Alcohol perruthenate-catalyzed

Practical Cr -catalyzed oxidations of alcohols have not been adopted widely, but Pr4N+ Ru04 (TPAP, tetrapropylammonium perruthenate) catalyzes the oxidation of alcohols to aldehydes by a stoichiometric oxidant such as NMO, H2O2, or O2 itself. The Ru(VII) complex oxidizes alcohols by the same mechanism described earlier for stoichiometric Cr species. The stoichiometric oxidant then reoxidizes the Ru(V) by-product back to Ru(VII). 

Indeed, only sparse attention has been paid to the mechanism of perruthenate-catalyzed alcohol oxidations. Although TRAP can act as a three-electron oxidant (Ru""" Ru ) the fact that it selectively oxidizes... 

Tab. 4.4 Perruthenate catalyzed oxidation of primary and secondary alcohols to aldehydes using...

Scheme 4.11 Aerobic alcohol oxidation catalyzed by perruthenate...

Sparse attention has been paid to the mechanism of perruthenate-catalyzed alcohol oxidations [55]. Although TPAP can act as a three-electron oxidant (Ru " Ru ) the fact that it selectively oxidizes cyclobutanol to cyclobutanone and tert-butyl phenyl-methanol to the corresponding ketone, militates against free radical intermediates and is consistent with a heterolytic, two-electron oxidation [55, 56]. Presumably, the key step involved P-hydride elimination from a high-valent, e. g., alkoxyruthenium (VII) intermediate followed by reoxidation of the lower valent rathenium by dioxygen. However, as shown in Scheme 4.12, if this involved the Ru "/Ru couple the reoxidation would require the close proximity of two ruthenium centers, which would seem unlikely in a polymer-supported catalyst A plausible alternative, which can occur at an isolated mthenium center involves the oxidation of a second molecule of alcohol, resulting in the reduction of mthenium(V) to rathenium(III), followed by reoxidation of the latter to ruthenium (VII) by dioxygen (see Scheme 4.12). 

Ley et al. reported oxidation of alcohols catalyzed by an ammonium perruthenate catalyst dissolved in [NEtJBr and [EMIM][PFg] [60]. Oxygen or N-methylmorpholine N-oxide is used as the oxidant and the authors describe easy product recovery by solvent extraction and mention the possibility of reusing the ionic catalyst solution. 

R. Lenz, S. V. Ley, Tetra-n-propyl ammonium perruthenate ( TPAP)-Catalyzed Oxidations of Alcohols using Molecular Oxygen as a Co-oxidant, J. Chem. Soc., Perkin Trans. 1, (1997) 3291-3292. 

M. Hasan, M. Musawir, P. N. Davey, I. V. Kozhevnikov Oxidation Of Primary alcohols to aldehydes with Oxygen Catalyzed by Tetra-n-propyl ammonium perruthenate,, J. Mol. Catal. A Chem. 180 (2002) 77-84. 

Polymer supported sodium ruthenate is able to catalyze the oxidation of alcohols with iodosobenzene or tetrabutylammonium periodate in CH2CI2.8 It is not clear whether the primary oxidant is ruthenate or perruthenate. 

Recently two heterogeneous TPAP-catalysts were developed, which could be recycled successfully and displayed no leaching In the first example the tetra-alkylammonium perruthenate was tethered to the internal surface of mesopor-ous silica (MCM-41) and was shown [153] to catalyze the selective aerobic oxidation of primary and secondary allylic and benzylic alcohols. Surprisingly, both cyclohexanol and cyclohexenol were unreactive although these substrates can easily be accommodated in the pores of MCM-41. The second example involves straightforward doping of methyl modified silica, denoted as ormosil, with tetra-propylammonium perruthenate via the sol-gel process [154]. A serious disadvantage of this system is the low-turnover frequency (1.0 and 1.8 h-1) observed for primary aliphatic alcohol and allylic alcohol respectively. 

The following Swern oxidation is an inexpensive, mild and fast transformation. It provides aldehydes starting from primary alcohols in the absence of water, exclusively. Other mild oxidation methods for the formation of aldehydes are known Dess-Martin periodinane (DMP), o-iodoxybenzoic acid (IBX), chromium(III) reagents, tetramethylpiperidine 7V-oxide and sodium hypochlorite (TEMPO/NaOCl), tetrapropylammonium perruthenate and N-methylmorpholine 7V-oxide (TPAP/NMO), " and palladium(II)-catalyzed oxidations are reported. ... 

Tony, K. J., Mahadevan, V., Rajaram, J., Swamy, C. S. Oxidation of secondary alcohols by N-methylmorpholine-N-oxide (NMO) catalyzed by a trans-dioxo ruthenium(VI) complex or perruthenate complex a kinetic study. React. Kinet. Catal. Lett. 1997, 62,105-116. 

Xie Y, Zhang Z, Hu S, Song J, Li W, Han B (2008) Aerobic oxidation of benzyl alcohol in supercritical CO catalyzed by perruthenate immobilized on polymer supported ionic liquid. Green Chem 10 278-282... 

A heterogeneous TPAP-catalyst was developed, which could be recycled successfully and displayed no leaching, by tethering the tetraalkylammonium perruthenate to the internal surface of mesoporous silica (MCM-41). It was shown [56] to catalyze the selective aerobic oxidation of primary and secondary allylic and benzylic alcohols (Figure 5.11). Surprisingly, both cyclohexanol and cyclohexenol were unreactive. 

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