N-Heterocyclic Carbene Boranes are Hydrogen Donors in Masamune-Bergman Reactions of Benzo[3,4]cyclodec-3-ene-1,5-diynes

Nucleophilic Addition of Enolates to 1,4-Dehydrobenzene Diradicals Derived from Enediynes: Synthesis of Functionalized Aromatics

Alkylation of aromatics and formation of a new C-C bond is usually achieved by the electrophilic attack of an activated carbon species on an electron-rich aromatic ring. Herein, we report an alternative method for alkylation of aromatics via nucleophilic addition of enolates of active methylene compounds to 1,4-dehydrobenzene diradicals derived from enediynes cyclodec-1,5-diyne-3-ene, benzo[3,4]-cyclodec-1,5-diyne-3-ene, and cyclohexeno[3,4]-cyclodec-1,5-diyne-3-ene. The benzo-substituted enediyne produces slightly higher yields of alkylation products than do the other two enediynes, but the differences are not substantial. The reaction produces a new C-C bonded aromatic alkylation product, which allows the construction of complex polyfunctional structures in a few steps. Moreover, this reaction provides solely C-arylated products, and no O-arylation products were observed.

Stereoselective hydrogen atom transfer to acyclic radicals: a switch enabling diastereodivergent borylative radical cascades

Radical cascade reactions are powerful tools to construct structurally complex molecules. However, the stereochemical control of acyclic radical intermediates remains a persistent challenge, due to the low differentiation between the two faces of these species. This hurdle further makes stereodivergent synthesis rather more difficult to be accomplished, in particular for intermediates resulted from complex cascades. Here we report an efficient strategy for stereoselective hydrogen atom transfer (HAT) to acyclic carbon radicals, which are generated via N-heterocyclic carbene (NHC)-boryl radicals triggered addition-translocation-cyclization cascades. A synergistic control by the NHC subunit and a thiol catalyst has proved effective for one facial HAT, while a ZnI2-chelation protocol allows for the preferential reaction to the opposite face. Such a stereoselectivity switch enables diastereodivergent construction of heterocycles tethering a boron-substituted stereocenter. Mechanistic studies suggest two complementary ways to tune HAT diastereoselectivity. The stereospecific conversions of the resulting boron-handled products to diverse functionalized molecules are demonstrated.

Advances in chemistry of N-heterocyclic carbene boryl radicals

Boron-centred radicals (boryl radicals) are potential and attractive species in main group chemistry and synthetic chemistry. Recently, the development of boron compounds ligated by N-heterocyclic carbenes (NHCs) has sparked off advavnces in boryl radical chemistry because NHCs can highly stabilise boryl radicals by electronic and steric factors. This review highlights recent synthesis and reactions of such NHC-boryl radicals. From the standpoint of main group chemistry, examples of isolation or detection of unique NHC-boryl radicals are presented. From the standpoint of synthetic chemistry, on the other hand, the development of reactions of user-friendly NHC-boryl radicals, which has contributed to radical chemistry, organoboron chemistry and polymer science, is comprehensively described.

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.

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.

DFT investigation of hydrogen atom-abstraction reactions of NHC-boranes by various carbon-centered radicals: barriers and correlation analyses

In this study, we employed a quantum-mechanical computational method to investigate the hydrogen-atom abstraction reactions of two nitrogen heterocyclic carbene boranes (NHC-boranes), NHC-BH3 and NHC-BH2CN, by a series of carbon-centered radicals bearing various substituents. We explored the degree of correlation of the activation and free energy barriers to their components. Furthermore, we also investigated the effects of the radical and substituent sizes, nucleophilicity/electrophilicity indices, and the spin density distribution of the radical reactants on the three fundamental barriers and the thermal contribution of the reaction energy to the kinetic barrier. Using the generated data, we assessed the abilities of the various radical reactants to abstract the hydrogen atom from NHC-boranes. Further, we performed a similar analysis after dividing those radical reactants into four groups, which were classified based on the dominant factor affecting their electronic density distribution, which involves the inductive effect, conjugation, hyperconjugation, and the feedback of lone-pair electrons. The results and conclusions of this investigation not only provide insight into the relationships between some of the key kinetic and thermodynamic parameters, which is useful for understanding the dynamics of such hydrogen-abstraction reactions, but also provide information for selecting suitable radical reactants for further experimental investigations.

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.

Regioselective Radical Hydroboration of gem-Difluoroalkenes: Synthesis of α-Borylated Organofluorines

A regioselective radical hydroboration of gem-difluoroalkenes was developed for the synthesis of α-difluoroalkylborons. The reaction features excellent regioselectivity, broad substrate scope, and good functional group capability. DFT calculations implicated the remarkable α-selectivity was driven from the kinetically and thermodynamically more favorable α-addition step. The resulting α-difluoroalkylborons could be readily converted into NHC-borane-tethered monofluoroalkenes, which demonstrated unique reactivity and applicability in the synthesis of monofluoroalkene derivatives through transformations of the boron unit.

Investigation of the rhodium-catalyzed hydroboration of NHC-boranes: the role of alkene coordination and the origin of enantioselectivity

The mechanism of the intramolecular enantioselective rhodium(i)-catalyzed hydroboration of NHC-boranes is investigated by experiments and calculations, using Density Functional Theory and Random Phase Approximation methods.