Oxidation aminohydroxylation

The asymmetric oxidation of organic compounds, especially the epoxidation, dihydroxylation, aminohydroxylation, aziridination, and related reactions have been extensively studied and found widespread applications in the asymmetric synthesis of many important compounds. Like many other asymmetric reactions discussed in other chapters of this book, oxidation systems have been developed and extended steadily over the years in order to attain high stereoselectivity. This chapter on oxidation is organized into several key topics. The first section covers the formation of epoxides from allylic alcohols or their derivatives and the corresponding ring-opening reactions of the thus formed 2,3-epoxy alcohols. The second part deals with dihydroxylation reactions, which can provide diols from olefins. The third section delineates the recently discovered aminohydroxylation of olefins. The fourth topic involves the oxidation of unfunc-tionalized olefins. The chapter ends with a discussion of the oxidation of eno-lates and asymmetric aziridination reactions. 

There have also been significant advances in the imido chemistry of ruthenium and osmium. A variety of imido complexes in oxidation states +8 to +6 have been reported. Notably, osmium (VIII) imido complexes are active intermediates in osmium-catalyzed asymmetric aminohydroxyl-ations of alkenes. Ruthenium(VI) imido complexes with porphyrin ligands can effect stoichiometric and catalytic aziridination of alkenes. With chiral porphyrins, asymmetric aziridination of alkenes has also been achieved. Some of these imido species may also serve as models for biological processes. An imido species has been postulated as an intermediate in the nitrite reductase cycle. " ... 

Unlike the impressive progress that has been reported with asymmetric catalysis in other additions to alkenes (i.e., the Diels-Alder cycloaddition, epoxidation, dihydroxylation, aminohydroxylation, and hydrogenation) so far this is terra incognita with nitrile oxide cycloadditions. It is easy to predict that this will become a major topic in the years to come. 

Osmium-catalysed dihydroxylation has been reviewed with emphasis on the use of new reoxidants and recycling of the catalysts.44 Various aspects of asymmetric dihydroxylation of alkenes by osmium complexes, including the mechanism, acceleration by chiral ligands 45 and development of novel asymmetric dihydroxylation processes,46 has been reviewed. Two reviews on the recent developments in osmium-catalysed asymmetric aminohydroxylation of alkenes have appeared. Factors responsible for chemo-, enantio- and regio-selectivities have been discussed.47,48 Osmium tetraoxide oxidizes unactivated alkanes in aqueous base. Isobutane is oxidized to r-butyl alcohol, cyclohexane to a mixture of adipate and succinate, toluene to benzoate, and both ethane and propane to acetate in low yields. The data are consistent with a concerted 3 + 2 mechanism, analogous to that proposed for alkane oxidation by Ru04, and for alkene oxidations by 0s04.49... 

Alkenes are oxidized to 2-amino ketones in an osmium-catalysed oxidation with CAT. The reaction can also be carried out as a sequential process consisting of asymmetric aminohydroxylation and subsequent oxidation to enantiopure 2-amino ketones.107... 

Much attention was given in CHEC-II(1996) <1996CHEC-II(4)409> to the synthesis of 1,2,3-dithiazoles. Cyclic and acyclic oximes were found to be important precursors to their preparation. The discovery of 4,5-dichloro-1,2,3-dithiazolium chloride (Appel salt 20) from the reaction of commercial and cheap acetonitrile and disulfur dichloride gave strong impulse to the synthesis of various 1,2,3-dithiazole derivatives. Formation of the 1,2,3-oxathiazole ring involved almost exclusively the conversion of m -aminohydroxyl compounds to the A-oxide derivatives. 

Angert, K. B. Sharpless, Angew. Chem. Int. Ed. Engl. 1996,35,2813 (c) H. C. Kolb, K. B. Sharpless, Asymmetric Aminohydroxylation in Transition Metals for Organic Synthesis, Vol. 2 M. Beller, C. Bolm (Eds.) WILEY-VCH, Weinheim, 1998,243 - 260 (d) G. Schlingloff, K. B. Sharpless, Asymmetric Aminohydroxylation in Asymmetric Oxidation Reactions A Practical Approach T. Katsuki (Ed.) Oxford University Press, Oxford, in press. 

Schlingloff G, Sharpless KB (2001) Asymmetric aminohydroxylation. In Katsuki T (ed) Asymmetric oxidation reactions. Oxford UP, Oxford, p 104... 

In 2001, K. B. Sharpless won the Nobel Prize in Chemistry for his work on asymmetric aminohydroxylation and asymmetric epoxidation °. These stereoselective oxidation reactions are powerful catalytic asymmetric methods that have revolutionized synthetic organic chemistry. 

Sharpless and co-workers first reported the aminohydroxyIation of alkenes in 1975 and have subsequently extended the reaction into an efficient one-step catalytic asymmetric aminohydroxylation. This reaction uses an osmium catalyst [K20s02(OH)4], chloramine salt (such as chloramine T see Chapter 7, section 7.6) as the oxidant and cinchona alkaloid 1.71 or 1.72 as the chiral ligand. For example, asymmetric aminohydroxylation of styrene (1.73) could produce two regioisomeric amino alcohols 1.74 and 1.75. Using Sharpless asymmetric aminohydroxylation, (IR)-N-ethoxycarbonyl-l-phenyl-2-hydroxyethylamine (1.74) was obtained by O Brien et al as the major product and with high enantiomeric excess than its regioisomeric counterpart (R)-N-ethoxycarbonyl-2-phenyl-2-hydroxyethylamine (1.75). The corresponding free amino alcohols were obtained by deprotection of ethyl carbamate (urethane) derivatives. 

Chiral amines and K20sC>2(0H)4 are also used to catalyze the Sharpless asymmetric aminohydroxylation. The stoichiometric oxidant in aminohydroxylation is a deprotonated /V-lialoamide, whose mechanistic behavior is very similar to NMO. The reaction proceeds by a mechanism essentially identical to that of Sharpless dihydroxylation. 

The Sharpless asymmetric hydroxylation can take one of two forms, the initially developed asymmetric dihydroxylation (AD)1 or the more recent variation, asymmetric aminohydroxylation (AA).2 In the case of AD, the product is a 1,2-diol, whereas in the AA reaction, a 1,2-amino alcohol is the desired product. These reactions involve the asymmetric transformation of an alkene to a vicinally functionalized alcohol mediated by osmium tetraoxide in the presence of chiral ligands (e.g., (DHQD)2-PHAL or (DHQ)2-PHAL). A mixture of these reagents (ligand, osmium, base, and oxidant) is commercially available and is sold under the name of AD-mix p or AD-mix a (vide infra). 

In 1996, Sharpless reported that modification of the AD reaction, by the inclusion of a nitrogen source, which also functions as an oxidant, gives an asymmetric aminohydroxylation (AA). > ... 

Natural product synthesis as a test of newly developed methodologies, for example, partial reduction of furans and pyrroles, nucleophilic addition to pyridinium salts. Os-catalyzed oxidative cyclization for the synthesis of tet-rahydrofurans and pyrrolidines, tethered aminohydroxylation of aUcenes, and ring-closing- and cross-metathesis for the constmction of heteroaromatics 12CC11924. 

The interesting step in a formal synthesis of (+)-monomorine I (1562) by Paderes and Chemler was the aminohydroxylation of the known (R)-alke-nylsulfonamide (+)-1642 with copper(II) 2-ethylhexanoate as the catalyst and TEMPO as the oxygen source (Scheme 208). When the reaction was performed in xylenes at 130 °C in a pressure tube, the 2,5-disubstituted pyrrolidine (+)-1643 was formed in 94% yield, and with a diastereoselectiv-ity of better than 20 1. Oxidation of1643 with w-CPBA produced the same aldehyde (+)-1610 that Backvall and his team had previously converted into (+)-monomorine (cf. Scheme 203). 

H.J. Kim, S.H. Cho, S. Chang, Intramolecular oxidative diamination and aminohydroxylation of olefins under metal-free conditions, Org. Lett. 14 (2012) 1424 -1427. 

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