Does the dehydrogenative coupling of aromatic compounds mediated by AlCl3 at high temperatures and also by FeCl3, MoCl5, PIFA, or K3[Fe(CN)6] at room temperature proceed by the same mechanism in all cases? With the growing importance of the synthesis of aromatic compounds by double C-H activation to give various biaryl structures, this question becomes pressing. Since some of these reactions proceed only in the presence of non-oxidizing Lewis acids and some only in the presence of certain oxidants, the authors venture the hypothesis that, depending on the electronic structure of the substrates and the nature of the "catalyst", two different mechanisms can operate. One involves the intermediacy of a radical cation and the other the formation of a sigma complex between the acid and the substrate. The goal of this Review is to encourage further mechanistic studies hopefully leading to an in-depth understanding of this phenomenon.

Oxidative aromatic coupling occupies a fundamental place in the modern chemistry of aromatic compounds. It is a method of choice for the assembly of large and bewildering architectures. Considerable effort was also devoted to applications of the Scholl reaction for the synthesis of chiral biphenols and natural products. The ability to form biaryl linkages without any prefunctionalization provides an efficient pathway to many complex structures. Although the chemistry of this process is only now becoming fully understood, this reaction continues to both fascinate and challenge researchers. This is especially true for heterocoupling, that is, oxidative aromatic coupling with the chemoselective formation of a C−C bond between two different arenes. Analysis of the progress achieved in this field since 2013 reveals that many groups have contributed by pushing the boundary of structural possibilities, expanding into surface‐assisted (cyclo)dehydrogenation, and developing new reagents.

No abstract

The syntheses and characterisation of the [a-(phosphany1)alkyl]cyclopentadienyl anions 2,3,6-8,10, and 11 are described. These anions form metallocenes 12-15 and 11-19 with FeC1, 2 THF and with ZrC1, . 2 THF, respectively. With ICO(CO)~ chelated carbonyl complexes 23-25, 28, and 29 are formed. The unchelated intermediate 20 has been detected by IR spectroscopy. The carbonyl chelate complexes are thermally stable. Under photochemical conditions, ligand exchange reactions are possible which in the case of 1,5-cyclooctadiene proceed with decomplexation of the phosphane arm. This does not prevent a reaction at the cobalt(1) atom, treatment of 35 with diphenylethyne gives the corresponding tetraphenylcyclobutadiene complex 36 in good yield, the phosphane arm remaining uncoordinated.Transition metal complexes with cyclopentadienyl ligands have been intensively studied since their initial synthesis in 1951 ['I. Special aspects of their chemistry are ring-slippage reactions1'] in which the usual -q5 bonding mode changes to an q3 bonding with temporary decomplexation of one double bond. This raised the question in how far chemical reactions with participation of the temporarily decoordinated double bond might be possible. One result of our work was the ring opening reaction of a (bicyclo[3.2.0]hepta-1,3-dienyl)cobalt(I) complex followed by a cycloaddition of the intermediate ortho-quinodimethane speciesL3]. In these ringslippage reactions the cyclopentadienyl ligand can formally be regarded as a bidentate "allyl-ene" ligand, whose "ene" fragment decoordinates in the course of the change in hapticity from q5 to q3 and is recoordinated later. The process reversibly generates a vacant coordination site, which is capable of participating in chemical reactionsr3].Vacant coordination sites are usually generated by decomplexation of a ligand. If the ligand is not present in large excess in the reaction mixture, it will normally not be recoordinated after use of the vacant coordination site for further reaction. However, if the decomplexed ligand is still attached to the complex by other means than by coordination to the metal atom, it will not leave the molecule as a whole and can be later recoordinated. The ring slippage of a cyclopentadienyl ("allyl-ene") ligand mentioned is a very special case, and more generally this line of thought leads to the use of bi-or multidentate ligands. Most bidentate ligands are combinations of two ligands of the same nature, e.g. diphosphanes, bipyridyl derivatives and similar systems. In contrast, heterobidentate ligands should allow use to be made of the different coordination properties of the two ligands involved. In this context we became interested in combinations of cyclopentadienyl and phosphane ligands, which are among the most thoroughly studied ligands in organometallic chemistry. To avoid any interference from resonance interactions we envisaged a connection of the ligands by an alkyl chain. The two partial ligands have rather different properties: whereas the cyclopentadienyl system ...

Molecular wires of the oligophenyleneethynylene (OPE)‐type are potentially rigid entities. The idea of the work reported herein was to replace some, not all, of the phenylene moieties with ferrocene units, thereby introducing limited conformational flexibility with the ferrocene units acting as hinges as a consequence of the facile rotation around the Cp–Fe–Cp axis. In this context, the syntheses of a number of 1,1′‐diaryl‐disubstituted ferrocene building blocks are described. The new terminal diynes 22 and 24 were used to construct the first representatives of ferrocene‐based molecular wires with three ferrocene hinges. The study includes an X‐ray structure analysis of the thiophene‐based diyne 24 as well as cyclovoltammetric analyses of a number of the compounds prepared.

No abstract

Chelate complex 1 (5 mol%) was found to catalyze the [2 + 2 + 2] cyclization of terminal alkynes in good yields in a 80/20 mixture of water and ethanol at room temperature without further activation.

Abstract:The hydrosilylation of alkynes is catalyzed by the di-tert-butylphosphanylethylcyclopentadienylcobalt chelate 1. While the reaction of internal alkynes exclusively affords syn hydrosilylation products with triethylsilane, the reaction with triethoxysilane shows predominant anti stereoselectivity. Reactions of terminal alkynes are less selective with triethylsilane and result in cyclotrimerization when triethoxysilane is used.