Stilbenes osmium tetroxide

Asymmetric dihydroxylation can be achieved using osmium tetroxide in conjunction with a chiral nitrogen ligand. " The very successful Sharpless procedure uses the natural cinchona alkaloids dihydroquinine (DHQ) and its diastereomer dihy-droquinidine (DHQD), as exemplified in the epoxidation of imni-stilbene... 

Other functionalized supports that are able to serve in the asymmetric dihydroxylation of alkenes were reported by the groups of Sharpless (catalyst 25) [88], Sal-vadori (catalyst 26) [89-91] and Cmdden (catalyst 27) (Scheme 4.13) [92]. Commonly, the oxidations were carried out using K3Fe(CN)g as secondary oxidant in acetone/water or tert-butyl alcohol/water as solvents. For reasons of comparison, the dihydroxylation of trons-stilbene is depicted in Scheme 4.13. The polymeric catalysts could be reused but had to be regenerated after each experiment by treatment with small amounts of osmium tetroxide. A systematic study on the role of the polymeric support and the influence of the alkoxy or aryloxy group in the C-9 position of the immobilized cinchona alkaloids was conducted by Salvadori and coworkers [89-91]. Co-polymerization of a dihydroquinidine phthalazine derivative with hydroxyethylmethacrylate and ethylene glycol dimethacrylate afforded a functionalized polymer (26) with better swelling properties in polar solvents and hence improved performance in the dihydroxylation process [90]. 

Grafting a modified cinchona alkaloid to hexagonal mesoporous molecular sieve SBA-15 afforded catalyst (27) with excellent activity. 1-Phenyl-1-propene was converted to the corresponding diol in 98% yield (98% ee), while trans-stilbene yielded the desired product in 97% yield (99% ee) [92]. Other examples in this field are the utilization of microencapsulated osmium tetroxide by Kobayashi [93] and the application of continuous dihydroxylation mns in chemzyme membrane reactors described by Woltinger [94]. 

Certain tertiary amines such as pyridine or a-quinuclidine accelerate the stoichiometric reaction between osmium tetroxide and olefins (86). An asymmetric olefin osmylation using stoichiometric amounts of cinchona alkaloids as the chiral ligands was described in 1980 (87a). Optical yields of up to 90% were attained with frans-stilbene as substrate. 

In the second approach, a chiral nitrogen-containing compound has most often been used as the ligand to achieve enantioselectivity. Thus, oxidation of ( )-stilbene (22 equation 9) with a stoichiometric quantity of osmium tetroxide in toluene at room temperature, in the presence of dihydroquinine acetate (23), yielded r/ireo-hydrobenzoins (24) after reductive hydrolysis, with an enantiomeric excess of 83.2% in favor of the (15,25)-(-)-isomer performing the reaction at -78 C increased the eiuuitiomeric excess to 89.7%. 

This procedure has been modified to become an effective catalytic procedure in which iV-methyl-moipholine A -oxide is used as the secondary oxidant. In this manner, ( )-stilbene has been converted into (+)-r/irco-hydrobenzoin (55% yield after two reciystallizations, >99% ee) on a one molar scale, by treatment with osmium tetroxide (0.002 mol equiv.) and iV-methylmoipholine 1 -oxide (1.2 mol equiv.) in aqueous acetone in the presence of dihydroquinidine p-chlorobenzoate (0.134 mol equiv.). The latter compound can be recovered in 91% yield. 

With diamine (25), 1-heptene afforded (/ )- ,2-heptanediol as the major ixoduct (86% ee) in 75% yield by this procedure but, curiously, oxidation of ( )-stilbene proceeded with lower optical yield (34% ee). Particularly efficient enantioface differentiation was achieved in the reaction of ( )-l-phe-nylpropene with a stoichiometric amount of osmium tetroxide in the presence of 1 mol equiv. of (-)-(27) when essentially optical pure (>99% ee) (15,2S)-l-phenylpropane-l,2-diol was obtained in 73% yield. This procedure is effective for mono-, ( )-di- and tri-substituted alkenes, with enantioface selection being as shown in Scheme 2 but, notably, the oxidation of (Z)-alkenes does not give satisfactory optical yields. 

Bioxazolines have also been employed in the enantioselec-tive dihydroxylation of alkenes with Osmium Tetroxide The best results have been obtained in the dihydroxylation of 1-phenylcyclohexene with a complex, formed between OSO4 and the diisobutylbioxazoline (4) (R=CH2CHMe2>, as a stoichiometric reagent (70% ee). Styrene and trans -stilbene afford enantioselec-tivities below 20% ee under these conditions (for highly enantios-elective dihydroxylation catalysts, see Dihydmquinine Acetate and Osmium Tetroxide). 

The continued fascination chemists possess with asymmetric synthesis provides the basis for the next four procedures. The synthesis of (R)-(-)-10-METHYL-l(9)-OCTALONE-2 is a nice demonstration of an asymmetric Michael addition by a chiral imine followed by an aldol—in short an asymmetric Robinson annulation. The asymmetric glycolization to STILBENE DIOL (R,R-l,2-DIPHENYL-I,2-ETHANEDIOL) represents an olefin oxidation using catalytic alkaloids in tandem with osmium tetroxide. As reagents for a variety of asymmetric alkylations, the preparation of 2-CYANO-6-PHENYLOXAZOLOPIPERIDINK is pavscnicd as well as another route to... 

It was reported [39] that osmium tetroxide promoted catalytic oxidative cleavage of ds-stilbene and other olefins. The ds-stilbene catalytic oxidative cleavage gave benzoic acid in 95% yield. The process for the preparation of substituted aromatic and heteroaromatic aldehydes and carboxylic acids by the oxidation of substituted stilbenes has been patented [40]. Stilbenes of various substituents (Ri, R2 = H, Cl-4 alkyl, Cl-4 alkoxy, OH, NO2, CN, COjH, CONH2, SO3H, halogen X = C, N Z = CHO, CO2H ... 

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