Mechanism of Nickel‐Catalyzed Suzuki–Miyaura Coupling of Amides

The Ni‐catalyzed Suzuki–Miyaura coupling of N ‐tert‐butoxycarbonyl (N ‐Boc)‐protected amides provides a versatile strategy for the construction of C−C bonds. In this study, density functional theory (DFT) methods have been used to elucidate the mechanism of this reaction, with particular emphasis on the roles of N ‐Boc, K₃PO₄ and H₂O. Our results corroborated those of previous reports, indicating that the overall catalytic cycle consists of three steps, including oxidative addition, transmetalation, and reductive elimination. Three of the possible transmetalation mechanisms were examined to interpret the effects of K₃PO₄ and H₂O. According to the most feasible of these transmetalation mechanisms, K₃PO₄ (acting as a Lewis base) would initially interact with the Lewis acid PhBpin to give a K₃PO₄‐PhBpin complex, which would readily undergo a hydrogen transfer step with H₂O. The H transfer in the transmetalation step was determined to be the rate‐determining step. Notably, the theoretical results showed good agreement with the experimental data.     

Palladium-catalyzed alkylation of sp2 and sp3 C-H bonds with methylboroxine and alkylboronic acids: two distinct C-H activation pathways

Palladium-catalyzed alkylations of sp2 and sp3 C-H bonds with either methylboroxine or alkylboronic acids were developed. Ag2O or AgCO3 is used as a crucial oxidant and promoter for the transmetalation step. Ether, ester, alcohol, and alkene functional groups are tolerated. A new C-H activation pathway differing from the cyclometalation process is elucidated using methylboroxine as the coupling partner.     

Transmetalation in the Suzuki–Miyaura Coupling: The Fork in the Trail

The Suzuki–Miyaura coupling is one of the few transition‐metal‐catalyzed C? C bond‐forming reactions that have been used in applications ranging from discovery chemistry to manufacturing processes. Although coupling proceeds through the generic three‐stage ‘oxidative addition, transmetalation, reductive elimination’ sequence, there are a number of features that differentiate the Suzuki–Miyaura process from other transition‐metal‐catalyzed cross‐couplings. Most of these features are centered around, or are a consequence of, activation of the boron reagent for transmetalation through one or both of two distinct pathways. This review focuses on the evidence that has been presented for this ‘fork in the trail′, and the potential to apply such mechanistic insight to the design of reaction conditions.     

Stereocontrolled Synthesis of Vinyl Boronates and Vinyl Silanes via Atom‐Economical Ruthenium‐Catalyzed Alkene–Alkyne Coupling

The synthesis of vinyl boronates and vinyl silanes was achieved by employing a Ru‐catalyzed alkene–alkyne coupling reaction of allyl boronates or allyl silanes with various alkynes. The double bond geometry in the generated vinyl boronates can be remotely controlled by the juxtaposing boron‐ and silicon groups on the alkyne substrate. The synthetic utility of the coupling products has been demonstrated in a variety of synthetic transformations, including iterative cross‐coupling reactions, and a Chan‐Lam‐type allyloxylation followed by a Claisen rearrangement. A sequential one‐pot alkene‐alkyne‐coupling/allylation‐sequence with an aldehyde to deliver a highly complex α‐silyl‐β‐hydroxy olefin with a handle for further functionalization was also realized.     

Bench‐Stable Stock Solutions of Silicon Grignard Reagents: Application to Iron‐ and Cobalt‐Catalyzed Radical C(sp3)–Si Cross‐Coupling Reactions

A robust method for the preparation of silicon‐based magnesium reagents is reported. The MgBr₂ used in the lithium‐to‐magnesium transmetalation step is generated in situ from 1,2‐dibromoethane and elemental magnesium in hot THF. No precipitation of MgBr₂ occurs in the heat, and transmetalation at elevated temperature leads to homogeneous stock solutions of the silicon Grignard reagents that are stable and storable in the fridge. This method avoids the preparation of silicon pronucleophiles such as Si?Si and Si?B reagents. The new Grignard reagents were applied to unprecedented iron‐ and cobalt‐catalyzed cross‐coupling reactions of unactivated alkyl bromides. The functional‐group tolerance of these magnesium reagents is excellent.     

Exploring Bis(cyclometalated) Ruthenium(II) Complexes as Active Catalyst Precursors: Room‐Temperature Alkene–Alkyne Coupling for 1,3‐Diene Synthesis

Described is the development of a new class of bis(cyclometalated) ruthenium(II) catalyst precursors for C? C coupling reactions between alkene and alkyne substrates. The complex [(cod)Ru(3‐methallyl)₂] reacts with benzophenone imine or benzophenone in a 1:2 ratio to form bis(cyclometalated) ruthenium(II) complexes ( 1 ). The imine‐ligated complex 1 a promoted room‐temperature coupling between acrylic esters and amides with internal alkynes to form 1,3‐diene products. A proposed catalytic cycle involves C? C bond formation by oxidative cyclization, β‐hydride elimination, and C? H bond reductive elimination. This Ru^II/Ru^IV pathway is consistent with the observed catalytic reactivity of 1 a for mild tail‐to‐tail methyl acrylate dimerization and for cyclobutene formation by [2+2] norbornene/alkyne cycloaddition.     

Theoretical Study on the Mechanism of Ni‐Catalyzed Alkyl–Alkyl Suzuki Cross‐Coupling

Ni‐catalyzed cross‐coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkyl–alkyl bonds. The mechanism of this reaction with the Ni/ L1 ( L1 =trans‐N,N′‐dimethyl‐1,2‐cyclohexanediamine) system was examined for the first time by using theoretical calculations. The feasible mechanism was found to involve a Ni^I–Ni^III catalytic cycle with three main steps: transmetalation of [Ni^I( L1 )X] (X=Cl, Br) with 9‐borabicyclo[3.3.1]nonane (9‐BBN)R₁ to produce [Ni^I( L1 )(R₁)], oxidative addition of R₂X with [Ni^I( L1 )(R₁)] to produce [Ni^III( L1 )(R₁)(R₂)X] through a radical pathway, and C? C reductive elimination to generate the product and [Ni^I( L1 )X]. The transmetalation step is rate‐determining for both primary and secondary alkyl bromides. KOiBu decreases the activation barrier of the transmetalation step by forming a potassium alkyl boronate salt with alkyl borane. Tertiary alkyl halides are not reactive because the activation barrier of reductive elimination is too high (+34.7 kcal mol^?1). On the other hand, the cross‐coupling of alkyl chlorides can be catalyzed by Ni/ L2 ( L2 =trans‐N,N′‐dimethyl‐1,2‐diphenylethane‐1,2‐diamine) because the activation barrier of transmetalation with L2 is lower than that with L1 . Importantly, the Ni⁰–Ni^II catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [Ni^II( L1 )(R₁)(R₂)] is very difficult.     

Synthesis and characterizations of arsine– and stibine–ligated Schiff base palladacycles and their applications in Suzuki–Miyaura cross‐coupling reactions

A series of arsine‐ and stibine‐ligated Schiff base palladacycles were synthesized by the reaction of μ‐Cl‐bridged Schiff base palladacycles [Pd(C₆H₄CH]NC₆H₂R)(μ‐Cl)]₂ (R = 2,4,6‐trimethyl or 2,6‐diisopropyl) with AsPh₃ or SbPh_3. The new arsine‐ and stibine‐ligated palladacycles were fully characterized using ¹H NMR, ¹³C NMR and infrared spectroscopies, high‐resolution mass spectrometry, elemental analysis and single‐crystal X‐ray diffraction. Further exploration of the catalytic application of the palladacycles for Suzuki–Miyaura cross‐coupling reactions of aryl bromides with arylboronic acids was carried out. It was found that the new palladacycles are considerably active for these coupling reactions. Copyright © 2016 John Wiley & Sons, Ltd.     

Dynamic PdII/CuI Multimetallic Assemblies as Molecular Models to Study Metal–Metal Cooperation in Sonogashira Coupling

Cooperation between two different metals plays a crucial role in many synergistic catalytic reactions, such as the Sonogashira C−C cross-coupling reaction, where an interaction between the Pd and Cu centers is proposed in the transmetalation step. Although several heterobimetallic Pd/Cu complexes were proposed as structural models of the active species in Sonogashira coupling, the detailed understanding of the metal–metal cooperation in transmetalation is still lacking in current systems. In this work, we report a stepwise and systematic approach to building heteromultimetallic Pd/Cu assemblies as a tool to study metal–metal cooperativity. We obtained fully characterized Pd/Cu multimetallic assemblies that show reactivity in alkyne activation, formation of catalytically relevant aryl/acetylide species, and C−C elimination, serving as functional models for Sonogashira reaction intermediates. The combined experimental and DFT studies highlight the importance of ligand-controlled coordination geometry, metal–metal distances and dynamics of the multimetallic assembly for transmetalation step.     

A density functional study of 1,1,5‐tris(4‐dimethylaminophenyl)‐3‐methyl‐divinylene

The optimized molecular structural parameters and UV‐vis, IR and Raman spectra of a dye molecule with an open formula of 1,1,5‐Tris(4‐dimethylaminophenyl)‐3‐methyl‐divinylene are determined by means of density functional theory (DFT) calculations using B3LYP/6‐31G** formalism. Neutral and cationically charged molecules are used for DFT computations. C? C, C?C, and N? C bond distances of dimethylaminophenyl groups of the dye molecule are in reasonable agreement with the experimental and theoretical C? C, C?C, and N? C bond lengths of the group reported in the literature. Because Vis‐NIR region is near UV‐Vis region in the electromagnetic spectra, calculated maximum wavelengths of UV‐vis spectra are close to those of the experimental VIS‐NIR spectra reported. C?C stretching frequencies calculated for dimethylaminophenyl, alkene, and benzene ring groups are also in good agreement with experimentally reported values. All calculated frequencies fall within 2.2% of the experimental frequency region. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Miktoarm star copolymers via combination of RAFT arm‐first technique and aldehyde–aminooxy click reaction

A facile synthetic pathway to miktoarm star copolymers with multiple arms has been developed by combining reversible addition–fragmentation chain transfer (RAFT) arm‐first technique and aldehyde–aminooxy “click” coupling reaction. Star polystyrene (PS) with aldehyde functionalized core was initially prepared by RAFT arm‐first technique via crosslinking of the preformed linear macro‐RAFT agents using a newly designed aldehyde‐containing divinyl compound 6,6′‐(ethane‐1,2‐diylbis(oxy))bis(3‐vinylbenzaldehyde) (EVBA). It was then used as a multifunctional coupling agent for the subsequent formation of the second generation poly(ethylene glycol) (PEG) arms via the click coupling reaction between its aldehyde groups and aminooxy‐terminated PEGs. The possible formation of PS‐PEG miktoarm star copolymer with Janus‐like segregated structure in cyclohexanone was also investigated. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3323–3330, 2010     

Cooperative Acid–Base Effects with Functionalized Mesoporous Silica Nanoparticles: Applications in Carbon–Carbon Bond‐Formation Reactions

Acid–base bifunctional mesoporous silica nanoparticles (MSN) were prepared by a one‐step synthesis by co‐condensation of tetraethoxysilane (TEOS) and silanes possessing amino and/or sulfonic acid groups. Both the functionality and morphology of the particles can be controlled. The grafted functional groups were characterized by using solid‐state ²⁹Si and ¹³C cross‐polarization/magic angle spinning (CP/MAS) NMR spectroscopy, thermal analysis, and elemental analysis, whereas the structural and the morphological features of the materials were evaluated by using XRD and N₂ adsorption–desorption analyses, and SEM imaging. The catalytic activities of the mono‐ and bifunctional mesoporous hybrid materials were evaluated in carbon–carbon coupling reactions like the nitroaldol reaction and the one‐pot deacetalization–nitroaldol and deacetalization–aldol reactions. Among all the catalysts evaluated, the bifunctional sample containing amine and sulfonic acid groups (MSN–NNH₂–SO₃H) showed excellent catalytic activity, whereas the homogeneous catalysts were unable to initiate the reaction due to their mutual neutralization in solution. Therefore a cooperative acid–base activation is envisaged for the carbon–carbon coupling reactions.     

Base‐Selective Five‐ versus Six‐Membered Ring Annulation in Palladium‐Catalyzed C–C Coupling Cascade Reactions: New Access to Electron‐Poor Polycyclic Aromatic Dicarboximides

Palladium‐catalyzed base‐selective annulation of dibromonaphthalimide to different aryl boronate esters by combined Suzuki–Miyaura cross‐coupling and direct C−H arylation afforded a series of new five‐ and six‐membered ring annulated electron‐poor polycyclic aromatic hydrocarbons. Cesium carbonate (Cs₂CO₃) as auxiliary base in these C−C coupling cascade reactions led exclusively to six‐membered ring annulation, while the use of organic base diazabicycloundecene (DBU) afforded the corresponding five‐membered ring annulated products. This base‐dependent selective mode of annulation is attributed to different mechanistic pathways directed by the applied base. The selective annulation was revealed by single crystal X‐ray analysis of the respective five‐ and six‐membered ring annulated products. The optical and redox properties of the new polycyclic aromatic dicarboximides were characterized by UV/Vis absorption and fluorescence spectroscopy and cyclic voltammetry.     

Hydrogen bonding structure and many‐body interactions in 1,3,5‐triazine–(water)3 and 1,2,4‐triazine–(water)3 complexes

The hydrogen bonding structure and many‐body interactions between 1,3,5‐triazine (1,2,4‐triazine) and three water molecules are studied using the density functional theory (DFT) B3LYP method and 6‐31++G** basis set. Various structures of 1,3,5‐triazine–(water)₃ and 1,2,4‐triazine–(water)₃ complexes are investigated, and the seven and eight stable structures are reported for 1,3,5‐triazine–(water)₃ and 1,2,4‐triazine–(water)₃, respectively. Many‐body analysis is also carried out to obtain relaxation energy and many‐body interaction energy (two‐, three‐, and four‐body), and the most stable conformer has the basis set superposition error corrected interaction energy of ?92.09 and ?99.53 kJ/mol. The two‐ and three‐body interactions have significant contribution to the total interaction energy, whereas the relaxation energy, four‐body interactions are very small for 1,3,5‐triazine–(water)₃ and 1,2,4‐triazine–(water)₃ complexes. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Mechanistic Dichotomy of Magnesium‐ and Zinc‐Based Germanium Nucleophiles in the C(sp3)−Ge Cross‐Coupling with Alkyl Electrophiles

Robust procedures for two mechanistically distinct C(sp³)?Ge bond formations from alkyl electrophiles and germanium nucleophiles are reported. The germanium reagents were made available as bench‐stable solutions by lithium‐to‐magnesium and lithium‐to‐zinc transmetalation, respectively. The germanium Grignard reagent reacts with various primary and secondary alkyl electrophiles by an ionic nucleophilic displacement. Conversely, the coupling of the corresponding zinc reagent requires a nickel catalyst, which then engages in radical bond formations with primary, secondary, and even tertiary alkyl bromides. Both methods avoid the regioselectivity issue of alkene hydrogermylation and enable the synthesis of a wide range of functionalized alkyl‐substituted germanes.     

2,4‐Dinitrophenol‐Catalyzed α‐C(sp3)−H and C(sp)−H Bond Functionalization of Cyclic Amines and Alkynes: Highly Regio‐/Diastereoselective Synthesis of α‐Alkynyl‐3‐Amino‐2‐Oxindoles

A transition‐metal‐ and oxidant‐free DNP (2,4‐dinitrophenol)‐catalyzed atom‐economical regio‐ and diastereoselective synthesis of monofunctionalized α‐alkynyl‐3‐amino‐2‐oxindole derivatives by C?H bond functionalization of cyclic amines and alkynes with indoline‐2,3‐diones has been developed. This cascade event sequentially involves the reductive amination of indoline‐2,3‐dione by imine formation and cross coupling between C(sp³)?H and C(sp)?H of the cyclic amines and alkynes. This reaction offers an efficient and attractive pathway to different types of α‐alkynyl‐3‐amino‐2‐oxindole derivatives in good yields with a wide tolerance of functional groups. The salient feature of this methodology is that it completely suppresses the homocoupling of alkynes. To the best of our knowledge, this is the first example of a DNP‐catalyzed metal‐free direct C(sp³)?H and C(sp)?H bond functionalization providing biologically active α‐alkynyl‐3‐amino‐2‐oxindole scaffolds.     

Gold‐Catalyzed Intermolecular Anti‐Markovnikov Hydroamination of Alkylidenecyclopropanes

The cationic gold phosphine complex [{PCy₂(o‐biphenyl)}Au(NCMe)]⁺SbF₆^? (Cy=cyclohexyl) catalyzes the intermolecular, anti‐Markovnikov hydroamination reaction of monosubstituted and cis‐ and trans‐disubstituted alkylidenecyclopropanes (ACPs) with imidazolidin‐2‐ones and other nucleophiles. This reaction forms 1‐cyclopropyl alkylamine derivatives in high yield and with high regio‐ and diastereoselectivity. NMR spectroscopic analysis of gold π‐ACP complexes and control experiments point to the sp hybridization of the ACP internal alkene carbon atom as controlling the regiochemistry of the ACP hydroamination reaction.     

Palladium supported on silica–chitosan hybrid material (Pd‐CS@SiO2) for Suzuki–Miyaura and Mizoroki–Heck cross‐coupling reactions

Palladium supported on silica–chitosan hybrid material was prepared and characterized using thermogravimetric and differential thermogravimetric analyses, scanning electron microscopy, and Fourier transform infrared, energy‐dispersive X‐ray and X‐ray photoelectron spectroscopies. The prepared Pd‐CS@SiO₂ catalyst (1 mol%) was used for the Suzuki–Miyaura cross‐coupling reaction of various aryl halides and arylboronic acids in 95% ethanol at 80 °C and the Mizoroki–Heck reaction in dimethylformamide at 110 °C using K₂CO₃ as a base. The developed catalyst is well suitable for the 3R approach (recoverable, robust, recyclable) for cross‐coupling reactions without appreciable loss of its activity. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical mechanism for selective catalysis of ruthenium complex catalyzed hydroboration of terminal alkynes to Z‐vinylboronates

Detailed mechanism of the hydroboration of terminal alkynes catalyzed by ruthenium complex was studied using density functional theory. The calculated results suggest that the reaction proceeds in two steps: alkyne rearrangement and catalyst regeneration. Vinylboronate products with E and Z configuration are formed in this reaction. Path A forming Z‐vinylboronate is the preferred pathway. Noncovalent interaction between B? H bond and Ru centre determines the preferred pathway of the reaction. The E_gap of HOMO‐LUMO for the reactant is lowered with the assistance of ruthenium–borane complex (Ru–Cat) formation. A hypothetical control model using 1, 2‐dimethyl acetylene (internal alkyne) and styrene (terminal alkene) as the reaction substrates was designed. The calculated results suggest that the activation barrier of the rate‐determining process is too high, which make the hydroboration reaction of styrene and 1, 2‐dimethyl acetylene (CH₃C‐CCH₃) hard to occur. The results uncover the selectivity of the ruthenium complex for hydroboration of terminal alkynes. © 2014 Wiley Periodicals, Inc.     

Suzuki–Miyaura Coupling of (Hetero)Aryl Sulfones: Complementary Reactivity Enables Iterative Polyaryl Synthesis

Ideal organic syntheses involve the rapid construction of C?C bonds, with minimal use of functional group interconversions. The Suzuki–Miyaura cross‐coupling (SMC) is a powerful way to form biaryl linkages, but the relatively similar reactivity of electrophilic partners makes iterative syntheses involving more than two sequential coupling events difficult to achieve without additional manipulations. Here we introduce (hetero)aryl sulfones as electrophilic coupling partners for the SMC reaction, which display an intermediate reactivity between those of typical aryl (pseudo)halides and nitroarenes. The new complementary reactivity allows for rapid sequential cross‐coupling of arenes bearing chloride, sulfone and nitro leaving groups, affording non‐symmetric ter‐ and quateraryls in only 2 or 3 steps, respectively. The SMC reactivity of (hetero)aryl sulfones is demonstrated in over 30 examples. Mechanistic experiments and DFT calculations are consistent with oxidative addition into the sulfone C?S bond as the turnover‐limiting step. The further development of electrophilic cross‐coupling partners with complementary reactivity may open new possibilities for divergent iterative synthesis starting from small pools of polyfunctionalized arenes.