Synthesis of Alkyl and Aryl Boronate Esters via CeO2-Catalyzed Borylation of Alkyl and Aryl Electrophiles Including Alkyl Chlorides

A recyclable protocol using a CeO2-nanorod catalyst for borylation of alkyl halides with B2pin2 (pin = OCMe2CMe2O) is reported. A wide range of synthetically useful alkyl boronate esters are readily obtained from primary and secondary alkyl electrophiles, including unactivated alkyl chlorides, demonstrating broad utility and functional group tolerance. Preliminary investigation revealed an involvement of in situ formed catalytically active boryl species. The catalyst can be reused for up to six runs without appreciable loss in activity. In addition, we have demonstrated the use of this recyclable catalyst for the borylation of aryl halides with B2pin2, providing valuable aryl boronate esters under neat conditions.

Cu-Catalyzed Asymmetric Kinetic Boron Conjugate Addition of γ-Substituted α,β-Unsaturated γ-Lactams

Chiral α,β-unsaturated γ-lactams bearing simple γ- substituents are found in biologically active molecules and natural products, however, their synthesis still remains difficult. Herein, we report an efficient kinetic resolution (KR) of γ-substituted α,β-unsaturated γ-lactams via a Cu-catalyzed asymmetric boron conjugate addition, which also leads to the efficient synthesis of chiral β-hydroxy-γ-lactams with β,γ-stereogenic carbon centers. The KR proceeded smoothly with a wide range of γ-alkyl or aryl substituted substrates including those bearing aromatic heterocycles and different N-protected substrates in up to 347 of s value. Their highly versatile transformations, synthetic utility in biologically active molecules, and inhibitory activities against cisplatin-sensitive ovarian cancer cell A2780 have also been demonstrated. Differing from the well-known mechanism involving Cu-B species in Cu-catalyzed boron conjugate additions, our mechanistic studies using density functional theory (DFT) calculations and experiments indicate that a Lewis acid CuI -catalyzed mechanism is the likely pathway in the catalytic reaction.

Enolates ambushed - asymmetric tandem conjugate addition and subsequent enolate trapping with conventional and less traditional electrophiles

Metal enolates are useful intermediates and building blocks indispensable in many organic synthetic transformations. Chiral metal enolates obtained by asymmetric conjugate additions of organometallic reagents are structurally complex intermediates that can be employed in many transformations. In this review, we describe this burgeoning field that is reaching maturity after more than 25 years of development. The effort of our group to broaden possibilities to engage metal enolates in reactions with new electrophiles is described. The material is divided according to the organometallic reagent employed in the conjugate addition step, and thus to the particular metal enolate formed. Short information on applications in total synthesis is also given.

anti-Selective synthesis of β-boryl-α-amino acid derivatives by Cu-catalysed borylamination of α,β-unsaturated esters

A copper-catalysed regio- and diastereoselective borylamination of α,β-unsaturated esters with B2pin2 and hydroxylamines has been developed to deliver acyclic β-boryl-α-amino acid derivatives with high anti-diastereoselectivity (up to >99 : 1), which is difficult to obtain by the established methods. A chiral phosphoramidite ligand also successfully induces the enantioselectivity, giving the optically active β-borylated α-amino acids. The products can be stereospecifically transformed into β-functionalised α-amino acids, which are of potent interest in medicinal chemistry.

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.

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.

Regioselective radical hydroboration of electron-deficient alkenes: synthesis of α-boryl functionalized molecules

A regioselective radical hydroboration of various electron-deficient alkenes is achieved by the employment of an NHC-boryl radical. A range of α-borylated nitriles, trifluoromethyl molecules, phosphonates, sulfones, and gem-diboron compounds have been prepared from readily available starting materials. Further synthetic applications of these products are also demonstrated.

Regioselective radical α-borylation of α,β-unsaturated carbonyl compounds for direct synthesis of α-borylcarbonyl molecules

Organoboron compounds are highly valuable in synthetic chemistry. In particular, α-borylcarbonyl compounds have shown versatile synthetic applications, owing to fruitful chemistries of both the boryl and carbonyl moieties. However, the synthesis of these molecules still remains tedious and time-consuming. Here we report a straightforward and practical route to synthesize α-borylcarbonyl molecules based on a regioselective radical α-borylation of α,β-unsaturated carbonyl compounds. The reaction features unusual α-regioselectivity and high functional-group compatibility. Further synthetic applications of new α-borylated products were also demonstrated. DFT and kinetic studies implicated that the α-regioselectivity of β-aryl-α,β-unsaturated carbonyl compounds was determined by the thermodynamically more favorable radical α-addition step, whereas the formation of α-addition products from β-alkyl-α,β-unsaturated carbonyl compounds was driven by an energetically favored hydrogen atom transfer step. Given that α,β-unsaturated carbonyl compounds can be easily obtained in abundance and variety, this method enjoys great advantages in diverse and economical synthesis of valuable α-borylcarbonyl molecules.

α-Borylcarbonyl compounds are versatile synthons in organic synthesis, however their preparation is often tedious. Here, the authors report a regioselective radical α-borylation of α,β-unsaturated carbonyl compounds to afford α-borylcarbonyl molecules with high selectivity and functional group compatibility.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.

Diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams

The conjugate addition reaction has been a useful tool in the formation of carbon-carbon bonds. The utility of this reaction has been demonstrated in the synthesis of many natural products, materials, and pharmacological agents. In the last three decades, there has been a significant increase in the development of asymmetric variants of this reaction. Unfortunately, conjugate addition reactions using α,β-unsaturated amides and lactams remain underdeveloped due to their inherently low reactivity. This review highlights the work that has been done on both diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams.

Organocatalytic functionalisation through boron chemistry

Diboron reagents can be activated both, from a metal or free-metal context, and consequently the addition of boryl units to unsaturated substrates proceeds sometimes with complementary selectivity. We highlight here the power of boron chemistry to functionalize molecules and provide new routes towards challenging compounds.

Tandem hydrogenation and condensation of fluorinated α,β-unsaturated ketones with primary amines, catalyzed by nickel

A simple homogeneous catalytic system based on nickel phosphine complexes has been developed for the transfer hydrogenation and condensation of α,β-unsaturated ketones to yield saturated ones and saturated imines using primary amines as hydrogen donors.

Borylation of primary and secondary alkyl bromides catalyzed by Cu2O nanoparticles

A Cu₂O nanoparticle catalyzed borylation of alkylboronic esters is developed, with mild reaction conditions, high yield and in the absence of ligands which otherwise are essential in homogenous catalysis, using bis(pinacolato)diboron as the boron source.

Asymmetric metal free β-boration of α,β-unsaturated imines assisted by (S)-MeBoPhoz

The adduct [MeO → Bpin–Bpin]⁻ efficiently mediates the β-boration of α,β-unsaturated imines formed in situ.

Metal-free borylative ring-opening of vinyl epoxides and aziridines

A rational approach towards the borylative ring-opening of vinylepoxides and vinylaziridines, by the in situ formed MeO(-)→bis(pinacolato)diboron adduct, has been developed. The enhanced nucleophilic character of the Bpin (sp(2)) moiety from the reagent favours the SN2' conjugated B addition with the concomitant opening of the epoxide and aziridine rings. The reaction proceeds with total chemoselectivity towards the polyfunctionalised (-OH or -NHTs) allyl boronate. Theoretical calculations have determined the transition states that come from the reaction of the vinylic substrates with the activated MeO(-)→bis(pinacolato)diboron adduct, and a plausible mechanism for the organocatalytic borylative ring opening reaction has been suggested.

A clear-cut example of selective Bpin-Bdan activation and precise Bdan transfer on boron conjugate addition

Activating the non-symmetrical Bpin-Bdan diboron reagent with alkoxide leads to the formation of two possible adducts: MeO(-) →Bpin-Bdan or MeO(-) →Bdan-Bpin. Experimental and theoretical investigation confirms that the MeO(-) →Bpin interaction is preferred and thus selective formation of a C-Bdan bond upon reaction with an activated C=C bond.

A selective transformation of enals into chiral γ-amino alcohols

A one-pot synthesis of chiral amino alcohols from α,β-unsaturated aldehydes is reported which circumvents competitive 1,2- versus 1,4-boryl addition, by means of using a sterically hindered amine-derived imine. In addition to the complete chemoselectivity, modification of the Cu(I) catalyst with readily available chiral diphosphines, such as (R)-DM-BINAP, gave the 1,4-boryl addition products with high levels of asymmetric induction.

Formal regiocontrolled hydroboration of unbiased internal alkynes via borylation/allylic alkylation of terminal alkynes

In accessing trisubstituted vinyl boronates from terminal alkynes, a propargyl directing (2-pyridyl)sulfonyl group allows terminal alkynes to undergo Cu-catalyzed B2(pin)2-borylation and subsequent Cu-catalyzed allylic alkylation with Grignard reagents without affecting the pinacolboronate moiety, thereby formally enabling a highly stereo- and regiocontrolled access to hydroboration products of unbiased dialkyl internal alkynes.

Catalytic methodologies for the β-boration of conjugated electron deficient alkenes

The area of boron conjugate addition via diboration (β-boration) has grown rapidly since the first examples appeared in the late 1990s. This article aims to give a comprehensive review of the current advances in β-boration (of electron deficient alkenes), providing a commentary upon the development of the asymmetric version. To date, many mechanistic models have been put forward to explain the experimental observations and this review surveys some of these key ideas. Recently, the development of organocatalytic methodologies that facilitate β-boration have also been demonstrated and current ideas regarding the mechanisms of such processes are examined.