Monobenzofused 1,4-azaborines: synthesis, characterization, and discovery of a unique coordination mode

Near-infrared phosphorescence in a ruthenium(II) complex equipped with a pyridyl-1,2-azaborine ligand

The 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene molecule Hazab-py has been successfully used, for the first time, as a ligand in a ruthenium(II) polypyridine complex A (with the formula [Ru(dtbbpy)2(azab-py)]+, where dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). This compound was characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and its electrochemical and photophysical properties were fully investigated and compared to those of its homoleptic analogue [Ru(dtbbpy)3]2+ (B), an archetypical mono-cationic cyclometalated complex C (with the formula [Ru(dtbbpy)2(ppy)]+, where Hppy = 2-phenylpyridine), and the more structurally similar analogue [Ru(dtbbpy)2(naft-py)]+ (D), where the B-N unit of the azaborine ligand is replaced by a standard CC one, resulting in the 2-(naphthalen-2-yl)pyridine ligand (Hnaft-py). The presence of the novel 1,2-azaborine ligand induces a 0.51 V decrease in the redox gap of complex A, compared to that of B, leading to electrochemical and photophysical properties that resemble those of C and D. Accordingly, the azaborine complex displays an emission band extending up to the near infrared region of the spectrum (with the maximum at 765 nm in room-temperature acetonitrile solution), arising from a triplet metal-to-ligand charge-transfer (3MLCT) state. As in the case of other mono-cationic cyclometalated ruthenium(II) complexes, A shows modest photoluminescence quantum yields (PLQYs), but higher PLQYs when compared to those of its direct CC analogue D (e.g., PLQY = 0.6 vs. 0.1% in a PMMA matrix at 298 K). Density functional theory (DFT) calculations were used to provide complete rationalization of the electronic properties of all the complexes and to identify lower-lying metal-centred triplets (3MC), responsible for the low PLQYs of such an azaborine-based ruthenium(II) complex.

The Tetrafluoroethylene-Mediated Ring-Closing Metathesis: Enabling Unfavored Reactions

While ring-closing metathesis (RCM) is a powerful method for constructing medium and large cyclic alkenes, its application to the synthesis of heterocycles faces considerable limitations. For instance, RCM of divinyloxyalkanes does not proceed under the conventional conditions of RCM. The challenge lies in the formation of stable Fischer-type carbene intermediates with heteroatom(s) bound to the carbene carbon, impeding subsequent metathesis. In the present research, we develop a novel reaction system termed "tetrafluoroethylene (TFE)-mediated RCM", which enables the RCM of divinyloxyalkanes. In this system, a divinyloxyalkane and a TFE molecule are converted to the corresponding ring-closing product (cyclic 1,2-dioxyethene) and two molecules of 1,1-difluoroethylene. This method facilitates the construction of six- to eight-membered rings with various functional groups. The addition of TFE has two key effects: thermodynamically, it renders the entire reaction exergonic, while kinetically, it ensures that all ruthenium-carbene intermediates in the catalytic cycle are Fischer-type.

Orienting Group Directed Cascade Borylation for Efficient One-Shot Synthesis of 1,4-BN-Doped Polycyclic Aromatic Hydrocarbons as Narrowband Organic Emitters

1,4-BN-doped polycyclic aromatic hydrocarbons (PAHs) have emerged as very promising emitters in organic light-emitting diodes (OLEDs) due to their narrowband emission spectra that may find application in high-definition displays. While considerable research has focused on investigating the properties of these materials, less attention has been placed on their synthetic methodology. Here we developed an efficient synthetic method for 1,4-BN-doped PAHs, which enables sustainable production of narrowband organic emitting materials. By strategically introducing substituents, such as methyl, tert-butyl, phenyl, and chloride, at the C5 position of the 1,3-benzenediamine substrates, we achieved remarkable regioselective borylation in the para-position of the substituted moiety. This approach facilitated the synthesis of a diverse range of 1,4-BN-doped PAHs emitters with good yields and exceptional regioselectivity. The synthetic method demonstrated excellent scalability for large-scale production and enabled late-stage transformation of the borylated products. Mechanistic investigations provided valuable insights into the pivotal roles of electron effect and steric hindrance effect in achieving highly efficient regioselective borylation. Moreover, the outstanding device performance of the synthesized compounds 10 b and 6 z, underscores the practicality and significance of the developed method.

Cobalt catalyzed practical hydroboration of terminal alkynes with time-dependent stereoselectivity

Stereodefined vinylboron compounds are important organic synthons. The synthesis of E-1-vinylboron compounds typically involves the addition of a B-H bond to terminal alkynes. The selective generation of the thermodynamically unfavorable Z-isomers remains challenging, necessitating improved methods. Here, such a proficient and cost-effective catalytic system is introduced, comprising a cobalt salt and a readily accessible air-stable CNC pincer ligand. This system enables the transformation of terminal alkynes, even in the presence of bulky substituents, with excellent Z-selectivity. High turnover numbers (>1,600) and turnover frequencies (>132,000 h-1) are achieved at room temperature, and the reaction can be scaled up to 30 mmol smoothly. Kinetic studies reveal a formal second-order dependence on cobalt concentration. Mechanistic investigations indicate that the alkynes exhibit a higher affinity for the catalyst than the alkene products, resulting in exceptional Z-selective performance. Furthermore, a rare time-dependent stereoselectivity is observed, allowing for quantitative conversion of Z-vinylboronate esters to the E-isomers.

Borataalkenes, boraalkenes, and the η2-B,C coordination mode in coordination chemistry and catalysis

Borataalkenes and boraalkenes are the boron-containing isoelectronic analogues of alkenes and vinyl cations respectively. Compared with alkenes, the borataalkene and boraalkene ligand motifs in transition metal coordination chemistry are relatively underexplored. In this review, the synthesis of borataalkene and boraalkene complexes and other transition metal complexes featuring the η2-B,C coordination mode is described. The diversity of coordination modes and geometry in these complexes, and the spectroscopic and structural evidence supporting their assignments is disclosed as well as computational analysis of bonding. The applications of the borataalkene ligand motif in synthetic organic homogeneous catalysis, especially those involving geminal bis(pinacolatoboronates) and 1,4-azaborines, are discussed.

Palladium-Catalyzed Atroposelective Kinetic C-H Olefination and Allylation for the Synthesis of C-B Axial Chirality

The direct C-H functionalization of 1,2-benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium-catalyzed enantioselective C-H olefination and allylation reactions of 1,2-benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C-B axially chiral 2-aryl-1,2-benzazaborines and 3-substituted 2-aryl-1,2-benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late-stage modification of complex molecules, scale-up reaction, and applications.

Synthesis of Allenes by Hydroalkylation of 1,3-Enynes with Ketones Enabled by Cooperative Catalysis

A method for the synthesis of allenes by the addition of ketones to 1,3-enynes by cooperative Pd(0)Senphos/B(C6F5)3/NR3 catalysis is described. A wide range of aryl- and aliphatic ketones undergo addition to various 1,3-enynes in high yields at room temperature. Mechanistic investigations revealed a rate-determining outer-sphere proton transfer mechanism, which was corroborated by DFT calculations.

Recent Advances in the Synthesis of Borinic Acid Derivatives

Borinic acids [R2B(OH)] and their chelate derivatives are a subclass of organoborane compounds used in cross-coupling reactions, catalysis, medicinal chemistry, polymer or optoelectronics materials. In this paper, we review the recent advances in the synthesis of diarylborinic acids and their four-coordinated analogs. The main strategies to build up borinic acids rely either on the addition of organometallic reagents to boranes (B(OR)3, BX3, aminoborane, arylboronic esters) or the reaction of triarylboranes with a ligand (diol, amino alcohol, etc.). After general practical considerations of borinic acids, an overview of the main synthetic methods, their scope and limitations is provided. We also discuss some mechanistic aspects.

trans-Hydroalkynylation of Internal 1,3-Enynes Enabled by Cooperative Catalysis

A cooperative catalyst system involving a Pd(0)/Senphos complex, tris(pentafluorophenyl)borane, copper bromide, and an amine base, is demonstrated to catalyze trans-hydroalkynylation of internal 1,3-enynes. For the first time, a Lewis acid catalyst is shown to promote the reaction involving the emerging outer-sphere oxidative reaction step. The resulting cross-conjugated dieneynes are versatile synthons for organic synthesis, and their characterization reveals distinct photophysical properties depending on the positioning of the donor/acceptor substituents along the conjugation path.

A syn outer-sphere oxidative addition: the reaction mechanism in Pd/Senphos-catalyzed carboboration of 1,3-enynes

We report a combined experimental and computational study of Pd/Senphos-catalyzed carboboration of 1,3-enynes utilizing DFT calculations, ³¹P NMR study, kinetic study, Hammett analysis and Arrhenius/Eyring analysis. Our mechanistic study provides evidence against the conventional inner-sphere β-migratory insertion mechanism. Instead, a syn outer-sphere oxidative addition mechanism featuring a Pd-π-allyl intermediate followed by coordination-assisted rearrangements is consistent with all the experimental observations.

Mechanism of Pd/Senphos-Catalyzed trans-Hydroboration of 1,3-Enynes: Experimental and Computational Evidence in Support of the Unusual Outer-Sphere Oxidative Addition Pathway

The reaction mechanism of the Pd/Senphos-catalyzed trans-hydroboration reaction of 1,3-enynes was investigated using various experimental techniques, including deuterium and double crossover labeling experiments, X-ray crystallographic characterization of model reaction intermediates, and reaction progress kinetic analysis. Our experimental data are in support of an unusual outer-sphere oxidative addition mechanism where the catecholborane serves as a suitable electrophile to activate the Pd0-bound 1,3-enyne substrate to form a Pd-η3-π-allyl species, which has been determined to be the likely resting state of the catalytic cycle. Double crossover labeling of the catecholborane points toward a second role played by the borane as a hydride delivery shuttle. Density functional theory calculations reveal that the rate-limiting transition state of the reaction is the hydride abstraction by the catecholborane shuttle, which is consistent with the experimentally determined rate law: rate = k[enyne]0[borane]1[catalyst]1. The computed activation free energy ΔG‡ = 17.7 kcal/mol and KIE (kH/kD = 1.3) are also in line with experimental observations. Overall, this work experimentally establishes Lewis acids such as catecholborane as viable electrophilic activators to engage in an outer-sphere oxidative addition reaction and points toward this underutilized mechanism as a general approach to activate unsaturated substrates.

The Rise of 1,4-BN-Heteroarenes: Synthesis, Properties, and Applications

BN-heteroarenes, which employ both boron and nitrogen in aromatic hydrocarbons, have gained great attention in the fields of organic chemistry and materials science. Nevertheless, the extensive studies on BN-heteroarenes are largely limited to 1,2-azaborine-based compounds with B-N covalent bonds, whereas 1,3- and 1,4-BN-heteroarenes are relatively rare due to their greater challenge in the synthesis. Recently, significant progresses have been achieved in the synthesis and applications of BN-heteroarenes featuring 1,4-azaborines, especially driven by their significant potential as multiresonant thermally activated delayed fluorescence (MR-TADF) materials. Therefore, it is timely to review these advances from the chemistry perspective. This review summarizes the synthetic methods and recent achievements of 1,4-azaborine-based BN-heteroarenes and discusses their unique properties and potential applications of this emerging class of materials, highlighting the value of 1,4-BN-heteroarenes beyond MR-TADF materials. It is hoped that this review would stimulate the conversation and cooperation between chemists who are interested in azaborine chemistry and materials scientists working in the fields of organic optoelectronics, metal catalysis, and carbon-based nanoscience etc.

(Z)-Selective Hydroboration of Terminal Alkynes Catalyzed by a PSP-Pincer Rhodium Complex

A highly (Z)-selective hydroboration of terminal alkynes was achieved using a thioxanthene-based PSP-pincer rhodium catalyst. This hydroboration exhibited good chemoselectivity toward alkynes over carbonyl compounds such as ketones and aldehydes. The mechanistic studies indicated the involvement of rhodium-vinylidene intermediates, and the high (Z)-selectivity could be attributed to the rigid and electron-rich nature of the PSP-rhodium catalyst.

1,4-Azaborines: Origin, Modern Synthesis, and Applications as Optoelectronic Materials

This short review summarizes the origins and recent progress in 1,4-azaborine research, focusing on synthetic methodologies. Academic laboratories have made significant efforts to generate boron-nitrogen-containing heteroaromatic compounds that mimic arenes. 1,2-, 1,3-, and 1,4-Azaborine motifs have provided breakthrough molecules in applications ranging from medicines to materials. Owing to recent advances in polyaromatic 1,4-azaborine synthesis and applications in industry, the research field is currently undergoing a renaissance. Photo- and electroluminescent properties driven by distinct structural variations are key components in the design of novel 1,4-azaborine structures. In this review, seminal reports on the synthesis of simple 1,4-azaborines to complex π-extended structures are briefly highlighted together along with key optoelectronic properties.

1 Introduction

2 Non-Fused 1,4-Azaborines

3 Monobenzo-Fused 1,4-Azaborines

4 Dibenzo-Fused 1,4-Azaborines and Their Derivatives

5 Ladder-Type 1,4-Azaborines

6 Complex 1,4-Azaborines

7 Optoelectronic Properties of Key 1,4-Azaborines

8 Conclusion and Outlook

C-Boron Enolates Enable Palladium Catalyzed Carboboration of Internal 1,3-Enynes

A new family of carbon-bound boron enolates, generated by a kinetically controlled halogen exchange between chlorocatecholborane and silylketene acetals, is described. These C-boron enolates are demonstrated to activate 1,3-enyne substrates in the presence of a Pd⁰ /Senphos ligand complex, resulting in the first examples of a carboboration reaction of an alkyne with enolate-equivalent nucleophiles. Highly substituted dienyl boron building blocks are produced in excellent site-, regio-, and diastereoselectivity by the described catalytic cis-carboboration reaction.

(Z)-Selective Hydrosilylation and Hydroboration of Terminal Alkynes Enabled by Ruthenium Complexes with an N-Heterocyclic Carbene Ligand

Metal-catalyzed trans-1,2-hydrosilylations and hydroborations of terminal alkynes that generate synthetically valuable (Z)-alkenylsilanes and (Z)-alkenylboranes remain challenging due to the (E)-selective nature of the reactions and the formation of the thermodynamically unfavorable (Z)-isomer. The development of new, efficient catalytic systems for the (Z)-selective hydrosilylation and hydroboration of terminal alkynes is thus highly desirable from a fundamental perspective as it would deepen our understanding of the metal-catalyzed (Z)-selective hydrosilylation and hydroboration of terminal alkynes. This personal account describes our research for developing a ruthenium complex that can efficiently catalyze the hydrosilylation and hydroboration of terminal alkynes, and for exploring the factors controlling (Z)-selectivity of the reactions. Our effort into the activation of B-protected boronic acids, R-B(dan) (dan=naphthalene-1,8-diaminato), that was believed not to participate in Suzuki-Miyaura cross-coupling, is also discussed.

Selectivity in the Elaboration of Bicyclic Borazarenes

Among aromatic compounds, borazarenes represent a significant class of isosteres in which carbon-carbon bonds have been replaced by B-N bonds. Described herein is a summary of the selective reactions that have been developed for known systems, as well as a summary of computationally-based predictions of selectivities that might be anticipated in reactions of yet unrealized substructures.

Phenylpyridyl-Fused Boroles: A Unique Coordination Mode and Weak B-N Coordination-Induced Dual Fluorescence

Using 4-phenylpyridine or 2-phenylpyridine in place of biphenyl, two electron-poor phenylpyridyl-fused boroles, [TipPBB1]4 and TipPBB2 were prepared. [TipPBB1]4 adopts a unique coordination mode and forms a tetramer with a cavity in both the solid state and solution. The boron center of TipPBB2 is 4-coordinate in the solid state but the system dissociates in solution, leading to 3-coordinate borole species. Compared to its borafluorene analogues, the electron-accepting ability of TipPBB2 is largely enhanced by the pyridyl group. TipPBB2 exhibits dual fluorescence in solution due to an equilibrium between free TipPBB2 and a weak intermolecular coordination adduct with a second molecule. This equilibrium was further investigated by low-temperature NMR spectroscopy and photophysical studies. Theoretical studies indicate that the highest occupied molecular orbital (HOMO) of TipPBB2 localizes at the Tip group, in contrast to its borafluorene derivatives, wherein the HOMOs are localized on the borafluorene cores.

Pd-Senphos Catalyzed trans-Selective Cyanoboration of 1,3-Enynes

The first trans-selective cyanoboration reaction of an alkyne, specifically a 1,3-enyne, is described. The reported palladium-catalyzed cyanoboration of 1,3-enynes is site-, regio-, and diastereoselective, and is uniquely enabled by the 1,4-azaborine-based Senphos ligand structure. Tetra-substituted alkenyl nitriles are obtained providing useful boron-dienenitrile building blocks that can be further functionalized. The utility of our method has been demonstrated with the synthesis of Satigrel, an anti-platelet aggregating agent.

Understanding the role of frustrated Lewis pairs as ligands in transition metal-catalyzed reactions

The role of frustrated Lewis pairs (FLPs) as ligands in gold(i) catalyzed-reactions has been computationally investigated by using state-of-the-art density functional theory calculations. To this end, the nature of (P,B)-FLP-transition metal interactions in different gold(i)-complexes has been first explored in detail with the help of the energy decomposition analysis method, which allowed us to accurately quantify the so far poorly understood AuB interactions present in these species. The impact of such interactions on the catalytic activity of gold(i)-complexes has been then evaluated by performing the Au(i)-catalyzed hydroarylation reaction of phenylacetylene with mesitylene. With the help of the activation strain model of reactivity, the factors governing the higher activity of Au(i)-complexes having a FLP as a ligand as compared to that of the parent PPh₃ system have also been quantitatively identified.

Intramolecular Phosphacyclization: Polyaromatic Phosphonium P-Heterocycles with Wide-Tuning Optical Properties

Rationally designed cationic phospha-polyaromatic fluorophores were prepared through intramolecular cyclization of the tertiary ortho-(acene)phenylene-phosphines mediated by CuII triflate. As a result of phosphorus quaternization, heterocyclic phosphonium salts 1 c-3 c, derived from naphthalene, phenanthrene, and anthracene cores, exhibited very intense blue to green fluorescence (Φem =0.38-0.99) and high photostability in aqueous medium. The structure-emission relationship was further investigated by tailoring the electron-donating functions to the anthracene moiety to give dyes 4 c-6 c with charge-transfer character. The latter significantly decreases the emission energy to reach near-IR region. Thus, the intramolecular phosphacyclization renders an ultra-wide tuning of fluorescence from 420 nm (1 c) to 780 nm (6 c) in solution, extended to 825 nm for 6 c in the solid state with quantum efficiency of approximately 0.07. The physical behavior of these new dyes was studied spectroscopically, crystallographically, and electrochemically, whereas computational analysis was used to correlate the experimental data with molecular electronic structures. The excellent stability, water solubility, and attractive photophysical characteristics make these phosphonium heterocycles powerful tools in cell imaging.

Chlorine-Substituted 9,10-Dihydro-9-aza-10-boraanthracene as a Precursor for Various Boron- and Nitrogen-Containing π-Conjugated Compounds

Chlorine-substituted 9,10-dihydro-9-aza-10-boraanthracene was synthesized. Derivatization of this compound by taking advantage of the transformable B-Cl moiety gave 9,10-dihydro-9-aza-10-boraanthracene derivatives with various aryl substituents. In addition, further functionalization on NH groups by alkylation and Buchwald-Hartwig amination was demonstrated. Photophysical properties of the resulting 9,10-dihydro-9-aza-10-boraanthracene derivatives were also discussed.

Formation of Reactive π-Conjugated Frustrated N/B Pairs by Borane-Induced Propargyl Amine Rearrangement

N-Propargyltetramethylpiperidine reacts with a series of trans-alkenyl-B(C₆F₅)₂ compounds to give the substituted alkenyl-bridged frustrated N/B Lewis pairs 5. Their structures and spectroscopic features indicate a pronounced participation of the mesomeric s-trans-iminium/borata-alkene resonance form. The compounds are thought to be formed in a stepwise addition/rearrangement process which is initiated by a trans-1,2-amine/borane FLP addition to the carbon-carbon triple bond to generate a reactive zwitterionic aziridinium/alkenylborate intermediate. Subsequent alkenylborate attack leads to opening of the activated three-membered heterocycle with clean formation of the products 5a-c. Treatment of the propargyl-TMP substrate with B(C₆F₅)₃ gave a stable example of such an aziridinium/borate betaine, which was isolated and amply characterized. The products 5a-c are active N/B FLPs. They split dihydrogen heterolytically under mild conditions to give the respective NH⁺/BH^- products 9a-c. These contain Z-configurated core C═C double bonds, which indicates rotational equilibration around the central C-C bond of 5a-c during this reaction. Structural and chemical features of the 5c system were analyzed by DFT calculations.

The State of the Art in Azaborine Chemistry: New Synthetic Methods and Applications

Boron-nitrogen heteroarenes hold great promise for practical application in many areas of chemistry. Enduring interest in realizing this potential has in turn driven perennial innovation with respect to these compounds' synthesis. This Perspective discusses in detail the most recent advances in methods pertaining to the preparation of BN-isosteres of benzene, naphthalene, and their derivatives. Additional focus is placed on the progress enabled by these syntheses toward functional utility of such BN-heterocycles in biochemistry and pharmacology, materials science, and transition-metal-based catalysis. The prospects for future research efforts in these and related fields are also assessed.

Synthesis, structure and aromaticity of carborane-fused carbo- and heterocycles

The results of molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and considerable conjugation between a 3-D carborane and a fused 2-D π-ring system.

Conjugation between a 3-D icosahedral carborane and a fused 2-D π-ring system is ambiguous. To address this issue, we prepared several carborane-fused carbo- and heterocycles. Detailed studies on their molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of (1) considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and (2) considerable conjugation between a 3-D carborane and a fused 2-D π-ring system. These results will shed some light on the design of new carborane-based materials.

Mechanism and Origin of the Stereoselectivity in the Palladium-Catalyzed trans Hydroboration of Internal 1,3-Enynes with an Azaborine-Based Phosphine Ligand

An azaborine-based phosphine-Pd catalyst was introduced by the Liu group to promote trans hydroboration of the C≡C triple bond of internal 1,3-enyne substrates. Despite the excellent yield and selectivity observed experimentally, the mechanism and the origin of this special trans selectivity remained unknown. Herein, a comprehensive theoretical investigation was performed to clarify these issues. Accordingly, two main mechanisms (inner- and outer-sphere) were proposed and examined. Different from the conventional inner-sphere mechanism, in which the transition metal is involved in H-B bond cleavage, this reaction follows an outer-sphere mechanism, in which Pd does not directly participate in H-B bond cleavage. More specifically, the favorable pathway followed a Tsuji-Trost type reaction, in which the H-B bond was weakened by the formation of a four-coordinate boron intermediate (i.e., the boron is attached to the terminal carbon of the alkyne group). It then underwent a hydride-transfer process with the assistance of a second borane molecule, and finally reductive elimination generated the trans hydroboration product. Further analysis ascribed the origin of the special trans selectivity to the unique steric effect and electronic effect introduced by the special κ¹ -P-η² -BC coordination pattern.

Mono BN-substituted analogues of naphthalene: a theoretical analysis of the effect of BN position on stability, aromaticity and frontier orbital energies

An insight into the electronic structure changes driven by CC → BN substitution in a naphthalene system has been given by quantum chemical calculations.

DFT Studies of Dimerization Reactions of Boroles

Boroles undergo dimerization reactions to give Diels-Alder (DA) dimers, bridged-bicyclic (BB) dimers or spiro dimers (SD) depending on the substituents on the borole. We performed DFT calculations to investigate how different substituents at the carbon atoms of the butadiene backbone as well as at the boron atom influence the dimerization reaction pathways. The DFT results show that, in general, both the DA and BB dimers are easily accessible kinetically, and the DA dimers are thermodynamically more stable than the BB dimers. When the substituent-substituent repulsive steric interactions are alleviated to a certain extent, the BB dimers are more stable than the DA dimer, and become accessible. The SD dimers are generally kinetically difficult to obtain. However, we found that aryl substituents promote the formation of the SD dimers.

Site-Selective and Stereoselective trans-Hydroboration of 1,3-Enynes Catalyzed by 1,4-Azaborine-Based Phosphine-Pd Complex

A concise synthesis of monobenzofused 1,4-azaborine phosphine ligands (Senphos) is described. These Senphos ligands uniquely support Pd-catalyzed trans-selective hydroboration of terminal and internal 1,3-enynes to furnish corresponding dienylboronates in high efficiency and diastereoselectivity. X-ray structural analysis of the Senphos-Pd(0) complex reveals a κ²-P-η²-BC coordination mode, and this isolated complex has been shown to serve as a competent catalyst for the trans-hydroboration reaction. This work demonstrates that the expanded chemical space provided by the BN/CC isosterism approach translates into the functional space in the context of stereoselective catalytic transformations.

Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-tert-butyliminoborane and Alkynes

Di-tert-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl(PiPr3)2]2, which gives a rhodium η(4)-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines. The substitution of terminal alkynes for acetylene also led to 1,4-azaborinines, enabling ring substitution at a predetermined location. We report the first general synthesis of this new methodology, which provides highly regioselective access to valuable 1,4-azaborinines in moderate yields. A mechanistic rationale for this reaction is supported by DFT calculations, which show the observed regioselectivity to arise from steric effects in the B-C bond coupling en route to the rhodium η(4)-1,2-azaborete complex and the selective oxidative cleavage of the B-N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene.

A Modular Synthetic Approach to Monocyclic 1,4-Azaborines

A simple and general method for the synthesis of a wide range of monocyclic 1,4-azaborines, including the first examples containing B heteroatoms is described. Post-heterocycle-formation olefin isomerization was employed as a key strategy. This new synthetic method provides fundamental insight into the resonance stabilization and photophysical properties of 1,4-azaborines.