Iridium oxidation, alcohols

The mechanism of the reaction is as shown in equations (13.139) and (13.140). This reaction is also catalyzed by compounds of other metals of groups 8 and 9 such as ruthenium and iridium. Higher alcohols EtOH, Pr"OH, Pr OH also undergo carbonylation to give corresponding carboxylic acids.However, the rate of the reaction is lower. It is assumed that in this case, the oxidative addition of alkyl iodide to the rhodium(I) complex proceeds according to a radical mechanism. Hydrocarboalkoxylation, carbonylation of esters, reductive carbonylation of... 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

Schemes 6-10 Oxidative addition of alcohol, phenol, and water to cationic iridium phosphine complex 67...

Alcohols will serve as hydrogen donors for the reduction of ketones and imi-nium salts, but not imines. Isopropanol is frequently used, and during the process is oxidized into acetone. The reaction is reversible and the products are in equilibrium with the starting materials. To enhance formation of the product, isopropanol is used in large excess and conveniently becomes the solvent. Initially, the reaction is controlled kinetically and the selectivity is high. As the concentration of the product and acetone increase, the rate of the reverse reaction also increases, and the ratio of enantiomers comes under thermodynamic control, with the result that the optical purity of the product falls. The rhodium and iridium CATHy catalysts are more active than the ruthenium arenes not only in the forward transfer hydrogenation but also in the reverse dehydrogenation. As a consequence, the optical purity of the product can fall faster with the... 

The oxidative dehydrogenation of secondary alcohols to ketones on iridium at 130°C has been measured by Le Nhu Thanh and Kraus (i-Zi), and the rates have been correlated by the Taft equation [series 112, four reactants of the structure R CH(OH)CH3, slope 4.7]. 

Complex 77 has also been reported to catalyze the oxidative dimerization of alcohols to esters when the reactions are performed in the presence of base [76]. The presence of base presumably encourages the reversible attack of the alcohol onto the initially formed aldehyde to give a hemiacetal, which is further oxidized to give the ester product. Alcohols 87 and 15 were converted into esters 88 and 89 with good isolated yields (Scheme 20). Alternative iridium catalysts have been used for related oxidative dimerization reactions, and the addition of base is not always a requirement for the reaction to favor ester formation over aldehyde formation [77, 78]. 

Scheme 11 Carbonyl tert-prenylation, crotylation, and allylation from the aldehyde or alcohol oxidation level under the conditions or iridium-catalyzed transfer hydrogenation...

Scheme 13 Enantioselective carbonyl tert-prenylation from the alcohol or aldehyde oxidation level via iridium-catalyzed C-C bond-forming transfer hydrogenation...

More recently, using the cyclometallated iridium C,(7-benzoate derived from allyl acetate, 4-methoxy-3-nitrobenzoic acid and BIPHEP, catalytic carbonyl crotylation employing 1,3-butadiene from the aldehyde, or alcohol oxidation was achieved under transfer hydrogenation conditions [274]. Carbonyl addition occurs with roughly equal facility from the alcohol or aldehyde oxidation level. However, products are obtained as diastereomeric mixtures. Stereoselective variants of these processes are under development. It should be noted that under the conditions of ruthenium-catalyzed transfer hydrogenation, conjugated dienes, including butadiene, couple to alcohols or aldehydes to provide either products of carbonyl crotylation or p,y-enones (Scheme 16) [275, 276]. 

As demonstrated in recent work by Obora and Ishii, alkynes serve as allyl donors in carbonyl allylations from the alcohol oxidation level [277]. Specifically, upon exposure to an iridium catalyst generated in situ from [lr(OH)(cod)]2 and P( -Oct)3, l-aryl-2-methylalkynes couple to primary alcohols to furnish homoallylic alcohols with complete branched regioselectivity and excellent levels of diastereoselectivity (Scheme 17). 

Scheme 17 Carbonyl arylallylation from the alcohol oxidation level via iridium-catalyzed transfer hydrogenation employing alkynes as allyl donors...

The BINAP derivative of the ort/io-cyclometallated iridium catalyst has been characterized by single crystal X-ray diffraction analysis [280]. Remarkably, although the reaction sequence depends upon oxidation of either the reactant alcohol or isopropanol, the enantiomeric purity of the homoallylic alcohol product... 

Scheme 22 1, n-Glycols as dialdehyde equivalents in iridium-catalyzed enantioselective carbonyl allylation from the alcohol oxidation level... 

A possible mechanism for the N-alkylation of primary amines is shown in Scheme 5.21. The first step of the reaction involves the oxidation of an alcohol to a carbonyl intermediate, accompanied by the generation of an iridium hydride. 

A possible mechanism for the P-alkylation of secondary alcohols with primary alcohols catalyzed by a 1/base system is illustrated in Scheme 5.28. The first step of the reaction involves oxidation of the primary and secondary alcohols to aldehydes and ketones, accompanied by the transitory generation of a hydrido iridium species. A base-mediated cross-aldol condensation then occurs to give an a,P-unsaturated ketone. Finally, successive transfer hydrogenation of the C=C and C=0 double bonds of the a,P-unsaturated ketone by the hydrido iridium species occurs to give the product. 

Hydrogen transfer oxidation of an alcohol to give an aldehyde and an iridium hydride. 

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... 

Rhodium complexes were generally found to be more effective than iridium, but on the whole they show moderate activity in alkene oxygenation reactions. Significantly, epoxides, a typical product of the oxidation of olefins catalyzed by the middle transition metals, have rarely been evoked as products [18-22]. Although allylic alcohols [23] or ethers [24] have sometimes been described as products, the above cited rhodium and iridiiun complexes are characterized by an excellent selectivity in the oxygenation of terminal alkenes to methyl ketones. 

Recent advances in alcohol oxidations by rhodium and iridium complexes have mainly focused on Oppenauer-type oxidations or reactions in which this type of oxidation is an intermediate step. An independent result is the oxidation of allyhc (Eq. 9) and benzyUc alcohols with f-BuOOH to the corresponding a,/l-unsaturated ketones [38] with [Rh2(p.-OAc)4]. The reactions were carried out at room temperature in dichloromethane and yields of up to 92% (by GC) in 24-48 h have been described. 

---