Primary alcohols efficient, aerobic, catalytic oxidation

Efficient, Aerobic, Catalytic Oxidation of Primary Alcohols... 

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. 

In situ generated NHC-Cu-TEMPO complex 34 [eqn (12.3)] was explored in the aerobic oxidation of primary alcohols to the corresponding aldehydes. " Surprisingly, the addition of a base (potassium tert-butoxide, triethylamine) had a detrimental effect on the catalytic reaction using 34 as catalyst. This fact contrasted with other reported Cu-N-based ligands/TEMPO catalytic systems, in which the addition of base favored the reaction. The reaction proceeded more efficiently in chlorobenzene than in other solvents. Unexpectedly, [(NHC)CuX] complexes, prepared in situ from Cu powder and the corresponding imidazolium salt, or their simple combination with TEMPO, were completely inactive in the oxidation of alcohols. It was proposed that TEMPO anchored to Cu-NHC complexes facilitated the intramolecular proton abstraction, promoting the oxidation of alcohols. However, the mechanism was not further explored, and the relation between structure and activity of the catalyst remained unclear. 

Efforts have been made since then to develop new catalytic protocols for the oxidation of alcohols with molecular oxygen as the sole oxidant in water. For instance, the water-soluble Pd(II)-biquinoUne 57 was demonstrated to be efficient for aerobic oxidation of primary and secondary alcohols in water [154]. With a catalyst loading of 1 mol%, secondary alcohols led to ketones in high yields (85-100%), while aliphatic primary alcohols were fully oxidized to the corresponding acids, and benzyl alcohols were transformed to pure benzoic acid or benzaldehydes with a relatively low amount of acids formed. 

Several catalytic systems involving two or more sequential catalytic cycles and utilizing hypervalent iodine species as catalysts have been developed. An efficient, catalytic aerobic oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones by using catalytic amounts of iodylbenzene, bromine and sodium nitrite in water (Scheme 4.55) has been reported by Liu and coworkers [86]. 

Sheldon and Arends found that the combination RuCl2(PPh3)3-TEMPO affords an efficient catalytic system for the aerobic oxidation of a broad range of primary and secondary alcohols at 100 °C, giving the corresponding aldehydes and ketones, respectively, in >99% selectivity in all cases [86]. The reoxidation of the ruthenium hydride species with TEMPO was proposed in the latter system [86c[. Allylic alcohols can be converted into a,(3-unsaturated aldehydes with 1 atm of molecular oxygen in the presence of RUO2 catalyst [87]. 

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