Asymmetric alkene epoxidation

Although the chiral ketoiminatomanganese(lll) complexes were reported to catalyze the asymmetric aerobic alkene epoxidations, an aldehyde such as pivalaldehyde is required as a sacrihcial reducing agent. Groves reported that the dioxo(porphyrinato)ruthenium complexes 31, prepared with m-chloroperoxyben-zoic acid, catalyzed the aerobic epoxidation without any reductant. " On the basis of these reports, Che synthesized the optically active D4-porphyrin 35 and applied it to the truly aerobic enantioselective epoxidation of alkenes catalyzed by the chiral frani-dioxo (D4-porphyrinato)ruthenium(Vl) complex. The dioxoruthenium complex catalyzed the enantioselective aerobic epoxidation of alkenes with moderate to good enantiomeric excess without any reductant. In the toluene solvent, the turnovers for the epoxidation of T-(3-methylstyrene reached 20 and the ee of the epoxide was increased to 73% ee. 

Of the numerous catalytic asymmetric methods developed for the functionalisation of alkenes, epoxidation has emerged as one of the most versatile and reliable methods... 

SCHEME 7. Sugar-derived ketones as catalysts for asymmetric alkene epoxidation... 

Asymmetric synthesis, chiral auxUiaries, 868 Asynchronous transition states, peracid alkene epoxidation, 50-8... 

Asymmetric Epoxidation. Asymmetric epoxidation of nonfunctionalized alkenes manifests a great synthetic challenge. The most successful method of asymmetric epoxidation, developed by Katsuki and Sharpless,332 employs a Ti(IV) alkoxide [usually Ti(OisoPr)4], an optically active dialkyl tartrate, and tert-BuOOH. This procedure, however, was designed to convert allylic alcohols to epoxy alcohols, and the hydroxyl group plays a decisive role in attaining high degree of enantiofa-cial selectivity.333,334 Without such function, the asymmetric epoxidation of simple olefins has been only moderately successful 335... 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

Scheme 10.1 Comparison of routes to epoxides from carbonyl compounds. Top Asymmetric carbonyl epoxidation. Bottom Wittig olefination followed by asymmetric alkene epoxidation.

Oxidation reactions - notably alkene epoxidations - were some of the first asymmetric organocatalytic processes to develop into generally useful synthetic methods applicable to a range of substrates [1], This chapter surveys these reactions, with emphasis placed on the most practical and general. Some recent, very useful oxidation reactions involving a-oxidation of carbonyl compounds are covered elsewhere (see Chapter 2). 

Closely related to the ketone/Oxone epoxidation system is the use of iminium salts as promoters. As isolated oxaziridinium salts are known to effect alkene epoxidation [38], these are presumed to be the reactive intermediates in this catalytic system (see Scheme 12.1 X = NR.2+). The first asymmetric example used the dihydroisoquinolinium-based system 15 (Fig. 12.7), which afforded 33% ee for the epoxidation of F-stilbene [39]. 

Scheme 12.10 Ammonium-catalyzed asymmetric alkene epoxidation.

Hie first of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with r-butyl hydroperoxide in Chapter 33, and this new reaction makes use of titanium, as titanium tetraisopropoxide, Ti(OiPr)4, to do the same thing. Sharpless surmised that, by adding a chiral ligand to the titanium catalyst, he might be able to make the reaction asymmetric. The ligand that works best is diethyl tartrate, and the reaction shown below is just one of many that demonstrate that this is a remarkably good reaction. 

The hrst of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with f-butyl hydroperoxide in Chapter 33,... 

Dalton, C. T., Ryan, K. M., Wall, V. M., Bousquet, C., Gilheany, D. G. Recent progress towards the understanding of metal-salen catalyzed asymmetric alkene epoxidation. Top. in Cat. 1998, 5, 75-91. 

S. Bhor, M. K. Tse, M. Klawonn, C. Dbbler, W. Magerlein, M. Beller, Ruthenium-catalyzed asymmetric alkene epoxidation with tert-butyl hydroperoxide as oxidant, Adv. Synth. Catal. 346 (2004) 263. 

Key Words Iminium, Oxaziridinium, Oxaziridine, Ketiminium, Oxone, Tetra-phenylphosphonium monoperoxysulphate, Isopinocampheylamine, Alkene, Epoxide, Enantiomeric excess. Asymmetric synthesis, Organocatalysis, 2-(2-Bromoethyl)benzaldehyde, Levcromakalim, Dihydroisoquinolinium, Spiro, Azepinium, Benzopyran, Dielectric constant, Binol. 2008 Elsevier B.v. 

It seems at first that regioselectivity is going to be a problem with diols such as 79 but this is not the case. A mixture of bromoesters 185 and 186 is indeed formed but both give the same epoxide 187 in base as each centre undergoes either no change or a double inversion. The result is again retention at both atoms and we have made the epoxide 187 from the alkene 79 asymmetrically.40... 

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