Ruthenium dihydroxylation

Unlike palladium(II), osmium tetraoxide and ruthenium tetraoxide catalyze the dihydroxylation of one or both double bonds of an allene. The osmium tetraoxide-catalyzed dihydroxylation of unsymmetrically substituted allenes 45 can lead to two different a-ketols, 46 and 47, depending on which of the double bonds is oxidized. David et al. studied this reaction using NMO as a stoichiometric oxidant and found good product selectivity in a few cases, but the yields were only moderate (Scheme 17.15) [16]. They showed that the most substituted double bond was oxidized preferably when the bulkiness of the allene substituents did not interfere. 

Ruthenium complexes catalyse the two main oxidative reactions for alkenes those in which oxygen atoms or hydroxyl groups span the erstwhile double bond without C=C rupture (e.g. epoxidation, ctT-dihydroxylation, ketohydroxylation), and cleavage reactions in which the C=C bond is broken. Although RuO has recently been shown to be effective for c/x-dihydroxylation and ketohdroxylation, epoxidations are in general effected by Ru complexes of lower oxidation states, while RuO excels at cleavage reactions. 

Ruthenium complexes alcohol oxidation, 788-9 alkene/alkyne vicinal dihydroxylation, 556 dioxetane decomposition, 1189-90... 

The oxidation of alkenes by ruthenium tetroxide is another example of a metal-oxo oxidation reaction where the mechansim was discussed. This compound is known to dihydroxylate alkenes and several groups have investigated its reactivity.59-67... 

Ruthenium catalysis allows dihydroxylation providing an easy access to syn-diols, but over-oxidation is a common side reaction. The improved protocol for the Ru-catalyzed syn-dihydroxylation uses only 0.5 mol% catalyst under acidic conditions that gave products in high yields with only minor formation of side products. ... 

The diols (97) from asymmetric dihydroxylation are easily converted to cyclic sulfite esters (98) and thence to cyclic sulfate esters (99). ° This two-step process, reaction of the diol (97) with thionyl chloride followed by ruthenium tetroxide catalyzed oxidation, can be done in one pot if desired and transforms the relatively unreactive diol into an epoxide mimic, i.e. the 1,2-cyclic sulfate (99), which is an excellent electrophile. A survey of reactions shows that cyclic sulfates can be opened by hydride, azide, fluoride, thiocyanide, carboxylate and nitrate ions. ° Benzylmagnesium chloride and the anion of dimethyl malonate can also be used to open the cyclic sulfates.Opening by a nucleophile leads to formation of an intermediate P-sulfate anion (100) which is easily hydrolyzed to a p-hydroxy compound (101). ° Conditions for catalytic acid hydrolysis have been developed that allow for selective removal of the sulfate ester in the presence of other acid sensitive groups such as acetals, ketals and silyl ethers. ... 

As an oxometal component, osmium tetroxide is the most reliable reagent on the laboratory scale to produce c/s-diols. Ruthenium tetroxide in the presence of NaI04 effects oxidative cleavage of olefins [4], but has been successfully employed for so-called lightning dihydroxylation reactions using a two-phase medium [6]. 

Norrby, P. O., Kolb, H. C., Sharpless, K. B. Calculations on the reaction of ruthenium tetroxide with olefins using density functional theory (DPT). Implications forthe possibility of intermediates in osmium-catalyzed asymmetric dihydroxylation. Organometallics 994,13, 344-347. 

In order to cleave the bicyclic C-glycoside to the desired functionalized dihydropyran, 284 was exposed to sodium amalgam. Syn-dihydroxylation of 285 (OSO4-NMO) gave a mixture of diastereomeric tetrads, which after separation, protection-deprotection procedure and oxidation of the product C-l alcohol, using generated in situ ruthenium tetraoxide, gave the acid 254. 

This reaction was employed to synthesize the perfumery compound, rosefuran 8.180 (Scheme 8.48) by the ruthenium-catalysed combination of alkyne 8.177 with an allylic alcohol. Dihydroxylation of the product 8.178 was followed by acid-catalysed dehydration. Hydrolysis of the ester and thermal elimination of water gave the natural product 8.180. Another synthesis of rosefuran can be found in Scheme 2.15. 

This article updates an earlier version in the first edition of this book and summarizes the recent developments in the area of osmium-catalyzed dihydroxylations which bring this transformation closer to a green reaction . Special emphasis is placed on the use of new reoxidants and recycling of the osmium catalyst Moreover, less toxic metal catalysts such as ruthenium and iron are also discussed. 

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