Benzyhc systems

The Sn2 rates for allylic and benzyhc systems are also increased (see Table 10.3), probably owing to resonance possibilities in the transition state. Evidence for this in benzylic systems is that the rate of the reaction was 8000 times slower than the rate with (PhCH2)2SEt". The cyclic 86 does not have the proper geometry for conjugation in the transition state. 

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... 

Besides hydroxylation of exogeneous phenol to the corresponding catechol, some imine and xylene-based Ugand systems have been shown to undergo (partial) hydroxylation of a central benzene-ring to yield a phenolate fragment in an intramolecular hydroxylation reaction [179,183-189]. Somewhat related reactivity was observed in the intramolecular hydroxylation at a benzyhc position of a sidearm on a specific aminopyridine ligand (compare Sect. 4.1). 

Nakata et al., 1996, 1999). TTie agreement between the theoretical dihedral angles 0caic and the empirical 0expi of the twisted benzyhc cations confirms that the observed decrease in the r value should be ascribed to a loss of resonance interaction caused by deviation from coplanarity of the carbocation centre and the benzene rr-system. 

Vanadium pentafluoride replaces benzyhc hydrogen by fluorine but also adds fluorine to the aromatic system, giving fluonnated cyclohexadienes and cyclohexenes [5] (equation 5)... 

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]. 

Several benzyhc cations have been obtained in solution as SbFg salts. Diaryl-methyl and triaryhnethyl cations are still more stable. Aryhnethyl cations are further stabihzed if they have electron-donating substituents in ortho or para positions. The stabihty of such cations can be further increased if electron-donating substituents feed into the it-system. Triaryknethyl cations are propeUer-shaped, even though the central carbon atom and the three ring carbons connected to it are in a plane. The three benzene rings cannot be all in the same plane because of steric hindrance. 

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. 

The Ritter-type reaction of adamantane is accomplished using the NHPI/NO system. In this section, we show that NHPI combined with cerium ammonium nitrate (CAN) serves as an efficient system for the generation of both PINO from NHPI and carbocations from alkyl radicals. Thus, benzyhc compounds first undergo the amidation with alkyl nitrile under mild conditions to form amides in good yields. The reaction of ethylbenzene in the presence of CAN and NHPI in EtCN under argon at 80 °C for 6h produced N-(l-phenylethyl)propionamide in 84% yield at 61% conversion (Eq. (6.32)). The NHPI/CAN system can apply to the Ritter-type reaction of various alkylbenzenes and adamanatanes. 

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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