title: 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.
abstract: 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.
authors.affiliation: UPMC Univ Paris 06, IPCM UMR CNRS 7201, 4 Place Jussieu, 75005 Paris, France.
authors.forename: David
authors.initials: D
authors.lastname: Leboeuf
authors.forename: Laura
authors.initials: L
authors.lastname: Iannazzo
authors.forename: Anaïs
authors.initials: A
authors.lastname: Geny
authors.forename: Max
authors.initials: M
authors.lastname: Malacria
authors.forename: K Peter C
authors.initials: KP
authors.lastname: Vollhardt
authors.forename: Corinne
authors.initials: C
authors.lastname: Aubert
authors.forename: Vincent
authors.initials: V
authors.lastname: Gandon