Osmium olefin

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

The most common oxidation states and the corresponding electronic configurations of osmium ate +2 and + (t5 ), which ate usually octahedral. Stable oxidation states that have various coordination geometries include —2 and 0 to +8 (P] The single most important appHcation is OsO oxidation of olefins to diols. Enantioselective oxidations have also been demonstrated. 

There are also reactions which show stereoselectivity primarily because of mechanism rather than spatial bias of substrate. For instance, the conversion of an olefin to a 1,2-diol by osmium tetroxide mechanistically is a cycloaddition process which is strictly suprafacial. The hydroxylation transform has elements of both substrate and mechanism control, as illustrated by the retrosynthetic conversion of 146 to 147. The validity of the retrosynthetic removal of both... 

Otsuka K-76, a fungal product with strong anticomplement activity, was synthesized from the A/B bicyclic precursor of stemodin. The aromatic subunit was retrosynthetically disconnected to a symmetrical precursor. A surprising non-selectivity of olefinic hydroxylation by osmium tetroxide was noted. 

Because osmium tetroxide is expensive, and its vapors are toxic, alternate methods have been explored for effecting vic-glycol formation. In the aliphatic series, olefins can be hydroxylated with hydrogen peroxide with the use of only a catalytic amount of osmium tetroxide. Anhydrous conditions are not necessary 30% hydrogen peroxide in acetone or acetone-ether is satisfactory. The intermediate osmate ester is presumably cleaved by peroxide to the glycol with regeneration of osmium tetroxide. When this reaction was tried on a A -steroid, the product isolated was the 20-ketone ... 

The low rate of reaction for trisubstituted olefins was shown to be a result of slow hydrolysis of the osmium glycolate 29. However, this hydrolysis can be accelerated by a factor of up to 50 simply by the addition of methanesulfonamide.26 This modification permits the AD to be performed at lower temperature, which nearly always results in an increase in the stereoselectivity of the reaction.27... 

Two methods that are particularly convenient for large-scale synthesis of aziridines are discussed below. Both utilize readily available chloramine salts, such as chloramine-T, as sources of nitrogen. The first method involves direct olefin azir-idination catalyzed by phenyltrimethylammonium tribromide (PhNMe3+Br3 PTAB) [42]. In the second method, 1,2-hydroxysulfonamides, conveniently obtained by osmium-catalyzed aminohydroxylation of olefins, are converted into aziridines by one-pot cyclodehydration. 

Diols are applied on a multimilhon ton scale as antifreezing agents and polyester monomers (ethylene and propylene glycol) [58]. In addition, they are starting materials for various fine chemicals. Intimately coimected with the epoxidation-hydrolysis process, dihydroxylation of C=C double bonds constitutes a shorter and more atom-efficient route to 1,2-diols. Although considerable advancements in the field of biomimetic nonheme complexes have been achieved in recent years, still osmium complexes remain the most efficient and reliable catalysts for dihydroxylation of olefins (reviews [59]). 

Dapprich, S., Ujaque, G., Maseras, F., Lledos, A., Musaev, D. G., Morokuma, K., 1996, Theory Does Not Support an Osmaoxetane Intermediate in the Osmium-Catalyzed Dihydroxylation of Olefins , J. Am, Chem. Soc., 118, 11660. 

A photo-induced dihydroxylation of methacryamide by chromium (VI) reagent in aqueous solution was recently reported and may have potential synthetic applications in the syn-dihydroxylation of electron-deficient olefins.63 Recently, Minato et al. demonstrated that K3Fe(CN)6 in the presence of K2C03 in aqueous rm-butyl alcohol provides a powerful system for the osmium-catalyzed dihydroxylation of olefins.64 This combination overcomes the disadvantages of overoxidation and low reactivity on hindered olefins related to previous processes (Eq. 3.14). 

Mono-, di-, and trisubstituted olefins undergo osmium-catalyzed enantioselective dihydroxylation in the presence of the (R)-proline-substituted hydroquinidine 3.9 to give diols in 67-95% yields and in 78-99% ee.75 Using potassium osmate(VI) as the catalyst and potassium carbonate as the base in a tm-butanol/water mixture as the solvent, olefins are dihydroxylated stereo- and enantioselectively in the presence of 3.9 and potassium ferricyanide with sodium chlorite as the stoichiometric oxidant the yields and enantiomeric excesses of the... 

Amino-Hydroxylation. A related reaction to asymmetric dihydroxylation is the asymmetric amino-hydroxylation of olefins, forming v/c-ami noalcohols. The vic-hydroxyamino group is found in many biologically important molecules, such as the (3-amino acid 3.10 (the side-chain of taxol). In the mid-1970s, Sharpless76 reported that the trihydrate of N-chloro-p-toluenesulfonamide sodium salt (chloramine-T) reacts with olefins in the presence of a catalytic amount of osmium tetroxide to produce vicinal hydroxyl p-toluenesulfonamides (Eq. 3.16). Aminohydroxylation was also promoted by palladium.77... 

The formation of these compounds has been rationalized according to Scheme 6. The reaction of Os (E )-CH=C 11 Ph C1 (C())( P Pr3)2 with n-BuLi involves replacement of the chloride anion by a butyl group to afford the intermediate Os (/i> CH=CHPh ( -Bu)(CO)(P Pr3)2, which by subsequent hydrogen (3 elimination gives OsH ( >CI I=CHPh (CO)( P Pr3)2. The intramolecular reductive elimination of styrene from this compound followed by the C—H activation of the o-aryl proton leads to the hydride-aryl species via the styrene-osmium(O) intermediate Os r 2-CH2=CHPh (CO)(P Pr3)2. In spite of the fact that the hydride-aryl complex is the only species detected in solution, the formation of OsH ( )-CH=CHPh L(CO)(P Pr3)2 and 0s ( )-CH=CHPh (K2-02CH)(C0)(P,Pr3)2 suggests that in solution the hydride-aryl complex is in equilibrium with undetectable concentrations of OsH ( )-CH=CHPh (CO)(P,Pr3)2. This implies that the olehn-osmium(O) intermediate is easily accessible and can give rise to activation reactions at both the olefinic and the ortho phenyl C—H bonds of the... 

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... 

More than sixty years ago, Criegee reported that the dihydroxylation of olefins by osmium tetroxide was accelerated by the addition of a tertiary amine.165 166 Later, this discovery prompted the study of asymmetric dihydroxylation, because the use of an optically active tertiary amine was expected to increase the reaction rate (kc > k0) and to induce asymmetry (Scheme 41).167... 

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]. 

Osmium(VIII) tetraoxide (0s04) is an effective reagent for the cis hydroxylation of olefins under stoichiometric conditions as well as in a variety of catalytic variants.213 Under both catalytic and stoichiometric conditions, the critical step is the formation of an osmium(VI) cycloadduct, the formation of which is dramatically accelerated in the presence of amine bases such as pyridine,214 i.e.,... 

An initial osmium tetraoxide/olefin complex (in Scheme 21) is not generally disputed and is considered a donor/acceptor complex between an alkene as electron donor and 0s04 as an electron acceptor (with a reversible reduction potential E ed = —0.06 V versus SEC),217 e.g.,... 

In our laboratory we have examined the reactivity pattern of [0s3(y-H)2(C0)10], an unsaturated cluster which can be represented as possessing an osmium-osmium double bond in its classical valence bond representation. We find (2,3) that this compound undergoes a number of reactions with metal carbonyls which in some cases can be formulated as proceeding through intermediates analogous to metal olefin complexes ... 

The history of asymmetric dihydroxylation51 dates back 1912 when Hoffmann showed, for the first time, that osmium tetroxide could be used catalytically in the presence of a secondary oxygen donor such as sodium or potassium chlorate for the cA-dihydroxylation of olefins.52 About 30 years later, Criegee et al.53 discovered a dramatic rate enhancement in the osmylation of alkene induced by tertiary amines, and this finding paved the way for asymmetric dihydroxylation of olefins. 

The first attempt to effect the asymmetric cw-dihydroxylation of olefins with osmium tetroxide was reported in 1980 by Hentges and Sharpless.54 Taking into consideration that the rate of osmium(VI) ester formation can be accelerated by nucleophilic ligands such as pyridine, Hentges and Sharpless used 1-2-(2-menthyl)-pyridine as a chiral ligand. However, the diols obtained in this way were of low enantiomeric excess (3-18% ee only). The low ee was attributed to the instability of the osmium tetroxide chiral pyridine complexes. As a result, the naturally occurring cinchona alkaloids quinine and quinidine were derived to dihydroquinine and dihydroquinidine acetate and were selected as chiral... 

In summary, the reaction of osmium tetroxide with alkenes is a reliable and selective transformation. Chiral diamines and cinchona alkakoid are most frequently used as chiral auxiliaries. Complexes derived from osmium tetroxide with diamines do not undergo catalytic turnover, whereas dihydroquinidine and dihydroquinine derivatives have been found to be very effective catalysts for the oxidation of a variety of alkenes. OsC>4 can be used catalytically in the presence of a secondary oxygen donor (e.g., H202, TBHP, A -methylmorpholine-/V-oxide, sodium periodate, 02, sodium hypochlorite, potassium ferricyanide). Furthermore, a remarkable rate enhancement occurs with the addition of a nucleophilic ligand such as pyridine or a tertiary amine. Table 4-11 lists the preferred chiral ligands for the dihydroxylation of a variety of olefins.61 Table 4-12 lists the recommended ligands for each class of olefins. 

The /Tamino alcohol structural unit is a key motif in many biologically important molecules. It is difficult to imagine a more efficient means of creating this functionality than by the direct addition of the two heteroatom substituents to an olefin, especially if this transformation could also be in regioselective and/ or enantioselective fashion. Although the osmium-mediated75 or palladium-mediated76 aminohydroxylation of alkenes has been studied for 20 years, several problems still remain to be overcome in order to develop this reaction into a catalytic asymmetric process. 

The catalytic asymmetric aminohydroxylation of a variety of styrene derivatives, vinyl aromatics, and some other olefins using osmium tetroxide... 

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