Ruthenium RuCl2 3-TEMPO

Table 2 Ruthenium/2,2, 6,6 -tetramethylpiperidinyl-W-oxyl (TEMPO) catalysed oxidation of primary and secondary alcohols to the corresponding aldehyde using molecular oxygen. 15 mmol substrate, 30 ml chlorobenzene, RuCl2(PPh3)3/TEMPO ratio of 1/3, 10 ml mur1 02/N2 (8/92 v/v), P=10 bar, T=100 °C...

Besides hypochlorite, oxygen can also be used as the oxidant.9,10 Unfortunately, in contrast to homogeneous TEMPO the combination of PIPO and RuCl2(PPh3)3 in chlorobenzene10 is not able to catalyse the aerobic oxidation of octan-2-ol, probably owing to coordination of ruthenium to the polyamine. On the other hand, in combination with CuCl in DMF,9 it catalyses the aerobic oxidation of benzylalcohol to benzaldehyde within 1.5 hours (entry 2 table 4).24 The activity of PIPO is comparable to that of TEMPO (entries 4 and 5) and is superior to that of the previously described heterogeneous TEMPO systems (entries 6,7 and 8). CuCl/PIPO also catalyses the aerobic oxidation of benzylalcohol under solvent-free conditions (entry 3). 

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]. 

Polystyrene-based immobilized ruthenium catalysts can be used in oxidation of alcohols with NMO, TEMPO/O2, or O2 [94]. Catalytic oxidative cleavage of vicinal diols to aldehydes with dioxygen was reported with RuCl2(PPh3)3 on active carbon [95]. Ionic liquids such as tetramethyl ammonium hydroxide and Aliquat 336 can be used as the solvent for the RuCl2(PPh3)3-catalyzed aerobic oxidation of alcohols [96]. 

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