Homolytic Cleavage of a B−B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

Photocatalytic and Electrochemical Borylation and Silylation Reactions

Due to their high versatility borylated and silylated compounds are inevitable synthons for organic chemists. To escape the classical hydroboration/hydrosilylation paradigm, chemists turned their attention to more modern and green methods such as photoredox chemistry and electrosynthesis. This account focuses on novel methods for the generation of boryl and silyl radicals to forge C-B and C-Si bonds from our group.

Electrooxidative Activation of B-B Bond in B2 cat2 : Access to gem-Diborylalkanes via Paired Electrolysis

This report describes the unprecedented electrooxidation of a solvent (e.g., DMF)-ligated B₂ cat₂ complex, whereby a solvent-stabilized boryl radical is formed via quasi-homolytic cleavage of the B-B bond in a DMF-ligated B₂ cat₂ radical cation. Cyclic voltammetry and density functional theory provide evidence to support this novel B-B bond activation strategy. Furthermore, a strategy for the electrochemical gem-diborylation of gem-bromides via paired electrolysis is developed for the first time, affording a range of versatile gem-diborylalkanes, which are widely used in synthetic society. Notably, this reaction approach is scalable, transition-metal-free, and requires no external activator.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

Metal-Free Reductive Coupling of para-Quinone Methides with 4-Cyanopyridines Enabled by Pyridine-Boryl Radicals: Access to Pyridylated Diarylmethanes with Anti-Cancer Activity

Reported herein is a streamlined protocol to produce pyridylated diarylmethanes through pyridine-boryl radical induced reductive coupling between para-quinone methides (p-QMs) and 4-cyanopyridines using bis(pinacolato)diboron (B₂ pin₂ ) as a templated reagent. The metal-free process is characterized by an operationally simple approach, excellent chemoselectivity (1,2- vs. 1,6-selectivity), and a broad substrate scope with good functional group compatibility. The mechanistic studies provided important insights into the reductive cross-coupling process between diarylmethyl radical and pyridine-boryl radical. Moreover, part of the obtained pyridylated diarylmethane products were screened against a panel of cancer cell lines, and 3 v was confirmed to significantly inhibit the proliferation of head and neck squamous cell carcinoma (HNSCC) cells. This method offers a platform for the preparation of new lead compounds with antitumor activity.

Copper-Photocatalyzed Hydroboration of Alkynes and Alkenes

The photocatalytic hydroboration of alkenes and alkynes is reported. The use of newly-designed copper photocatalysts with B2 Pin2 permits the formation a boryl radical, which is used for hydroboration of a large panel of alkenes and alkynes. The hydroborated products were isolated in high yields, with excellent diastereoselectivities and a high functional group tolerance under mild conditions. The hydroboration reactions were developed under continuous flow conditions to demonstrate their synthetic utility. The reaction mechanism was studied and suggested an oxidation reaction between an in situ formed borate and the Cu-photocatalyst in its excited state for the boryl radical formation.

Ni-Catalyzed Borylation of Aryl Sulfoxides

A nickel/N-heterocyclic carbene (NHC) catalytic system has been developed for the borylation of aryl sulfoxides with B2 (neop)2 (neop=neopentyl glycolato). A wide range of aryl sulfoxides with different electronic and steric properties were converted into the corresponding arylboronic esters in good yields. The regioselective borylation of unsymmetric diaryl sulfoxides was also feasible leading to borylation of the sterically less encumbered aryl substituent. Competition experiments demonstrated that an electron-deficient aryl moiety reacts preferentially. The origin of the selectivity in the Ni-catalyzed borylation of electronically biased unsymmetrical diaryl sulfoxide lies in the oxidative addition step of the catalytic cycle, as oxidative addition of methoxyphenyl 4-(trifluoromethyl)phenyl sulfoxide to the Ni(0) complex occurs selectively to give the structurally characterized complex trans-[Ni(ICy)2 (4-CF3 -C6 H4 ){(SO)-4-MeO-C6 H4 }] 4. For complex 5, the isomer trans-[Ni(ICy)2 (C6 H5 )(OSC6 H5 )] 5-I was structurally characterized in which the phenyl sulfinyl ligand is bound via the oxygen atom to nickel. In solution, the complex trans-[Ni(ICy)2 (C6 H5 )(OSC6 H5 )] 5-I is in equilibrium with the S-bonded isomer trans-[Ni(ICy)2 (C6 H5 )(SOC6 H5 )] 5, as shown by NMR spectroscopy. DFT calculations reveal that these isomers are separated by a mere 0.3 kJ/mol (M06/def2-TZVP-level of theory) and connected via a transition state trans-[Ni(ICy)2 (C6 H5 )(η2 -{SO}-C6 H5 )], which lies only 10.8 kcal/mol above 5.

Electrochemical Hydroboration of Alkynes

Herein we reported the electrochemical hydroboration of alkynes by using B₂Pin₂ as the boron source. This unprecedented reaction manifold was applied to a broad range of alkynes, giving the hydroboration products in good to excellent yields without the need of a metal catalyst or a hydride source. This transformation relied on the possible electrochemical oxidation of an in situ formed borate. This anodic oxidation performed in an undivided cell allowed the formation of a putative boryl radical, which reacted on the alkyne.

Decarboxylative Borylation of Stabilized and Activated Carbon Radicals

Redox-active esters (RAEs) as active radical precursors have been extensively studied for C-B bond formations. However, the analogous transformations of stabilized radicals from the corresponding acid precursors remain challenging owing to the strong preference towards single-electron oxidation to the stable carbocations. This work describes a general strategy for rapid access to various aliphatic and aromatic boronic esters by mild photoinduced decarboxylative borylation. Both aryl and alkyl radicals could be generated from the leaving group-assisted N-hydroxybenzimidoyl chloride esters, even α-CF₃ substituted substrates could be activated for further elaboration.

New Radical Borylation Pathways for Organoboron Synthesis Enabled by Photoredox Catalysis

Radical borylation using N-heterocyclic carbene (NHC)-BH₃ complexes as boryl radical precursors has emerged as an important synthetic tool for organoboron assembly. However, the majority of reported methods are limited to reaction modes involving carbo- and/or hydroboration of specific alkenes and alkynes. Moreover, the generation of NHC-boryl radicals relies principally on hydrogen atom abstraction with the aid of radical initiators. A distinct radical generation method is reported, as well as the reaction pathways of NHC-boryl radicals enabled by photoredox catalysis. NHC-boryl radicals are generated via a single-electron oxidation and subsequently undergo cross-coupling with the in-situ-generated radical anions to yield gem-difluoroallylboronates. A photoredox-catalyzed radical arylboration reaction of alkenes was achieved using cyanoarenes as arylating components from which elaborated organoborons were accessed. Mechanistic studies verified the oxidative formation of NHC-boryl radicals through a single-electron-transfer pathway.

Photoinduced Single-Electron Transfer as an Enabling Principle in the Radical Borylation of Alkenes with NHC-Borane

A photoinduced SET process enables the direct B-H bond activation of NHC-boranes. In contrast to common hydrogen atom transfer (HAT) strategies, this photoinduced reaction simply takes advantage of the beneficial redox potentials of NHC-boranes, thus obviating the need for extra radical initiators. The resulting NHC-boryl radical was used for the borylation of a wide range of α-trifluoromethylalkenes and alkenes with diverse electronic and structural features, providing facile access to highly functionalized borylated molecules. Labeling and photoquenching experiments provide insight into the mechanism of this photoinduced SET pathway.

Selective C-N Borylation of Alkyl Amines Promoted by Lewis Base

An efficient method for the metal-free deaminative borylation of alkylamines, using bis(catecholato)diboron as the boron source, to directly synthesize various alkyl potassium trifluoroborate salts is introduced. The key to this high reactivity is the utilization of pyridinium salt activated alkylamines, with a catalytic amount of a bipyridine-type Lewis base as a promoter. This transformation shows good functional-group compatibility (e.g., it is unimpeded by the presence of a ketone, indole, internal alkene, or unactivated alkyl chloride) and can serve as a powerful synthetic tool for borylation of amine groups in complex compounds. Mechanistic experiments and computations suggest a mechanism in which the Lewis base activated B₂ cat₂ unit intercepts an alkyl radical generated by single-electron transfer (SET) from a boron-based reductant.

Synthesis of Functionalized Hydrazines: Facile Homogeneous (N-Heterocyclic Carbene)-Palladium(0)-Catalyzed Diboration and Silaboration of Azobenzenes

The bis(N-heterocyclic carbene)(diphenylacetylene)palladium complex [Pd(ITMe)2(PhC≡CPh)] (ITMe=1,3,4,5-tetramethylimidazol-2-ylidene) acts as a highly active pre-catalyst in the diboration and silaboration of azobenzenes to synthesize a series of novel functionalized hydrazines. The reactions proceed using commercially available diboranes and silaboranes under mild reaction conditions.