of new chemotypes that have distinct and practical functions. To expand the structural diversity of boron-containing heterocycles, we report herein the preparation of novel monocyclic hemiboronic acids, diazaborines. These compounds have enabled the study of a pseudoaromatic boranol-containing (B-OH) ring free of influence from an appended aromatic system. Synthetic and spectroscopic studies have provided insight into the aromatic character, Lewis acidic nature, chemical reactivity, and unique ability of the exocyclic B-OH unit to participate in hydroxy exchange, suggesting their use in organocatalysis and as reversible covalent inhibitors. Moreover, density functional theory and nucleus-independent chemical shift calculations reveal that the aromatic character of the boroheterocyclic ring is increased significantly in comparison to known bicyclic benzodiazaborines (naphthoid congeners), consequently leading to attenuated Lewis acidity. Direct structural comparison to a well-established biaryl isostere, 2-phenylphenol, through X-ray crystallographic analysis reveals that N-aryl derivatives are strikingly similar in size and conformation, with attenuated logP values underscoring the value of the polar BNN unit. Their potential application as low-molecular-weight scaffolds in drug discovery is demonstrated through orthogonal diversification and preliminary antifungal evaluation (Candida albicans), which unveiled analogs with low micromolar inhibitory concentration.

Modular Synthesis of Polysubstituted α-Phosphorylated Arenes via the Catellani Strategy

In this paper, we described a palladium/norbornene-catalyzed ortho-C-H phosphormethylation of aryl iodides using XCH2P(O)RR', offering a reliable method for the modular synthesis of polysubstituted α-phosphorylated arenes. Alkenylation, hydrogenation, cyanation, methylation, and arylation were all viable termination steps compatible with the reaction. This method demonstrates excellent functional group tolerance and can be extended to the late-stage modification of bioactive molecules. Furthermore, the synthetic transformations of the products demonstrate the practical utility of this reaction.

Switchable Site-Selective Benzanilide C(sp2)-H Bromination via Promoter Regulation

Regioselective benzanilide bromination that generates either regioisomer from the same starting material is desirable. Herein, we develop switchable site-selective C(sp2)-H bromination by promoter regulation. This protocol leads to regiodivergent brominated benzanilide starting from the single substrate via selection of promoters. The protocol demonstrates excellent regioselectivity and good tolerance of functional groups with high yields. The utility effectiveness of this method has been well exemplified in the late-stage modification of biologically important molecules.