Substitution reactions, osmium

When 35 was heated in acetic acid containing hydrogen bromide, the tribromide 46 was obtained as a single product in 74% yield. Debromina-tion of 46 with zinc dust in acetic acid furnished the cyclohexene derivative 47, which was converted into compound 48 by osmium tetraoxide hydroxyl-ation and acetylation. The substitution reaction of 48 with acetate ions provided carba-a-DL-glucopyranose pentaacetate (49), which gave the carba-sugar 50 on hydrolysis. ... 

The osmium-catalyzed dihydroxylation reaction, that is, the addition of osmium tetr-oxide to alkenes producing a vicinal diol, is one of the most selective and reliable of organic transformations. Work by Sharpless, Fokin, and coworkers has revealed that electron-deficient alkenes can be converted to the corresponding diols much more efficiently when the pH of the reaction medium is maintained on the acidic side [199]. One of the most useful additives in this context has proved to be citric acid (2 equivalents), which, in combination with 4-methylmorpholine N-oxide (NMO) as a reoxidant for osmium(VI) and potassium osmate [K20s02(0H)4] (0.2 mol%) as a stable, non-volatile substitute for osmium tetroxide, allows the conversion of many olefinic substrates to their corresponding diols at ambient temperatures. In specific cases, such as with extremely electron-deficient alkenes (Scheme 6.96), the reaction has to be carried out under microwave irradiation at 120 °C, to produce in the illustrated case an 81% isolated yield of the pure diol [199]. 

The substitution reactions can be accompanied by subsequent reactions. Thus, Ru3(C0)i2 reacts with azobenzene (61) or fluorinated azobenzenes (60) to yield products like [47], and the pyrolysis of Ru3(CO)9L3 complexes leads to reactions similar to those discussed in Chapter 3.4. for the corresponding osmium clusters. Rearrangements and orthometalations were observed (65, 66), and one cluster formulated as [42] was isolated (65). 

On the other hand, when 39 was heated with hydrogen bromide in acetic acid, 1,2-di-<9-acetyl-( 1,3/2,6)-3,4-dibromo-6-(bromomethyl)-l,2-cyclohexanediol (50) was obtained, which was converted into 1,2-di-0-acetyl-( 1,3/2)-3(bromomethyl)-5-cyclo-hexene-l,2-diol (51) by debromination with zinc dust in glacial acetic acid [21]. Hydro-xylation of 51 with osmium tetroxide, and successive acetylation yielded 1,2,3,4-tetra-C>-acetyl-6-bromo-6-deoxy-pseudo-a-DL-glucopyranose (52). Nucleophilic substitution reactions of 52 with sodium acetate gave pseudo-a-DL-glucopyranose pentaacetate (55), which gave pseudo-a-DL-glucopyranose (54) by usual hydrolysis [22]. Alternatively, the pentaacetate 55 was obtained as a minor component in a poor yield by nucleophilic substitutions of 2,3,4-tri-0-acetyl-l,6-dibromo-l,6-dideoxy-pseudo-... 

Table VI lists the known trifluoroacetato complexes of iron, ruthenium, and osmium. Photochemical substitution reactions of various tricovalent phosphorus-donor ligands with (7r-allyl)Fe(C0)2(02CCF3) and (iT-Cp)Fe(C0)3(02CCF3), analogous to those described in the preceding for Mn(C0)5(02CCF3), have afforded (ir-allyl)Fe(CO)(cis-Ph2PCH=CHPPh2)(02CCF3), (7r-Cp)Fe(C0)(PR3)(02CCF3) (where...

Figure 5.11 Scheme for the synthesis of a pyridinylimidazolyl ligand, its copolymerization with acrylic acid (AA) and butyl acrylate (BA), and subsequent ligand substitution reaction with an osmium complex to yield a redox polymer. From [147] with permission from Elsevier. 

This reoxidation of the relatively substitution inert osmium(VI) ester to a substitution labile osmium (Vni) ester is the rate-limiting step [22], This step would be expected to be slower with the more sterically demanding PHP than with TBHP. This is indeed what we observed 0s04-catalyzed dihydroxylation of cyclohexene (1, Table 4) gave a faster reaction with TBHP than with PHP, the final conversion being reached in 6h and 24h respectively. 

The mechanism of this activation of the C-H bond is unknown although the reaction may proceed by an oxidative addition. Generally, the pentaammine-osmium(II) system is known to activate phenols, anilines, and anisoles toward electrophilic addition and substitution reactions by binding the aromatic ligand in an T -fashion. Protonation, for example, results in the formation of a heterotriene system [30b] ... 

The most extensive studies of the chemistiy of cluster complexes have been associated with the trinuclear cluster unit, as may be anticipated. A wide range of substitution reactions has been demonstrated for both Ru3(CO)i2 and Os3(CO)i2, with the full range of ligands normally employed in the study of metal carbonyl chemistry. In genera 1, the trinuclear osmium cluster is more readily maintained, ruthenium often giving rise to cluster breakdown, yielding mononuclear and binu-clear adducts. This reflects the increased bond enei of the metal-metal bond on descending the triad (see Table X later in this section). 

Dihydroxylated by-products might indeed result from dihydroxylation mediated by the imido osmium compound so2 Toig. l s a very low kinetic selectivity of only 0.9 kcal/mol (14.3-13.4 kcal/mol) has been obtained. In case of the methyl-substituted imido osmium, another pathway becomes more probable, which is the dihydroxylation by in situ formed OSO4 NH3. It was predicted to have a reaction barrier of only 17.0 kcal/mol and might be responsible for the experimental observation of a significant amount of dihydroxylated by-product in case of R = tert-Bu [18, 90]. 

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