Rhodium allylic oxidation catalyst

Wilkinson s catalyst has also been utilized for the hydroboration of other alkenes. Sulfone derivatives of allyl alcohol can be hydroborated with HBcat and subsequently oxidized to give the secondary rather than primary alcohol. This reactivity proves to be independent of substituents on the sulfur atom.36 Similarly, thioalkenes undergo anti-Markovnikoff addition to afford a-thioboronate esters.37 The benefits of metal-catalyzed reactions come to the fore in the hydroboration of bromoalkenes (higher yields, shorter reaction times), although the benefits were less clear for the corresponding chloroalkenes (Table 3).38,39 Dienes can be hydroborated using both rhodium and palladium catalysts [Pd(PPh3)4] reacts readily with 1,3-dienes, but cyclic dienes are more active towards [Rh4(CO)i2].40... 

In comparison to the bismuth molybdate and cuprous oxide catalyst systems, data on other catalyst systems are much more sparse. However, by the use of similar labeling techniques, the allylic species has been identified as an intermediate in the selective oxidation of propylene over uranium antimonate catalysts (20), tin oxide-antimony oxide catalysts (21), and supported rhodium, ruthenium (22), and gold (23) catalysts. A direct observation of the allylic species has been made on zinc oxide by means of infrared spectroscopy (24-26). In this system, however, only adsorbed acrolein is detected because the temperature cannot be raised sufficiently to cause desorption of acrolein without initiating reactions which yield primarily oxides of carbon and water. 

The fust example of rhodium catalysis for this purpose utilized chlorotiis(triphenylphosphine)rho-diumG) to catalyze the allylic oxidation of a range of alkenes. 47oxidize cyclic allylsilanes " to afford -silyl-2-cycloalkoiones in very good yields and with exclusive fearrangemrat (equation 43). 

The avermectins also possess a number of allylic positions that are susceptible to oxidative modification. In particular the 8a-methylene group, which is both allylic and alpha to an ether oxygen, is susceptible to radical oxidation. The primary product is the 8a-hydroperoxide, which has been isolated occasionally as an impurity of an avermectin B1 reaction (such as the catalytic hydrogenation of avermectin B with Wilkinson s rhodium chloride-triphenylphosphine catalyst to obtain ivermectin). An 8a-hydroxy derivative can also be detected occasionally as a metabolite (42) or as an impurity arising presumably by air oxidation. An 8a-oxo-derivative can be obtained by oxidizing 5-O-protected avermectins with pyridinium dichromate (43). This also can arise by treating the 8a-hydroperoxide with base. 

Exposure of anilide 151 and allyl carbonate 152 to a rhodium catalyst resulted in a tandem C-H allylation/oxidative cyclization to afford indole 153.The route is fairly economical the allyl group is incorporated into the indole core and only CO2 and methanol are generated as by-products. A variety of electron-donating and -withdrawing groups as well as halogens can be present as substituents (130L4576). 

Rhodium catalysts have also been used with increasing frequency for the allylic etherification of aliphatic alcohols. The chiral 7r-allylrhodium complexes generated from asymmetric ring-opening (ARO) reactions have been shown to react with both aromatic and aliphatic alcohols (Equation (46)).185-188 Mechanistic studies have shown that the reaction proceeds by an oxidative addition of Rh(i) into the oxabicyclic alkene system with retention of configuration, as directed by coordination of the oxygen atom, and subsequent SN2 addition of the oxygen nucleophile. 

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acryhc acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubihty in water and retain this property after functionahzation with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by coohng the solution to 0 °C [92]. Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Monohydrides play an important role in the following rhodium-complex-catalyzed hydrogenations in aqueous media. The catalyst precursor is [RhCl(PTA)3], which gives the catalytically active species (HRh(PTA)3] formed by dehydrochlorination of the primary product of H2 oxidative addition (88). The complex is an active catalyst for several reactants, e.g., olefinic and oxo adds, allyl alcohol, and sulfostyrene. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Although C—H insertion reactions rarely occur in intermolecular reactions with diazoacetates, these are common side reactions with diazomalonates3132 (equation 10) and diazo ketones (with a-allyl vinyl ethers).33 Several mechanistic pathways are available to generate the products of an apparent direct C—H insertion reaction and these include dipolar intermediates, ir-allyl complexes and ring opening of cyclopropanes.1 Oxidative problems due to the presence of oxygen are common with copper catalysts, but these are rarely encountered with rhodium catalysts except in systems where the carbenoid is ineffectively captured.34... 

Intramolecular hydrosilylations of functionalized alkenes followed by hydrogen peroxide oxidation provide powerful methods for organic syntheses86-88. The reactions of allylic O-dimethylsilyl ethers 59 promoted by platinum catalysts, e.g. Karstedt s catalyst and Pt(PPh3)2(CH2=CH2), or rhodium catalysts, e.g. Rh(acac)(COD) and [RhCl(CH2=CH2)2]2> proceed via 5-endo cyclization to give oxasilacyclopentanes 60 with a couple of exceptions in which siloxatanes 61 are formed (Scheme ll)87,89. 

Multiatomic [6] as well as cationic [7] rhodium catalysts also display a high preference for linear hydroformylation products. However, a catalyst system which generally yields branched hydroformylation products has not yet been found. Vinylarenes, such as styrene (16), form preferentially the (.vo-aldehyde 20 and not the n-aldehydes. The possibility to form a relatively stable Rh- -allyl complex 18 is most likely the decisive factor for this result [8]. Subsequent oxidation of 20 leads to 2-arylpropionic acids 21, of which some derivatives like 22-24 are of great importance as non-steroidal inflammatory drugs (NSID) (Scheme 3) [9]. For their synthesis by the hydroformylation of styrenes, not only a regioselective but also an enantioselective reaction process is... 

Isomerisation of allyl ethers to enol ethers by Wilkinson s catalyst in refluxing aqueous ethanol is accompanied by competing reduction of the double bond to the propyl ether409-412 However, treatment of Wilkinson s catalyst with butyl-lithium results in a red rhodium catalyst that is able to isomerise a wide range of substituted and unsubstituted allylic ethers without competing reduction.413-414 In the example shown in Scheme 4.215, the unpurified enol ether product was cleaved by treatment with a mixture of mercury(Il) chloride and mercury(II) oxide in acetone to liberate the anomeric centre in 91% yield for the two steps.413-414... 

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