Cu/Pd-catalyzed arylboration of a 1-silyl-1,3-cyclohexadiene for stereocontrolled and diverse cyclohexane/ene synthesis

The synthesis and Cu/Pd-catalyzed arylboration of 1-silyl-1,3-cyclohexadiene is described. This diene is significant as it allows for synthesis of polyfunctional cyclohexane/enes. To achieve high levels of diastereoselectivity, the use of a pyridylidene Cu-complex was employed. In addition, through the use of a chiral catalyst, an enantioselective reaction was possible. Due to the presence of the silyl and boron substituents, the products can be easily diversified into a range of valuable cyclohexane/ene products.

Bis(Iminophosphorano)-Substituted Pyridinium Ions and their Corresponding Bispyridinylidene Organic Electron Donors

Optimized synthetic procedures for pyridinium ions featuring iminophosphorano (-N=PR₃ ; R=Ph, Cy) π-donor substituents in the 2- and 4- positions are described. Crystallographic and theoretical studies reveal that the strongly donating substituents severely polarize the π-electrons of the pyridyl ring at the expense of aromaticity. Moreover, the pyridinium ions are readily deprotonated to generate powerful bispyridinylidene (BPY) organic electron donors. Electrochemical studies show exceptionally low redox potentials for the two-electron BPY/BPY²⁺ couples, ranging from -1.71 V vs the saturated calomel electrode for 3PhPh (with four Ph₃ P=N- groups) to -1.85 V for 3CyCy (with four Cy₃ P=N- groups). These new compounds represent the most reducing neutral organic electron donors (OEDs) currently known. Some preliminary reductions involving 3CyCy showed enhanced capability owing to its low redox potential, such as the thermally activated reduction of an aryl chloride, but purification challenges were often encountered.

Synthesis of Bisheteroarylalkanes by Heteroarylboration: Development and Application of a Pyridylidene-Copper Complex

The development of pyridylidene-Cu-complexes and their application in Cu/Pd-catalyzed heteroarylboration of alkenylheteroarenes is reported. The significance of 1,1'-heteroarylalkanes as building blocks for drug discovery, as well as the straightforward and modular sequence to prepare the pyridylidene-Cu-complexes, makes this catalyst and it applications attractive for chemical synthesis. Furthermore, chiral variants of the pyridylidene-Cu-complexes have been prepared and utilized in the enantioselective arylboration of E-alkenes, further demonstrating the value and potential of this class of catalysts.

Diaryl-λ3-chloranes: Versatile Synthesis and Unique Reactivity as Aryl Cation Equivalent

We have developed a versatile, high-yield synthesis of diarylchloroniums/λ³-chloranes through the reaction of various chloroarenes with readily prepared mesityldiazonium tetrakis(pentafluorophenyl)borate under mild conditions. The scope of the reaction is broad, including ArCl, ArBr, and ArI. The diarylchloroniums/λ³-chloranes prepared here show unique reactivity in various respects, enabling intermolecular electrophilic arylation reaction of weak nucleophiles, and chlorane-halogane exchange reaction.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Pyridylidene ligand facilitates gold-catalyzed oxidative C-H arylation of heterocycles

Triaryl-2-pyridylidene effectively facilitates the gold-catalyzed oxidative C-H arylation of heteroarenes with arylsilanes as a unique electron-donating ligand on gold. The employment of the 2-pyridylidene ligand, which is one of the strongest electron-donating N-heterocyclic carbenes, resulted in the rate acceleration of the C-H arylation reaction of heterocycles over conventional ligands such as triphenylphosphine and a classical N-heterocyclic carbene. In situ observation and isolation of the 2-pyridylidene-gold(III) species, as well as a DFT study, indicated unusual stability of gold(III) species stabilized by strong electron donation from the 2-pyridylidene ligand. Thus, the gold(I)-to-gold(III) oxidation process is thought to be facilitated by the highly electron-donating 2-pyridylidene ligand.

Anti-Markovnikov hydroimination of terminal alkynes in gold-catalyzed pyridine construction from ammonia

Gold-catalyzed hydroimination of terminal alkynes, giving rise to anti-Markovnikov adducts concomitant with unstable Markovnikov adducts is described. The elementary step can be applied for the construction of pyridine derivatives from ammonia and alkynes.

Pyridylidene-Mediated Dihydrogen Activation Coupled with Catalytic Imine Reduction

In recent years, dihydrogen activation at non-metallic centers has received increasing attention. A system in which dihydrogen is trapped by a pyridylidene intermediate that is generated from a pyridinium salt and a base is now reported. The dihydropyridine formed in this process can act as reducing agent towards organic electrophiles. By coupling the hydrogen-activation step with subsequent hydride transfer from the dihydropyridine to an imine, a catalytic process was established. Treatment of the N-phenylimine of phenyl trifluoromethyl ketone with 5-20 mol% of N-mesityl-3,5-bis(2,6-dimethylphenyl)pyridinium triflate and 0.3-1.0 equivalents of LiN(SiMe3)2 under 50 bar of hydrogen gas resulted in high conversion into the corresponding amine.

Computational evaluation of tris(carbene)borate donor properties in {NiNO}(10) complexes

Electronic structure calculations are performed to characterise the structures, energies, and spectroscopic data for a series of four-coordinate tris(carbene)borate {NiNO}(10) complexes. There is excellent agreement between the computational and experimental results for known complexes, allowing for structure-function relationships to be delineated. Calculations that provide insights into the synthetic accessibility of nickel(iv) nitrides by oxygen atom abstraction from these complexes are also reported.

Expanding the palette of phenanthridinium cations

5,6-Disubstituted phenanthridinium cations have a range of redox, fluorescence and biological properties. Some properties rely on phenanthridiniums intercalating into DNA, but the use of these cations as exomarkers for the reactive oxygen species (ROS), superoxide, and as inhibitors of acetylcholine esterase (AChE) do not require intercalation. A versatile modular synthesis of 5,6-disubstituted phenanthridiniums that introduces diversity by Suzuki–Miyaura coupling, imine formation and microwave-assisted cyclisation is presented. Computational modelling at the density functional theory (DFT) level reveals that the novel displacement of the aryl halide by an acyclic N-alkylimine proceeds by an S(N)Ar mechanism rather than electrocyclisation. It is found that the displacement of halide is concerted and there is no stable Meisenheimer intermediate, provided the calculations consistently use a polarisable solvent model and a diffuse basis set.

Pyridine-based dicarbene ligand: synthesis and structure of a bis-2-pyridylidene palladium complex

We herein report the synthesis of a bis-2-pyridylidene palladium(II) complex as an isoelectronic structure of the well-known 2,2'-bipyridine complexes. One-pot or stepwise C-Cl oxidative addition followed by base-mediated C-H palladation of the chlorobipyridine dication to palladium(0) yielded the bis-2-pyridylidene palladium(II) carbonate smoothly.