Addition reactions osmium! VIII

The active reagent is based on osmium(VIII) and is used in just catalytic amounts. This means that there has to be a stoichiometric quantity of another oxidant to reoxidize the osmium after each catalytic cycle—KiFeCCN) is most commonly used. Because O5O4 is volatile and toxic, the osmium is usually added as which forms Os04 in the reaction mixture. The other additives include... 

As with the mechanism of osmium tetroxide addition, the origin of the enantioseiectivity in the AD process has also been a subject of much debate. An early proposal by Corey et al. [48], suggested that the AD reaction proceeded through the p-oxo-bridged bis-osmium(VIII) complex shown in Fig. 11. 

Oxidative addition to ruthenium and osmium four-coordinate complexes occurs readily. These complexes are excellent starting materials for group VIII complexes. Addition of formaldehyde to complexes M(CO)L(PPh3)2 (L = CO or PPhs selection of L is metal dependent) leads to oxidative addition products, a reaction of relevance to Fischer-Tropsch processes. The ruthenium complex is proposed as an intermediate only the osmium complex has been isolated ... 

One of the problems encountered in the development of the AD reaction was a second cycle.23 The first cycle is the same as the cycle we saw in the Upjohn procedure with one modification. It is possible for the osmium in the osmate (VI) ester to become oxidised before hydrolysis to form an osmate (VIII) ester. If this species simply hydrolyses to diol and 0s04 then there is no problem but otherwise this is where the trouble starts. In the second cycle the osmate (VIII) ester reacts with another olefin instead of being hydrolysed. It does so with low enantioselectivity (and sometimes in the opposite sense to the first cycle) which results in the poorer enantiopurity of the product (step D rather than step C below). One way round this is to keep the olefin concentration as low as possible which can be done by adding the olefin slowly.22,23 Although slow olefin addition resulted in a profound improvement in enantiomeric excess, it was inconvenient and no way near as effective as the alternative which was to alter the conditions. The use of two phases makes it impossible for the second cycle to occur.19,24... 

New Reoxidants for Dihydroxylation. Backvall has introduced some well thought out reoxidants for the dihydroxylation and asymmetric dihydroxylation processes. Hydrogen peroxide is the terminal oxidant that oxidizes osmium(VI) back to os-mium(VIII) by first oxidizing flavin (the product flavin-OOH in turn oxidizes an amine to its A -oxide in situ this A -oxide then acts conventionally in the dihydroxylation reaction). Originally, A -methylmorpholine was the amine of choice and this additive could be added in catalytic amounts. Recent developments have... 

What is the mechanism of this transformation The initial reaction of the ir bond with osmium tetroxide constitutes a concerted (Section 6-4) addition in which three electron pairs move simultaneously to give a cyclic ester containing Os(VI). This process can be viewed as an electrophilic attack on the alkene Two electrons flow from the alkene onto the metal, which is reduced [Os(VIII) Os(VI)]. For steric reasons, the product can form only in a way that introduces the two oxygen atoms on the same face of the double bond—syn. This intermediate is usually not isolated but converted upon reductive work-up into the free diol. 

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