Benzyhc systems oxidation

Typical examples are listed in Table 2.1. A few oxidations are effected by RuO but in general it is too powerful an oxidant for this purpose. The system RuCyaq. NaCl-CCy Pt anode oxidised benzyl alcohol to benzaldehyde and benzoic acid and p-anisaldehyde to p-anisic acid [24], and a wide range of primary alcohols and aldehydes were converted to carboxylic acids, secondary alcohols to ketones, l, -diols to lactones and keto acids from RuOj/aq. NaCl pH 4/Na(H3PO )/Pt electrodes (Tables 2.1-2.4). The system [RuO ] "/aq. K3(S303)/Adogen /CH3Cl3 oxidised benzyhc alcohols to aldehydes [30]. The oxidation catalyst TPAP (( Pr N)[RuO ]) (cf. 1.3.4) is extremely useful as an oxidant of primary alcohols to aldehydes and secondary alcohols to ketones without... 

The mechanism of the reaction was not studied, but a catalytic cycle similar to that in Scheme 15.6a is likely. Subsequently, there have been numerous other reports of nitroxyl/NOj catalytic systems for aerobic alcohol oxidation [30], including the chemoselective oxidation of primary over secondary ahphatic alcohols [31], and application to the oxidation of hgnin, in which secondary benzyhc alcohols are oxidized in preference to primary aliphatic alcohols [32]. 

Pd-catalyzed benzylation shares some fundamental features with Pd-catalyzed allylation. However, it is less comphcated and generally more favorable than allylation, even though oxidative addition of benzyhc electrophiles with Pd is kinetically less favorable than that of allylic electrophiles. Much of these differences between benzyl and allyl may be attributable to the fact that the li,y rr-bond in benzyl is part of an aromatic ring system and is hence less reactive toward Pd than that in allyl. Some fundamental features of the ben-zylic reagents in Pd-catalyzed cross-coupling are summarized in Table 6. 

In a follow-up study, the same group applied a similar approach to another tandem biocatalyst system. Therein, activated methylene groups (benzyhc positions) were transformed into the corresponding achiral ketones by double oxidation (Scheme 3.8) [27]. 

The results obtained in the oxidation of representative primary and secondary aliphatic alcohols and allylic and benzyhc alcohols using this system are shown in Tables 1 and 2. 

However, these methods suffer from low activities and/or narrow scope. Uemura and coworkers reported an inproved procedure involving the use of Pd(OAc>2 (5m%) in combination with pyridine (20m%) and 3A molecular sieves (500 mg per mmol of substrate) in toluene at 80°C. This system catalyzed the smooth aerobic oxidation of primary and secondary aliphatic alcohols to the corresponding aldehydes and ketones, respectively, in addition to benzyhc and allyhc alcohols. Representative examples are summarized in Table 6. 1,4- and 1,5-Diols afforded the corresponding lactones. 

SemmeUiack et al. [104] reported that the combination of CuCl and 4-hydroxy TEMPO catalyzes the aerobic oxidation of alcohols. However, the scope was limited to active benzyhc and allylic alcohols and activities were low (10 mol% of catalyst was needed for smooth reaction). They proposed that the copper catalyzes the reoxidation of TEMPO to the oxoammonium cation. Based on our results with the Ru/TEMPO system we doubted the validity of this mechanism. Hence, we subjected the Cu/ TEMPO to the same mechanistic studies described above for the Ru/TEMPO system [105]. The results of stoichiometric experiments under anaerobic conditions, Hammett correlations and kinetic isotope effect studies showed a similar pattern to those with the Ru/TEMPO system, i.e., they are inconsistent with a mechanism involving an oxoammonium species as the active oxidant. Hence, we propose the mechanism shown in Scheme 4.18 for Cu /TEM PO-catalyzed aerobic oxidation of alcohols. 

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