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.

Recent Advances in Boron‐Substituted 1,3‐Dienes Chemistry: Synthesis and Application

“…One of the B−B bifunctionalized trienes 397 , which were synthesized using a cobalt‐mediated 2:1 co‐oligomerization of a diyne 395 with an enol ether 396 (Scheme ; Path I), was involved in a palladium‐catalyzed Suzuki coupling with an aryl iodide 399 (Scheme ; Path II) …”

Intramolecular [2+2+2] Cycloaddition Reactions of Yne‐ene‐yne and Yne‐yne‐ene Enediynes Catalysed by RhI: Experimental and Theoretical Mechanistic Studies

“…In agreement with the discovery of this new reaction pathway, Aubert, Gandon and co-workers [9] reported the cycloaddition reactions of diynes with enol ethers catalysed by stoichiometric [CpCoL 2 ] (Cp= cyclopentadienyl).…”

“…[12a] (E)-N,N'-Bis(4-methylphenylsulfonyl)-2-butene-1,4-diamine (6) [12b] and N-[(E)-4-bromo-2-butenyl]-N-(tert-butyloxycarbonyl)-4-methylphenylsulfonamide (9) [12b] were prepared as previously reported. www.chemeurj.org 1,16-Bis((tert-butyloxycarbonyl)-1,6,11,16-tetrakis(4-methylphenylsulfonyl)-1,6,11,16-tetraazahexadeca-8-ene-3,13-diyne (1): A stirred mixture of 6 (0.60 g, 1.52 mmol), potassium carbonate (1.08 g, 7.61 mmol) and acetonitrile (40 mL) was heated at reflux for 10 min.…”

Alkenes in [2+2+2] Cycloadditions

“…The trienes formed in these reactions may undergo a thermal cyclization to complete a formal [2+2+2] process. Minor variations such as lengthening the diyne tether or introducing electron‐withdrawing groups (EWGs) as substituents caused reversion to the standard [2+2+2] mechanism (Scheme ) …”