Osmyl esters

Osmylation of Ceo with OSO4, in the presence of 4-t-butyl-pyridine (L), yields the osmyl ester Ceo[Os04L2] (12), the first stmcturally characterized Ceo derivative. The OSO4 adds across a six-six ring junction, in a similar manner to the dioxolane derivative (9), and the two sp hybridized carbon atoms are moved away from the center of the cage center-to-C(sp ) distances, 3.80 A average center-to-C(sp ) distance. 

A. The bis-osmyl ester C6o[Os04L2]2 has been prepared as a mixture of five isomers and probable structures for all five compounds have been deduced from H and NMR studies of the separated components. ... 

The reaction of OSO4 with Buckminsterfullerene Ceo gives the corresponding osmyl ester species see Carbon Fullerenes). 

Despite the extensive utility of osmylation reactions, the mechanism remains controversial - generally focusing on either a concerted [3 + 2] cycloaddition to directly yield the cyclic osmate ester or a two-step mechanism where [2 + 2] cycloaddition forms a 4-membered osmaoxetane which then undergoes a ligand-assisted ring expansion to yield the cyclic osmate ester216 (Scheme 21). 

Pentitol synthesis An asymmetric synthesis of L-arabinitol involves condensation of the (E)-a,fJ-unsaturated ester (2) with the anion of methyl (R)-p-tolyl sulfoxide (1). The resulting p-keto sulfoxide (3) is reduced stereoselectively by ZnCl2/DIBAH (13, 115-116) to 4. Osmylation of 4 with (CH,)3NO and a catalytic amount of 0s04 (13, 224-225) yields essentially a single triol (5). Finally, a Pum-merer rearrangement of the sulfoxide followed by reduction of an intermediate... 

Asymmetric catalytic osmylation.s Chiral cinchona bases are known to effect asymmetric dihydroxylation with 0s04 as a stoichiometric reagent (10, 291). Significant but opposite stereoselectivity is shown by esters of dihydroquinine (1) and of dihydroquinidine (2), even though these bases are diastereomers rather than enantiomers. 

A stereoselective osmylation approach was applied to the synthesis of C(l)—C(7) and C(7)—C(13) subunits of erythronolide A41. A key synthon of the erythronolide A seco acid, 30, was prepared in an enantiomerically pure form by utilizing a stereoselective osmylation of the chiral hydroxy (Z, )-diene ester 31 and subsequent hydrogenation of the resulting butenolide 32 (equation 24). 

Osmylation of the olefmic ester 151 in an acetone-water mixture proceeds with high diastereoselectivity to produce the lactone 152 in a one-pot procedure (Equation 52) <1999TL5791>. 

When the secondary reaction cycle shown in Scheme 6D.3 was discovered, it became clear that an increase in the rate of hydrolysis of trioxogly colate 10 should reduce the role played by this cycle. The addition of nucleophiles such as acetate (tetraethylammonium acetate is used) to osmylations is known to facilitate hydrolysis of osmate esters. Addition of acetate ion to catalytic ADs by using NMO as cooxidant was found to improve the enantiomeric purity for some diols, presumably as a result of accelerated osmate ester hydrolysis [16]. The subsequent change to potassium ferricyanide as cooxidant appears to result in nearly complete avoidance of the secondary cycle (see Section 4.4.2.2.), but the turnover rate of the new catalytic cycle may still depend on the rate of hydrolysis of the osmate ester 9. The addition of a sulfonamide (usually methanesulfonamide) has been found to enhance the rate of hydrolysis for osmate esters derived from 1,2-disubstituted and trisubstituted olefins [29]. However, for reasons that are not yet understood, addition of a sulfon-amide to the catalytic AD of terminal olefins (i.e., monosubstituted and 1,1-disubstituted olefins) actually slows the overall rate of the reaction. Therefore, when called for, the sulfonamide is added to the reaction at the rate of one equivalent per equivalent of olefin. This enhancement in rate of osmate hydrolysis allows most sluggish dihydroxylation reactions to be mn at 0°C rather than at room temperature [29]. 

The [2+2] Mechanism Already in 1977 Sharpless proposed a stepwise [2+2] mechanism for the osmylation of olefins in analogy to related oxidative processes with d°-metals such as alkene oxidations with CrO,Cl2 [23, 24], Metallaoxetanes [25] were suggested to be formed by suprafacial addition of the oxygens to the olefinic double bond. In the case of osmylation the intermediate osmaoxetane would be derived from an olefm-osmium(VIII) complex that subsequently would rearrange to the stable osmium(VI) ester. 

Asymmetric dihydroxylation of alkenes (14, 235-239). Further study1 of this reaction reveals that the optical yields of products can be markedly improved by slow addition (5-26 hours) of the alkene to the catalyst in acetone-water at 0° with stirring. The enantioselectivity can also be increased by addition of tetraethylam-monium acetate, which facilitates hydrolysis of osmate esters. The report suggests that the first product (1) of osmylation can undergo a second osmylation to provide 2, with reverse enantioselectivity of the first osmylation. 

In the area of osmium oxo chemistry there are three main topics of interest the tetroxide itself, the osmyl complexes which contain the irons 0=0svi=0 unit, and the cyclic oxo ester species which contain the OsVI moiety. This is perhaps the most appropriate point at which to comment briefly on the reasons for the stability of these three systems. 

As mentioned above the chemistry here is dominated by that of the osmyl species (see p. 581) and by oxo ester complexes (p. 584). 

As mentioned above (p. 579) this is one of the most important classes of osmium complexes, rivalling in number and diversity the oxo esters to be discussed in the next section. The trivial name osmyl denotes the trans 0s=0svl=0 grouping and, like uranyl for 0=UVI=0, has passed into common parlance and will be used here. 

The oxo esters Os02(02R)py2 are considered on p. 584 but since they are osmyl complexes we list X-ray data for four of these in Table 20 there are also IR data for them.490,497... 

Oxo ester complexes with nitrogenous bases (L). Although most of these species contain the osmyl unit there are so many of them that it is appropriate to deal with them in a separate reaction. Most of the complexes have an Os L ratio of 1 2 (the majority taking the form 0s02(02R)py2) and so we consider such species first and include those complexes of the type 0s02(02R)(L—L), where L —L is a bidentate N donor (usually 2,2 -bipyridyl). Within this first section we consider first the species derived from alkenes R or diols R(OH)2, viz. 0s02(02R)L2 or 0s02(02R)L—L, and then species derived from dienes, trienes and alkynes. We then deal with the much smaller body of complexes where the Os L ratio is 1 1. 

The conformation of the central ring is essentially that of a skewed chair-boat form. The osmyl units (Os=0 = 1.72 A) are substantially bent (OOsO angle = 163.5(7)°) from the ester group (Os —O (ester) = 1.95 A) towards the nitrogen atoms (Os—N = 2.19 A),522 as in other osmyl complexes (p. 583). 

It is a consequence of the unusual structure of these complexes in both the solid (dimer) and solution (monomer) states that the IR and Raman spectra show v(0s=0) frequencies to be higher than in osmyl complexes.519,545 In particular the IR band assigned to vas(OsOz) lies near 890 cm 1 in all these oxo esters containing Os L units rather than the 830 cm-1 absorption typical of those containing OsL2 or Os(L) units such behaviour is more typical461 of cis rather than of trans dioxo complexes. 

Osmylation afforded a mixture of two cis glycols 255, which was cleaved into the corresponding hydroxyaldehydes 256. Subsequent oxidation with chromic trioxide in methanol afforded a mixture of two hydroxy esters 257 and 258 that were separated. Contrary to the result in the model compounds (56), dehydration of both hydroxy esters 257 and 258 afforded the identical... 

---