The application of bidentate phosphine ligands in cobalt-catalyzed transformations of cyclic alkenes such as cyclopentene and cycloheptene with internal alkynes led to a chemoselective Alder-ene or a [2 + 2] cycloaddition reaction depending on the electronic nature of the alkyne and the bite angle of the ligand used.
A cobalt(I)-catalyzed [2 + 2 + 2] cycloaddition reaction between an internal acceptor-substituted alkyne and a terminal alkene leads to the formation of regiochemically enriched polysubstituted 1,3-cyclohexadiene derivatives in acceptable yields when methyl butynoate is used, whereas regiochemically pure products are formed in good yields form phenyl propyonate. The concurrent cyclotrimerization reaction of the alkyne to the corresponding benzene derivative is dependent on the sterical bulk of the alkyne and is considerably reduced with the sterically more hindered alkyne.
This article reports on the applications of electrochemistry for the generation of radicals as neutral, cationic, and anionic intermediates and their use in preparative reactions for the formation of carbon–carbon and carbon–heteroatom bonds. The focus of this article is on electrochemical oxidation reactions of enol ethers, ketene acetals, and electron‐rich aromatic compounds. Furthermore, recent advances in the electrochemically induced synthesis of four‐membered ring systems and biaryl compounds are covered. The oxidative carbon–carbon bond formation by Kolbe electrolysis concludes the section dealing with oxidative transformations. The section describing reductive processes consists of indirect electrochemical reduction of alkylhalides and thioesters, as well as the reductive interconversion of functional groups described recently.
The cobalt-catalysed [6+2] cycloaddition of cycloheptatriene with internal alkynes as well as with terminal alkenes is described. The cycloaddition process is most efficiently catalysed by cobalt dibromide[bis(triisopropylphosphite)] complexes in dichloromethane as the solvent of choice. In the cobalt-catalysed reaction of cycloheptatriene with norbornene an unexpected tricyclic [2+2] cycloaddition product was isolated in excellent yields.The application of cobalt complexes in cycloaddition reactions and other carbon-carbon bond formation processes is well documented. 1 While the predominant application of cobalt catalysts lies in the formation of five-2 and sixmembered-ring systems, 3 recent reports describe the applications of cobalt catalysts for the formation of four-4 as well as eight-membered-ring systems. 5 Only a few years ago Buono established a methodology for the first example of a cobalt-catalysed [6+2] cycloaddition process utilising cycloheptatriene 6 (Scheme 1) as well as cyclooctatetraene or cyclooctatriene 7 as starting materials in cobaltcatalysed cycloadditions applying mostly terminal alkynes such as phenylacetylene as starting materials. Scheme 1 First cobalt-catalysed [6+2] cycloaddition reactionOur own research led us primarily to an unprecedented cobalt-catalysed formation of an eight-membered ring system by a [4+2+2] cycloaddition process. Simultaneously we found a very reactive catalyst system, which performed the [6+2] cycloaddition as outlined in Scheme 1 when internal alkynes, such as hex-3-yne (Scheme 2), were used leading to the generation of bridged eight-membered cyclooctatriene derivatives of type 3.As we have shown recently, cobalt phosphine (or phosphite) based catalyst systems are of limited use for the [2+2+2] cyclotrimerisation of terminal as well as internal alkynes under reductive conditions. 3b Accordingly, in the presence of a 1,3-diene or a cycloheptatriene as starting material a [4+2] or a [6+2] cycloaddition process becomes more favourable compared to the [2+2+2] cyclotrimerisation reaction pathway. In order to achieve further optimisation of the reaction and to minimise the amount of side products, a limited number of solvents and ligands were screened in the reaction of cycloheptatriene with hex-1-ene as test substrate to generate 5a (see Scheme 4; R = nBu). We chose this test system although in GCMS analysis three isomers (most likely: endo/exo-[6+2] cyclisation isomers of 5a and the hydrovinylation product) were detectable. Separation of these hydrocarbon molecules by column chromatography was very tedious with only limited success because of their highly unpolar nature.In a brief survey of the solvents which had been most efficient over the course of our previous investigations concerning cobalt-catalysed reactions utilising triisopropyl phosphite as ligand, dichloromethane proved to be the solvent of choice resulting in a quantitative yield of 5a (>99%). Other solvents such as THF or acetonitrile gave inferior results. In the investigation concerning the applica...
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