Asymmetric Oxidations and Related Reactions

Abstract

GENERAL INTRODUCTIONOxidation is a fundamental technology for converting bulk chemicals into valuable materials and is also a useful tool for sophisticated functionalization of organic molecules. Thus, intensive research efforts have been devoted to the development of selective and practical oxidation methods, and a wide variety of chiral metal -based catalysts and organocatalysts have been developed for catalytic asymmetric oxidation reactions in industry and academia in the last half -century [1] . In contrast to the rapid improvement in stereoselectivity, the enhancement of atom economy [2] falls behind. While atom effi ciency (especially active oxygen content in oxygenation reactions) of stoichiometric oxidants is a factor that should be considered, most of the catalytic asymmetric oxidations still use conventional stoichiometric oxidants of low atomeffi ciency such as peracids, alkyl hydroperoxides, hypervalent iodine reagents, hypochlorite, and N -oxide compounds (Table 11.1 ). The use of such oxidants causes the formation of large amounts of undesirable waste. From the viewpoint of ecological sustainability, oxidation with a higher atom -effi cient, safe, abundant, and preferably inexpensive oxidant is favorable. Considering the requirements, molecular oxygen and hydrogen peroxide are the oxidants of choice [3] . Molecular oxygen offers a large advantage because it is abundant in air and is inexpensive. Aerobic oxidation that directly uses ambient air as an oxidant is similar to respiration in living organisms. Hydrogen peroxide is also recognized as a green oxidant. It is almost as equally atomeffi cient as molecular oxygen, and the by -product is safe and clean water. Moreover, its aqueous solution (typically 30 -35%) is inexpensive and easy to handle. Consequently, the development of catalytic asymmetric oxidations with molecular oxygen Catalytic Asymmetric Synthesis, Third Edition, Edited by Iwao Ojima

Enantioselective Epoxidations of Olefins with Various Oxidants on Bioinspired Mn Complexes: Evidence for Different Mechanisms and Chiral Additive Amplification

“…Catalytic asymmetric epoxidation of olefins remains the major synthetic tool to access chiral epoxides, the catalysts scope including enzymes, organocatalysts, and metal complexes. − Many transition metal catalysts have been suggested for asymmetric selective epoxidations. − A remarkable trend of the past decade has been the consideration of metal-complex based epoxidation catalysts as synthetic models of naturally occurring metalloenzymes capable of conducting this challenging oxidative transformation. In early 2000s, Shul’pin and co-workers discovered that the 1,4,7-trimethyl-1,4,7-triazacyclononane Mn complex can catalyze the epoxidation of olefins with hydrogen peroxide in the presence of acetic acid. , Subsequently, enantioselectivities up to 17% ee were reported for the epoxidation of indene with H 2 O 2 in the presence of Mn complexes with chiral triazacyclononane derived ligands .…”

Enantioselective Epoxidation of β,β-Disubstituted Enamides with a Manganese Catalyst and Aqueous Hydrogen Peroxide

“…Chiral epoxides are useful building blocks for a number of transformations of interest in organic synthesis . Because of that, methods for asymmetric epoxidation have been actively investigated and are currently known, covering a large range of olefin typology .…”

Iron Catalyzed Highly Enantioselective Epoxidation of Cyclic Aliphatic Enones with Aqueous H2O2

“…Asymmetric epoxidation is a valuable reaction because chiral epoxides are versatile building blocks in synthetic organic chemistry. − Catalytic epoxidation methodologies based on iron complexes and peroxides (especially H 2 O 2 ), which can be considered as biologically inspired, are interesting because of the availability and low environmental impact of these reagents. − Despite their appeal, the approach is challenging because it requires the design of iron coordination complexes that can activate the OO bond of peroxides to create selective metal-based oxidants and avoid the often facile production of hydroxyl radicals via the Fenton reaction. ,,, Recent reports have disclosed successful examples where asymmetric epoxidation is accomplished, in some cases producing high levels of stereoselectivity (Figure ). − Highly enantioselective epoxidation of difficult substrates such as β,β-disubstituted aromatic enones and α-alkyl styrenes, not accessible by other methods, have also been described. , However, a major limitation still resides in the fact that iron catalyzed asymmetric epoxidations have been limited in scope to olefins conjugated to aromatic rings − and remains to be accomplished for aliphatic substrates.…”

“…Asymmetric epoxidation is a valuable reaction because chiral epoxides are versatile building blocks in synthetic organic chemistry. [1][2][3][4] Catalytic epoxidation methodologies based on iron complexes and peroxides (especially H 2 O 2 ), which can be considered as biologically inspired, are interesting because of the availability and low environmental impact of these reagents. [5][6][7][8][9][10][11][12][13][14][15]16 Despite appealing, the approach is challenging because it requires the design of iron coordination complexes that can activate the O-O bond of peroxides to create selective metal based oxidants, and avoid the often facile production of hydroxyl radicals via the Fenton reaction.…”