Synthesis and Applications of Unquaternized C-Bound Boron Enolates

A Boron Scan of Ethyl Acetoacetate Leads to Versatile Building Blocks

Discovered in the 19th century, ethyl acetoacetate has been central to the development of organic chemistry, including its pedagogy and applications. In this study, we present borylated derivatives of this venerable molecule. A boron handle has been installed at either α ${{\rm \alpha }}$ - or β ${\beta }$ -position of acetoacetate by homologation of acyl-MIDA (N-methyliminodiacetic acid) boronates with diazoacetates. Either alkyl or boryl groups were found to migrate with regiochemistry being a function of the steric bulk of the diazo species. Boryl β ${{\rm \beta }}$ -ketoesters can be further modified into borylated pyrazolones and oximes, thereby expanding the synthetic toolkit and offering opportunities for additional modifications.

Visible-Light-Mediated Synthesis of α-Aryl Ester Derivatives via an EDA Complex

We report an efficient radical-based and photocatalyst-free method for the C(sp2)-C(sp3) cross-coupling reaction to synthesize α-aryl ester derivatives. The process starts from a β-keto ester and an electron-deficient halogenated aryl halide under alkaline conditions to form an electron donor-acceptor complex and is driven by visible light. From the synthetic point of view, this newly established method represents a simple way to access arylpropionic acids from commercially available and cheap starting materials.

A syn outer-sphere oxidative addition: the reaction mechanism in Pd/Senphos-catalyzed carboboration of 1,3-enynes

We report a combined experimental and computational study of Pd/Senphos-catalyzed carboboration of 1,3-enynes utilizing DFT calculations, ³¹P NMR study, kinetic study, Hammett analysis and Arrhenius/Eyring analysis. Our mechanistic study provides evidence against the conventional inner-sphere β-migratory insertion mechanism. Instead, a syn outer-sphere oxidative addition mechanism featuring a Pd-π-allyl intermediate followed by coordination-assisted rearrangements is consistent with all the experimental observations.

Diastereo- and Enantioselective Reductive Mannich-type Reaction of α,β-Unsaturated Carboxylic Acids to Ketimines: A Direct Entry to Unprotected β2,3,3 -Amino Acids

Stereoselective construction of unprotected β-amino acids is a significant challenge owing to the lack of methods for the catalytic generation of highly enantioenriched carboxylic acid enolates. In this study, a novel copper-catalyzed diastereo- and enantioselective reductive Mannich-type reaction of α,β-unsaturated carboxylic acids was developed, which provides a direct and scalable synthetic method for enantioenriched β^2,3,3 -amino acids with vicinal stereogenic centers. The protocol features in situ generation of transiently protected carboxylic acids by a hydrosilane and their diastereo- and enantioselective reductive coupling with ketimines. The synthetic utility of this process was demonstrated by a gram-scale reaction and the transformation of β-amino acids.

Synthesis of β-Silyl-α-amino Acid Derivatives by Cu-Catalyzed Regio- and Enantioselective Silylamination of α,β-Unsaturated Esters

A copper-catalyzed silylamination of α,β-unsaturated esters with silylboranes and hydroxylamines has been developed to afford the corresponding β-silyl-α-amino acid derivatives, which are of great interest in medicinal and pharmaceutical chemistry. Additionally, by using a suitable chiral bisphosphine ligand, the asymmetric induction is possible, delivering the optically active β-silyl-α-amino acids with synthetically acceptable diastereomeric ratios (55:45-82:18 dr) and high enantiomeric ratios (81:19-99:1 er).

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Copper-Catalyzed Reductive Ireland-Claisen Rearrangements of Propargylic Acrylates and Allylic Allenoates

The copper-catalyzed reductive Ireland-Claisen rearrangement of propargylic acrylates led to 3,4-allenoic acids. The use of silanes or pinacolborane as stoichiometric reducing agents and triethylphosphite as a ligand facilitated the divergent and complementary selectivity for the synthesis of diastereomeric anti- and syn-rearranged products, respectively. Copper-catalyzed reductive Ireland-Claisen rearrangement of allylic 2,3-allenoates proceeded effectively only when pinacolborane was used as a reductant to generate various 1,5-dienes in excellent yields and with good diastereoselectivities in some cases. Mechanistic studies showed that the silyl and boron enolates, rather than the copper enolate, underwent a stereospecific rearrangement via a chairlike transition state to afford the corresponding Claisen rearrangement products.

Mechanistic Insight into the 1,3,2-Diazaphospholene-Catalyzed Reductant (HBpin/NH3BH3)-Controlled Reaction of Allyl 2-Phenylacrylate: Claisen Rearrangement or Hydrogenation?

Mechanistic study on the 1,3,2-diazaphospholene (1)-catalyzed reduction reaction of allyl 2-phenylacrylate 4 with HBpin or ammonia borane (AB) was systematically performed by the density functional theory (DFT) method. When HBpin is employed as the reductant, the reductive Ireland-Claisen (IC) rearrangement reaction occurs. First, the active species P-hydrido-1,3,2-diazaphospholene 3 is generated through the metathesis reaction of 1 with HBpin. Next, the terminal C═C double bond of 4 is inserted into the P-H bond of 3 to produce 6a through the 1,2-addition (Markovnikov) step, which is followed by the pinB-H bond activation to afford key boron enolate 8. Then, 8 undergoes the [3,3] rearrangement that is followed by the alcoholysis reaction with methanol leading to the final product γ,δ-unsaturated carboxylic acid. The [3,3] rearrangement step is the rate-determining step with the Gibbs energy barrier (ΔG^≠) and Gibbs reaction energy (ΔG) of 23.9 and -27.5 kcal/mol, respectively. When AB is employed as the reductant, the transfer hydrogenation reaction occurs through two comparable pathways, 1,2- and 1,4-transfer hydrogenation pathways. The former pathway directly leads to the hydrogenation product with the ΔG^≠ and ΔG values of 22.4 and -27.7 kcal/mol, respectively. The latter pathway produces an enolate intermediate (rate-determining step, ΔG^≠/ΔG = 24.1/-0.3 kcal/mol) first, which then prefers to undergo the enol-keto tautomerism instead of the [3,3] rearrangement to afford the hydrogenation product. Obviously, the generation of the boron enolate plays a crucial role in the reductive IC rearrangement reaction because it prevents the enol-keto tautomerism.

C-Boron Enolates Enable Palladium Catalyzed Carboboration of Internal 1,3-Enynes

A new family of carbon-bound boron enolates, generated by a kinetically controlled halogen exchange between chlorocatecholborane and silylketene acetals, is described. These C-boron enolates are demonstrated to activate 1,3-enyne substrates in the presence of a Pd⁰ /Senphos ligand complex, resulting in the first examples of a carboboration reaction of an alkyne with enolate-equivalent nucleophiles. Highly substituted dienyl boron building blocks are produced in excellent site-, regio-, and diastereoselectivity by the described catalytic cis-carboboration reaction.

Pd/Cu Dual-Catalyzed Asymmetric Synthesis of Highly Functional All-Carbon Quaternary Stereocenters from Vinyl Carbonates

The challenging metal-catalyzed asymmetric synthesis of highly functional quaternary carbon centers using decarboxylative C(sp³ )-C(sp³ ) bond formation reactions is reported. The key substrate, a vinyl cyclic carbonate, is activated to provide concomitantly both the requisite nucleophile (by formal umpolung) and electrophile reaction partner preceding the asymmetric cross-coupling process. A wide screening of reaction conditions, additives and catalyst precursors afforded a protocol that gave access to a series of compounds featuring densely functionalized, elusive quaternary carbon stereocenters in appreciable yield and with enantiomeric ratios (er's) of up to 90 : 10.

Stereoselective Diels-Alder Reactions of gem-Diborylalkenes: Toward the Synthesis of gem-Diboron-Based Polymers

Although gem-diborylalkenes are known to be among the most valuable reagents in modern organic synthesis, providing a rapid access to a wide array of transformations, including the construction of C-C and C-heteroatom bonds, their use as dienophile-reactive groups has been rare. Herein we report the Diels-Alder (DA) reaction of (unsymmetrical) gem-diborylalkenes. These reactions provide a general and efficient method for the stereoselective conversion of gem-diborylalkenes to rapidly access 1,1-bisborylcyclohexenes. Using the same DA reaction manifold with borylated-dienes and gem-diborylalkenes, we also developed a concise, highly regioselective synthesis of 1,1,2-tris- and 1,1,3,4-tetrakis(boronates)cyclohexenes, a family of compounds that currently lack efficient synthetic access. Furthermore, DFT calculations provided insight into the underlying factors that control the chemo-, regio-, and stereoselectivity of these DA reactions. This method also provides stereodivergent syntheses of gem-diborylnorbornenes. The utility of the gem-diborylnorbornene building blocks was demonstrated by ring-opening metathesis polymerization (ROMP), providing a highly modular approach to the first synthesis of the gem-diboron-based polymers. Additionally, these polymers have been successfully submitted to postpolymerization modification reactions. Given its simplicity and versatility, we believe that this novel DA and ROMP approach holds great promise for organoboron synthesis as well as organoboron-based polymers and that it will result in more novel transformations in both academic and industrial research.

Synthetic Study Aiming at the Tricyclic Core of 12-epi-JBIR-23/24

The synthetic study toward highly enantio- and diastereoselective synthesis of the tricyclic framework of 12-epi-JBIR-23/24, a natural product analogue showing inhibitory activity against four malignant pleural mesothelioma cell lines, is presented herein. In this synthesis, a rhodium-catalyzed asymmetric three-component Michael/aldol reaction introduces three consecutive tertiary carbon centers, while the unique epoxyquinol core motif is successfully forged via [3,3]-sigmatropic rearrangement of an allylic xanthate, vinylogous Pummerer rearrangement, and a selective mesylation/epoxidation cascade of a triol.

Applications of diazaphospholene hydrides in chemical catalysis

Diazaphospholenes have recently emerged as main-group hydride transfer catalysts. This review will briefly summarize the common structural variants of diazaphospholenes, and the properties that make them superb hydride transfer catalysts, followed by a critical examination of the various preparative routes toward diazaphospholenes. Finally, an in-depth examination of the reactivity of diazaphospholenes in contemporary catalysis, including asymmetric catalysis will be undertaken.

Ketone Synthesis from Benzyldiboronates and Esters: Leveraging α-Boryl Carbanions for Carbon-Carbon Bond Formation

An alkoxide-promoted method for the synthesis of ketones from readily available esters and benzyldiboronates is described. The synthetic method is compatible with a host of sterically differentiated alkyl groups, alkenes, acidic protons α to carbonyl groups, tertiary amides, and aryl rings having common organic functional groups. With esters bearing α-stereocenters, high enantiomeric excess was maintained during ketone formation, establishing minimal competing racemization by deprotonation. Monitoring the reaction between benzyldiboronate and LiOtBu in THF at 23 °C allowed for the identification of products arising from deborylation to form an α-boryl carbanion, deprotonation, and alkoxide addition to form an "-ate" complex. Addition of 4-trifluoromethylbenzoate to this mixture established the α-boryl carbanion as the intermediate responsible for C-C bond formation and ultimately ketone synthesis. Elucidation of the role of this intermediate leveraged additional bond-forming chemistry and enabled the one-pot synthesis of ketones with α-halogen atoms and quaternary centers with four-different carbon substituents.

Organophosphorus chemistry based on elemental phosphorus: advances and horizons

The results of studies on the application of elemental phosphorus for the synthesis of important organophosphorus compounds are surveyed and summarized. Currently, this trend represents a synthetically, environmentally and technologically attractive alternative to classical organophosphorus chemistry based on toxic and corrosive phosphorus chlorides. Direct phosphination and phosphinylation of organic compounds with elemental phosphorus (discussed in the first part of the review) basically extend the range of available phosphines, phosphine chalcogenides and phosphinic acids and provides further development of their synthetic potential (discussed in the second part of the review). It is shown that the breakthrough in this area is largely due to the discovery of reactions of elemental phosphorus (white and red) with various electrophiles in superbasic suspensions and emulsions derived from alkali metal hydroxides and to the development of electrochemical, electrocatalytic and catalytic activation of white phosphorus.

The bibliography includes 299 references.

Iridium-catalyzed B-H insertion of sulfoxonium ylides and borane adducts: a versatile platform to α-boryl carbonyls

Iridium-catalyzed boron-hydrogen bond insertion reactions of trimethylamine-borane and sulfoxonium ylides have been demonstrated, furnishing α-boryl ketones in moderate to excellent yields in most cases (51 examples; up to 84%). This practical and scalable insertion reaction showed broad substrate scope, high functional-group compatibility and could be applied in late-stage modification of structurally complex drug compounds. Further synthetic applications were also demonstrated.

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.

Kinetic Resolution of Racemic Aldehydes through Asymmetric Allenoate γ-Addition: Synthesis of (+)-Xylogiblactone A

A synthesis of (+)-xylogiblactone A has been achieved from t-butyl 2-methylbuta-2,3-dienoate in a linear three-step sequence. The key elements of the synthesis include a kinetic resolution of racemic 2-silyoxyaldehyde through the allenoate γ-addition to yield the γ-adduct as a single isomer and the subsequent gold catalysis to form the butenolide core. For a general method, the kinetic resolution of several racemic 2-silyloxyaldehydes is also performed to provide products in high levels of stereoselectivity with unusual anti-Felkin-Anh addition fashion.

Site-Selective Mono-Oxidation of 1,2-Bis(boronates)

Site-selective oxidation of vicinal bis(boronates) is accomplished through the use of trimethylamine N-oxide in 1-butanol solvent. The reaction occurs with good efficiency and selectivity across a range of substrates, providing 2-hydro-1-boronic esters which are shown to be versatile intermediates in the synthesis of chiral building blocks.

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.

Reductive α-borylation of α,β-unsaturated esters using NHC-BH3 activated by I2 as a metal-free route to α-boryl esters

Useful α-boryl esters can be synthesized in one step from α,β-unsaturated esters using just a simple to access NHC-BH₃ (NHC = N-heterocyclic carbene) and catalytic I₂. The scope of this reductive α-borylation methodology is excellent and includes a range of alkyl, aryl substituted and cyclic and acyclic α,β-unsaturated esters. Mechanistic studies involving reductive borylation of a cyclic α,β-unsaturated ester with NHC-BD₃/I₂ indicated that concerted hydroboration of the alkene moiety in the α,β-unsaturated ester proceeds instead of a stepwise process involving initial 1,4-hydroboration; this is in contrast to the recently reported reductive α-silylation. The BH₂(NHC) unit can be transformed into electrophilic BX₂(NHC) moieties (X = halide) and the ester moiety can be reduced to the alcohol with the borane unit remaining intact to form β-boryl alcohols. The use of a chiral auxiliary, 8-phenylmenthyl ester, also enables effective stereo-control of the newly formed C-B bond. Combined two step ester reduction/borane oxidation forms diols, including excellent e.e. (97%) for the formation of S-3-phenylpropane-1,2-diol. This work represents a simple transition metal free route to form bench stable α-boryl esters from inexpensive starting materials.

Borylated reagents for multicomponent reactions

Over the past two decades, there has been an increasing interest in the therapeutic potential of organoboron reagents due to the discovery of bortezomib (Velcade). This has motivated synthetic chemists to develop novel routes for the preparation of heteroatom-rich boron-containing molecules (BCMs). In particular, the development of borylated building blocks has provided facile access to difficult-to-access heteroatom-rich BCMs. In this review, we will discuss the methods used to prepare boron-containing molecules of biological relevance from multicomponent reactions with borylated building blocks.

Difunctionalization of ketones via gem-bis(boronates) to synthesize quaternary carbon with high selectivity

A transformation of gem-bis(boronate) compounds with different electrophiles through a tertiary boronate intermediate to concurrently introduce aldehyde and allylic groups, which provides an efficient protocol to difunctionalize ketones, was reported.