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.

Radical Addition Enables 1,2-Aryl Migration from a Vinyl-Substituted All-Carbon Quaternary Center

An efficient method for photocatalytic perfluoroalkylation of vinyl-substituted all-carbon quaternary centers involving 1,2-aryl migration has been developed. The rearrangement reactions use fac-Ir(ppy)₃ , visible light and commercially available fluoroalkyl halides and can generate valuable multisubstituted perfluoroalkylated compounds in a single step that would be challenging to prepare by other methods. Mechanistically, the photoinduced alkyl radical addition to an alkene leads to the migration of a vicinal aryl substituent from its adjacent all-carbon quaternary center with the concomitant generation of a C-radical bearing two electron-withdrawing groups that is further reduced by a hydrogen donor to complete the domino sequence.

Synthesis of 1,3-Bis-(boryl)alkanes through Boronic Ester Induced Consecutive Double 1,2-Migration

A general and efficient approach for the preparation of 1,3-bis-(boryl)alkanes is introduced. It is shown that readily generated vinylboron ate complexes react with commercially available ICH2 Bpin to valuable 1,3-bis-(boryl)alkanes. The introduced transformation, which is experimentally easy to conduct, shows broad substrate scope and high functional-group tolerance. Mechanistic studies reveal that the reaction does not proceed via radical intermediates. Instead, an unprecedented boronic ester induced sequential bis-1,2-migration cascade is suggested.

Radical-Induced 1,2-Migrations of Boron Ate Complexes

1,2-Boron ate rearrangements represent a fundamental class of transformations to establish new C-C bonds while retaining the valuable boron moiety in the product. In established ionic processes, the boron ate complex is activated by an external electrophile to induce a 1,2-migration from boron to an adjacent sp 3 or sp 2 carbon atom. Recently, two complementary radical polar crossover approaches have been explored for both classes, 1,2-migrations to sp 2 and sp 3 carbon centers. This review describes the general concepts in this emerging research field and summarizes recent developments of radical-induced 1,2-migrations from boron to carbon.

Catalytic Enantioselective Synthesis of anti-Vicinal Silylboronates by Conjunctive Cross-Coupling

Chiral 1,2-bimetallic reagents are useful motifs in synthetic chemistry. Although syn-1,2-bimetallic compounds can be prepared by alkene dimetallation, anti-1,2-bimetallics are still rare. The stereospecific 1,2-metallate shift that occurs during conjunctive cross-coupling is shown to enable a practical and modular approach to the catalytic synthesis of enantioenriched anti-1,2-borosilanes. In addition to reaction development, the synthetic utility of anti-1,2-borosilanes was investigated, including applications to the synthesis of anti-1,2-diols and anti-1,2-amino alcohols.

Recent Advances in Radical Addition to Alkenylboron Compounds

The addition of carbon-centered radicals to alkenylboron compounds provides a useful method for the construction of organoboron reagents which are versatile reagents in chemical synthesis. While the first examples of this type or process appeared 70 years ago, until recently, attention to this type of reaction has been limited. A number of examples of this reactivity have been demonstrated recently, and strategies are emerging that allow for tuning the reagent structure in order to modulate reactivity. This review surveys recent advances in this class of reaction.

Enantioselective Radical Addition/Cross-Coupling of Organozinc Reagents, Alkyl Iodides, and Alkenyl Boron Reagents

A hybrid transition-metal/radical process is described that results in the addition of organozinc reagents and alkyl halides across alkenyl boron reagents in an enantioselective catalytic fashion. The reaction can be accomplished both intermolecularly and intramolecularly, providing useful product yields and high enantioselectivities in both manifolds.

Catalytic protodeboronation of pinacol boronic esters: formal anti-Markovnikov hydromethylation of alkenes

Pinacol boronic esters are highly valuable building blocks in organic synthesis. In contrast to the many protocols available on the functionalizing deboronation of alkyl boronic esters, protodeboronation is not well developed. Herein we report catalytic protodeboronation of 1°, 2° and 3° alkyl boronic esters utilizing a radical approach. Paired with a Matteson-CH2-homologation, our protocol allows for formal anti-Markovnikov alkene hydromethylation, a valuable but unknown transformation. The hydromethylation sequence was applied to methoxy protected (-)-Δ8-THC and cholesterol. The protodeboronation was further used in the formal total synthesis of δ-(R)-coniceine and indolizidine 209B.

An Olefinic 1,2-Boryl-Migration Enabled by Radical Addition: Construction of gem-Bis(boryl)alkanes

A series of in situ formed alkenyl diboronate complexes from alkenyl Grignard reagents (commercially available or prepared from alkenyl bromides and Mg) with B₂ Pin₂ (bis(pinacolato)diboron) react with diverse alkyl halides by a Ru photocatalyst to give various gem-bis(boryl)alkanes. Alkyl radicals add efficiently to the alkenyl diboronate complexes, and the adduct radical anions undergo radical-polar crossover, specifically, a 1,2-boryl-anion shift from boron to the α-carbon sp² center. This transformation shows good functional-group compatibility and can serve as a powerful synthetic tool for late-stage functionalization in complex compounds. Measurements of the quantum yield reveal that a radical-chain mechanism is operative in which the alkenyl diboronates acts as reductive quencher for the excited state of the photocatalyst.

Catalyst-Free Enantiospecific Olefination with In Situ Generated Organocerium Species

Described is the in situ formation of triorganocerium reagents and their application in catalyst-free Zweifel olefinations. These unique cerium species were generated through novel exchange reactions of organohalides with n-Bu₃ Ce reagents. The adequate electronegativity of cerium allowed for compensating the disadvantages of both usually functional-group-sensitive organolithium species and less reactive organomagnesium reagents. Exchange reactions were performed on aryl and alkenyl bromides, enabling enantiospecific transformations of chiral boron pinacol esters. Finally, these new organocerium species were engaged in selective 1,2-additions onto enolisable and sterically hindered ketones.

Enantioselective Construction of Tertiary Boronic Esters by Conjunctive Cross-Coupling

Catalytic enantioselective conjunctive cross-coupling has been developed to construct tertiary alkylboronic esters. These reactions occur with good yield and enantioselectivity for a range of substrates. Mechanistic experiments reveal aspects of the catalytic cycle that allow hindered substrates to react without significant complicating side reactions.

Radical-Triggered Three-Component Coupling Reaction of Alkenylboronates, α-Halocarbonyl Compounds, and Organolithium Reagents: The Inverse Ylid Mechanism

An operationally simple protocol to affect a radical addition to alkenylboronates that spontaneously undergo a [1,2]-metalate shift is described. Overall, the reaction is a three-component coupling of an organolithium, alkenylboronic ester, and halide which takes place with broad scope and good to excellent yields. Experimental mechanistic investigations support the formation of a boron inverse ylid intermediate.