Accessing an azaborine building block: synthesis and substitution reactions of 2-chloromethyl-2,1-borazaronaphthalene

The Versatile Reaction Chemistry of an Alpha-Boryl Diazo Compound

The first α-boryl diazo compound that is capable of engaging in classic synthetic organic diazo reaction chemistry is described. The diazomethyl-1,2-azaborine 1, which is a BN isostere of phenyldiazomethane, is significantly more stable than phenyldiazomethane; its reaction chemistry ranges from C-H activation, O-H activation, [3+2] cycloaddition, and halogenation, to Ru-catalyzed carbonyl olefination. The demonstrated broad range of reactivity of diazomethyl-1,2-azaborine 1 makes it an exceptionally versatile synthetic building block for the 1,2-azaborine heterocyclic motif.

One tool to bring them all: Au-catalyzed synthesis of B,O- and B,N-doped PAHs from boronic and borinic acids

The isoelectronic replacement of C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds with ⁻BN⁺ bonds in polycyclic aromatic hydrocarbons (PAHs) is a widely used tool to prepare novel optoelectronic materials. Far less well explored are corresponding B,O-doped PAHs, although they have a similarly high application potential. We herein report on the modular synthesis of B,N- and B,O-doped PAHs through the [Au(PPh₃)NTf₂]-catalyzed 6-endo-dig cyclization of BN–H and BO–H bonds across suitably positioned C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds in the key step. Readily available, easy-to-handle o-alkynylaryl boronic and borinic acids serve as starting materials, which are either cyclized directly or first converted into the corresponding aminoboranes and then cyclized. The reaction even tolerates bulky mesityl substituents on boron, which later kinetically protect the formed B,N/O-PAHs from hydrolysis or oxidation. Our approach is also applicable for the synthesis of rare doubly B,N/O-doped PAHs. Specifically, we prepared 1,2-B,E-naphthalenes and -anthracenes, 1,5-B₂-2,6-E₂-anthracenes (E = N, O) as well as B,O₂-containing and unprecedented B,N,O-containing phenalenyls. Selected examples of these compounds have been structurally characterized by X-ray crystallography; their optoelectronic properties have been studied by cyclic voltammetry, electron spectroscopy, and quantum-chemical calculations. Using a new unsubstituted (B,O)₂-perylene as the substrate for late-stage functionalization, we finally show that the introduction of two pinacolatoboryl (Bpin) substituents is possible in high yield and with perfect regioselectivity via an Ir-catalyzed C–H borylation approach.

Singly and doubly B,E-doped PAHs were synthesized using a protocol that starts from easy-to-handle boronic and borinic acids and offers the possibility to choose between the preparation of B,O- and B,N-PAHs in the final reaction step.

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.

Synthesis and characterization of an unnatural boron and nitrogen-containing tryptophan analogue and its incorporation into proteins

A boron and nitrogen containing unnatural analogue of tryptophan is synthesized and incorporated into proteins.

A boron and nitrogen containing unnatural analogue of tryptophan is synthesized through the functionalization of BN-indole. The spectroscopic properties of BN-tryptophan are reported with respect to the natural tryptophan, and the incorporation of BN-tryptophan into proteins expressed in E. coli using selective pressure incorporation is described. This work shows that a cellular system can recognize the unnatural, BN-containing tryptophan. More importantly, it presents the first example of an azaborine containing amino acid being incorporated into proteins.

Aromatic Metamorphosis of Indoles into 1,2-Benzazaborins

Among the plethora of aromatic compounds, indoles represent a privileged class of substructures that is ubiquitous in natural products and pharmaceuticals. While numerous exocyclic functionalizations of indoles have provided access to a variety of useful derivatives, endocyclic transformations involving the cleavage of the C2-N bond remain challenging due to the high aromaticity and strength of this bond in indoles. Herein, we report the "aromatic metamorphosis" of indoles into 1,2-benzazaborins via the insertion of boron into the C2-N bond. This endocyclic insertion consists of a reductive ring-opening using lithium metal and a subsequent trapping of the resulting dianionic species with organoboronic esters. Considering that 1,2-azaborins have attracted increasing academic and industrial attention as BN isosteres of benzene, the counterintuitive aromatic metamorphosis presented herein can feasibly be expected to substantially advance the promising chemistry of 1,2-azaborins.

Photoredox Catalysis Enables Access to N-Functionalized 2,1-Borazaronaphthalenes

The synthesis and utilization of a class of 2,1-borazaronaphthyltrifluoroborate reagents that provide a general solution to the challenge of N-functionalization of the 2,1-borazaronaphthalene core is described. By adorning the nitrogen of this core with a trifluoroboratomethyl unit, a suite of odd-electron processes can be executed, installing motifs that would otherwise be inaccessible using a two-electron approach. In addition, this process enables rapid annulation, furnishing a heretofore unknown polycyclic B-N species.

Expanding the functional group tolerance of cross-coupling in 1,2-dihydro-1,2-azaborines: installation of alkyl, alkenyl, aryl, and heteroaryl substituents while maintaining a B-H bond

Palladium-catalyzed Negishi cross-coupling of 3-bromo-1-(tert-butyldimethylsilyl)-1,2-dihydro-1,2-azaborine while maintaining the B-H functionality has been demonstrated. 17 examples, including dialkylzinc, alkyl-, alkenyl-, aryl-, as well as nitrogen-, sulfur-, and oxygen-containing heteroaryl-zinc halide reagents have been coupled to generate new C(3) substituted 1,2-azaborines in moderate to excellent yields.

Late-stage functionalization of BN-heterocycles

BN/CC isosterism has emerged as a viable strategy to expand the chemical space of organic molecules. In particular, the application of BN/CC isosterism to arenes has received significant attention due to the vast available chemical space provided by aromatic hydrocarbons. The synthetic efforts directed at assembling novel aromatic BN heterocycles have resulted in the discovery of new properties and functions in a variety of fields including biomedical research, medicinal chemistry, materials science, catalysis, and organic synthesis. This tutorial review specifically covers recent advances in synthetic technologies that functionalize assembled boron-nitrogen (BN) heterocycles and highlights their distinct reactivity and selectivity in comparison to their carbonaceous counterparts. It is intended to serve as a state-of-the-art compendium for readers who are interested in the reaction chemistry of BN heterocycles.

One-Step Conversion of Potassium Organotrifluoroborates to Metal Organoborohydrides

This letter describes the one-step conversion of heteroatom-substituted potassium organotrifluoroborates (KRBF₃) to metal monoorganoborohydrides (MRBH₃) using alkali metal aluminum hydrides. The method tolerates a variety of functional groups, expanding MRBH₃ diversity. Hydride removal with Me₃SiCl in the presence of dimethylaminopyridine (DMAP) affords the organoborane·DMAP (RBH₂·DMAP) adducts.

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.

Divergent reactivity of nucleophilic 1-bora-7a-azaindenide anions

The reactions of 1-bora-7a-azaindenide anions, prepared in moderate to excellent yields by reduction of the appropriate 1-bora-7a-azaindenyl chlorides with KC₈ in THF, with alkyl halides and carbon dioxide were studied. With alkyl halides (CH₂Cl₂, CH₃I and BrCH(D)CH(D)^tBu), the anions behave as boron anions, alkylating the boron centre via a classic S_N2 mechanism. This was established with DFT methods and via experiments utilizing the neo-hexyl stereoprobe BrCH(D)CH(D)^tBu. These reactions were in part driven by a re-aromatization of the six membered pyridyl ring upon formation of the product. Conversely, in the reaction of the 1-bora-7a-azaindenide anions with CO₂, a novel carboxylation of the C-2 carbon alpha to boron was observed. Computations indicated that while carboxylation of the boron centre was kinetically feasible, the products of B-carboxylation were not thermodynamically favored relative to the observed C-2 carboxylated species, which were formed preferably due to the generation of both C-C and B-O bonds. In these products, the pyridyl ring remains non-aromatic, in part accounting for the observed reversibility of carboxylation.

Regioselective Diversification of 2,1-Borazaronaphthalenes: Unlocking Isosteric Space via C-H Activation

Methods for the regioselective C–H borylation and subsequent cross-coupling of the 2,1-borazaronaphthalene core are reported. Azaborines are dependent on B–N/C=C isosterism when employed in strategies for developing diverse heterocyclic scaffolds. Although 2,1-borazaronaphthalene is closely related to naphthalene in terms of structure, the argument is made that the former has electronic similarities to indole. Based on that premise, iridium-mediated C–H activation has enabled facile installation of a versatile, nucleophilic coupling handle at a previously inaccessible site of 2,1-borazaronaphthalenes. A variety of substituted 2,1-borazaronaphthalene cores can be successfully borylated and further cross-coupled in a facile manner to yield diverse C(8)-substituted 2,1-borazaronaphthalenes.

Gram-scale free radical polymerization of an azaborine vinyl monomer

The gram-scale polymerization of a novel azaborine vinyl monomer is reported.

Method for Accessing Nitrogen-Containing, B-Heteroaryl-Substituted 2,1-Borazaronaphthalenes

The azaborine motif provides a mimic of aromatic systems through replacement of a C═C bond with a B-N bond. In particular, 2,1-borazaronaphthalenes, accessible through robust methods of synthesis and subsequent functionalization, afford an ideal platform to use for a variety of applications. However, the scope of substructures for this archetype has been limited by the lack of nitrogen-containing heteroaryls that can be incorporated within them. In this study, modified reaction conditions were developed to provide access to a wider range of substructures.

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.

Accessing Elaborated 2,1-Borazaronaphthalene Cores Using Photoredox/Nickel Dual-Catalytic Functionalization

A highly effective method for derivatizing 2,1-borazaronaphthalene cores using ammonium alkylbis(catecholato)silicates via photoredox/nickel dual catalysis is reported. By forging C(sp)(3)-C(sp)(2) bonds via this approach, alkyl fragments with various functional groups can be introduced to the azaborine core, affording previously inaccessible heterocyclic isosteres in good to excellent yields. The base-free, room-temperature conditions outlined allow sensitive functional group tolerance, even permitting the cross-coupling of unprotected primary and secondary amines.

Efficient synthetic methods for the installation of boron–nitrogen bonds in conjugated organic molecules

New synthetic methods for preparing gram quantities BN analogs of polycyclic aromatic hydrocarbons are highlighted. Such methods are key to proper evaluation of these materials in device applications.

The synthesis of BN-embedded tetraphenes and their photophysical properties

A series of intensely blue fluorescent BN-embedded tetraphene derivatives have been synthesized by the catalytic cyclization of BN-naphthalenes with alkynes.

Oxygen insertion into boroles as a route to 1,2-oxaborines

The synthesis of 1,2-oxaborines is accomplishedviathe reaction of pentaarylboroles withN-methylmorpholine-N-oxideviaa 1,1-insertion reaction.

Toward Efficient Nucleophilic Azaborine Building Blocks for the Synthesis of B-N Naphthyl (Hetero)arylmethane Isosteres

To develop a method for the synthesis of a class of azaborines, potassium 2-(trifluoroboratomethyl)-2,1-borazaronaphthalenes have been synthesized to serve as nucleophilic building blocks. In palladium-catalyzed cross-coupling reactions with (hetero)aryl chlorides they serve to produce a variety of pseudobenzylic (hetero)aryl substituted azaborines. Potassium 2-(trifluoroboratomethyl)-2,1-borazaronaphthalenes are crystalline solids that are more stable than 2-(chloromethyl)-2,1-borazaronaphthalenes and have a broader substrate scope in cross-coupling reactions compared to their pseudobenzylic chloride counterparts.

Synthesis of 1,2-borazaronaphthalenes from imines by base-promoted borylation of C-H bond

A new route from benzylic imines permits the synthesis of 1,2-borazaronaphthalenes in good yields. The reaction involves formation of the enamidyl dibromoborane, which undergoes base-promoted borylation of the nearby aromatic C-H bond. Electrophilic attack of the boron species onto the benzylic arene is supported by the slow borylation of arenes substituted with electron-withdrawing groups. The resultant boron bromides can be easily substituted with lithium reagents to provide a range of products. The electronic properties of these 1,2-borazaronaphthalene derivatives have been investigated by UV-vis and fluorescence spectroscopy.

Accessing 2-(hetero)arylmethyl-, -allyl-, and -propargyl-2,1-borazaronaphthalenes: palladium-catalyzed cross-couplings of 2-(chloromethyl)-2,1-borazaronaphthalenes

The synthesis of 2-(chloromethyl)-2,1-borazaronaphthalene has provided an opportunity to expand dramatically the functionalization of the azaborines. This azaborinyl building block can serve as the electrophile in palladium-catalyzed cross-coupling reactions to form sp(3)-sp and sp(3)-sp(2) bonds. The cross-coupling reactions of 2-(chloromethyl)-2,1-borazaronaphthalene with potassium (hetero)aryl- and alkenyltrifluoroborates as well as terminal alkynes provides access to a variety of novel azaborines, allowing a library of pseudobenzylic substituted azaborines to be prepared from one common starting material.