Hydroboration of Enynes and Mechanistic Insights

Pd-catalyzed ortho-/meta-C-H-annulation of biphenyl amines with enynes through non-rollover cyclometallation

Annulations through dual C-H activation represent a powerful tool to selectively assemble multi-cyclic scaffolds. We present herein a palladium-catalyzed ortho-/meta-C-H-annulation of biphenyl amines with 1,6-enynes. This regioselective non-rollover cyclometallation was achieved through meticulous tuning of electronic factors of both the partners. This method is applicable to a wide range of protected o-arylanilines and enynes, and results in the regioselective preparation of benzo[f]isoindolyl derivatives in high yields with good diastereoselectivity (with respect to two types of stereogenic elements). Certain essential control experiments and kinetic isotope effect (KIE) studies were undertaken to elucidate the reaction mechanism, while subsequent transformations and a scale-up reaction were performed to substantiate the sturdiness of the transformation.

Electrochemically Enabled Hydroxyphosphorylation of 1,3-Enynes to Access Phosphinyl-Substituted Propargyl Alcohols

Catalytic difunctionalization with the direct activation of (O)P-H bonds has been recently established as a potentially robust platform to generate valuable organophosphorus compounds. In terms of 1,3-enynes, despite of the various catalytic methods developed for hydrophosphorylation, the radical-mediated hetero-functionalization of two different atoms has been less explored. In this study, we disclosed an electrochemically induced hydroxyphosphorylation of 1,3-enynes for the construction of phosphinyl-substituted propargyl alcohols. The system involves the direct activation of both arylphosphine oxides and oxygen in ambient air with no external metal or additive needed. The use of electrochemistry ensures the regioselective, atom-economic and eco-friendly for the difunctionalization process. This strategy highlights the advantages of mild reaction conditions, readily available starting materials and broad substrate scope, showing its practical synthetic value in organic synthesis.

Reagent- and Ligand-Dependent Mechanistic Trichotomy in Fe-Catalyzed Borylative Cyclizations of Enynes

Based on some very recent results on Fe-catalyzed boron-source-dependent regiodivergent hydroborations of internal alkynes, we report here a boron-source-dependent but also ligand-dependent mechanistic trichotomy in borylative cyclizations. The choice of ligand plus boron source allows the synthesis of three isomeric borylative cyclization products starting from a common substrate and using the same precatalyst ((Ph3P)2Fe(CO)(NO)H) and sets the stage for the development of a unifying concept in Fe-catalyzed borylative cyclizations.

Chemo-, Regio-, and Stereoselective cis-Hydroboration of 1,3-Enynes: Copper-Catalyzed Access to (Z,Z)- and (Z,E)-2-Boryl-1,3-dienes

A copper-catalyzed alkyne-selective hydroboration of 1,3-enynes is disclosed, providing access to the previously elusive 2-boryl-1,3-dienes. Using CuOAc, Xantphos, and HBpin, Bpin was installed on the internal carbon of a series of symmetric and nonsymmetric 1,3-enynes, affording products with excellent Z:E selectivity. The utility of the 2-boryl-1,3-diene products was demonstrated by transformation to useful functional groups.

Efficient and selective external activator-free cobalt catalyst for hydroboration of terminal alkynes enabled by BiPyPhos

Herein, a robust catalyst system, composed of a bipyridine-based diphosphine ligand (BiPyPhos) and a cobalt precursor Co(acac)2, is successfully developed and applied in the hydroboration of terminal alkynes, exclusively affording various versatile β-E-vinylboronates in high yields at room temperature.

Enantio- and Regioselective Cascade Hydroboration of Methylenecyclopropanes for Facile Access to Chiral 1,3- and 1,4-Bis(boronates)

Chiral 1, n-bis(boronate) plays a crucial role in organic synthesis and medicinal chemistry. However, their catalytic and asymmetric synthesis has long posed a challenge in terms of operability and accessibility from readily available substrates. The recent discovery of the C═C bond formation through β-C elimination of methylenecyclopropanes (MCP) has provided an exciting opportunity to enhance molecular complexity. In this study, the catalyzed asymmetric cascade hydroboration of MCP is developed. By employing different ligands, various homoallylic boronate intermediate are obtained through the hydroboration ring opening process. Subsequently, the cascade hydroboration with HBpin or B2pin2 resulted in the synthesis of enantioenriched chiral 1,3- and 1,4-bis(boronates) in high yields, accompanied by excellent chemo- and enantioselectivities. The selective transformation of these two distinct C─B bonds also demonstrated their application potential in organic synthesis.

Asymmetric Total Synthesis of Anti-HBV Drug Entecavir: Catalytic Strategies for the Stereospecific Construction of Densely Substituted Cyclopentene Cores

We have successfully accomplished a catalytic asymmetric total synthesis of entecavir, a first-line antihepatitis B virus medication. The pivotal aspect of our strategy lies in the utilization of a Pd-catalyzed enyne borylative cyclization reaction, enabling the construction of a highly substituted cyclopentene scaffold with exceptional stereoselectivity. Additionally, we efficiently accessed the crucial 1,3-diol enyne system early in our synthetic route through a diarylprolinol organocatalyzed enantioselective cross-aldol reaction and Re-catalyzed allylic alcohol relocation. By strategically integrating these three catalytic protocols, we established a practical pathway for acquiring valuable densely heteroatom-substituted cyclopentene cores.

Cobalt-Catalyzed Selective Hydroboration of 1,3-Enynes with HBpin toward 1,3-Dienylboronate Esters

An efficient cobalt-catalyzed selective hydroboration of 1,3-enynes with HBpin toward 1,3-dienylboronate esters is disclosed. With a commercially available catalytic system of Co(acac)2 and dppf, the hydroboration reactions proceeded well to afford a wide range of 1,3-dienylborates in moderate to high yields. This protocol features a cheap base-metal catalytic system, broad substrate scope, excellent selectivity, easy gram-scale preparation, and good functional group tolerance and provides access to synthetically valuable 1,3-dienylborates.

Metal-free C-H Borylation and Hydroboration of Indoles

The C-H borylation and hydroboration reactions have emerged as promising synthetic tools to construct organoboron compounds. Organoboron compounds of N-heterocycles, particularly indole derivatives, have found widespread application in a variety of fields. As a result, considerable advancement in the area of C-H borylation and hydroboration reactions of indoles was observed in the last few decades. Among the various synthetic methods applied, the metal-free approach has received special attention. This mini-review discusses the recent progress in the area of C-H borylation and hydroboration reactions of indoles under metal-free conditions, their scope, and brief mechanistic studies.

Selective synthesis of boron-substituted enynes via a one-pot diboration/protodeboration sequence

An efficient and facile one-pot protocol to access enynylboronates via a Pt-catalyzed diboration/protodeboration strategy has been developed. The reaction is suitable for various silylsubstituted symmetrical and unsymmetrical 1,3-diynes, leading to π-conjugated organoboron compounds with excellent regio- and stereoselectivity.

Notable Catalytic Activity of Transition Metal Thiolate Complexes against Hydrosilylation and Hydroboration of Carbon-Heteroatom Bonds

Chemists tend to use transition metal hydride complexes rather than thiolate complexes to catalyse chemical transformations because the hydride complexes possess diverse catalytic reactivity, although most of them are air/moisture-sensitive and difficult to prepare. By comparing the catalytic performances of pincer ligated group 10 metal thiolate and hydride complexes in catalysing the hydroboration and hydrosilylation of C=O and C=N bonds, we demonstrate in this review that transition metal thiolate complexes are much better catalysts than the corresponding hydride complexes in catalysing this type of reactions. Many hydroboration and hydrosilylation reactions catalysed by pincer ligated group 10 metal hydride complexes can also be catalysed by the corresponding thiolate complexes and the thiolate systems are far more active. Therefore, the applications of transition metal thiolate complexes in the catalytic hydroboration and hydrosilylation of unsaturated carbon-heteroatom bonds deserve special attention in future work.

Gold-Catalyzed Cyclization/Hydroboration of 1,6-Enynes: Synthesis of Bicyclo[3.1.0]hexane Boranes

The gold-catalyzed cyclization/hydroboration of 1,6-enynes offers facile, versatile, and atom-economical one-step access to bicyclo[3.1.0]hexane boranes. This new protocol proceeds in moderate to good yields under mild conditions. Different from bicyclo[3.1.0]hexane borates, these products are stable in air and during chromatography. Moreover, the borane moiety of the products can readily undergo a diverse array of transformations. The kinetic isotope effect experiment indicates that the hydrogen-transfer step is a fast process, which is not involved in the rate-limiting step.

Ruthenium-Catalyzed Geminal Hydroborative Cyclization of Enynes

Disclosed here is the first geminal (gem-) hydroborative cyclization of enynes. Different from known hydroborative cyclizations, this process adds hydrogen and boron to the same position, leading to a new reaction mode. With [Cp*RuCl]₄ as catalyst, a range of gem-hydroborated bicyclic products bearing a cyclopropane unit could be rapidly assembled from simple enyne substrates. Control experiments and density functional theory (DFT) calculations provided important insights into the reaction mechanism. Notably, two major competing pathways may operate with substrate-dependence. 1,6-Enynes favor initial oxidative cyclometalation to form a ruthenacyclopentene intermediate prior to engaging hydroborane, while other enynes (e.g., 1,7-enynes) that lack strong propensity toward cyclization prefer initial alkyne gem-(H,B)-addition to form an α-boryl ruthenium carbene followed by intramolecular olefin cyclopropanation. This process also represents the first ruthenium-catalyzed enyne hydroborative cyclization.

Radical modulated regioselective difunctionalization of vinyl enynes: tunable access to naphthalen-1(2H)-ones and allenic alcohols

A novel and efficient radical-modulated difunctionalization of vinyl enynes has been disclosed using TEMPO as a radical regulator. Facile access to structurally diverse 3-bromo-naphthalen-1(2H)-ones and 4-bromo-allenic alcohols was realized via 1,2-addition/1,2-migration or 1,4-addition, respectively. This protocol represents the first example of radical-modulated metal-free difunctionalization of 1,3-enynes with high regioselectivity.

Hydroelementation of diynes

This review highlights the hydroelementation reactions of conjugated and separated diynes, which depending on the process conditions, catalytic system, as well as the type of reagents, leads to the formation of various products: enynes, dienes, allenes, polymers, or cyclic compounds. The presence of two triple bonds in the diyne structure makes these compounds important reagents but selective product formation is often difficult owing to problems associated with maintaining appropriate reaction regio- and stereoselectivity. Herein we review this topic to gain knowledge on the reactivity of diynes and to systematise the range of information relating to their use in hydroelementation reactions. The review is divided according to the addition of the E-H (E = Mg, B, Al, Si, Ge, Sn, N, P, O, S, Se, Te) bond to the triple bond(s) in the diyne, as well as to the type of the reagent used, and the product formed. Not only are the hydroelementation reactions comprehensively discussed, but the synthetic potential of the obtained products is also presented. The majority of published research is included within this review, illustrating the potential as well as limitations of these processes, with the intent to showcase the power of these transformations and the obtained products in synthesis and materials chemistry.

Mechanism and Origins of Enantioselectivity of Cobalt-Catalyzed Intermolecular Hydroacylation/Cyclization of 1,6-Enynes with Aldehydes

Density functional theory calculations were performed to investigate the cobalt-catalyzed intermolecular hydroacylation/cyclization of 1,6-enynes. The computations show that the initial oxidative cyclization constitutes the rate-determining step of the overall reaction....

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.