OSMIUM TETROXIDE 1 1 O R R O Osmium Tetroxide O R O OsO4 + NR3 Os O Os NR3 O O NR3 R O OsO4 OH R (3) [20816-12-0] O4Os (MW 254.20) R InChI = 1S/4O.Os OH InChIKey = VUVGYHUDAICLFK-UHFFFAOYSA-N Due to the electrophilic nature of osmium tetroxide, electron- withdrawing groups connected to the alkene double bond re- (cis dihydroxylation of alkenes; osmylation; asymmetric and tard the dihydroxylation.2 This is in contrast to the oxidation diastereoselective dihydroxylation; oxyamination of alkenes) of alkenes by Potassium Permanganate, which preferentially at- Physical Data: mp 39.5–41 ◦C; bp 130 ◦C; d 4.906 g cm−3; tacks electron-deficient double bonds. However, in the presence of chlorine- or ozone-like odor. a tertiary amine such as pyridine, even the most electron-deficient 3 Solubility: soluble in water (5.3% at 0 ◦C, 7.24% at 25 ◦C); solu- alkenes can be osmylated by osmium tetroxide (eq 4). The more highly substituted double bonds are preferentially oxidized (eq 5). ble in many organic solvents (toluene, t-BuOH, CCl4, acetone, methyl t-butyl ether). F C OH H2O 3 Form Supplied in: pale yellow solid in glass ampule, as 4 wt % OH R R = CF F C solution in water, and as 2.5 wt % in t-BuOH. R OO 3 3 OsO4 py R Os (4) Handling, Storage, and Precautions: vapor is toxic, causing dam- py py O O age to the eyes, respiratory tract, and skin; may cause temporary R O H2O OH blindness; LD50 14 mg/kg for the rat, 162 mg/kg for the mouse. R = F HO Because of its high toxicity and high vapor pressure, it should be –HF handled with extreme care in a chemical fume hood; chemical- resistant gloves, safety goggles, and other protective clothing should be worn; the solid reagent and its solutions should be 1. OsO4, py stored in a refrigerator. (5) 2. Na SO 2 3 HO HO Original Commentary HO OH Under stoichiometric and common catalytic osmylation condi- Yun Gao tions, alkene double bonds are hydroxylated by osmium tetroxide Sepracor, Marlborough, MA, USA without affecting other functional groups such as hydroxyl groups, aldehyde and ketone carbonyl groups, acetals, triple bonds, Dihydroxylation of Alkenes. The cis dihydroxylation (os- and sulfides (see also Osmium Tetroxide–N-Methylmorpholine mylation) of alkenes by osmium tetroxide to form cis-1,2-diols N-Oxide). (vic-glycols) is one of the most reliable synthetic transformations The cis dihydroxylation can be performed either stoichiometri- 1 (eq 1). cally, if the alkene is precious, or more economically and con- O O veniently with a catalytic amount of osmium tetroxide (or its R1 R3 Os HO OH precursors such as osmium chloride or potassium osmate) in con- O O R1 R3 + OsO4 R1 R3 (1) junction with a cooxidant. In the stoichiometric dihydroxylation, R2 R4 2 4 the diol product is usually obtained by the reductive hydroly- 2 4 R R R R sis of the osmate ester with a reducing agent such as Lithium The reaction has been proposed to proceed through a [3 + 2] or Aluminum Hydride, Hydrogen Sulfide,K2SO3 or Na2SO3, and [2 + 2] pathway to give the common intermediate osmium(VI) KHSO3 or NaHSO3. The reduced osmium species is normally monoglycolate ester (osmate ester), which is then hydrolyzed removed by filtration. Osmium can be recovered as osmium tetrox- ide by oxidation of low-valent osmium compounds with hy- reductively or oxidatively to give the cis-1,2-diol (eq 2). The 4 cis dihydroxylation of alkenes is accelerated by tertiary amines drogen peroxide. In the catalytic dihydroxylation, the osmate such as Pyridine, quinuclidine, and derivatives of dihydroquini- ester is usually hydrolyzed under basic aqueous conditions to dine (DHQD) or dihydroquinine (DHQ) (eq 3). produce the diol and osmium(VI) compounds, which are then reoxidized by the cooxidant to osmium tetroxide to continue the OH catalytic cycle. Normally 0.01% to 2% equiv of osmium tetroxide R R O [3 + 2] O R or precursors are used in the catalytic dihydroxylation. Common + OsO4 Os R (2) R O O cooxidants are metal chlorates, N-Methylmorpholine N-Oxide R OH (NMO), Trimethylamine N-Oxide, Hydrogen Peroxide, t-Butyl [2 + 2] Hydroperoxide, and Potassium Ferricyanide. Oxygen has also R been used as cooxidant in dihydroxylation of certain alkenes.5 O O Excess cooxidant and osmium tetroxide are reduced with a re- Os O R ducing agent such as those mentioned above during the workup. O The stoichiometric dihydroxylation can be carried out in almost 2 OSMIUM TETROXIDE CO Me any inert organic solvent, including most commonly MTBE, 2 CO2Me HO toluene, and t-BuOH. In the catalytic dihydroxylation, in order to O OsO , py O OTBS 4 OTBS OH dissolve the inorganic cooxidant and other additives, a mixture (10) MeO O MeO O of water and an organic solvent are often used. The most com- OBz OBz mon solvent combinations in this case are acetone–water and t- de > 25:1 BuOH–water. Because of the high cost and toxicity of osmium tetroxide, the stoichiometric dihydroxylation has been mostly OH OH OH OH OH replaced by the catalytic version in preparative organic chem- OsO4 OBn OBn (11) istry (see also Osmium Tetroxide–tert-Butyl Hydroperoxide, NMO Osmium Tetroxide–N-Methylmorpholine N-Oxide, and Osmium HO Tetroxide–Potassium Ferricyanide). de = 35:1 Diastereoselective Dihydroxylation. Dihydroxylation of OH HO OH OsO4 OH acyclic alkenes containing an allylic, oxygen-bearing stereocenter (12) proceeds with predictable stereochemistry. In general, regardless MeO2C NMO MeO2C of the double-bond substitution pattern and geometry, the rela- de > 100:1 tive stereochemistry between the pre-existing hydroxyl or alkoxyl group and the adjacent newly formed hydroxyl group of the major diastereomer will be erythro (i.e. anti if the carbon chain is drawn OTBS OH OTBS OH OsO4 in the zig-zag convention) (eq 6).6,7 R R R NMO R OTBS OH OTBS OH OsO4 OH 3 OsO de > 99:1 H R 1 4 4 OsO4 or R R NMO (13) 3 (6) OsO4 1 4 OsO4, NMO R R R (R2)HO OH TBSO OH NMO 2 OH(R ) R R R = CO2Et, CH2OAc In the osmylation of 1,2-disubstituted allylic alcohols and TBSO OH de > 99:1 derivatives, cis-alkenes provide higher diastereoselectivity than the corresponding trans-alkenes (eqs 7 and 8).6 Opposite se- lectivities have been observed in the osmylation of (Z)-enoate The diastereoselective osmylation has been extended to 8 and (E)-enoate esters (eqs 9 and 10). High selectivity has also oxygen-substituted allylic silane systems, and the general rule 9 been observed in the osmylation of 1,1-disubstituted and (E)- observed for the allylic alcohol system also applies (eq 14).12 10 trisubstituted allylic alcohols and derivatives and bis-allylic High selectivity is also observed in the osmylation of allylsilanes 11 compounds (eqs 11–13). where the substituent on the chiral center bearing the silyl group 13 OH is larger than a methyl group (eq 15). These diastereoselec- OBn tivities have been achieved in both stoichiometric and catalytic BnO dihydroxylations. Slightly higher selectivity has been observed in the stoichiometric reaction than in the catalytic reaction; this may OBn OH OBn OH be due to less selective bis-osmate ester formation in the catalytic BnO OH + BnO OH (7) reaction using NMO as the cooxidant. Use of K3Fe(CN)6 may solve this discrepancy. Several rationales have been proposed for OH OH the observed selectivity.14 The conclusion appears to be that the OsO4 8.0:1.0 osmylation of these systems is controlled by steric bias, rather OsO , NMO 7.0:1.0 4 than by the electronic nature of the allylic system, and osmylation OBn will occur from the sterically more accessible face. The high di- BnO OH astereoselectivity of osmium tetroxide in the dihydroxylation of chiral unsaturated compounds has been applied widely in organic synthesis.8,15 OBn OH OBn OH BnO OH + BnO OH (8) OH OsO OH OH PhMe2Si 4 R OBn OBn OsO4 4.2:1.0 OH OH OH OsO4, NMO 3.1:1.0 de = 97:3 Ac2O (14) CO2Me HO OH O OsO4, py O OsO OTBS CO2Me OTBS OH PhMe2Si 4 R (9) OBn OBn MeO O MeO O OBz OBz OAc OH OH de > 18:1 de = 4.4:1 OSMIUM TETROXIDE 3 SiMe Ph PhMe Si OH PhMe Si OH 1 2 OsO4 2 2 as chiral ligands for the asymmetric dihydroxylation (AD). The + (15) AD can be classified into two types: (a) noncatalytic reaction, R py R R OH OH where stoichiometric amounts of ligand and osmium tetroxide R = Me 34:65 are used, and (b) catalytic reaction, where catalytic amounts of R = i-Pr 67:33 ligand and osmium tetroxide are employed in conjunction with R = Ph 92:8 stoichiometric amounts of cooxidant. Generally, in the stoichio- metric AD systems, chiral chelating diamines are used as chiral Sulfoxide groups direct the dihydroxylation of a remote dou- auxiliaries with osmium tetroxide for the introduction of asymme- ble bond in an acyclic system perhaps by prior complexation of try to the diol products.1,19 Although high asymmetric inductions 16 the sulfoxide oxygen with osmium tetroxide (eqs 16 and 17). have been achieved in these systems, the stoichiometric ADs have Chiral sulfoximine-directed diastereoselective osmylation of cy- limited use in practical organic synthesis because of the cost of cloalkenes has been used for the synthesis of optically pure di- both ligand and osmium tetroxide. The discovery of the ligand- 17 hydroxycycloalkanones (eq 18).