Ru-Catalyzed Geminal Hydroboration of Silyl Alkynes via a New gem-Addition Mechanism

Defect-engineering improves the activity of Metal-Organic frameworks for catalyzing hydroboration of Alkynes: A combination of experimental investigation and Density functional theory calculations

Defect-engineered metal-organic frameworks (DEMOFs) are emerging advanced materials. The construction of DEMOFs is of great significance; however, DEMOF-based catalysis remains unexplored. (E)-vinylboronates, an important building block for asymmetric synthesis, can be synthesized via the hydroboration of alkynes. However, the lack of high-performance catalysts considerably hinders their synthesis. Herein, a series of DEHKUST-1 (HKUST = Hong Kong University of Science and Technology) (Da-f) catalysts with missing occupation of linkers at Cu nodes were designed by partially replacing benzene-1,3,5-tricarboxylate (H3BTC) with defective connectors of pyridine-3,5-dicarboxylate (PYDC) to efficiently promote the hydroboration of alkynes. Results showed that the Dd containing 0.8 doping ratio of PYDC exhibited remarkable catalytic activity than the defect-free HKUST-1. This originated from the improved accessibility for reactants towards the Lewis acid active Cu sites of DEHKUST-1 due to the presence of plenty of rooms next to the Cu sites and enhanced coordination ability in such 'defective' HKUST-1. Dd had high selectivity (>99 %) and yield (>96 %) for (E)-vinylboronates and extensive functional group compatibility for terminal alkynes. Density functional theory (DFT) calculations were performed to elucidate the mechanism of hydroboration. Compared with that of defect-free HKUST-1, the low energy barrier of DEHKUST-1 can be attributed to the lower coordination number of Cu sites and enhanced accessibility of Cu active sites towards reagents.

Running Wild through Dirhodium Tetracarboxylate-Catalyzed Combined CH(C)-Functionalization/Cope Rearrangement Landscapes: Does Post-Transition-State Dynamic Mismatching Influence Product Distributions?

A special type of C-H functionalization can be achieved through C-H insertion combined with Cope rearrangement (CHCR) in the presence of dirhodium catalysts. This type of reaction was studied using density functional theory and ab initio molecular dynamics simulations, the results of which pointed to the dynamic origins of low yields observed in some experiments. These studies not only reveal intimate details of the complex reaction network underpinning CHCR reactions but also further cement the generality of the importance of nonstatistical dynamic effects in controlling Rh2L4-promoted reactions.

Ligand-controlled regiodivergent Ni-catalyzed trans-hydroboration/carboboration of internal alkynes with B2pin2

Unprecedented regioselective trans-hydroboration and carboboration of unbiased electronically internal alkynes were realized via a nickel catalysis system with the aid of the directing group strategy. Furthermore, the excellent α- and β-regioselectivity could be accurately switched by the nitrogen ligand (terpy) and phosphine ligand (Xantphos). Mechanistic studies provided an insight into the rational reaction process, that underwent the cis-to-trans isomerization of alkenyl nickel species. This transformation not only expands the scope of transition-metal-catalyzed boration of internal alkynes but also, more particularly, portrays the vast prospects of the directing group strategy in the selective functionalization of unactivated alkynes.

Mild Stereoselective Synthesis of Densely Substituted [3]Dendralenes via Ru-Catalyzed Intermolecular Dimerization of 1,1-Disubstituted Allenes

Described here is a mild and stereoselective protocol for the synthesis of [3]dendralenes via the intermolecular dimerization of allenes. With the proper choice of a ruthenium catalyst, a range of unactivated 1,1-disubstituted allenes, without prefunctionalization in the allylic position, reacted efficiently to provide rapid access to densely substituted [3]dendralenes. An intermolecular C-C bond and three different types of C═C double bonds (di-, tri-, and tetrasubstituted) embedded in an acyclic structure were constructed with good to high E/Z stereocontrol. This is in contrast to the known catalytic protocols that focus on allenes with prefunctionalization at the allylic position and/or monosubstituted allenes, which would proceed by a different mechanism or require less stereocontrol. The silyl-substituted dendralene products are precursors of other useful dendralene molecules. Density functional theory (DFT) studies and control experiments supported a mechanism involving oxidative cyclometalation, β-H elimination (the rate-determining step), and reductive elimination.

Exploring the Divergent Reactivity of Vinyl Radicals Emanating from Alkynes and Thiols via Photoredox Catalysis

Organic chemistry has seen a surge in visible-light-driven transformations, which offer unique reaction pathways and access to new synthetic possibilities. We aim to provide a comprehensive understanding of state-of-the-art photo-mediated alkyne functionalization, with a focus on the reactive behavior of vinyl radicals. This review outlines our contributions to the field, including developing new methods for forming carbon-carbon and carbon-heteroatom bonds.

Asymmetric anti-Selective Borylalkylation of Terminal Alkynes by Nickel Catalysis

Selective transformation of alkyne triple bonds to double bonds serves as an efficient platform to construct substituted alkenes. While significant advances have been made in its spatiotemporal regulation, achieving a multicomponent enantioselective reaction that requires multifaceted selectivity issues to be overcome is still uncommon. Here, we report an unprecedented asymmetric anti-stereoselective borylcarbofunctionalization of terminal alkynes by nickel catalysis. The utilization of an inexpensive chiral diamine ligand enables the three-component cross-coupling of terminal alkynes, a diboron reagent, and prochiral alkyl electrophiles with high levels of regio-, stereo-, and enantioselectivities. This reaction provides an efficient protocol to access enantioenriched alkenyl esters bearing an α-stereogenic center, is remarkably practical, and has a broad scope and an outstanding functional group compatibility. In addition, the value of this method has been highlighted in a diversity of follow-up stereoretentive derivatizations and the stereoselective concise synthesis of complex drug molecules.

1,3-Difunctionalization of Propargyl Silanes with Concomitant 1,2-Silyl Shift: Synthesis of Allyl Functionalized Vinyl Silanes

Terminal alkynes with a silyl group at the propargylic position upon activation with electrophiles such as N-bromosuccinimide undergo (E)-selective 1,2-silyl group migration. Subsequently, an allyl cation is formed that is intercepted by an external nucleophile. This approach provides allyl ethers and esters with stereochemically defined vinyl halide and silane handles for further functionalization. The scope of propargyl silanes and electrophile-nucleophile pairs are investigated, and various trisubstituted olefins are prepared in up to 78% yield. The obtained products have been demonstrated to serve as building blocks for transition-metal-catalyzed cross-couplings of vinyl halides, silicon-halogen exchange, and allyl acetate functionalization reactions.

Ir-Catalyzed Regioselective Dihydroboration of Thioalkynes toward Gem-Diboryl Thioethers

While 1,1-diboryl (gem-diboryl) compounds are valuable synthetic building blocks, currently, related studies have mainly focused on those 1,1-diboryl alkanes without a hetero functional group in the α-position. gem-Diboryl compounds with an α-hetero substituent, though highly versatile, have been limitedly accessible and thus rarely utilized. Herein, we have developed the first α-dihydroboration of heteroalkynes leading to the efficient construction of gem-diboryl, hetero-, and tetra-substituted carbon centers. This straightforward, practical, mild, and atom-economic reaction is an attractive complement to the conventional multistep synthetic strategy relying on deprotonation of gem-diborylmethane by a strong base. Specifically, [Ir(cod)(OMe)]₂ was found to be uniquely effective for this process of thioalkynes, leading to excellent α-regioselectivity when delivering the two boryl groups, which is remarkable in view of the many competitive paths including monohydroboration, 1,2-dihydroboration, dehydrodiboration, triboration, tetraboration, etc. Control experiments combined with DFT calculations suggested that this process involves two sequential hydroboration events. The second hydroboration requires a higher energy barrier due to severe steric repulsion in generating the highly congested α-sulfenyl gem-diboryl carbon center, a structural motif that was almost unknown before.

An ionic Fe-based metal-organic-framework with 4'-pyridyl-2,2':6',2''-terpyridine for catalytic hydroboration of alkynes

An ionic metal-organic-framework (MOF) containing nanoscale channels was readily assembled from ditopic 4'-pyridyl-2,2':6',2''-terpyridine (pytpy) and a simple iron(ii) salt. X-ray structural analysis revealed a two-dimensional grid-like framework assembled by classic octahedral (pytpy)2FeII cations as linkers (with pytpy as a new ditopic pyridyl ligand) and octa-coordinate FeCl2 centers as nodes. The layer-by-layer assembly of the 2-D framework resulted in the formation of 3-D porous materials consisting of nano-scale channels. The charges of the cationic framework were balanced with anionic Cl3FeOFeCl3 in its void channels. The new Fe-based MOF material was employed as a precatalyst for syn-selective hydroboration of alkynes under mild, solvent-free conditions in the presence of an activator, leading to the synthesis of a range of trans-alkenylboronates in good yields. The larger scale applicability and recyclability of the new MOF catalyst was further explored. This represents a rare example of an ionic MOF material that can be utilized in hydroboration catalysis.

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.

Ru-Catalyzed Hydroboration of Ynones Leads to a Nontraditional Mode of Reactivity

Although hydroboration of simple ketones and alkynes have been well-established, little is known about the unique hydroboration reactivity for ynones, a family of important building blocks. Herein we report a new reaction mode of ynones leading to structurally novel and synthetically useful but previously inaccessible products, vinyl α-hydroxylboronates, under mild ruthenium-catalyzed hydroboration conditions. This reaction features high efficiency, a broad scope, and complete chemo-, regio-, and stereoselectivity, in spite of many possible competitive pathways. Both control experiments and detailed DFT studies suggested a two-step mechanism, involving initial rate-determining conjugate addition of hydroborane to form the key boryl allenolate intermediate followed by a fast second hydroboration of the enolate motif of the allenolate. Notably, direct 1,4-addition of hydroborane to carbonyl-conjugated alkynes also represents a new mode of reactivity. Despite the overwhelming complexity of this process, which involves selectivity control in almost every step, a thorough and detailed computation on a large set of possible transition states explained the unusual reactivity and intrinsic origin of selectivity.

Borole/Borapyramidane Relationship

Boroles and borapyramidanes are classical and nonclassical constitutional isomers, respectively. It is here shown that they can indeed be interconverted. Treatment of the bis(alkynyl)B(C₆F₅) SMe₂ adduct 3·SMe₂ with HB(C₆F₅)₂ gave borole 1·SMe₂, featuring trimethylsilyl substituents in both α positions to boron, by means of a 1,1-hydroboration/alkenylboration sequence. Photolysis of the classical borole adduct 1·SMe₂ resulted in rearrangement to its nonclassical structural isomer, borapyramidane 2, in high yield, which exhibits a vicinal pair of trimethylsilyl substituents at the square pyramidane base. Neutral borapyramidane 2 is a rare example of an isoster of the (CH)₅⁺ pyramidane cation. Thermolysis of borapyramidane 2 in the presence of SMe₂ at 60 °C re-formed borole 1·SMe₂, which converted at 100 °C to 2,3-bis-silyl-substituted borole isomer 8·SMe₂. Its photolysis also gave borapyramidane 2. Prolonged photolysis of 2 at elevated temperatures converted this to borapyramidane isomer 10 containing a pair of trimethylsilyl groups in 1,3-position at its square C₄-pyramidal base. The borole and borapyramidane isomers were characterized by X-ray diffraction, and the system was analyzed by density functional theory (DFT) calculations.

New Insights and Predictions into Complex Homogeneous Reactions Enabled by Computational Chemistry in Synergy with Experiments: Isotopes and Mechanisms

Homogeneous catalysis and biocatalysis have been widely applied in synthetic, medicinal, and energy chemistry as well as synthetic biology. Driven by developments of new computational chemistry methods and better computer hardware, computational chemistry has become an essentially indispensable mechanistic "instrument" to help understand structures and decipher reaction mechanisms in catalysis. In addition, synergy between computational and experimental chemistry deepens our mechanistic understanding, which further promotes the rational design of new catalysts. In this Account, we summarize new or deeper mechanistic insights (including isotope, dispersion, and dynamical effects) into several complex homogeneous reactions from our systematic computational studies along with subsequent experimental studies by different groups. Apart from uncovering new mechanisms in some reactions, a few computational predictions (such as excited-state heavy-atom tunneling, steric-controlled enantioswitching, and a new geminal addition mechanism) based on our mechanistic insights were further verified by ensuing experiments.The Zimmerman group developed a photoinduced triplet di-π-methane rearrangement to form cyclopropane derivatives. Recently, our computational study predicted the first excited-state heavy-atom (carbon) quantum tunneling in one triplet di-π-methane rearrangement, in which the reaction rates and ¹²C/¹³C kinetic isotope effects (KIEs) can be enhanced by quantum tunneling at low temperatures. This unprecedented excited-state heavy-atom tunneling in a photoinduced reaction has recently been verified by an experimental ¹²C/¹³C KIE study by the Singleton group. Such combined computational and experimental studies should open up opportunities to discover more rare excited-state heavy-atom tunneling in other photoinduced reactions. In addition, we found unexpectedly large secondary KIE values in the five-coordinate Fe(III)-catalyzed hetero-Diels-Alder pathway, even with substantial C-C bond formation, due to the non-negligible equilibrium isotope effect (EIE) derived from altered metal coordination. Therefore, these KIE values cannot reliably reflect transition-state structures for the five-coordinate metal pathway. Furthermore, our density functional theory (DFT) quasi-classical molecular dynamics (MD) simulations demonstrated that the coordination mode and/or spin state of the iron metal as well as an electric field can affect the dynamics of this reaction (e.g., the dynamically stepwise process, the entrance/exit reaction channels).Moreover, we unveiled a new reaction mechanism to account for the uncommon Ru(II)-catalyzed geminal-addition semihydrogenation and hydroboration of silyl alkynes. Our proposed key gem-Ru(II)-carbene intermediates derived from double migrations on the same alkyne carbon were verified by crossover experiments. Additionally, our DFT MD simulations suggested that the first hydrogen migration transition-state structures may directly and quickly form the key gem-Ru-carbene structures, thereby "bypassing" the second migration step. Furthermore, our extensive study revealed the origin of the enantioselectivity of the Cu(I)-catalyzed 1,3-dipolar cycloaddition of azomethine ylides with β-substituted alkenyl bicyclic heteroarenes enabled by dual coordination of both substrates. Such mechanistic insights promoted our computational predictions of the enantioselectivity reversal for the corresponding monocyclic heteroarene substrates and the regiospecific addition to the less reactive internal C═C bond of one diene substrate. These predictions were proven by our experimental collaborators. Finally, our mechanistic insights into a few other reactions are also presented. Overall, we hope that these interactive computational and experimental studies enrich our mechanistic understanding and aid in reaction development.

Metal-free synthesis of β-aminoketones by the reductive hydroamination of ynones

This study describes a cascade method for the synthesis of β-aminoketones through the reductive hydroamination of alkynes under very mild metal-free conditions. It allows for the rapid conversion of ynones and amines into corresponding β-aminoketones with a broad substrate scope and diverse functionalities. This straightforward and easy-to-handle reaction process can be successfully applied for the synthesis of Proroxan and Propipocaine, offering a potential option for the synthesis of drug molecules with the β-aminoketone skeleton.

C-H Insertion via Ruthenium Catalyzed gem-Hydrogenation of 1,3-Enynes

gem-Hydrogenation of an internal alkyne with the aid of [Cp*RuCl]₄ as the precatalyst is a highly unorthodox transformation, in which one C atom of the triple bond is transformed into a methylene group, whereas the second C atom gets converted into a ruthenium carbene. In the case of 1,3-enynes bearing a propargylic steering substituent as the substrates, the reaction occurs regioselectively, giving rise to vinyl carbene complexes that adopt interconverting η¹/η³-binding modes in solution; a prototypical example of such a reactive intermediate was characterized in detail by spectroscopic means. Although both forms are similarly stable, only the η³-vinyl carbene proved kinetically competent to insert into primary, secondary, or tertiary C–H bonds on the steering group itself or another suitably placed ether, acetal, orthoester, or (sulfon)amide substituent. The ensuing net hydrogenative C–H insertion reaction is highly enabling in that it gives ready access to spirocyclic as well as bridged ring systems of immediate relevance as building blocks for medicinal chemistry. Moreover, the reaction scales well and lends itself to the formation of partly or fully deuterated isotopologues. Labeling experiments in combination with PHIP NMR spectroscopy (PHIP = parahydrogen induced polarization) confirmed that the reactions are indeed triggered by gem-hydrogenation, whereas kinetic data provided valuable insights into the very nature of the turnover-limiting transition state of the actual C–H insertion step.

Hydroboration and Diboration of Internal Alkynes under Iridium Catalysis

Here we demonstrate the feasibility and efficiency of simple iridium-based catalytic systems in the synthesis of multisubstituted alkenyl boronates from internal alkynes with high selectivities. A variety of alkynes were smoothly decorated with HBpin under a mild [Ir(cod)Cl]₂/dppm/acetone condition to afford trisubstituted alkenyl boronic esters with up to >99:1 regioselectivity. The diboration reaction could effectively occur in the presence of [Ir(cod)Cl]₂/DCM. Plausible mechanisms were provided to illustrate these two catalytic processes, in which the intrinsic functional group of the alkyne was supposed to be important in facilitating these reactions as well as the regioselectivity.

Directed cis-hydrosilylation of borylalkynes to borylsilylalkenes

Directed by the choice of catalyst cis-hydrosilylation of borylalkynes leads to novel borylsilylalkenes which are crucial synthons for the introduction of the carbon–carbon double bonds in organic synthesis.

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.

Quasiclassical Direct Dynamics Trajectory Simulations of Organometallic Reactions

Homogeneous metal-mediated organometallic reactions represent a very large and diverse reaction class. Density functional theory calculations are now routinely carried out and reported for analyzing organometallic mechanisms and reaction pathways. While density functional theory calculations are extremely powerful to understand the energy and structure of organometallic reactions, there are several assumptions in their use and interpretation to define reaction mechanisms and to analyze reaction selectivity. Almost always it is assumed that potential energy structures calculated with density functional theory adequately describe mechanisms and selectivity within the framework of statistical theories, for example, transition state theory and RRKM theory. However, these static structures and corresponding energy landscapes do not provide atomic motion information during reactions that could reveal nonstatistical intermediates without complete intramolecular vibrational redistribution and nonintrinsic reaction coordinate (non-IRC) pathways. While nonstatistical intermediates and non-IRC reaction pathways are now relatively well established for organic reactions, these dynamic effects have heretofore been highly underexplored in organometallic reactions. Through a series of quasiclassical density functional theory direct dynamics trajectory studies, my group has recently demonstrated that dynamic effects occur in a variety of fundamental organometallic reactions, especially bond activation reactions. For example, in the C-H activation reaction between methane and [Cp*(PMe₃)Ir^III(CH₃)]⁺, while the density functional theory energy landscape showed a two-step oxidative cleavage and reductive coupling mechanism, trajectories revealed a mixture of this two-step mechanism and a dynamic one-step mechanism that skipped the [Cp*(PMe₃)Ir^V(H)(CH₃)₂]⁺ intermediate. This study also showed that despite a methane σ-complex being located on the density functional theory surface before oxidative cleavage and after reductive coupling, this intermediate is always skipped and should not be considered an intermediate during reactive trajectories. For non-IRC reaction pathways, quasiclassical direct dynamics trajectories showed that for the isomerization of [Tp(NO)(PMe₃)W(η²-benzene)] to [Tp(NO)(PMe₃)W(H)(Ph)], there are many dynamic reaction pathway connections due to a relatively flat energy landscape and π coordination is not necessary for C-H bond activation through oxidative cleavage. Trajectories also showed that dynamic effects are important in selectivity for ethylene C-H activation versus π coordination in reaction with Cp(PMe₃)₂Re, and trajectories provide a more quantitative model of selectivity than transition state theory. Quasiclassical trajectories examining Au-catalyzed monoallylic diol cyclizations showed dynamic coupling of several reaction steps that include alkoxylation π bond addition, proton shuttling, and water elimination reaction steps. Overall, these studies highlight the need to use direct dynamics trajectory simulations to consider atomic motion during reactions to understand organometallic reaction mechanisms and selectivity.

A Chan-Evans-Lam approach to trisubstituted vinyl ethers

Trisubstituted vinyl ethers were accessed via Chan-Evans-Lam coupling of vinyl trifluoroborates and primary aliphatic alcohols. This approach complements prior methods that required the use of neat liquid alcohol coupling partners. A palladium-catalyzed redox-relay Heck reaction was used to convert several vinyl ethers into aldehyde-functionalized 1,3-dihydroisobenzofurans.

Cobalt-Catalyzed Hydroboration of Terminal and Internal Alkynes

A novel methodology to access synthetically versatile vinylboronic esters through a ligand-controlled cobalt-catalyzed hydroboration of terminal and internal alkynes is reported. The approach relies on the in situ reduction of Co(II) by H-BPin in the presence of bisphosphine ligands generating catalytically active Co(I) hydride complexes. This procedure avoids the use of stoichiometric amounts of base, and no boron-containing byproducts are generated which is translated into high functional group tolerance and atom economy.

Bottom-up modular synthesis of well-defined oligo(arylfuran)s

Oligofurans have attracted great attention in the field of materials over the last decades because of their several advantages, such as strong fluorescence, charge delocalization, and increased solubility. Although unsubstituted or alkyl-substituted oligofurans have been well-established, there is an increasing demand for the development of the aryl decorated oligofuran with structural diversity and unrevealed properties. Here, we report the bottom-up modular construction of chemically and structurally well-defined oligo(arylfuran)s by de novo synthesis of α,β′-bifuran monomers and late-stage bromination, stannylation and subsequent coupling reaction. The preliminary study of the photophysical properties demonstrated that the polarity-sensitive fluorescence emission and high quantum yields in THF solution could be achieved by modulating the aryl groups on the oligo(arylfuran)s. These twisted molecules constitute a new class of oligofuran backbone useful for structure−activities relationship studies. Meanwhile, the experimental studies and calculations showed that tetrafurans have appropriate HOMO energy levels, and could therefore potentially be high-performance p-type semiconductors.

Oligofurans have attracted great attention because of their strong fluorescence, charge delocalization, and increased solubility. Here the authors show a bottom-up modular construction of chemically and structurally well-defined oligo(arylfuran)s by de novo synthesis of α,β′-bifuran monomers and late-stage bromination, stannylation and subsequent coupling reaction.

Regioselective trans-Hydrostannation of Boron-Capped Alkynes

Alkynyl‐B(aam) (aam=anthranilamidato) derivatives are readily available bench‐stable compounds that undergo remarkably selective reactions with Bu₃SnH in the presence of [Cp*RuCl]₄ as the catalyst. The addition follows a stereochemically unorthodox trans‐selective course; in terms of regioselectivity, the Bu₃Sn‐ unit is delivered with high fidelity to the C‐atom of the triple bond adjacent to the boracyclic head group (“alpha,trans‐addition”). This outcome is deemed to reflect a hydrogen bonding interaction between the protic −NH groups of the benzo‐1,3,2‐diazaborininone ring system and the polarized [Ru−Cl] bond in the loaded catalyst, which locks the substrate in place in a favorable orientation relative to the incoming reagent. The resulting isomerically (almost) pure gem‐dimetalated building blocks are amenable to numerous downstream functionalizations; most remarkable is the ability to subject the −B(aam) moiety to Suzuki‐Miyaura cross coupling without need for prior hydrolysis while keeping the adjacent Bu₃Sn‐ group intact. Alternatively, the tin residue can be engaged in selective tin/halogen exchange without touching the boron substituent; the fact that the two ‐NH entities of −B(aam) do not protonate organozinc reagents and hence do not interfere with Negishi reactions of the alkenyl halides thus formed is another virtue of this so far underutilized boracycle. Overall, the ruthenium catalyzed trans‐hydrostannation of alkynyl‐B(aam) derivatives opens a practical gateway to isomerically pure trisubstituted alkenes of many different substitution patterns by sequential functionalization of the 1‐alkenyl‐1,1‐heterobimetallic adducts primarily formed.

(Z)-Selective Hydrosilylation and Hydroboration of Terminal Alkynes Enabled by Ruthenium Complexes with an N-Heterocyclic Carbene Ligand

Metal-catalyzed trans-1,2-hydrosilylations and hydroborations of terminal alkynes that generate synthetically valuable (Z)-alkenylsilanes and (Z)-alkenylboranes remain challenging due to the (E)-selective nature of the reactions and the formation of the thermodynamically unfavorable (Z)-isomer. The development of new, efficient catalytic systems for the (Z)-selective hydrosilylation and hydroboration of terminal alkynes is thus highly desirable from a fundamental perspective as it would deepen our understanding of the metal-catalyzed (Z)-selective hydrosilylation and hydroboration of terminal alkynes. This personal account describes our research for developing a ruthenium complex that can efficiently catalyze the hydrosilylation and hydroboration of terminal alkynes, and for exploring the factors controlling (Z)-selectivity of the reactions. Our effort into the activation of B-protected boronic acids, R-B(dan) (dan=naphthalene-1,8-diaminato), that was believed not to participate in Suzuki-Miyaura cross-coupling, is also discussed.

Alkyne 1,1-Hydroboration to a Reactive Frustrated P/B-H Lewis Pair

The Mes₂ P-C≡C-SiMe₃ alkyne reacts with the borane H₂ B-Fmes by means of a rare 1,1-hydroboration reaction to give an unsaturated C₂ -bridged frustrated P/B-H Lewis pair. Most of its reactions are determined by the presence of the B-H functionality at the FLP function and the activated connecting carbon-carbon double bond. It reduces carbon monoxide to the formyl stage. With nitriles it reacts in an extraordinary way: it undergoes a reaction sequence that eventually results in the formation of a P-substituted dihydro-1,2-azaborole derivative. Several similar examples were found. In one case a P-ylide was isolated that was related to an intermediate of the reaction sequence. It subsequently opened in an alternative way to give an alkenyl borane product.

Samarium(iii) catalyzed synthesis of alkenylboron compounds via hydroboration of alkynes

The homoleptic lanthanide complex Sm[N(TMS)₂]₃ is an efficient rare-earth catalyst for the hydroboration of alkynes to the corresponding alkenylboron compounds.

Iron-Catalyzed Regiodivergent Hydrostannation of Alkynes: Intermediacy of Fe(IV)-H versus Fe(II)-Vinylidene

We report an iron system, Cp*Fe(1,2-R₂PC₆H₄X), which controls the Markovnikov and anti-Markovnikov hydrostannation of alkynes by tuning the ionic metal-heteroatom bonds (Fe-X) reactivity. The sequential addition of nBu₃SnH to the iron-amido catalyst (1, X = HN^-, R = Ph) affords a distannyl Fe(IV)-H species responsible for syn-addition of the Sn-H bond across the C≡C bond to produce branched α-vinylstannanes. Activation of the C(sp)-H bond of alkynes by an iron-aryloxide catalyst (2, X = O^-, R = Cy) affords an iron(II) vinylidene intermediate, allowing for gem-addition of the Sn-H to the terminal-carbon producing β-vinylstannanes. These catalytic reactions exhibit excellent regioselectivity and broad functional group compatibility and enable the large-scale synthesis of diverse vinylstannanes. Many new reactions have been established based on such a synthetic Fe-X platform to demonstrate that the initial step of the catalysis is conveniently controlled by the activation of either the tin hydride or the alkyne substrate.

Insertion of Alkylidene Carbenes into B-H Bonds

We have developed a protocol for insertion of alkylidene carbenes into the B-H bonds of amine-borane adducts, enabling, for the first time, the construction of C(sp²)-B bonds by means of carbene-insertion reactions. Various acyclic and cyclic alkenyl borane-amine adducts were prepared from readily accessible starting materials in good to high yields and were subsequently subjected to a diverse array of functional group transformations. The unprecedented spiro B-N heterocycles prepared in this study have potential utility as building blocks for the synthesis of pharmaceuticals. Preliminary mechanistic studies suggest that insertion of the alkylidene carbenes into the B-H bonds of the amine-borane adducts proceeds via a concerted process involving a three-membered-ring transition state.