Die Cobalt-katalysierte selektive Isomerisierung von terminalen Alkenen hin zum thermodynamisch weniger stabilen (Z)-2-Alken bei Raumtemperatur läuft über einen neuartigen Mechanismus ab, der die Übertragung eines Wasserstoffs von einem Ph₂PH-Liganden zur Ausgangsverbindung und die Bildung eines Phospheniumkomplexes, aus dem durch eine 1,2-H-Wanderung der Ph₂PH-Komplex regeneriert wird, einschließt.

The cobalt-catalyzed selective isomerization of terminal alkenes to the thermodynamically less-stable (Z)-2-alkenes at ambient temperatures takes place by a new mechanism involving the transfer of a hydrogen atom from a Ph₂PH ligand to the starting material and the formation of a phosphenium complex, which recycles the Ph₂PH complex through a 1,2-H shift.

The hydrogen-bonding strength of a variety of commonly employed thiourea catalysts was quantified by using a trialkylphosphine oxide as a ³¹P NMR probe. Simple diarylthioureas and more complex bifunctional amine- and hydroxy-substituted thiourea derivatives were examined. Their catalytic activity was determined in a Diels–Alder reaction, and the obtained pseudo-first-order rate constants were correlated with the ³¹P NMR chemical shifts. A linear correlation between both variables was observed throughout the functionalized thioureas. The ³¹P NMR probe correlation fared better in comparison to a pK_a correlation. Accordingly, the quantification presented herein by using a ³¹P NMR probe offers an elegant way to estimate the catalytic activity of thiourea catalysts in hydrogen-bond-activated reactions such as the Diels–Alder reaction.

1,4-Cyclohexadiene derivatives are easily accessed via transition-metal cycloadditions of 1,3-dienes with alkynes. The mild reaction conditions of several transition-metal-catalysed reactions allows the incorporation of various functional groups to access functionalised 1,4-cyclohexadienes. The control of the regiochemistry in the intermolecular cobalt-catalysed Diels–Alder reaction is realised utilising different ligand designs. The functionalised 1,4-cyclohexadiene derivatives are valuable building blocks in follow-up transformations. Finally, the oxidation of the 1,4-cyclohexadienes can be accomplished under mild conditions to generate the corresponding arene derivatives.

The cobalt-catalyzed isomerization of 1,3-dienes to 2Z,4E-dienes was realized for the very challenging substrates with an additional double bond in the side chain. An isomerization to the conjugated 3,5,7-triene derivative was not observed, which is in stark contrast to observations with many other isomerization catalysts. Accordingly, the synthesis of the natural product urushiol, which has a sensitive 2Z,4E,7Z-triene subunit in the side chain, was investigated. The O-protected urushiol derivative was generated selectively without isomerization to the conjugated 3,5,7-triene or Z/E-isomerization of the double bond at position 7.

Dibromocyclopropanes can be ring-opened by utilizing zinc dibromide in catalytic amounts and iron powder in substoichiometric amounts. The presence of the carbonyl group of aldehydes, ketones, glyoxylic esters, or imines is necessary for the ring-opening reaction. The ring opening leads to a bromo-functionalized 1,3-diene, which then reacts in situ with the carbonyl group in a hetero-Diels–Alder reaction. The products are formed with high control of the regiochemistry and in good yields for electron-deficient aldehydes and imines, such as carbamates. The application of (E)-ethyl 2-(methoxycarbonylimino)acetate led to a cyclic α-amino acid derivative. The use of unsymmetrical dibromo- and dihalocyclopropanes is also reported.

The highly regioselective cobalt-catalysed 1,4-hydrovinylation of terminal alkenes with 2-trimethylsilyloxy-1,3-butadiene generates in a stereospecific fashion unsaturated E-configured silyl enol ether intermediates that are suitable for diastereoselective Mukaiyama-aldol reactions with bulky aliphatic aldehydes. The acidic hydrolysis of the enol ethers to γ,δ-unsaturated ketones followed by ozonolysis can be used for the synthesis of various 1,4-diketones and polycarbonyl derivatives. The 1,4-diketones and polycarbonyl derivatives were successfully tested for the synthesis of some mono- and bis-pyrrole derivatives. The γ,δ-unsaturated ketones are useful building blocks (e.g., in natural product synthesis) and can be generated in a one-pot procedure.

The intermolecular carbon–carbon bond formation between two alkenes also known as 1,2-hydrovinylation reaction can be realised with different transition metal catalysts. The application of styrene derivatives, norbornenes and other alkenes in asymmetric catalysis with a variety of chiral ligands leads to α-chiral alkene products in an atom-economic transformation. Accordingly, the 1,2-hydrovinylation is one of just a few asymmetric transformations which produce stereogenic centres in the absence of polarised functional groups. The 1,4-hydrovinylation of terminal alkenes and 1,3-dienes can be controlled by the electronic nature of the alkene starting material for the selective formation of linear or branched 1,4-dienes. These adducts can be used for the synthesis of 1,3- as well as 1,4-dicarbonyl derivatives upon ozonolysis of suitable intermediates. As an extension of the 1,4-hydrovinylation reaction a cobalt-catalysed 1,4-hydrobutadienylation reaction is reported where two different 1,3-dienes react selectively for the formation of 1,3,6-trienes.

The straightforward synthesis of new bicyclopropyl-substituted alkynes and 1,3-dienes and their application in cobalt-catalyzed Diels–Alder reactions are described. The cycloaddition processes generated the desired bicyclopropyl-substituted arene derivatives in moderate to good yields, depending on the steric congestion of the reaction partners. The regioselectivity of the cycloaddition was controlled by the ligand coordinated to the cobalt center. The cyclopropyl moiety remained unchanged over the course of the Diels–Alder reaction, indicating that no radical type intermediates were formed. Only in a single case did the DDQ oxidation of the primarily formed dihydroaromatic product lead to ring opening of a cyclopropyl subunit. In all of the other cases, cyclopropyl-modified arenes with various functionalities were obtained.

A ruthenium-catalyzed enyne metathesis reaction followed by a cobalt-catalyzed Diels–Alder reaction and oxidation of the dihydroaromatic intermediate generates the two regioisomeric aromatic products in good overall yields in a one-pot procedure. The regiochemistry of the cycloaddition can be controlled by the ligand choice on the cobalt catalyst to generate the product with the 1,3,5- or the 1,2,4-substitution pattern predominantly. The high functional group tolerance was used to generate bifunctionalized building blocks and finally an unprecedented regioselective Scholl reaction is described.

The Lewis acidity of various silyl triflates was quantified by utilizing [D₅]pyridine as a ²H NMR spectroscopy probe. The chemical shifts of the ²H NMR signals for pyridine–silyl adducts are reported. The rate constants of silyl triflate catalyzed Diels–Alder reactions were determined by using UV/Vis spectroscopy. A correlation of the magnitude of the ²H NMR chemical shifts with the rate constants for most silyl triflates investigated was observed, but Me₃SiOTf exhibits a surprisingly large deviation. Control experiments indicate that proton catalysis of the Diels–Alder reaction can be excluded.

The quantification of Lewis acidity is accomplished by a deuterated quinolizidine probe utilizing ²H NMR spectroscopy. The chemical shifts of the ²H NMR signals for aluminum, boron, titanium, and zinc halides are reported. The rate constants for Lewis acid catalyzed reactions were determined by utilizing UV/Vis spectroscopy for Diels–Alder and Povarov reactions under pseudo-first-order conditions. The magnitude of the ²H NMR chemical shifts correlates with the rate constants of the organic transformations for many Lewis acids investigated in the same order, whereas some deviations are identified.

The anodic oxidation of 1,3-diisopropylbenzene in methanol gave exclusively the desired dimethoxy-functionalized product in excellent yield when potassium bromate was utilized as supporting electrolyte. Similar electrochemical conversions in other alcoholic solvents were not successful based on the low conductivity of the mixture. The introduction of other alcohols could then be realized when the dimethoxy derivative was converted under S_N1 conditions in alcohols used as solvent. Thereby, higher alcohols could be introduced in moderate yields and acceptable selectivities.

The intermolecular cobalt-catalysed cyclotrimerisation of phenylacetylene as a benchmark alkyne with [1,2-bis(4-methoxyphenylthio)ethane]cobalt dibromide gave the symmetrical 1,3,5-triphenylbenzene as the major product when dichloromethane was used as the solvent, whereas the unsymmetrical 1,2,4-trisubstituted product was obtained in acetonitrile in quantitative yield. Optimisation of the aromatic and aliphatic disulfide ligands in different solvents is discussed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)