Oxidative transformations, benzene

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

In the presence of oxygen, however, intermediate 94 may further react to form a peroxy radical (97), which is oxidized to hydroquinone749,750 (Scheme 9.20). Each intermediate may be transformed to phenol as well. Selective formation of either phenol or hydroquinone was achieved by running the oxidation at proper pH (phenol hydroquinone = 71 29 at pH = 1.3, and 14 86 at pH = 3.5).750 A direct route to hydroquinone was also recognized when the oxidation of benzene was performed in a H2S04—CH3CN—HzO solution.751... 

The discovery of an intermediate, benzene-cis-dihydrodiol, from the oxidation of benzene to C02 in a mutant of Pseudomonas putida (Gibson, 1968) led to a big effort to find corresponding new transformations and to utilize the diol products for other syntheses. 

Secondary amines can be converted into the corresponding imines, in a highly efficient single step, upon treatment with 2 equiv. of t-BuOOH in benzene in the presence of RuCl2(PPh3)3 catalyst at room temperature [134]. This is the first catalytic oxidative transformation of secondary amines to imines, which are hardly accessible by conventional methods. A 4A molecular sieve is needed to prevent the hydrolysis of product imines in some cases. The oxidations of tetrahydroisoquinoline 61 and allylamine 63 gave the corresponding cyclic imine 62 and azadiene 64 in 98% and 80% yields, respectively (Eqs. 3.75 and 3.76). 

A few years later, Hubert Mimoun discovered the rather selective oxidation of benzene by a few vanadium(V) peroxo complexes [10], Using an excess of hydrogen peroxide under phase-transfer conditions transformed this stoichiometric reaction into a true catalytic process, but the turnover numbers remained very low [11]. 

The direct oxidation of benzene into phenol constitutes one of the challenges in chemistry to substitute the cumene process at the industrial level. Such oxidation has also been achieved with several TpfCu complexes as catalysts, leading to moderate yields and high selectivity toward phenol, in a transformation using hydrogen peroxide as the oxidant and at moderate temperatures. The same catalytic system has been employed for the selective oxidation of anthracenes into anthraquinones (Scheme 24). 

Use of the chiral pool typically requires a series of subsequent transformations to achieve the substitution pattern desired and sometimes may be limited by the availability of only one enantiomer. Microbial oxidations of benzene derivatives have provided an excellent route to cyclohexadienediols in enantiomerically pure form. Although this provides only one enantiomer, synthetic methods have been devised to circumvent this problem [36]. Far fewer methods exist for the enantioselective synthesis of cycloheptenes for which there exists no reaction analagous to the Diels-Alder process [37,38,39,40,41,42]. The enantioselective hydroalumination route to dihydronapthalenols may prove to be particularly important. Only one other method has been reported for the enantioselective synthesis of these compounds microbial oxidation of dihydronaphthalene by P. putida generates the dihydronaphthalenol in >95% ee and 60% yield... 

In order to simplify as far as possible the very complex data dealing with the oxidation of benzene and its various derivatives, the phenomena will be considered in the following order (a) oxidations involving the formation of diphenyl and its derivatives (b) oxidations involving the formation of phenols and quinones (c) oxidations accompanied by a break-down in the ring structure and (d) oxidations which primarily involve the side chains present in the homologs of benzene and their respective phenolic or other derivatives. In the case of each of these principal types of oxidation, it has been deemed advisable to review briefly the main facts which have been established as the result of investigations which have been carried out in the liquid phase. In this way, it is hoped that the important chemical relationships of the substances whose transformations are to be considered may be kept well in mind. 

A facile and efficient synthesis of lactols 363 via an oxidative rearrangement reaction of 2,3-epoxy alcohols 362 with [bis(trifluoroacetoxy)iodo]benzene has been reported (Scheme 3.145) [463-465], This hypervalent-iodine-induced oxidative transformation has been utilized in the synthesis of several lactones and in the asymmetric synthesis of the marine y-lactone metabolite (+)-tanikolide [463,464]. 

Iglesias-Arteaga and coworkers have reported several (diacetoxyiodo)benzene-promoted oxidative transformations of steroidal substrates (Schemes 3.148 and 3.149) [468 71]. In particular, the treatment of (25f )-3a-acetoxy-5p-spirostan-23-one (370) with (diacetoxyiodo)benzene in basic methanol leads to F-ring contraction via Favorskii rearrangement to afford product 371 (Scheme 3.148) [468],... 

The mechanism (Scheme 5) of the oxidative C-H transformation of allyl arenes to alkenyl aldehydes by 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (37) and catalysed by FeCl2 in C2H4CI2 had the initial reaction in which (37) oxidized allyl benzene (38), or [(Z)-prop-l-enyl]benzene (39), assisted by FeCl2, through a singleelectron transfer to the corresponding allyl radical (40) and Fe(IIl) complex (41). Next, (41) oxidized (40) to the allyl cation (42) and was reduced to Fe(I) complex... 

Figure 4.29. (a) The speculated reaction path for the oxidative transformation of benzene to phenol in the Panov reaction , (b) Rates of phenol formation and N2O decomposition as a function of Fe content in ZSM-5... 

ZeoUte-supported Re clusters prepared by chemical vapor deposition (CVD) of CH ReO onto zeohte HZSM-5 catalyzed efficiently the gas-phase oxidation of benzene with in the presence of ammonia [68]. Phenol selectivities of 91.6-93.9% at 1.7-9.9% conversions and 82.4— 87.7% at 0.8-5.8% conversions were achieved in pulse reactions and steady-state reactions, respectively. The process involves structural transformation between Re ji clusters and Re monomers (Scheme 14.11). EXAFS confirmed the formation of the Re clusters after catalyst treatment with NH, [68]. 

Recently, we have discovered [8], for the first time, that ferrocene (catalyst 1.1) is an efficient (pre)catalyst for several types of oxidative transformations, namely, the oxidation of alkanes and benzene by H2O2 or tert-butyl hydroperoxide. The oxidation of gaseous and liquid alkanes to alkyl hydroperoxides by H2O2 proceeds in MeCN at 50 °C. An obligatory cocatalyst is pyrazine-2-carboxylic acid (PCA, or Hpca, where H is a proton and pea is the anion of PCA). In the cyclohexane oxidation, the yield and TON after 1.5 h attained 32% and 1200, respectively. In the ethane oxidation, TON reached 970. Maximum yield (58% based on the alkane) was obtained for the n-butane oxidation after 4 h. 

Another typical example for the direct oxidative transformation of methyl arenes were developed by Wang and co-workers [118]. In this palladium catalyzed reaction, tert-butyl nitrite (TBN) was employed as both nitrogen source and oxidant N-hydroxyphthalimide (NHPI) was used as precursor of the active phthalimide N-oxyl (PINO) radical, which initiates the reaction to give benzylic radical A by grabbing hydrogen atom from the substrate. In the interaction between TBN and NHPI, TBN decomposes to NO radical and tert-butyl alcohol. NO radical would trap benzylic radical A to give nitrosomethyl benzene B, which isomerizes to aldoxime C [119]. Finally, nitrile product would be generated fitom C by palladium catalysis (Scheme 4.21). 

The oxidation of organic compounds by manganese dioxide has recently been reviewed. It is of limited application for the introduction of double bonds, but the advantages of mildness and simple workup make it attractive for some laboratory-scale transformations. Manganese dioxide is similar to chloranil in that it will oxidize A -3-ketones to A -dienones in refluxing benzene. Unfortunately, this reaction does not normally go to completion, and the separation of product from starting material is difficult. However, Sondheimer found that A -3-alcohols are converted into A -3-ketones, and in this instance separation is easier, but conversions are only 30%. (cf. Harrison s report that manganese dioxide in DMF or pyridine at room temperature very slowly converts A -3-alcohols to A -3-ketones.)... 

UV irradiation of a mixture of hexafluorobenzene in the presence of oxygen gives Dewar benzene oxide also as a minor product, which undergoes thermal transformation to hexafluorocyclohexa-2,4-dienone [J46] (equation 36)... 

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