RuCl2 3, alcohol oxidant

Figure 9.4 Aerobic oxidation of primary and secondary alcohols catalyzed by RuCl2 (Ph3P)3/TEMPO in PhCl at 100°C.

In the presence of a [RuCl2(PPh3)3] catalyst N-methylmorphoHne-N-oxide (MMO) reacts with alcohols in dichloroethane or 1,2-dichloroethylene to afford mostly aldehydes together with carboxylic acids. Instead of the rather expensive MMO as reagent, a combination of N-methylmorphoHne and aqueous H2O2 (35 w%) could be used with similar results for the oxidation of long chain alcohols (1-octanol to 1-hexadecanol) [16]. At the end of the reaction the aqueous phase, containing the mthenium catalyst and methylmorphoHne could be recycled with no apparent loss of activity. 

In another example, RuCl2(PPh3)3 in [BMIMjCl was shown to be a much less active catalyst than this complex in the ionic liquids [R4N]C1 or [R4N]OH for the aerobic oxidation of alcohol (199). The weaker association of the bulky tetraalkyl-ammonium cation than the planar [BMIM] cation with the anion appears to be the principal reason for the difference. 

The effect of cations and anions of several ionic liquids was investigated for the performance of several catalyst precursors, including RuCh and RuCl2(PPh3)3, for aerobic oxidation of alcohols 199).The catalysts in the neutral ionic liquids... 

RuCl2(picphen)]Cl (picphen=few(picolinaldehyde)-(9-phenylenedi-imine) is made by condensation of picolinaldehyde and o-phenylenediamine) and the resulting Schiff base then treated with RuClj in ethanol. Kinetics were followed of the oxidation of secondary alcohols (benzhydrol, 1-phenylethanol and a-tetralol) to the corresponding ketones by [RuCl2(picphen)] "/NMO or Tl(OAc)3/water/30°C. The intermediacy of a Ru(V) oxo species was suggested [800]. 

Aerobic oxidation of primary alcohols to aldehydes and secondary alcohols to ketones was accomplished in ionic liquids (bmim, l-butyl-3-methyl-imidazolium cation) as RuCl2(PPh3)j/(bmim)V80°C RuClj or [RuCl Cp-cymene)] were also used... 

It is noteworthy that allylic alcohols are oxidized to products with retained configuration of the olefinic bond. Geraniol and nerol were oxidized to the corresponding ( )-and (Z)-a-enals, respectively. As expected, primary alcohols were oxidized faster than secondary ones with the RuCl2(PPh3)3/BTSP system with relative rates from 20-40 1. The new system was applicable for the selective oxidation of primary-secondary diols... 

Silicon and gennaninm peroxides TABLE 8. Oxidation of alcohols with BTSP in the presence of RuCl2(PPh3)3... 

Quinolines can be prepared from the oxidative coupling and cyclation of the 2-aminobenzyl alcohol and ketones (Scheme 4) catalyzed by the system [RhCl(PPh3)3]/KOH [58]. The reactions were carried out in dioxane at 80 °C with 85% yield in 24 h (alcohol/Rh = 100/1). However, better yields are obtained with the related ruthenium system [RuCl2(=CHPh)(PCy3)2 [59]. 

The aerobic oxidation of alcohols mediated by TEMPO, used in combination with other catalysts, such as CuBr Me2S,31 RuCl2(PPh3)332 or the enzyme laccase,33 must also be mentioned. 

In 1981, Oshima et al.21 reported that stoichiometric RuCl2(PPh3)3 in benzene shows a remarkable selectivity for the oxidation of primary alcohols in the presence of secondary ones. This was confirmed by other researchers in the preparation of complex organic compounds.22... 

Selective oxidations with stoichiometric RuCl2(PPh3)3 are normally carried out simply by stirring a solution of the alcohol in benzene at room temperature in the presence of the oxidant. The addition of 2 equivalents of K2C03 may improve the reaction.228... 

IOB alone can oxidize some alcohols, but catalysed oxidations are much more efficient. Thus, in the presence of RuCl2(PPh3)2 primary aliphatic alcohols were oxidized cleanly to aldehydes, at room temperature the use of m-iodosylbenzoic acid instead of IOB considerably increased the yields for example, hexanal was formed from hexanol quantitatively (by GC) [19], Another catalytic system involved the use of simple lanthanide salts such as ytterbium triacetate [20]. Cyclic y-stannyl alcohols, readily available from cyclic vinyl ketones and Bu3SnLi, underwent oxidation accompanied by carbon-carbon bond cleavage (Grab fragmentation), when treated with IOB.BF3 and DCC. The products were unsaturated aldehydes or ketones. 

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