Specific Z-Selectivity in the Oxidative Isomerization of Allyl Ethers to Generate Geometrically Defined Z-Enol Ethers Using a Cobalt(II)(salen) Complex Catalyst

Abstract

Enol ether structural motifs exist in many highly oxygenated biologically active natural products and pharmaceuticals. The synthesis of the geometrically less stable Z-enol ethers is challenging. An efficient Z-selective oxidative isomerization process of allyl ethers catalyzed by a cobalt­(II) (salen) complex using N-fluoro-2,4,6-trimethylpyridinium trifluoromethanesulfonate (Me3NFPY•OTf) as an oxidant has been developed. Thermodynamically less stable Z-enol ethers were prepared in excellent yields with high geometric control. This methodology also demonstrates the effectiveness in controlling the Z-selective isomerization reaction of diallyl ethers at room temperature. This catalytic system provides an alternative pathway to extend the traditional reductive isomerization of allyl ethers.

Alkyl Enol Ethers: Development in Intermolecular Organic Transformation

“…Recently, Chen and co‐workers introduced Z ‐enol ethers synthesis from oxidative isomerization of allyl ethers with high geometric control via Co‐catalyzed reaction (path I, Scheme 10). [21] Besides, Lithium diisopropyl amide (LDA) also promotes transformation of allyl ethers to 27 (path II, Scheme 10). [22] …”

Interactions Between Two Cobalt Salen Centers Bridged by Non‐Conjugated Linkers**

“…The use of this and similar “privileged catalysts” [7] is not limited to oxygen transfer. Highly cis ‐ and enantio‐ or diastereoselective cyclopropanation, [10,11] cross‐coupling, [12] electroreductive alkylation of olefins, [13] epoxide ring opening, [14] aziridination, [15] carbonyl cyanosilylation, [16] or oxidative isomerization of allyl ethers to Z ‐enol ethers [17] have been reported recently, just to name a few.…”

Highly Z-Selective Double Bond Transposition in Simple Alkenes and Allylarenes through a Spin-Accelerated Allyl Mechanism

“…Alkene transposition (isomerization) is a synthetic tool that has numerous applications in organic synthesis. − Phenylpropenoids are a particularly popular target based on their numerous applications in fragrance industry, pharmaceuticals, and as building blocks for complex products. , Moreover, there are several tandem transformations involving this transposition of CC double bonds, which enable rapid diversification to functionalized compounds. ,− Regioselective − and stereoselective − alkene transposition reactions are atom-economical, but only if there is high selectivity for the desired isomer.…”

“…Dobereiner reported an air-stable Mo precatalyst for Z -selective isomerization ( Z / E = 84:16 for 1-decene), which is activated by p -toluenesulfonic acid . Hilt reported in situ generated cobalt­(II) and nickel­(II) phosphine mixtures as catalysts for Z -selective isomerization (Co gives Z / E = 85:15 for 1-hexadecene, Ni gives Z / E = 72:28 for 1-decene). , Chen recently reported a cobalt­(II) salen complex capable of isomerizing allyl ethers to Z -enol ethers with high Z -selectivity ( Z / E = 99:1) . Unfortunately, none of these catalysts achieve high selectivity for allylbenzene, which gives biorelevant phenylpropenoid products.…”

“…16,17 Chen recently reported a cobalt(II) salen complex capable of isomerizing allyl ethers to Z-enol ethers with high Z-selectivity (Z/E = 99:1). 30 Unfortunately, none of these catalysts achieve high selectivity for allylbenzene, which gives biorelevant phenylpropenoid products.…”