Olefin dihydroxylation osmium-mediated

Another useful method for the asymmetric oxidation of enol derivatives is osmium-mediated dihydroxylation using cinchona alkaloid as the chiral auxiliary. The oxidation of enol ethers and enol silyl ethers proceeds with enantioselectivity as high as that of the corresponding dihydroxylation of olefins (vide infra) (Scheme 30).139 It is noteworthy that the oxidation of E- and Z-enol ethers gives the same product, and the E/Z ratio of the substrates does not strongly affect the... 

An extended addition increases the effective mole ratio of reaction components, which can have a beneficial impact on reaction selectivity. This is particularly true for reactions using catalysts. In the dihydroxylations in Figure 8.7, olefin 13 was added over 6 hours in order to ensure high enantiomeric purity of the product [6], Excess olefin, which accumulates if the addition rate is faster than the reaction rate, is thought to coordinate with the catalyst in a fashion that leads to non-enantioselective dihydroxylation [6]. In the osmium-mediated dihydroxylation of... 

The discussion about the possible formation of metalla-2-oxetanes in transition metal-mediated oxidation reactions began with the ground breaking work of Sharpless in the field of enantioselective dihydroxylation of olefins with osmium tetraoxide using cinchona alkaloids as ligands [6]. The transfer of the stereochemical information of the chiral ligand to the substrate was explained by Sharpless with a two-step mechanism for the addition reaction, which should occur rather than a concerted [3+2] addition as originally proposed [110] (Fig. 15). 

Abstract The oxidative functionalization of olefins is an important reaction for organic synthesis as well as for the industrial production of bulk chemicals. Various processes have been explored, among them also metal-catalyzed methods using strong oxidants like osmium tetroxide. Especially, the asymmetric dihydroxylation of olefins by osmium(Vlll) complexes has proven to be a valuable reaction for the synthetic chemist. A large number of experimental studies had been conducted, but the mechanisms of the various osmium-catalyzed reactions remained a controversial issue. This changed when density functional theory calculations became available and computational studies helped to unravel the open mechanistic questions. This mini review will focus on recent mechanistic studies on osmium-mediated oxidation reactions of alkenes. 

The oxidative functionalization of olefins mediated by transition metal oxides leads to a variety of products including epoxides, 1,2-diols, 1,2-aminoalcohols, and 1,2-diamines [1]. Also the formation of tetrahydrofurans (THF) from 1,5-dienes has been observed, and enantioselective versions of the different reactions have been developed. Although a lot of experimental data has been available, the reaction mechanisms have been a subject of controversial discussion. Especially, osmium (VIII) complexes play an important role there, as the proposal of a stepwise mechanism [2] for the dihydroxylation (DH) of olefins by osmium tetroxide (OSO4) had started an intense discussion about the mechanism [2—11],... 

The cis dihydroxylation of olefins mediated by osmium tetroxide represents an important general method for olefin functionalization [1,2]. For the purpose of introducing the subject of this chapter, it is useful to divide osmium tetroxide mediated cis dihydroxylations into four categories (1) the stoichiometric dihydroxylation of olefins, in which a stoichiometric equivalent of osmium tetroxide is used for an equivalent of olefin (2) the catalytic dihydroxylation of olefins, in which only a catalytic amount of osmium tetroxide is used relative to the amount of olefin in the reaction (3) the stoichiometric, asymmetric dihydroxylation of olefins, in which osmium tetroxide, an olefinic compound, and a chiral auxiliary are all used in equivalent or stoichiometric amounts and (4) the catalytic, asymmetric dihydroxylation of olefins. The last category is the focus of this chapter. Many features of the reaction are common to all four categories, and are outlined briefly in this introductory section. 

Asymmetric induction also occurs during osmium tetroxide mediated dihydroxylation of olefinic molecules containing a stereogenic center, especially if this center is near the double bond. In these reactions, the chiral framework of the molecule serves to induce the diastereoselectivity of the oxidation. These diastereoselective reactions are achieved with either stoichiometric or catalytic quantities of osmium tetroxide. The possibility exists for pairing or matching this diastereoselectivity with the face selectivity of asymmetric dihydroxylation to achieve enhanced or double diastereoselectivity [25], as discussed further later in the chapter. 

This brief outline of historical developments in osmium tetroxide-mediated olefin hydroxy-lation brings us to our main subject, catalytic asymmetric dihydroxylation. The transition from stoichiometric to catalytic asymmetric dihydroxylation was made in 1987 with the discovery by Sharpless and co-workers that the stoichiometric process became catalytic when N-methyl-... 

It should also be noted that epoxidation of olefins followed by ring opening with OH provides 1,2 diols with different stereochemistry from that observed with osmium-tetroxide-mediated 5yn-dihydroxylation. Therefore, even when only one diastereomeric olefin is available, it is possible to prepare both diastereomeric 1,2-diols, as shown below. 

Osmium-catalyzed dihydroxylation of olefins involves an Os(VIII)/Os(VI) substrate-selective redox system [36c-gj. In this system, N-methylmorpholine N-oxide (NMMO) can be used for the reoxidation of Os(VI) to Os(VIII), with NMMO being reduced to NMM. In cyclohexene oxidation catalyzed by Os with HP, the yield to cis-1,2-cyclohexandiol can be improved remarkably by the use of specific mediators for NMM oxidation to NMMO, for instance by means of catalytic fiavin/HP. In this case, a yield to the cis-diol of 91% was obtained, as compared to 50% with the OSO4/HP system alone [36hj. Mixtures of aqueous HP and acetic acid or formic acid are also effective reagents for the dihydroxylation of olefins, but neutralization of the acid solvent is necessary for the recovery of the product. 

More recently Backvall and coworkers developed a novel and robust system for osmium-catalyzed asymmetric dihydroxylation of olefins by H2O2 with methyltrioxo-rhenium (MTO) as the electron transfer mediator [19]. Interestingly, here MTO catalyzes oxidation of the chiral ligand to its mono-N-oxide, which in turn reoxidizes Os to Os ". This system gives vicinal diols in good yields and high enantiomeric excess up to 99%. 

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