A-Hydroperoxide intermediate

Strong evidence has been provided that the photoxygenation proceeds via a hydroperoxide intermediate which is transformed into an epoxide. The epoxide is then opened by the nucleophilic solvent. 

In the postulated bioluminescence mechanism, firefly luciferin is adenylated in the presence of luciferase, ATP and Mg2+. Luciferyl adenylate in the active site of luciferase is quickly oxygenated at its tertiary carbon (position 4), forming a hydroperoxide intermediate (A). 

Although the cofacial diporphyrins represent a vibrant and innovative direction in dioxygen activation, simple porphyrins and their derivatives also remain an important research area. The dichlorophenyl-substituted porphyrin tdcpp [5,10,15,20-tetrakis(2,6-dichlorophenyl)-porphyrin] forms a complex with cobalt(II), [Co(tdcpp)], and catalyzes the oxidation of conjugated olefins to (after experimental workup) ketones in the presence of dioxygen and triethylsilane (80) a hydroperoxide intermediate has been isolated from these reactions (81). 

Simple alkyl-substituted dihydro-1,4-thiazines are readily converted into the corresponding 5-monoxides atmospheric oxidation is sometimes sufficient, although side reactions may also occur. The cyclohexene derivative (99), for instance, affords not only the oxide (100) but also the spiro compound (101) probably generated by ring contraction of a hydroperoxide intermediate (Scheme 42) <73US(B)(8)34l>. 

Considerable evidence exists that indicates the selective oxidation of propylene proceeds via the formation of a symmetrical allyl species. Subsequent steps may vary as a function of the catalyst. Some catalyst systems may abstract a second hydrogen atom before the insertion of oxygen. Others may add molecular oxygen, forming a hydroperoxide intermediate, which may then subsequently decompose into acrolein and water. 

Catalytic oxidation ofl-alkenes.2 The Co(II) complex catalyzes the oxygenation of terminal alkenes to the methyl ketone and the corresponding alcohol. The reaction is not a radical-initiated autoxidation it probably involves a hydroperoxide intermediate. 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

If the reaction is four-centered (5 - 7), a silicon hydroperoxide should be formed. The reaction of triethylsilane with ozone was monitored by NMR at —57°C. The only species observed under these conditions were the silane and the silanol no evidence for a hydroperoxide intermediate which might have been stable at that temperature (35) was detected, and chemical tests for peroxides proved negative (5, 6). As the concentration of ozone was increased from zero to saturation, the spectrum of the silane completely disappeared with the concurrent appearance of the silanol NMR spectrum. 

The reaction of lithium enolates with molecular oxygen has been used for the a-hydroxylation of several substrates. The carbanion generated in the reaction of N,N-dialkylamides or esters with alkyl lithium reagents undergoes rapid oxidation under mild conditions when treated with molecular oxygen. The reaction produces an a-hydroperoxide intermediate which is cleanly reduced with sodium sulphite to the a-hydroxo derivatives" (equation 1). 

O Reduced flavin adenine dinucleotide reacts with molecular oxygen to give a hydroperoxide intermediate. 

Figure 20.6. A Wasteful Side Reaction. The reactive enediolate intermediate on rubisco also reacts with molecular oxygen to form a hydroperoxide intermediate, which then proceeds to form one molecule of 3-phosphogly cerate and one molecule of phosphoglycolate.

Mechanistically, it has been proposed that the reaction proceeds predominantly via epoxidation of the aromatic species 28, which leads to unstable arene-oxides 29-31 (equation 47). Rearrangement of the arene-oxides 29-31 involving the migration of a hydride anion (NIH-shift) forms the phenolic product 32 or 33. Alternative flavin-dependent oxidases have been proposed to involve a hydroperoxide intermediate. ... 

Keulks et al. (7) discussed evidence for a model in which the selective oxidation of propylene proceeded via the formation of a symmetric allylic species with subsequent steps depending on the nature of the catalyst. In some catalytic systems the abstraction of a second hydrogen atom seemed to precede the insertion of oxygen, while others appeared to add molecular oxygen to form a hydroperoxide intermediate which subsequently decomposed into acrolein and water. 

The bulk of free reduced flavin does not go through a hydroperoxide intermediate when reacting with O2 (Scheme 20). Instead, the reaction is autocatalytic." ° Oxidized flavin, initially formed through the slow... 

The reaction is postulated to proceed through a hydroperoxide intermediate, [Cu2(L-H)(OOH)]. Experimental studies into Fenton and Fenton-like reactions point out that the detailed mechanism involves the formation of a low-valent cation.However, while [(H20)5Cr " 02H ] decomposes directly to [Cr(H20)6] " and OH in the presence of >0.1 M EtOH, 2-PrOH, and 2-BuOH, [(H20), Cu 02H ] reacts directly with the alcohols. In the absence of organic substrate, the latter complex does yield [Cu(H20) ] and OH. 

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