Biomimetic aerobic oxidation

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

A reoxidation of the catalytic amounts of hydroquinone (HQ) to benzoquinone (BQ) in Scheme 8-11 by molecular dioxygen was realized by the use of an oxygen-activating macrocyclic metal complex as cocatalyst [53,62-65]. This leads to a mild biomimetic aerobic oxidation which is now based on a triple catalytic system (Scheme 8-12). With this system cyclohexa-1,3-diene is oxidized to frans-l,4-diacetoxycyclohex-2-ene at room temperature in 85-89% (>91% tmns) [62]. With the use of 2-phenylsulfonyl-l,4-benzoquinone as quinone, the trans selectivity of this process was >97% [53]. 

Scheme 8-12 Biomimetic aerobic oxidation of 1,3-dienes MB = Co(TPP), Fe(phthalocyanine) or Co(salophen) [62a].

A breakthrough was reported by Stack and co-workers in 1998 (212) who reported the first biomimetic catalytic system for the oxidation of primary alcohols by air. Independently, in the same year Chaudhuri, et al. (216) reported efficient aerobic oxidation of primary and secondary alcohols by the dinuclear catalyst [Cu2 2(Ls )2]C12 (216). Next, we will briefly review the salient features of these two systems. 

Akkilagunta VK, Reddy VP et al (2010) Aqueous-phase aerobic oxidation of alcohols by ru/c in the presence of eyelodextrin one-pot biomimetic approach to quinoxaline synthesis. Synlett 2571-2574... 

Wigington BN, Drummond ML, Cundari TR, Thom DL, Hanson SK, Scott SL. A biomimetic pathway for vanadium-catalyzed aerobic oxidation of alcohols evidence for a base-assisted dehydrogenation mechanism. Chem Eur J. 2012 18 ... 

Subsequent deprotection of the THP group in 198 followed by oxidation led to the enal 201 (Scheme 12.49). Trauner and coworkers [96] then elegandy employed an intramolecular Diels-Alder cycloaddition followed by a retro-hetero Diels-Alder cycloaddition to unleash ( )-pin-natal 202 and ( )-sterekunthal A 203 [95], respectively. The ( )-sterekunthal A 203 was treated with Wilkinson s catalyst for deformylation to give cyclohexadiene followed by aerobic oxidative aromatization to yield the anthrakunthone 205 (Scheme 12.50) [95b]. This endeavor further reinforced the biomimetic aspect of the oxa-[3+3] formal cycloaddition. 

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