The drastic effect of cobalt and chromium catalysts in the borylation of arylzinc reagents

Cr-catalyzed borylation of C(aryl)-F bonds using a terpyridine ligand

The defluoroborylation of fluoroarenes by chromium-catalyzed cleavage of unactivated C-F bonds is described. The reaction uses HBpin as the boron source, low-cost and commercially available chromium salt as the precatalyst, and terpyridine as a crucial ligand, providing a protocol with atom-efficient benefits and a wide range of applicable substrates for the functionalization of aryl C-F bonds. Preliminary mechanistic studies indicate that an unprecedented Cr-catalyzed magnesiation of the unactivated C-F bond occurred. The generated arylmagnesium intermediates then participated in the subsequent borylation reaction. The application of the strategy in the preparation of valuable derivatives is demonstrated by the late-stage functionalization of boronate ester groups.

Etherification of Fluoroarenes with Alkoxyboronic Acid Pinacol Esters via C-F Bond Cleavage

Potassium-base-mediated defluoroetherification of aryl and heteroaryl fluorides with alkoxyboronic acid pinacol esters under transition-metal-free conditions is reported. This protocol efficiently and safely provides a wide variety of aryl ethers in high yields without using metal catalysts, specific ligands, and harsh conditions to selectively forge C_sp2-O bonds via the C_sp2-F cleavage. This method can be applied to the late-stage etherification of structurally complex C_sp2-fluorides and bioactive alcohols, such as β-estradiol, calciferol, and tocopherol.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Visible-Light-Induced Ni-Catalyzed Radical Borylation of Chloroarenes

A highly selective and general photoinduced C-Cl borylation protocol that employs [Ni(IMes)₂] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the radical borylation of chloroarenes is reported. This photoinduced system operates with visible light (400 nm) and achieves borylation of a wide range of chloroarenes with B₂pin₂ at room temperature in excellent yields and with high selectivity, thereby demonstrating its broad utility and functional group tolerance. Mechanistic investigations suggest that the borylation reactions proceed via a radical process. EPR studies demonstrate that [Ni(IMes)₂] undergoes very fast chlorine atom abstraction from aryl chlorides to give [Ni^I(IMes)₂Cl] and aryl radicals. Control experiments indicate that light promotes the reaction of [Ni^I(IMes)₂Cl] with aryl chlorides generating additional aryl radicals and [Ni^II(IMes)₂Cl₂]. The aryl radicals react with an anionic sp²-sp³ diborane [B₂pin₂(OMe)]^- formed from B₂pin₂ and KOMe to yield the corresponding borylation product and the [Bpin(OMe)]^•- radical anion, which reduces [Ni^II(IMes)₂Cl₂] under irradiation to regenerate [Ni^I(IMes)₂Cl] and [Ni(IMes)₂] for the next catalytic cycle.

Three-component coupling of aryl iodides, allenes, and aldehydes catalyzed by a Co/Cr-hybrid catalyst

The cobalt/chromium-catalyzed three-component coupling of aryl iodides, allenes, and aldehydes has been developed to afford multi-substituted homoallylic alcohols in a diastereoselective manner. Control experiments for understanding the reaction mechanism reveal that the cobalt catalyst is involved in the oxidative addition and carbometalation steps in the reaction, whereas the chromium salt generates highly nucleophilic allylchromium intermediates from allylcobalt species, without the loss of stereochemical information, to allow the addition to aldehydes.

Highly nucleophilic vitamin B12-assisted nickel-catalysed reductive coupling of aryl halides and non-activated alkyl tosylates

Reductive cross-coupling of aryl halides with ubiquitous alkyl tosylates was developed using a combination of nickel and vitamin B₁₂ (VB₁₂: cyanocobalamin) catalysts. The tosylate was activated by reduced VB₁₂ to form alkyl cobalt(iii), which served as a good alkylating agent for aryl-nickel species, leading to C(sp³)-C(sp²) bond formation.

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.