Organoboron Research Articles

Preparation of Chitosan-Composite-Film-Supported Copper Nanoparticles and Their Application in 1,6-Hydroboration Reactions of p-Quinone Methides.

Here, we describe the preparation of copper nanoparticles that are stabilized on a chitosan composite film (CP@Cu). This material could catalyze the 1,6-hydroboration reactions of p-quinone methides with B2pin2 as a boron source under mild conditions. This reaction exhibited very good functional group compatibility, and the organoboron compounds that were formed could easily be converted into corresponding hydroxyl products with good to excellent yields. This newly developed methodology provides an efficient and sequential pathway for the synthesis of gem-disubstituted methanols.

Recent Advances in the Chemistry of Iminoborane Derivatives

AbstractIminoboranes are important unsaturated organoboron compounds with the formula RB≡NR’. They are isoelectronic with alkynes, yet normally exhibit higher reactivity due to their polarity, thus enabling versatile applications in coordination, synthetic and materials chemistry. This review briefly describes the most recent progress in iminoborane derivatives with the focus on new persistent species and their diverse reactivity, which greatly enrich the iminoborane chemistry. In addition, the promising research directions are also provided.

Copper‐Catalyzed Simultaneous Installation of Boryl and Carbonyl Functionalities to Unsaturated Hydrocarbons

AbstractCu‐catalyzed borylative functionalization of unsaturated hydrocarbons is an effective method for synthesizing various organoboron compounds under mild reaction conditions. In particular, installing carbonyl functionalities such as acyl, formyl, and carboxy groups is interesting because of the essential role of these functionalities in organic synthesis. This review focuses on the installation of boryl and carbonyl functionalities in unsaturated hydrocarbons using Cu catalysts. First, formylation and acylation reactions using formate esters, acyl halides, or acid anhydrides are addressed. Second, the simultaneous incorporation of a boryl group and a carboxy group using carbon dioxide as the electrophile is summarized. Third, the borocyanation of unsaturated hydrocarbons using electrophilic cyanation reagents is described.

Multicomponent synthesis of α-chloro alkylboronic esters via visible-light-mediated dual catalysis

<h2>Summary</h2> α-Chloro alkylboronic (α-CAB) esters are a class of stable bifunctional molecules wherein the intrinsic differences of a carbon–chlorine and carbon–boron bond can be used to unlock distinct reactivity through carbon-electrophile, carbon-nucleophile, and carbon-centered radical intermediates. Despite their apparent utility, complex variants are not trivial to prepare via established methods. We present a new visible-light-mediated dual catalytic process for the multicomponent coupling of diaryliodonium salts, alkenes, and a simple metal chloride. The broad scope of the reaction combined with the capacity of their synthetic diversification will make this transformation attractive for practitioners of synthetic and medicinal chemistry in academic and industrial environments.

1,2‐Boronate Rearrangement: An Efficient Tool for the Opening, Functionalization and Formation of Strained Cycles

AbstractThe 1,2‐metallate rearrangement occurring with boronate complexes is a strategy of choice for the synthesis of organoboron compounds, giving a direct access to highly functionalized molecules. Lately, significant progress has been made in the use of this reaction for the opening, the functionalization, and the formation of strained cycles. In this review, we will highlight these recent advances, focusing on cycloalkanes and oxygen‐ or nitrogen‐containing strained cycles.

N-Coordinated Organoboron in Polymer Synthesis and Material Science

Organoboron chemistry has been widely explored and developed in synthetic chemistry for over half a century and provides various elegant synthetic protocols in polymer synthesis. Compared with most trivalent bare organoboron compounds, N-coordinated organoboron shows better performances, such as air and moisture stability. This review summarizes the application of various N-coordinated boranes and boronic acid/esters in polymer synthesis and materials science. We introduce the significance of N-coordinated boranes and boronate esters for controllable polymer synthesis and systematically summarize the structures and properties of polymers containing N-coordinated boronate esters. Furthermore, we highlight the effect of N→B dative bonds on improving the performance of self-healing materials. We hope that, through this review, more researchers will realize the advantages of N-coordinated organoboron and promote the development of this direction in polymer synthesis and materials science.

Copper-Catalyzed Ligand-Controlled Selective Borocarbonylation of α-Substituted Styrenes Toward β-Boryl Aldehydes and Cyclopropyl Boronate Esters

Copper-Catalyzed Ligand-Controlled Selective Borocarbonylation of α-Substituted Styrenes Toward β-Boryl Aldehydes and Cyclopropyl Boronate Esters

Unsymmetric Molecular Hinges Showing Intramolecular Charge Transfer Excitation: Strong Photoluminescence Properties in the Red to Near-Infrared Region Exhibited by Tri-BF2Complexes

Rationally designed unsymmetric tri-BF2 red to near-infrared (NIR) emitters were first reported in this study via a facile one-pot synthetic approach. The single-crystal structures and density functional theory calculations suggest their tunable conformation change around the bipyrrole part and unsymmetric structures in which three boron atoms are coordinated with the existing four different bidentate binding pockets, leaving two of the uncoordinated nitrogens in the pyrrole part. These novel tri-BF2 emitters in the red to NIR region demonstrated ca. 30% of photoluminescence quantum yield (ΦPL) in toluene at room temperature and highly intense fluorescence at 77 K (up to 89% of ΦPL in 2-MeTHF), which is attributed to increased molecular flexibility and enhanced intramolecular charge transfer interactions resulting from symmetry breaking. The photophysical modulation in multi-BF2 complexes was achieved by the coordinated number of BF2 units, providing a step forward to understanding and guiding the design of the most efficient versatile emissive organoboron compounds.

Regioselective Transition-Metal-Free C(sp2 )-H Borylation: A Subject of Practical and Ongoing Interest in Synthetic Organic Chemistry.

Considerable advances have been made in the area of C-H functionalization in the last few decades. A number of approaches including both directed and nondirected strategies have been developed thus far. Among the various C-H functionalizations, C-H borylation is of special interest due to the wide applications of organoboron compounds. In this regard, various transition-metal-catalyzed regioselective strategies have been developed. However, the major concern regarding metal-catalyzed C-H borylation procedures is the requirement of a precious metal as well as the contamination by metal precursors in the desired products, which limit the application of this process in large-scale synthesis. Therefore, recent trends have involved the use of transition-metal-free systems. We summarize recent developments in transition-metal-free regioselective C-H borylation. We believe that this Review will help to increase interest in this field and stimulate further progress.

Regioselective Transition‐Metal‐Free C(sp2)−H Borylation: A Subject of Practical and Ongoing Interest in Synthetic Organic Chemistry

AbstractConsiderable advances have been made in the area of C−H functionalization in the last few decades. A number of approaches including both directed and nondirected strategies have been developed thus far. Among the various C−H functionalizations, C−H borylation is of special interest due to the wide applications of organoboron compounds. In this regard, various transition‐metal‐catalyzed regioselective strategies have been developed. However, the major concern regarding metal‐catalyzed C−H borylation procedures is the requirement of a precious metal as well as the contamination by metal precursors in the desired products, which limit the application of this process in large‐scale synthesis. Therefore, recent trends have involved the use of transition‐metal‐free systems. We summarize recent developments in transition‐metal‐free regioselective C−H borylation. We believe that this Review will help to increase interest in this field and stimulate further progress.

Stereoselective ZnCl2-Catalyzed B-H Bond Insertion of Vinyl Carbenes Generated from Cyclopropenes for the Synthesis of Allylboranes.

Zinc-catalyzed insertion of vinyl carbenes generated from cyclopropenes into the B-H bonds of Lewis base-borane adducts for concise and efficient access to allylboranes has been developed. This protocol represents the first zinc-catalyzed B-H bond insertion of carbenes for organoborane compounds. In this protocol, inexpensive ZnCl2, with low toxicity, is used as the catalyst. This simple ligand-free catalytic system affords allylboranes in yields up to 92%, with E/Z ratios of >20:1. Besides, this new protocol offers a broad scope of Lewis base-borane adducts, which are not easily obtained by other catalytic methods for metal carbene insertion into B-H bonds. The potential synthetic applicability of this new methodology is exemplified by a gram-scale experiment and synthetic transformation of the products.

The Impact of Boron Hybridisation on Photocatalytic Processes.

Recently the fruitful merger of organoboron chemistry and photocatalysis has come to the forefront of organic synthesis, resulting in the development of new technologies to access complex (non)borylated frameworks. Central to the success of this combination is control of boron hybridisation. Contingent on the photoactivation mode, boron as its neutral planar form or tetrahedral boronate can be used to regulate reactivity. This Minireview highlights the current state of the art in photocatalytic processes utilising organoboron compounds, paying particular attention to the role of boron hybridisation for the target transformation.

The Impact of Boron Hybridisation on Photocatalytic Processes

AbstractRecently the fruitful merger of organoboron chemistry and photocatalysis has come to the forefront of organic synthesis, resulting in the development of new technologies to access complex (non)borylated frameworks. Central to the success of this combination is control of boron hybridisation. Contingent on the photoactivation mode, boron as its neutral planar form or tetrahedral boronate can be used to regulate reactivity. This Minireview highlights the current state of the art in photocatalytic processes utilising organoboron compounds, paying particular attention to the role of boron hybridisation for the target transformation.

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Synthesis of Tri‐ and Tetrasubstituted Alkenyl Boronates from Alkynes

AbstractThe synthesis of organoboron compounds have attracted the attention of the synthetic community. In particular, molecules with C(sp2)‐B bonds enable the transformation to new C−C or C‐heteroatom bonds by well‐established methodologies. Alkenyl boronates have the possibility for further conversion of the boron moiety or functionalization of the double bond. This review gives an overview on the recent methodologies for the selective preparation of the challenging highly substituted alkenyl boronates from alkynes.

Ligand-controlled Cobalt-Catalyzed formation of Carbon–Boron bonds: Hydroboration vs C[sbnd]H/B–H dehydrocoupling

Herein, we describe the functionalization of silylacetylenes with hydroboranes catalyzed by cobalt (pre)catalysts. The direction of the reaction is determined depending on the type of the substituent at the aromatic ring in triazine PN5P pincer ligands. Thus, our catalytic approach has been successfully applied to obtain both hydroboration and dehydrogenative coupling products. Finally, this versatile protocol implies the use of commercially available pinacolborane as a simultaneous substrate and (pre)catalyst’s activator. Moreover, the described procedure leads to an interesting group of compounds with many potential industrial and synthetic applications.

Understanding of Photo-Induced Reversible Rearrangement from Borepin to Borirane.

The first reversible photoisomerization between a borepin and a borirane was reported in the photo-induced reactions of B(npy)Ar2 (npy=2-(naphthalen-1-yl) pyridine, Ar=phenyl or electron rich aryl; S. Wang, et al. Angew. Chem. Int. Ed. 2019, 58, 6683-6687). In this work, the detailed mechanisms of the unprecedented reversible photoisomerization between the borepin (compound a) and the borirane (compound b) of B(npy)Ph2 in the first excited singlet (S1 ) state and the ground (S0 ) state were studied by carrying out calculations with the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT). The calculation results show that photoexcitation of a-S0 at 365 nm and b-S0 at 450 nm populate their S1 state with evident charge transfer characteristics. The photoisomerization is triggered in the S1 state and ends in the S0 state, at which the intersection points in a (S1 /S0 )x intersection seam participate in and promote phenyl migration and ring-closure processes. Furthermore, we reveal that the not large energy difference (less than 0.6 eV) and similar conjugation properties of π electrons between a-S0 and b-S0 are responsible for their unique photo-reversible reactivity, compared with those of the isomers of the thermally reversible compound B(ppy)Mes2 . Our results contribute to an understanding of the excited-state reactivity of organoboron compounds and will be useful to support the design of new boron-based photo-responsive materials.

Enantioselective Copper-Catalyzed sp2/sp3 Diborylation of 1-Chloro-1-Trifluoromethylalkenes.

Fluorine-containing organoboron compounds have emerged as novel building blocks in chemical synthesis; among them, fluorinated sp2/sp3 diborylated compounds are particularly appealing, since they might undergo chemoselective and diversified transformations of different C–B bonds with fluorinated functionality, thus bringing versatility and complexity to the eventual products. However, expedient, synthetic strategies for the construction of such fluorinated diborylative compounds are very sparse. Herein, we disclose enantioselective Cu-catalyzed sp2/sp3 diborylations of 1-chloro-1-trifluoromethylalkenes, leading to diborylated compounds bearing a gem-difluoroalkenyl moiety; most intriguingly, the new formed C–B bonds include one stereoselective and optically pure Csp3–B bond. Further transformations on the eventual products demonstrated the values of our presented strategy.

Dual Nickel/Photoredox-Catalyzed Site-Selective Cross-Coupling of 1,2-Bis-Boronic Esters Enabled by 1,2-Boron Shifts.

Site‐selective transition‐metal‐catalyzed mono‐deboronative cross‐couplings of 1,2‐bis‐boronic esters are valuable methods for the synthesis of functionalized organoboron compounds. However, such cross‐couplings are limited to reaction of the sterically less hindered primary boronic ester. Herein, we report a nickel/photoredox‐catalyzed mono‐deboronative arylation of 1,2‐bis‐boronic esters that is selective for coupling of the more sterically hindered secondary/tertiary position. This is achieved by taking advantage of a 1,2‐boron shift of primary β‐boryl radicals to the thermodynamically favored secondary/tertiary radicals, which are subsequently intercepted by the nickel catalyst to enable arylation. The mild conditions are amenable to a broad range of aryl halides to give β‐aryl boronic ester products in good yields and with high regioselectivity. This method also allows stereodivergent coupling of cyclic cis‐1,2‐bis‐boronic esters to give trans‐substituted products.

Dual Nickel/Photoredox‐Catalyzed Site‐Selective Cross‐Coupling of 1,2‐Bis‐Boronic Esters Enabled by 1,2‐Boron Shifts

AbstractSite‐selective transition‐metal‐catalyzed mono‐deboronative cross‐couplings of 1,2‐bis‐boronic esters are valuable methods for the synthesis of functionalized organoboron compounds. However, such cross‐couplings are limited to reaction of the sterically less hindered primary boronic ester. Herein, we report a nickel/photoredox‐catalyzed mono‐deboronative arylation of 1,2‐bis‐boronic esters that is selective for coupling of the more sterically hindered secondary/tertiary position. This is achieved by taking advantage of a 1,2‐boron shift of primary β‐boryl radicals to the thermodynamically favored secondary/tertiary radicals, which are subsequently intercepted by the nickel catalyst to enable arylation. The mild conditions are amenable to a broad range of aryl halides to give β‐aryl boronic ester products in good yields and with high regioselectivity. This method also allows stereodivergent coupling of cyclic cis‐1,2‐bis‐boronic esters to give trans‐substituted products.