Gif type systems

Barton and coworkers [121-123] have discovered multicomponent systems which have come to be known as Gif systems . They oxidize saturated hydrocarbons with molecular oxygen under ambient conditions. The main components of the systems are acetic acid (or other acid) in pyridine as solvent, iron powder as a source of electrons and a precursor of an iron catalyst. The compositions of the different variants and the probable functions of the individual components are listed in Table VI. 

With only a few exceptions Gif systems are based on non-porphyrin type iron catalysts. 

Typical substrates that can be oxidized by the Gif systems are adamantane, cyclohexane, methylcyclohexane, substituted 

Over-oxidation of cyclohexane beyond cyclohexanol and cyclohexanone by the Gif system has recently been reported [124]. Cyclododecanone is overoxidized to cyclododecanediones [125]. 

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Towards the end of this section it may be worthwhile to point out some new reactions with high-valent metals and TBHP. The first is a pyridinium dichromate PDC-TBHP system. Nonsubstituted or alkyl-substituted conjugated dienes, such as 1,3-cyclooctadiene (87) and others (also linear dienes), yield keto allyl peroxides 88 (equation 18), whereas phenyl-substituted dienes such as 1,4-diphenylbutadiene (89) gave diketo compounds, 90 (equation 19). In further research into a GIF-type system with iron and TBHP, limonene gave a mixture of products with carvone as the major product. The mechanism is thought to proceed initially by formation of a Fe(V)-carbon... 

The selective functionalization of saturated hydrocarbons is an important and challenging academic and industrial objective. Our own contributions to this area are based on Gif-type systems. Recent studies have shown that fert-butyl hydroperoxide (TBHP) is an efficient oxidizing agent in these systems. 

A new series of Gif-type systems has recently been introduced, distinguished by the suffix Agg (referring to "Aggieland", Texas A M... 

The oxidation of adamantane and other saturated hydrocarbons to ketones by Gif-type systems does not involve secondary adamantyl radicals as the corresponding pyridine adducts 10 and 11 are absent [137,1381 if sufficient 0 is available. This is in support of an... 

Information of the Gif system has been summarized,1055 1123 and new results, including new oxidants such as bis(trimethylsilyl) peroxide,1124 the synergistic oxidation of saturated hydrocarbons and H2S,1125 studies with the Fe3+-picolinate complex encapsulated within zeolites,1126 and the use of Udenfriend s system under Gif conditions1127 were disclosed. Gif-type oxidations were found to be moderately stereoselective.1128 Iron/zinc-containing species involved in Gif-type chemistry were synthesized, and their reactivity and catalytic behavior were studied.1129... 

This section deals with Gif and GoAgg systems that were discovered by Barton and coworkers in the 1980s. After the presentation of the various systems, we will focus on the mechanism of the reaction. The last section will focus on the latest applications of Gif and GoAgg type systems to alkane oxidation. 

Significant efforts have been made to reproduce functional aspects of the nonheme di-iron enzymes. A few examples are now available in which well-characterized high-valent di-iron species participate in the key C H/O activation steps. Differentiation of radical-free vs. free-radical processes is an important issue to be addressed for the catalytic systems that employ hydrogen peroxide or alkyl peroxides as terminal oxidants. In the following sections, selected nonheme di-iron systems that effect either stoichiometric or catalytic oxidation of organic substrates are described. Detailed accounts of Gif chemistry and Gif-type reagents can be found elsewhere. Reviews of related C—activation by Fenton-type processes are also available. " ... 

The chemical (Gif system) and the electrochemical conversion (Gif-Orsay system) have been compared in the oxidation of six saturated hydrocarbons (cyclohexane, 3-ethylpentane, methylcyclopentane, cis- and traus-decalin and adamantane). The results obtained for pyridine, acetone and pyridine-acetone were similar for both systems. Total or partial replacement of pyridine for acetone affects the selectivity for the secondary position and lowers the ratio ketone secondary alcohol. The formation of the same ratio of cis- and traws-decal-9-ol from either cis- or trans-deca in indicates that tertiary alcohols result from a mechanism essentially radical in nature. The C /C ratio between 6.5 and 32.7 rules out a radical mechanism for the formation of ketones and secondary alcohols. Ratios of 0.14 and 0.4 were reported for radical-type oxidations of adamantane and cis-decalin. Partial replacement of pyridine by methanol, ethanol or f-propanol results in diminished yields and a lower selectivity. Acetone gives comparable yields however, the C /C ratio drops to 0.2-10.7. 

Hydrocarbon functionalisation by ruthenium and rhodium systems is covered in two monographs,87 while Gif-type chemistry is the subject of a review. Theoretical studies of methane C-H oxidative additions by (FeCH4) and Ni(PH3)2 are of note. 

Barton et al. used zinc (Zn) as a reductant. They isolated a trinuclear iron complex, Fe"Fe 20(OAc)6(py)i,s, from a reaction system containing iron powder, carboxylic acids, and H2S in aqueous pyridine under 1 atm O2 and found that it oxidized alkanes in the presence of Zn, pyridine, and acetic acid under 1 atm (Gif system). Later, the Gif type oxygenations were performed mostly by using monoiron salts, e.g. FeCl2 4H20 or FeCI,i 6H20. Barton et al reported a number of experiments for application of this system to the more complicated compounds such as steroids " and for clarification of mechanism, specially participation of the radical and nonradical processes. Intermediate formation of Fe -... 

Several lines of evidence have recently confirmed that the major oxidant in Gif solutions is hydroxyl radicals rather than the originally proposed Fe =0 species. Hydroxyl radicals are generated via Fenton-like interactions between Fe(II) sites and hydrogen peroxide in GoAgg systems. Gif -type chemistry relies on Fe(II)/O2 interactions and reducing equivalents provided by Zn dust to afford hydroxyl radicals. As the substrate-derived alkyl-hydroperoxide builds up, especially under dioxygen-rich conditions, a secondary oxidant becomes apparent by means of its enhanced selectivity for the activation of tertiary C-H bonds. The... 

This was also accomplished with BaRu(0)2(OH)3. The same type of conversion, with lower yields (20-30%), has been achieved with the Gif system There are several variations. One consists of pyridine-acetic acid, with H2O2 as oxidizing agent and tris(picolinato)iron(III) as catalyst. Other Gif systems use O2 as oxidizing agent and zinc as a reductant. The selectivity of the Gif systems toward alkyl carbons is CH2 > CH > CH3, which is unusual, and shows that a simple free-radical mechanism (see p. 899) is not involved. ° Another reagent that can oxidize the CH2 of an alkane is methyl(trifluoromethyl)dioxirane, but this produces CH—OH more often than C=0 (see 14-4). ... 

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Several other organizations are considering SMRs of integral type that share some design features with IRIS, within the IPSR group (Integral Primary System Reactors) of the INTD/GIF. 

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