Alkene asymmetric osmium-catalyzed

X-Ray, NMR, kinetic analyses, and theoretical approaches have provided insight into the mechanism for both the achiral and asymmetric osmium-catalyzed oxidations of alkenes [64, 77, 81, 108-119]. 

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. " ... 

Scheme 1 Proposed mechanism of the osmium-catalyzed asymmetric dihydioxylation of alkenes...

Because most olefins are prochiral starting materials, the dihydroxylation reaction creates one or two new stereogenic centers in the products. Since the discovery of the first stoichiometric asymmetric dihydroxylations [7], catalytic versions with considerable improvements in both scope and enantioselectivity have been developed [8]. From the standpoint of general applicability, scope, and limitations, the osmium-catalyzed asymmetric dihydroxylation (AD) of alkenes has reached a level of effectiveness which is unique among asymmetric catalytic methods. As there are recent reviews in this field [9], this section is primarily oriented toward a summary of aspects of fundamental understanding and interesting practical application of catalytic dihydroxylations. 

Lohay, B. B., Bhushan, V. Mechanism of osmium-catalyzed asymmetric dihydroxylation (ADH) of alkenes. Tetrahedron Lett. 1992, 33, 5113-5116. 

For oxidation reactions, the cinchona alkaloids have been mainly employed to control the osmium-catalyzed conversion of an alkene to give a 1,2-diol or vicinal functionalized alcohol. As these are important asymmetric reactions, they have been the subject of a number of reviews [1-18]. This chapter discusses the uses of these alkaloids as chiral ligands in asymmetric oxidation reactions. Oxidation reactions where an alkaloid is used in a phase-transfer sense are discussed in Chapter 5. 

From the standpoint of general applicability, and scope the osmium-catalyzed asymmetric dihydroxylation of alkenes (Sharpless dihydroxylation) has reached a level of effectiveness which is unique among asymmetric catalytic methods . In the presence of an optimized catalyst ligand system nearly every class of olefin can be dihydroxylated with high enantioselectivities. 

An obvious extension of the AD-process would be the asymmetric transfer of heteroatoms other than oxygen to a carbon carbon double bond. Indeed, the osmium catalyzed [3] or palladium mediated [4] aminohydroxylation of alkenes has been known for 20 years. The resulting jff-amino alcohols are an important structural element in biologically active compounds as well as the starting point in the design of many chiral ligands. However, to develop this reaction into a catalytic, asymmetric process several problems had to be overcome. 

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. 

Of the quinuclidine-ligated transition metal catalyzed processes, the osmium-catalyzed asymmetric dihydroxylation of alkenes, developed by Sharpless, has had the greatest impact on synthetic chemistry (67). In a recent synthesis, 1,3-dienoates were dihydroxylated with high enantioselectivities in the presence of (DHQD)2PHAL (hydroquinidine 1,4-phthalazinediyl diether) (Fig. 19) (183). 

Since the publication of the Upjohn procedure in 1976, the use of N-methylmorpho-line N-oxide (NMO) based oxidants has become one of the standard methods for osmium-catalyzed dihydroxylations. However, NMO has not been fully appreciated in the asymmetric dihydroxylation for a long time since it was difficult to obtain high enantiomeric excess (ee). This drawback was significantly improved by slow addition of the alkene to the aqueous tert-BuOH reaction mixture, in which 97% ee was achieved with styrene [15]. 

It was later found that the chiral amine can be used as the tertiary amine generating the Notdde needed for reoxidation of Os(VI). Thus, a simplified procedure for osmium-catalyzed asymmetric dihydroxylation of alkenes by H2O2 was developed in which the tertiary amine NMM is omitted [133]. A robust version of this reaction, where the flavin 19 has been replaced by MeRe03 (MTO), was reported (Scheme 8.8) [134]. The chiral ligand has a dual role in these reactions it acts as... 

Choudary and coworkers [138] have also used the principle of in situ generation of N-oxide in catalytic amounts in osmiumcatalyzed asymmetric dihydroxylation ofalkenes was reported in which the N-oxide of N-methyl morpholine (NMM) was generated from H2O2 with CO2 as a catalyst [139]. In the latter system, NMM is not used in catalytic amounts instead, 2 equiv. relative to the alkene was required. 

Studies undertaken in connection with the amine-catalyzed asymmetric alkene osmylation have recently clarified the peculiar mechanism of the hexacyanoferrate/CO2 mediated osmium re-oxidation in biphasic conditions36. Reversal of the osmate oxidation/hydrolysis sequence with respect to the previously described R3NO-mediated conditions was noted with this system. Thus, the monodiolate(amine) osmium(VI) ester 9 appears to be first hydrolyzed, releasing the diol and the amine ligand to the organic phase, and the resulting [0s02(0H)4]2 into the aqueous phase. 

Dawes and Hutcheson [933] examined the asymmetric cyclopropanation of alkenes with methyl or ethyl vinyldiazoesters 7.140 in the presence of a rhodium catalyst bearing 3.60 as ligand. E-Cyclopropanecarboxylates 7.141 are obtained with a high enantioselectivity when using styrenes or simple alkenes (Figure 7.88). Bulkier alkyl diazoesters yield less useful selectivities. The use of the same catalyst in the cyclopropanation of styrene with ethyl diazoacetate gives low selectivities. Rhodium- or osmium-porphyrin-catalyzed cyclopropanations of alkenes by diazoesters also yield poor selectivities [1502,1502a]. 

---