Studies on the formation and kinetic resolution of cyclohexadienones

Date

2018-08

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Abstract

Iodine(III)-mediated regents have been widely used for natural products synthesis thanks to their low toxicity, high stability and commercial availability. With time these reagents are being frequently employed in the synthesis of various natural and complex molecules. Iodine (III) reagents are also employed in steroselective formation of 2,5-cyclohexadienones through oxidative dearomatization of phenols. It’s necessary to understand the mechanism for improving the reaction performance and designing new catalysts. Two proposed mechanisms are commonly discussed in the literature based on indirect evidene. Herein, the Hammett study of oxidative dearomatization of substituted 4-phenyphenols was developed to explore the mechanism. The negative slop generated in the Hammett plots suggest and corroborateds the cationic phenoxenium ion proposed as the rate-determining step in the mechanistic pathway for this oxidation. Desymmetrization of prochiral substrates can also provide enantiopure 2,5- cyclohexadienones. In this regard, kinetic resolution of para-quinols via thiol-Michael addition reactions was explored. A series of chiral thiourea catalysts were used in an effort to induce enantioselectivity. Enantiopurity of 2,5-cyclohexadienones is highly dependent on substituents on the substrates. As the reaction does not follow absolute first-order kinetics, desired enantio-purity cannot be achieved based on the calculation of selectivity factor.

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Keywords

Hypervalent iodine chemistry, Hammett plot, Kinetic resolution

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