Dihydroxylation , of alkenes, with

Catalytic asymmetric dihydroxylation of alkenes with participation of insoluble polymer-bound cinchonine alkaloids 99SLI181. 

With this reaction, two new asymmetric centers can be generated in one step from an achiral precursor in moderate to good enantiomeric purity by using a chiral catalyst for oxidation. The Sharpless dihydroxylation has been developed from the earlier y -dihydroxylation of alkenes with osmium tetroxide, which usually led to a racemic mixture. 

The work by E.J. Corey [37], M. Hirama [38] and K. Tomioka [39], and their associates, on asymmetric dihydroxylation of alkenes with chiral diamine-osmium tetroxide complexes also deserves to be mentioned. 

A reacts with the alkene in a 1,3-dipolar cycloaddition to form a pyridine-associated cyclic osmium(VI)acid diester B. The latter decomposes to pyridine and the same osmium(VI) acid diester that is directly generated in a pyridine-free reaction as an intermediate (A in Figure 17.19). However, the formation of the osmium(VI) acid diester via these three steps is much faster than via the one-step reaction shown in Figure 17.19. In other words, tertiary amines like pyridine are potent catalysts of CM-vic-dihydroxylations of alkenes with 0s04. 

Thus, in cw-vic-dihydroxylations of alkenes with 0s04 tertiary amines, like pyridine, have ligand acceleration effects (this term was introduced in Section 3.4.6, using the Sharpless epoxidation as an example). 

Fig. 14.16. cis-vic Dihydroxylation of alkenes with Os(VIII) and pertinent mechanistic insights obtained thus far. 

The possibility cannot be excluded that the cis-vic dihydroxylation of alkenes with 0s04 in the presence of water proceeds via intermediate D and perhaps even via E. Hydrolysis of either of these species would namely yield the same diol as the hydrolysis of B. 

Figure 6 Enantiofacial selectivity in the dihydroxylation of alkenes with osmium tetroxide/alkaloid ester/NMO...

In an extension of the cis dihydroxylation of alkenes with osmium tetroxide. Sharpless has developed a series of reactions, reviewed by Case et al., in which an osmium imine species is added to an alkene. 

Bioxazolines have also been employed in the enantioselec-tive dihydroxylation of alkenes with Osmium Tetroxide The best results have been obtained in the dihydroxylation of 1-phenylcyclohexene with a complex, formed between OSO4 and the diisobutylbioxazoline (4) (R=CH2CHMe2>, as a stoichiometric reagent (70% ee). Styrene and trans -stilbene afford enantioselec-tivities below 20% ee under these conditions (for highly enantios-elective dihydroxylation catalysts, see Dihydmquinine Acetate and Osmium Tetroxide). 

Because of relatively low yields, oxidations are among the reactions which deserve special cultivation. Early aqueous-phase oxidations have been described with fatty alcohols (to aldehydes or acids [174, 175]). Sheldon reviewed the scene in 1998 [239]. Under the heading greener oxidations actual work concentrates on alcohol oxidation toward ketones with ligand-modified Pd catalysts or with PhI=0/KBr [218 m, 240]. Much work has been done in dihydroxylation of alkenes with ligand-modified Os catalysts and dioxygen [218 s]. 

Related to the dihydroxylation of alkenes with osmium tetroxide is the direct conversion of alkenes into 1,2-amino alcohols. Treatment of an alkene with osmium tetroxide in the presence of A -chloramine-T (TsNClNa) provides the 1,2-hydroxy toluene-/ -sulfonamide (5.94). The sulfonylimido osmium compound 94 is believed to be the active reagent, and is continuously regenerated during the reaction. The sulfonamide products can be converted to their free amines by cleavage of the tosyl group with sodium in liquid ammonia. 

Alkenes, Arenes, and Alkanes. The catalytic cis-dihydroxylation of alkenes with iron complexes and Oxone as terminal oxidant has been reported by Che and coworkers. A wide range of electron-rich, electron-poor, aliphatic, aromatic, terminal, and o , -unsaturated alkenes were tolerated under the optimized reaction conditions. For example, methyl cinnamate could be transformed into the diol 9 on a 30 mmol scale with traces of undesired products 10 and 11 being isolated as side products (eq 98). Oxone has also been employed in the diastere-oselective dihydroxylation of alkenes in a catalytic approach to the Prevost-Woodward reaction. Up to 77% yield was obtained in the dihydroxylation of styrene however, NaI04 was found to be a superior oxidant affording the diol in 87% yield (eq 99). 

Scheme 1.11 Proposed catalytic cycle for the dihydroxylation of alkenes with OSO4 and oxygen as the terminal oxidant.

The familiar cw-dihydroxylation of alkenes with OSO4 or KMn04 creates two new stereogenic centres. Normally, this is of no consequence since in most applications the diol is cleaved or the alkene has a plane of symmetry. 

Fluorinated alcohols catalyse the dihydroxylation of alkenes with cyclopropyl mal-onoyl peroxide. Addition of perfluoro t-butyl alcohol to a toluene solution of alkene and the peroxide increases the rate of product formation and the stereoselectivity. The reaction is postulated to proceed via a dioxonium species (Scheme 19). ... 

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