The intermolecular cyclotrimerisation of terminal and internal alkynes can be catalysed by simple cobalt complexes such as a CoBr2(diimine) under mild reaction conditions when treated with zinc and zinc iodide with high regioselectivity in excellent yields.

The Diels-Alder reaction has belonged to the arsenal of synthetic organic chemists for decades as can be seen in the large number of applications in natural-product synthesis and of biologically active compounds.[1] The usefulness of the Diels-Alder reaction is based on the cyclic transition state which allows the transfer of the stereochemical information in the starting material E-or Z-selectively into cis-or transconfigured products. Furthermore, the regiochemistry of the intermolecular Diels-Alder reaction can be predicted by the Woodward-Hoffmann rules and the therein implied ortho/ para rules.[2] While the ortho/para-selectivity of the thermal Diels-Alder reaction allows access to ortho-or para-substituted products (such as 1, Scheme 1) the intermolecular reaction is limited to the generation of para-substituted products when starting from isoprene, the meta-substituted products of type 2 (Scheme 1) can not be accessed. Only the thermal Diels-Alder reaction of alkenyl boron compounds, such as the vinyl-9-borabicyclo[3.3.1]nonane, with unsymmetrical 1,3-dienes, such as isoprene, poses an exception. In this case the regioselective meta-directing Diels-Alder reaction generates the product of type 2 in a ratio of meta:para = 90:10.[3]To our knowledge an analogous meta-directing DielsAlder reaction to produce 4 starting from a 1,3-diene and an alkyne has not been described. Nevertheless, the metasubstituted dihydroaromatic systems 4 have been detected and isolated as side products in transition-metal-catalyzed cycloaddition reactions.[4] In these transition-metal-catalyzed reactions the para-substituted product 3 is formed as the major product.In our investigations of cobalt-catalyzed cyclizations we reported the use of a simple cobalt catalyst system consisting of [CoBr 2 dppe] (dppe = 1,2-bis(diphenylphosphino)ethane), ZnI 2 , Zn, or Bu 4 NBH 4 as reducing agent for the Diels-Alder reaction of unactivated starting materials under mild conditions for the production of the para-substituted Diels-Alder adducts 3.[5] Recently, we also identified a simple cobalt catalyst system consisting of [CoBr 2 (diimine)] (5 Mol %), ZnI 2 , Zn, and Fe powder (each 10 Mol %) which performs the cobalt catalyzed Diels-Alder reaction between phenyl acetylene and isoprene, gives very good yields, and forms the meta-substituted isomer 5 in excellent regioselectivity. The corresponding aromatic product 6 was then generated by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and its identity determined by NMR spectroscopy. Among several ligands tested the best chemo-and regioselectivity was obtained for cobalt complexes with pyridine donor ligands, such as 7 (Scheme 2) or 2,2'-bipyridine (8, Scheme 3) or 1,10-phenanthroline.[6] Also symmetrical aliphatic diimine ligands (9) as well as mixed pyridine imine ligands (10 and 11) gave good results. In addition, by using a combination of zinc-and iron powder as the reducing agent, the formation of the [2+2+2]-cyclotrimerization product from the alkyne was completely suppressed...

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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)

Alkenyl‐substituted dihydroaromatic boronic esters, generated by neutral cobalt(I)‐catalysed Diels–Alder reactions, reacted under palladium catalysis conditions with diiodobenzene, bromoiodobenzene and iodoaniline derivatives for the synthesis of regioselectively substituted phenanthrene and phenanthridine derivatives. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)