Cobalt cyclopentadienyl complexes incorporating a fumarate and a CO ligand (see picture) efficiently catalyze inter- and intramolecular [2+2+2] cycloadditions of alkynes, nitriles, and/or alkenes to give benzenes, pyridines, or 1,3-cyclohexadienes. Unlike catalysts such as [CpCo(CO)(2)] or [CpCo(C(2)H(4))(2)] (Cp = C(5)H(5)), they are air-stable, easy to handle, compatible with microwave conditions, and do not necessarily require irradiation to be active.

A series of 1,3- and 1,4-diboryl-1,3-cyclohexadienes have been prepared by intermolecular CoCp-mediated [2+2+2] cocyclizations of alkynylboronic pinacolate esters with alkenes, followed by oxidative demetallation with iron(III) chloride. The effect of substitution at the borylated alkyne on chemo- and regioselectivities has been studied, suggesting steric control. The proper choice of substituents allowed the preparation of 1,3-diborylated cyclohexadienes in a highly selective manner. Alternatively, 1,4-diborylated cyclohexadienes could be prepared from diborylated diynes. The scope of this reaction has been examined and found to include electron-poor, electron-rich, linear, and cyclic alkenes. The diborylated cyclohexadienes were submitted to single or double Suzuki-Miyaura cross-coupling reactions with haloarenes to afford polyarylated systems. The mechanism of the title reaction, including the regioselectivity of the cycloaddition steps, has been analyzed by means of DFT computations.

DFT computations have been executed aimed at illuminating the variety of pathways by which pyridones react with alkynes in the presence of [CpCoL(2)]: NH-2-pyridones furnish N-dienylated ligands (N-H activation pathway), N-methyl-2-pyridones are converted into ligated cyclohexadienes ([2+2+2] cocycloaddition pathway), and N-alkynyl-2-pyridones may undergo either [2+2+2] cocycloaddition or C-dienylation (C-H activation), depending on the length of the tether. The calculations predict the formation of the experimentally observed products, including their regio- and stereochemical make up. In addition, the unusual regiochemical outcome of the all-intramolecular [2+2+2] cycloaddition of N,N'-dipentynylpyrazinedione was rationalized by computation, which led to the discovery of a new mechanism.

A variety of 1,6-heptadiynes and certain borylalkynes co-oligomerize with enol ethers in the presence of [CpCo(C(2)H(4))(2)] (Cp=cyclopentadienyl) to furnish the hitherto elusive acyclic 2:1 products, 1,3,5-trien-1-ol ethers, in preference to or in competition with the alternative pathway that leads to the standard [2+2+2] cycloadducts, 5-alkoxy-1,3-cyclohexadienes. Minor variations, such as lengthening the diyne tether, cause reversion to the standard mechanism. The trienes, including synthetically potent borylated derivatives, are generated with excellent levels of chemo-, regio-, and diastereoselectivity, and are obtained directly by decomplexation of the crude mixtures during chromatography. The cyclohexadienes are isolated as the corresponding dehydroalkoxylated arenes. In one example, even ethene functions as a linear cotrimerization partner. The alkoxytrienes are thermally labile with respect to 6pi-electrocyclization-elimination to give the same arenes that are the products of cycloaddition. The latter, regardless of the mechanism of their formation, can be viewed as the result of a formal [2+2+2] cyclization of the starting alkynes with acetylene. One-pot conditions for the exclusive formation of arenes are developed. DFT computations indicate that cyclohexadiene and triene formation share a common intermediate, a cobaltacycloheptadiene, from which reductive elimination and beta-hydride elimination compete.

Enediynes undergo intramolecular [2 + 2 + 2] cycloaddition in the presence of cobalt(II) iodide (CoI 2 ), manganese and an N-heterocyclic carbene (IPr) generated in situ from the corresponding imidazolium salt and butyllithium (BuLi). Polycyclic cyclohexadienes are obtained selectively. This new method represents an interesting alternative to those employing air-sensitive cyclopentadi-Moreover, the N-heterocyclic carbene can be used catalytically, which is a significant improvement compared to the corresponding phosphine-based system which requires an excess of ligand.

Inter‐ und intramolekulare [2+2+2]‐Cycloadditionen von Alkinen, Nitrilen und/oder Alkenen zu Benzolen, Pyridinen oder 1,3‐Cyclohexadienen werden effizient von Cyclopentadienylcobalt‐Komplexen mit einem Fumarat‐ und einem CO‐Liganden katalysiert (siehe Bild). Anders als [CpCo(CO)2] oder [CpCo(C2H4)2] sind die Komplexe luftstabil, leicht handhabbar, mikrowellengeeignet und müssen zur Aktivierung nicht unbedingt bestrahlt werden.

Give 1-Alkoxy-1,3,5-trienes: A Missing Mode of Reactivity. -A variety of diynes and some borylalkynes undergo the title reaction with enol ethers (II), (IV) or (XIII) in the presence of cobalt complexes to furnish trienes with excellent levels of chemo-, regio-, and diastereoselectivity. These products are formed in preference to or in competition with the alternative pathway leading to the standard [2 + 2 + 2] cycloadducts, cyclohexadienes, which are isolated as the corresponding dehydroalkoxylated arenes. One-pot conditions for the exclusive formation of arenes are also developed. -(LEBOEUF, D.; IANNAZZO, L.; GENY, A.; MALACRIA, M.; VOLLHARDT, K. P. C.; AUBERT*, C.; GANDON, V.; Chem. Eur.

Cycloaddition reactions O 0070A Straightforward Procedure for the [2 + 2 + 2] Cycloaddition of Enediynes. -In the presence of stoichiometric amounts of cobalt iodide and manganese, and a catalytic amount of carbene IPr, the enediynes are chemoselectively transformed into polycyclic fused 1,3-cyclohexadienes. -(GENY, A.; GAUDREL, S.; SLOWINSKI, F.; AMATORE, M.; CHOURAQUI, G.; MALACRIA, M.; AUBERT*, C.; GANDON, V.; Adv. Synth. Catal. 351 (2009) 1-2, 271-275; Lab. Chim. Org., CNRS, Univ. P. et M. Curie, F-75252 Paris, Fr.; Eng.) -Klein 23-037