Green Organocatalytic Oxidative Methods using Activated Ketones

Abstract

Alkenes are very important moieties in organic chemistry and have been employed as starting materials for the synthesis of important organic compounds. In this review, we present organocatalytic reactions in which alkenes play a central role, especially their organocatalytic oxidation to epoxides. This topic plays a pivotal role in our recent research, for which we utilize H2O2 as a green oxidant and a ketone as the catalyst. Extension to other oxidative transformations is also presented.

Aldehyde-catalyzed epoxidation of unactivated alkenes with aqueous hydrogen peroxide

“…Epoxides are versatile intermediates for the synthesis of countless irreplaceable compounds. 1 Since the introduction of various versatile metal-catalyzed epoxidations pioneered by Sharpless, Jacobsen, Katsuki and others, 2 alkene epoxidation has become one of the most studied reactions in organic synthesis, both in industry and academia. 2 After the successful developments employing metal catalysts, much effort has been devoted to the development of protocols where a more sustainable metal-free catalyst is employed.…”

“… 1 Since the introduction of various versatile metal-catalyzed epoxidations pioneered by Sharpless, Jacobsen, Katsuki and others, 2 alkene epoxidation has become one of the most studied reactions in organic synthesis, both in industry and academia. 2 After the successful developments employing metal catalysts, much effort has been devoted to the development of protocols where a more sustainable metal-free catalyst is employed. The organocatalytic epoxidation has flourished 1 a ,1 b ,2 in particular with ketone, 2 acid, 3 nitrile 4 or iminium salt 2 catalysts ( Scheme 1 , A ).…”

“… 2 After the successful developments employing metal catalysts, much effort has been devoted to the development of protocols where a more sustainable metal-free catalyst is employed. The organocatalytic epoxidation has flourished 1 a ,1 b ,2 in particular with ketone, 2 acid, 3 nitrile 4 or iminium salt 2 catalysts ( Scheme 1 , A ). Pioneered by Adam, 5 Curci, 6 Yang, 7 Denmark, 8 Shi, 9 Miller, 10 Page 11 and others, a variety of oxidants, such as Oxone, tert -butyl hydroperoxide (TBHP) or less commonly hydrogen peroxide (H 2 O 2 ) were utilized to in situ generate the key dioxirane, peracid, imidoperoxoic acid, or oxaziridine intermediates.…”

“…Over the last few years, the epoxidation of alkenes with environmentally friendly aqueous H 2 O 2 12 as the oxidant is drawing increasing attention, since it is inexpensive, relatively safe and affords water as byproduct. However, hydrogen peroxide by itself is a poor oxidant for organic oxidations 2,13 and thus has to be activated by a catalyst to form a reactive intermediate in order to enable the epoxidation of unactivated alkenes. As one of the most common functionalities in organic chemistry, a wide range of aldehydes are readily available and have been employed as catalysts.…”

Oxidative Dearomatization of Phenols and Polycyclic Aromatics with Hydrogen Peroxide Triggered by Heterogeneous Sulfonic Acids

“…Most of the reported methods use relatively strong and expensive oxidants, including hypervalent iodine reagents, [23] NBS [24] and organic peroxides. [25] On the contrary, reports with hydrogen peroxide, [26] which is a cheaper and more benign chemical, are at present limited to the use of transition-metal based catalysts. [27,28] Notable exceptions are represented by the use of diphenyldiselenide as precursor of the mediator for the homogeneous oxidation of 1-naphthols [28] and a recent report that uses 1,1,1,3,3,3-hexafluoroisopropanol (HFIP).…”