This paper describes mechanistic studies on the functionalization of arenes with the diboron reagent B(2)pin(2) (bis-pinacolato diborane(4)) catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and olefin-ligated iridium halide or olefin-ligated iridium alkoxide complexes. This work identifies the catalyst resting state as [Ir(dtbpy)(COE)(Bpin)(3)] (COE = cyclooctene, Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl). [Ir(dtbpy)(COE)(Bpin)(3)] was prepared by independent synthesis in high yield from [Ir(COD)(OMe)](2), dtbpy, COE, and HBpin. This complex is formed in low yield from [Ir(COD)(OMe)](2), dtbpy, COE, and B(2)pin(2). Kinetic studies show that this complex reacts with arenes after reversible dissociation of COE. An alternative mechanism in which the arene reacts with the Ir(I) complex [Ir(dtbpy)Bpin] after dissociation of COE and reductive elimination of B(2)pin(2) does not occur to a measurable extent. The reaction of [Ir(dtbpy)(COE)(Bpin)(3)] with arenes and the catalytic reaction of B(2)pin(2) with arenes catalyzed by [Ir(COD)(OMe)](2) and dtbpy occur faster with electron-poor arenes than with electron-rich arenes. However, both the stoichiometric and catalytic reactions also occur faster with the electron-rich heteroarenes thiophene and furan than with arenes, perhaps because eta(2)-heteroarene complexes are more stable than the eta(2)-arene complexes and the eta(2)-heteroarene or arene complexes are intermediates that precede oxidative addition. Kinetic studies on the catalytic reaction show that [Ir(dtbpy)(COE)(Bpin)(3)] enters the catalytic cycle by dissociation of COE, and a comparison of the kinetic isotope effects of the catalytic and stoichiometric reactions shows that the reactive intermediate [Ir(dtbpy)(Bpin)(3)] cleaves the arene C-H bond. The barriers for ligand exchange and C-H activation allow an experimental assessment of several conclusions drawn from computational work. Most generally, our results corroborate the conclusion that C-H bond cleavage is turnover-limiting, but the experimental barrier for this bond cleavage is much lower than the calculated barrier.

A series of studies have been conducted by experimental and theoretical methods on the synthesis, structures, and reactions of CpRh boryl complexes that are likely intermediates in the rhodium-catalyzed regioselective, terminal functionalization of alkanes. The photochemical reaction of CpRh(eta(6)-C(6)Me(6)) with pinacolborane (HBpin) generates the bisboryl complex CpRh(H)(2)(Bpin)(2) (2), which reacts with neat HBpin to generate CpRh(H)(Bpin)(3) (3). X-ray diffraction, density functional theory (DFT) calculations, and NMR spectroscopy suggest a weak, but measurable, B-H bonding interaction. Both 2 and 3 dissociate HBpin and coordinate PEt(3) or P(p-Tol)(3) to generate the conventional rhodium(III) species CpRh(PEt(3))(H)(Bpin) (4) and CpRh[P(p-tol)(3)](Bpin)(2) (5). Compounds 2 and 3 also react with alkanes and arenes to form alkyl- and arylboronate esters at temperatures similar to or below those of the catalytic borylation of alkanes and arenes. Further, these compounds were observed directly in catalytic reactions. The enthalpies and free energies for generation of the 16-electron intermediate and for the C-H bond cleavage and B-C bond formation have been calculated with DFT. These results strongly suggest that the C-H bond cleavage process occurs by a metal-assisted sigma-bond metathesis mechanism to generate a borane complex that isomerizes if necessary to place the alkyl group cis to the boryl group. This complex with cis boryl and alkyl groups then undergoes B-C bond formation by a second sigma-bond metathesis to generate the final functionalized product.

Cycloaddition reactions compose one of the most important classes of reactions when it comes to the simultaneous formation of several bonds in one reaction step. The de novo construction of carbocyclic aromatic systems from acetylenes was also found as an excellent possibility for the assembly of heteroaromatic systems. The transition metal-catalysed [2 + 2 + 2] cycloaddition reaction constitutes a fascinating tool for the synthesis of pyridines from nitriles and the most recent developments demonstrate the ability to control the substitution pattern as well as the possibility of introducing chirality by the use of achiral substrates and a chiral catalyst under mild conditions. In this tutorial review we are focusing on the de novo construction of pyridine ring systems by the transition metal-catalysed [2 + 2 + 2] cycloaddition reaction. After surveying the mechanistic features and intermediates of the reaction depending on the different metal complexes used, we depict the preparation of achiral pyridine derivatives. The last section describes the advances in the synthesis of chiral pyridines and biaryls using the cyclotrimerization method. The various possibilities of introducing chirality by catalytic means are presented and illustrated by instructive examples. This review will be of interest for people active in: Organic Chemistry, Organometallic Chemistry, Transition Metal Chemistry, Stereoselective Synthesis, Heterocyclic Chemistry.

The asymmetric synthesis of a range of axially chiral 2-arylpyridines by a cobalt-catalyzed [2 + 2 + 2] cycloaddition reaction is described. The use of a planar chiral (1-neomenthylindenyl)cobalt(COD) complex under photochemical conditions is the key for reacting the 1-naphthyldiynes with a range of differently functionalized nitriles, giving the enantiomeric atropoisomers with high chemical yields and enantiomeric excesses of up to 94% ee.

Chiral cavities with inward‐pointing functional groups are afforded by chiral bis(bipyridine) ligands. Upon coordination to suitable transition‐metal ions, these self‐assemble in a diastereoselective manner to form double‐ and triple‐stranded helicates (left and right pictures, respectively).

The [2+2+2] cycloaddition reaction has become an invaluable tool for the synthetic chemistry in recent years, due to the increase of complexity of the accessed target molecules by this methodology. Over several decades, the members of group 9 of the periodic table have contributed significantly to the development and understanding of this particular cycloaddition reaction. On the other hand, catalyst complexes containing alkenes as ligands are widely used today as either catalysts or as precursor molecules for the facile generation of catalytically active metal species. In this tutorial review we want to describe the catalytically relevant chemistry of group 9 metal-alkene complexes and compile some recent applications in [2+2+2] cycloaddition reactions.

2,2'-Bipyridines are among the most widely used classes of chelating ligands that have found applications in the fields of coordination chemistry, supra-, nano- and macromolecular chemistry, analytical and photochemistry, but also in asymmetric synthesis and natural product chemistry. Hence, there is a huge demand for efficient synthetic approaches to functionalized derivatives. Modern cross-coupling procedures have been proven to be particularly effective in this context. This critical review will give an overview about the advances made so far focussing mainly on cross-coupling reactions with tin, zinc and boron compounds for the achievement of interesting, versatile and unusual substitution patterns (156 references).

The reduction of cobaltocene with metallic potassium in the presence of trimethylvinylsilane yielded the new CpCo(I) complex [CpCo(H 2 CdCHSiMe 3 ) 2 ] (6). Complex 6 turned out to be an ideal starting material for substitution reactions with functionalized mono-and diolefins, giving access to either new [CpCo(olefin) 2 ] complexes such as 7 (trichlorovinylsilane) and 12 (dimethyl fumarate) or CpCo(diolefin) complexes such as 8 (diallyl ether), 9 (1,5-hexadiene), 10 (1,4-pentadiene), and 11 (1,1,3,3-tetramethyldivinyldisiloxane) with essentially quantitative yields. These complexes are rarely accessible by other methods or the direct reduction method using alkali metals. The complexes 6-9 were unambiguously characterized by X-ray structure analysis, which is reported for the first time for these types of complexes, giving way for the structural comparison. Computational calculations on the olefin ligand exchange processes with 6 display the different stabilities and reactivity trends of the different CpCo(I)-olefin complexes. The extraordinarily high reactivity and the application of complexes 6 and 9 were demonstrated in [2þ2þ2] cycloaddition reactions of different diynes with nitriles, yielding the substituted isoquinoline derivatives with good yields within short reaction times and under very mild conditions. The presented complexes show higher activities compared to [CpCo(COD)] (3) while possessing advantages in preparation and handling compared to [CpCo(H 2 CdCH 2 ) 2 ] (4).