Transition-Metal-Free Borylation of Alkyl Iodides via a Radical Mechanism

Borylation of Stable C(sp3)-O Bonds of Alkyl Esters over Supported Au Catalysts

We report herein that supported gold catalysts efficiently promote the borylation of stable C(sp3)-O bonds of alkyl esters. The use of a disilane as an electron source and gold nanoparticles as a single-electron transfer catalyst is the key to generating alkyl radicals via the homolysis of stable C(sp3)-O bonds, thereby enabling cross-coupling between bis(pinacolato)diboron and linear and cyclic alkyl esters to afford the diverse alkyl boronates.

Photo-Triggered, Copper(II) Chloride-Catalyzed Radical Hydroalkylation and Hydrosilylation of Vinylboronic Esters To Access Alkylboronic Esters

Alkyl boronic acids and their derivatives constitute vital building blocks in organic synthesis and are important motifs identified in medicinal chemistry. Herein, we present a phototriggered, CuCl2-catalyzed radical hydroalkylation and hydrosilylation of vinylboronic esters to alkylboronic esters. This approach exhibits mild reaction conditions, utilization of easily accessible reagents, and scalability up to a gram scale. Further synthetic transformations of the hydrosilylation products and mechanistic studies are also demonstrated.

Enhancing Ultrasound-Assisted Iodine-Mediated Reversible-Deactivation Radical Polymerization by Piezoelectric Nanoparticles

The development of mechanochemical tools for regulating the polymerization process has received an increasing amount of attention in recent years. Herein, we report the example of the mechanically controlled iodine-mediated reversible-deactivation radical polymerization (mechano-RDRP) using piezoelectric tetragonal BaTiO3 nanoparticles (T-BTO) as mechanoredox catalyst and alkyl iodide as the initiator. We demonstrated a more efficient mechanochemical initiation and reversible deactivation process than sonochemical activation via a mechanoredox-mediated alkyl iodide cleavage reaction. The mechanochemical activation of the C-I bond was verified by density functional theory (DFT) calculations. Theoretical calculations together with experimental results confirmed the more efficient initiation and polymerization than the traditional sonochemical approach. The influence of BaTiO3, initiator, and solvent was further examined to reveal the mechanism of the mechano-RDRP. The results showed good controllability over molecular weight and capacity for a one-pot chain extension. This work expands the scope of mechanically controlled polymerization and shows good potential in the construction of adaptive materials.

Transition metal-free visible light photoredox-catalyzed remote C(sp3)-H borylation enabled by 1,5-hydrogen atom transfer

The borylation of unreactive carbon-hydrogen bonds is a valuable method for transforming feedstock chemicals into versatile building blocks. Here, we describe a transition metal-free method for the photoredox-catalyzed borylation of unactivated C(sp³)-H bond, initiated by 1,5-hydrogen atom transfer (HAT). The remote borylation was directed by 1,5-HAT of the amidyl radical, which was generated by photocatalytic reduction of hydroxamic acid derivatives. The method accommodates substrates with primary, secondary and tertiary C(sp³)-H bonds, yielding moderate to good product yields (up to 92%) with tolerance for various functional groups. Mechanistic studies, including radical clock experiments and DFT calculations, provided detailed insight into the 1,5-HAT borylation process.

Iodine-Catalyzed Borylation of Benzylic Alcohols

Direct borylation of benzylic alcohols has been achieved via an iodine-catalyzed process. This transition-metal-free borylation transformation is compatible with various functional groups and provides a practical and convenient method to access important and useful benzylic boronate esters from widely available benzylic alcohols. Preliminary mechanistic investigations indicated that benzylic iodide and radicals are involved as the key intermediates in this borylation reaction.

Exploiting the sp2 character of bicyclo[1.1.1]pentyl radicals in the transition-metal-free multi-component difunctionalization of [1.1.1]propellane

Strained bicyclic substructures are increasingly relevant in medicinal chemistry discovery research because of their role as bioisosteres. Over the last decade, the successful use of bicyclo[1.1.1]pentane (BCP) as a para-disubstituted benzene replacement has made it a highly valuable pharmacophore. However, various challenges, including limited and lengthy access to useful BCP building blocks, are hampering early discovery research. Here we report a single-step transition-metal-free multi-component approach to synthetically versatile BCP boronates. Radicals derived from commonly available carboxylic acids and organohalides perform additions onto [1.1.1]propellane to afford BCP radicals, which then engage in polarity-matched borylation. A wide array of alkyl-, aryl- and alkenyl-functionalized BCP boronates were easily prepared. Late-stage functionalization performed on natural products and approved drugs proceeded with good efficiency to generate the corresponding BCP conjugates. Various photoredox transformations forging C-C and C-N bonds were demonstrated by taking advantage of BCP trifluoroborate salts derived from the BCP boronates.

Selective, Transition Metal-free 1,2-Diboration of Alkyl Halides, Tosylates, and Alcohols

Defunctionalization of readily available feedstocks to provide alkenes for the synthesis of multifunctional molecules represents an extremely useful process in organic synthesis. Herein, we describe a transition metal-free, simple and efficient strategy to access alkyl 1,2-bis(boronate esters) via regio- and diastereoselective diboration of secondary and tertiary alkyl halides (Br, Cl, I), tosylates, and alcohols. Control experiments demonstrated that the key to this high reactivity and selectivity is the addition of a combination of potassium iodide and N,N-dimethylacetamide (DMA). The practicality and industrial potential of this transformation are demonstrated by its operational simplicity, wide functional group tolerance, and the late-stage modification of complex molecules. From a drug discovery perspective, this synthetic method offers control of the position of diversification and diastereoselectivity in complex ring scaffolds, which would be especially useful in a lead optimization program.

Transition Metal Free Stannylation of Alkyl Halides: The Rapid Synthesis of Alkyltrimethylstannanes

A transition metal free stannylation reaction of alkyl bromides and iodides with hexamethyldistannane has been developed. This protocol is operationally convenient and features a rapid reaction and good functional group tolerance. A wide range of functionalized primary and secondary alkyl and benzyl trimethyl stannanes are prepared in moderate to excellent yields. The success of the gram-scale procedure and tandem Stille coupling reaction has allowed this protocol to demonstrate potential for application in organic synthesis. Both experimental and theoretical studies reveal the mechanistic details of this stannylation reaction.

A tandem process for the synthesis of β-aminoboronic acids from aziridines with haloamine intermediates

An unprecedented synthetic strategy is devised to generate β-aminoboronic acids from aziridines via a sequential process involving 1,2-iodoamine formation and radical borylation under light irradiation. A variety of aziridines including multiply substituted aziridines have been successfully employed as synthetic precursors, expanding their synthetic utility compared to previous methods. Mechanistic studies suggest that the boron source plays a unique role in the borylation step, and in the formation of haloamine intermediates.

Base-Mediated Radical Borylation of Alkyl Sulfones

A practical and direct method was developed for the production of versatile alkyl boronate esters via transition metal-free borylation of primary and secondary alkyl sulfones. The key to the success of the strategy is the use of bis(neopentyl glycolato) diboron (B2 neop2 ), with a stoichiometric amount of base as a promoter. The practicality and industrial potential of this protocol are highlighted by its wide functional group tolerance, the late-stage modification of complex compounds, no need for further transesterification, and operational simplicity. Radical clock, radical trap experiments, and EPR studies were conducted which show that the borylation process involves radical intermediates.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

Electrochemical borylation of carboxylic acids

A simple electrochemically mediated method for the conversion of alkyl carboxylic acids to their borylated congeners is presented. This protocol features an undivided cell setup with inexpensive carbon-based electrodes and exhibits a broad substrate scope and scalability in both flow and batch reactors. The use of this method in challenging contexts is exemplified with a modular formal synthesis of jawsamycin, a natural product harboring five cyclopropane rings.

Transition metal- and light-free radical borylation of alkyl bromides and iodides using silane

We report operationally simple and neutral conditions for borylation of alkyl bromides and iodides to alkyl boronic esters under transition metal- and light-free conditions. A series of substrates with a wide range of functional groups were effectively transformed into the borylation products in moderate to good yields. Mechanistic studies, including radical clock experiments and DFT calculations, gave detailed insight into the radical borylation process.

Straightforward Synthesis of Fluorinated Enals via Photocatalytic α-Perfluoroalkenylation of Aldehydes

(Per)fluorinated substances represent an important compound class with regard to drug design and material chemistry. We found a mild, operationally simple, and inexpensive photocatalytic perfluoroalkenylation reaction giving tetrasubstituted, highly electron-deficient enals straight from aldehydes. This one-step reaction tolerates various functional groups and can be applied to a wide range of substrates giving the products in yields of 52-84%.

Copper-Catalyzed Carbonylative Coupling of Alkyl Halides

Transition metal-catalyzed carbonylation reactions represent a direct and atom-economical approach to introduce oxygen functionality into organic compounds, with CO acting as an inexpensive and readily available C1 feedstock. Despite the long history of carbonylation catalysis, including many processes that have been industrialized at bulk scale, there remain several challenges to tackle. For example, noble metals such as Pd, Rh, and Ir are typically used as catalysts for carbonylation reactions, rather than earth-abundant alternatives. Additionally, while carbonylation of C(sp²)-hybridized substrates (e.g., aryl halides) is well-known, carbonylation of unactivated alkyl electrophiles, especially where β-hydride elimination can compete with desired CO migratory insertion at the catalyst site, remains challenging for many systems. Recently, base metal catalysis based on Mn, Co, and other metals has enabled advances in carbonylative coupling of alkyl electrophiles, though the nucleophiles are often limited to alcohols or amines to generate esters or amides as products. Thus, we have targeted base metal-catalyzed carbonylative C-C and C-E (E = N, H, Si, B) coupling reactions as a method for approaching diverse carbonyl compounds of synthetic importance.Initially, we designed a heterobimetallic catalyst platform for carbonylative C-C coupling of alkyl halides with arylboronic esters (i.e., carbonylative Suzuki-Miyaura coupling) to generate aryl alkyl ketones. Subsequently, we developed multicomponent carbonylation reactions of alkyl halides using NHC-Cu catalysts (NHC = N-heterocyclic carbene). These reactions operate by radical mechanisms, converting alkyl halides into either acyl radical or acyl halide intermediates that undergo subsequent C-C or C-E coupling at the Cu site. This mechanistic paradigm is relatively novel in the metal-catalyzed carbonylation area, allowing us to discover a previously unexplored chemical space in carbonylative coupling catalysis. We have successfully developed the following reactions: (a) hydrocarbonylative coupling of alkynes with alkyl halides; (b) borocarbonylative coupling of alkynes with alkyl halides; (c) reductive aminocarbonylation of alkyl halides with nitroarenes; (d) reductive carbonylation of alkyl halides; (e) carbonylative silylation of alkyl halides; (f) carbonylative borylation of alkyl halides. These reactions provide a broad range of valuable products including ketones, allylic alcohols, β-borylenones, amides, alcohols, acylsilanes, and acylborons in an efficient manner. Notably, the preparation of some of these products has previously required multistep syntheses, harsh conditions, or specialized reagents. By contrast, the multicomponent coupling platform that we have developed requires only readily available building blocks and rapidly increases molecular complexity in a single synthetic manipulation.

Progress in the medicinal chemistry of organoboron compounds

The review aims to draw attention to the latest advances in the organoboron chemistry and therapeutic use of organoboron compounds. The synthetic strategies towards boron-containing compounds with proven in vitro and/or in vivo biological activities, including derivatives of boronic acids, benzoxaboroles, benzoxaborines and benzodiazaborines, are summarized. Approaches to the synthesis of hybrid structures containing an organoboron moiety as one of the pharmacophores are considered, and the effect of this modification on the pharmacological activity of the initial molecules is analyzed. On the basis of analysis of the published data, the most promising areas of research in the field of organoboron compounds are identified, including the latest methods of synthesis, modification and design of effective therapeutic agents.

The bibliography includes 246 references.

(o-Phenylenediamino)borylstannanes: Efficient Reagents for Borylation of Various Alkyl Radical Precursors

(o-Phenylenediamino)borylstannanes were newly synthesized to achieve radical boryl substitutions of a variety of alkyl radical precursors. Dehalogenative, deaminative, decharcogenative, and decarboxylative borylations proceeded in the presence of a radical initiator to give the corresponding organic boron compounds. Radical clock experiments and computational studies have provided insights into the mechanism of the homolytic substitution (S_H 2) of the borylstannanes with alkyl radical intermediates. DFT calculation disclosed that the phenylenediamino structure lowered the LUMO level including the vacant p-orbital on the boron atom to enhance the reactivity to alkyl radicals in S_H 2. Moreover, C(sp³ )-H borylation of THF was accomplished using the triplet state of xanthone.

One-Step Synthesis of Acylboron Compounds via Copper-Catalyzed Carbonylative Borylation of Alkyl Halides*

A copper-catalyzed carbonylative borylation of unactivated alkyl halides has been developed, enabling efficient synthesis of aliphatic potassium acyltrifluoroborates (KATs) in high yields by treating the in situ formed tetracoordinated acylboron intermediates with aqueous KHF₂ . A variety of functional groups are tolerated under the mild reaction conditions, and primary, secondary, and tertiary alkyl halides are all applicable. In addition, this method also provides facile access to N-methyliminodiacetyl (MIDA) acylboronates as well as α-methylated potassium acyltrifluoroborates in a one-pot manner. Mechanistic studies indicate a radical atom transfer carbonylation (ATC) mechanism to form acyl halide intermediates that are subsequently borylated by (NHC)CuBpin.

Evaluation of the role of graphene-based Cu(i) catalysts in borylation reactions

A methodical experimental and theoretical analysis of different carbon-based Cu(i) materials in the context of the development of an efficient, general, scalable, and sustainable borylation reaction of aliphatic and aromatic halides has been performed.

Transition-Metal-Free Borylation of Aryl Bromide Using a Simple Diboron Source

In this study, we developed a simple transition-metal-free borylation reaction of aryl bromides. Bis-boronic acid (BBA), was used, and the borylation reaction was performed using a simple procedure at a mild temperature. Under mild conditions, aryl bromides were converted to arylboronic acids directly without any deprotection steps and purified by conversion to trifluoroborate salts. The functional group tolerance was considerably high. The mechanism study suggested that this borylation reaction proceeds via a radical pathway.

Transition metal-free synthesis of alkyl pinacol boronates

This review systematically outlined the research in the area of transition metal free synthesis of alkyl pinacol boronates, which are versatile and important scaffolds to construct diverse organic compounds.

Photoinduced Deoxygenative Borylations of Aliphatic Alcohols

A photochemical method for converting aliphatic alcohols into boronic esters is described. Preactivation of the alcohol as a 2-iodophenyl-thionocarbonate enables a novel Barton-McCombie-type radical deoxygenation that proceeds efficiently with visible light irradiation and without the requirement for a photocatalyst, a radical initiator, or tin or silicon hydrides. The resultant alkyl radical is intercepted by bis(catecholato)diboron, furnishing boronic esters from a diverse range of structurally complex alcohols.

Zn+-O- Dual-Spin Surface State Formation by Modification of ZnO Nanoparticles with Diboron Compounds

ZnO semiconductor oxides are versatile functional materials that are used in photoelectronics, catalysis, sensing, etc. The Zn⁺-O^- surface electronic states of semiconductor oxides were formed on the ZnO surface by Zn 4s and O 2p orbital coupling with the diboron compound's B 2p orbitals. The formation of spin-coupled surface states was based on the spin-orbit interaction on the interface, which has not been reported before. This shows that the semiconductor oxide's spin surface states can be modulated by regulating surface orbital energy. The Zn⁺-O^- surface electronic states were confirmed by electron spin resonance results, which may help in expanding the fundamental research on spintronics modulation and quantum transport.

Modification of TiO2 Nanoparticles with Organodiboron Molecules Inducing Stable Surface Ti3+ Complex

As one of the most promising semiconductor oxide materials, titanium dioxide (TiO₂) absorbs UV light but not visible light. To address this limitation, the introduction of Ti³⁺ defects represents a common strategy to render TiO₂ visible-light responsive. Unfortunately, current hurdles in Ti³⁺ generation technologies impeded the widespread application of Ti³⁺ modified materials. Herein, we demonstrate a simple and mechanistically distinct approach to generating abundant surface-Ti³⁺ sites without leaving behind oxygen vacancy and sacrificing one-off electron donors. In particular, upon adsorption of organodiboron reagents onto TiO₂ nanoparticles, spontaneous electron injection from the diboron-bound O²⁻ site to adjacent Ti⁴⁺ site leads to an extremely stable blue surface Ti³⁺‒O^−· complex. Notably, this defect generation protocol is also applicable to other semiconductor oxides including ZnO, SnO₂, Nb₂O₅, and In₂O₃. Furthermore, the as-prepared photoelectronic device using this strategy affords 10³-fold higher visible light response and the fabricated perovskite solar cell shows an enhanced performance.

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Organodiborons are used to reshape the surface electronic state of semiconductor oxides

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Diboron adsorption leads to spontaneous charge transfer and reduced surface metal ions

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Photodetector based on diboron material affords 10³ fold higher visible light response

New avenues for C-B bond formation via radical intermediates

This perspective gives an overview on recent findings in the emerging area of C-radical borylation using diborons as radical trapping reagents. Aryl, vinyl and alkyl boronic esters can be accessed via such an approach under mild conditions. These processes are complementary to established transition metal catalysed cross coupling reactions. Radical borylations can be conducted in the absence of a transition metal but some processes require transition metals as catalysts. It will be shown that various readily available C-radical precursors can be used to run these borylations. For a better understanding of the chemistry, mechanistic discussions are also presented and an outlook on this topic will be provided at the end of the article.