Solvent-free peroxidative oxidations

Effective solvent-free peroxidative oxidations of 1-phenylethanol (Scheme 18.1) and/or some secondary aliphatic alcohols toward the corresponding ketones with tert-butylhydroperoxide (TBHP) under MW irradiation, catalyzed by copper(II)-alkoxy-triazapentadienato (Cu -TAP) [10, 11] complexes 1, 2, dicopper(II)-aminopolyalcoholate (Cu -APA) [12] complexes 3, 4, arylhydrazone-j8-diketonate (CuII-AHBD) complex 5 [13], mixed-N,S copper(II) and iron(II) complexes 6-11 [14] and by the tetranuclear copper(II) aryUiydrazone of malononitrile complex 12 [15], have been achieved (Scheme 18.2). 

A Co(II) complex supported on SBA-15 has been employed as a highly active and reusable catalyst in the selective oxidation of various alcohols, in which the improved rates of reactions (from hours to minutes), yields and even selectivities in some cases, illustrate the usefulness of MW protocols as alternative methodologies in organic synthesis.A MW-assisted solvent-free peroxidative oxidation of benzyl alcohol to benzaldehyde catalyzed by magnetic Ni-doped MgFc204 NPs was also successfully performed.The catalyst is reusable and eco-friendly, is apphed in a small amount and the reaction time is short. ... 

Borthakur R, AsthanaM, Kumar A, Koch A, Lai RA. Solvent free selective oxidation of alcohols catalyzed by a trinuclear complex with a dicopper(II)-monozinc(II) centre using hydrogen peroxide as an oxidant. RSC Adv. 2013 3 22957-22962. 

A mixture of 1,4-dioxane and water is often used as the solvent for the conversion of aldehydes and ketones by H2Se03 to a-dicarbonyl compounds in one step (Eq. 8.117).331 Dehydrogenation of carbonyl compounds with selenium dioxide generates the a, (i-unsaturated carbonyl compounds in aqueous acetic acid.332 Using water as the reaction medium, ketones can be transformed into a-iodo ketones upon treatment with sodium iodide, hydrogen peroxide, and an acid.333 Interestingly, a-iodo ketones can be also obtained from secondary alcohol through a metal-free tandem oxidation-iodination approach. 

The oxidation of sulfides to the corresponding sulfoxides and sulfones proceeds under rather strenuous conditions requiring strong oxidants such as nitric acid, hydrogen peroxide, chromic acid, peracids, and periodate. Using MW irradiation, this oxidation is achievable under solvent-free conditions and with desired selectivity to either sulfoxides or sulfones using 10% sodium periodate on silica (Scheme 6.34)... 

In 2002, Ichihara and coworkers reported on the utilization of a Keggin-type phos-phomolybdate (NH4)3PMoi2O40 on apatite as catalyst for the solvent-free epoxidation of olefins (see Section III.A.3.c) and the oxidation of sulfldes and sulfoxides in the presence of urea hydrogen peroxide (Scheme 104) . Chemoselectivities of the oxidation of the sulfides were good [product ratios (sulflde/sulfoxide) 84/16 up to 91/9] depending on the substrate, temperature and reaction time. 

At least two systems can be cited as catalysts of peroxide oxidation the first are the iron (III) porphyrins (44) and the second are the Gif reagents (45,46), based on iron salt catalysis in a pyridine/acetic acid solvent with peroxide reagents and other oxidants. The author s opinion is that more than systems for stress testing these are tools useful for the synthesis of impurities, especially epoxides. From another point of view, they are often considered as potential biomimetic systems, predicting drug metabolism. Metabolites are sometimes also degradation impurities, but this is not a general rule, because enzymes and free radicals have different reactivity an example is the metabolic synthesis of arene oxides that never can be obtained by radical oxidation. 

Direct activation methods are not normally successful for the oxidation of alcohols using hydrogen peroxide. An advantage of this is that alcohols can often be used as solvents for other oxidations, which do not use direct activation methods. However, hydroxy ketals can be cleaved with aqueous hydrogen peroxide, and the ene-diol of L-ascorbic acid can be oxidized with sodium perborate.185 Under the conditions employed, it is likely that the active oxidant is free hydrogen peroxide rather than any boron species. 

The reaction can be operated as a two-phase system in this case the process can be employed at the reflux temperature of the solvent. It should be noted that in some cases, no solvent need be used296 and this is especially efficacious where mono-brominated products are the target. An example of the system s versatility in solvent-free mode can be seen in Table 3.7. Here we see that at a molar ratio of 1 1.1 1 (hydrogen peroxide/hydrogen bromide/2-nitrotoluene) over 90% selectivity to the benzyl bromide is observed at a conversion of 76%. However, increasing the oxidant ratio to 3 3.2 1 (hydrogen peroxide/hydrogen bromide/2-nitrotoluene), a selectivity of 98% to the benzal bromide is afforded. Diphenylmethane can be fully converted to oxidized products with 96% selectivity to the benzophenone compound.297... 

In a first series of experiments, an aqueous solution of hydrogen peroxide was applied as oxidant. As can be seen in Table 1, only low turnover numbers (T.O.N. = nunol products formed / mmol Ti sites in the catalyst) could be realised under solvent free conditions. As both reagent phases hardly mix. the blank reaction is completely negligible. 

---