Cobalt-Mediated Linear 2:1 Co-oligomerization of Alkynes with Enol Ethers to Give 1-Alkoxy-1,3,5-Trienes: A Missing Mode of Reactivity

• Published in: Chemistry : a European journal 2010-08, Vol.16 (29), p.8904-8913
• Main Authors: Lebœuf, David, Iannazzo, Laura, Geny, Anaïs, Malacria, Max, Vollhardt, K. Peter C., Aubert, Corinne, Gandon, Vincent
• Format: Article
• Language: English
• Subjects: Alkynes; Aromatic compounds; boron; Chemistry; cobalt; coupling reactions; Cycloaddition; CH activation; electrocyclic reactions; Ethene; Ethers; Formations; Oligomerization; Trienes
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201000486

A variety of 1,6‐heptadiynes and certain borylalkynes co‐oligomerize with enol ethers in the presence of [CpCo(C2H4)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 6π‐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 β‐hydride elimination compete. Competing pathways: Trienol ethers, including diborylated derivatives, are constructed by cobalt‐mediated linear co‐oligomerization of alkynes with enol ethers. This formal alkyne coupling/CH activation sequence proceeds in a highly regio‐ and diastereoselective fashion. Thermal electrocyclic ring closure, coupled with dehydroalkoxylation, leads to arene derivatives (see figure), the sequence constituting a formal [2+2+2] cycloaddition of alkynes to acetylene.