Double-Bond Isomerization: Highly Reactive Nickel Catalyst Applied in the Synthesis of the Pheromone (9Z,12Z)-Tetradeca-9,12-dienyl Acetate.

A highly reactive nickel catalyst comprising NiCl2(dppp) or NiCl2(dppe) with zinc powder, ZnI2 and Ph2PH, was applied in the isomerization of terminal alkenes to Z-2-alkenes. The double-bond geometry of the 2-alkene can be controlled via the reaction temperature to yield the 2-Z-alkenes in excellent yields and high Z-selectivities. The formation of other constitutional isomers, such as 3-alkenes, is suppressed on the basis of the proposed mechanism via a 1,2-hydride shift from the metal to the Ph2P ligand. The nickel-catalyzed isomerization reaction was then applied in the synthesis of (9Z,12Z)-tetradeca-9,12-dienyl acetate, a pheromone with a 2Z,5Z-diene subunit.

An alternative mechanism for the cobalt-catalyzed isomerization of terminal alkenes to (Z)-2-alkenes.

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.

Unprecedented cobalt-catalyzed isomerization reactions to single skipped 2,4,7-trienes applied in the synthesis of urushiol.

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.

Immobilisation of photoswitchable diarylcyclohexenes synthesised via cobalt-mediated Diels-Alder reaction.

Functionalised photoswitches--photochromic dithienylcyclohexenes--were prepared in two steps by a cobalt-mediated Diels-Alder reaction of internal alkynes with the isoprenylpinacolboronic ester. The three-component one-pot reaction sequence provides the photochromic dithienylcyclohexenes in up to 67% overall yield.

Scope and limitations of 1,3,5-hexatriene derivatives in regioselective cobalt-catalyzed reactions.

Applications of 1,3,5-hexatriene derivatives in atom-economic cobalt-catalyzed transformations, such as the Diels-Alder reaction with alkynes, the 1,4-hydrovinylation reaction with terminal alkenes, and the 1,4-hydrohexatrienylation reaction, are investigated. In all cases, regioselective transformations were found to generate cyclic derivatives such as stilbenes or acyclic products with a high control of the double bond geometry in the skipped trienes derived from the 1,4-hydrovinylation process or the tetraenes generated in the so far unprecedented 1,4-hydrohexatrienylation reaction.