Oxidative activation competition experiments

The background for the development of VK69 was a need for reduction of S02 emissions from double-absorption plants by installing a more active catalyst at low temperature downstream from the intermediate absorption tower. Clearly, the catalytic solution should be more competitive than the alternatives, e.g. tail gas scrubbing or triple-absorption layout, in terms of capital and operating costs. In the following, the required technical performance of the catalyst with respect to S02 oxidation activity, mechanical strength and pressure drop is discussed, and input from the literature and from practical experience in the field is presented. Reviews of the extensive literature published on sulphuric acid catalysts can be found in [2-5],... 

Recently, Sen has reported two catalytic systems, one heterogeneous and the other homogeneous, which simultaneously activate dioxygen and alkane C-H bonds, resulting in direct oxidations of alkanes. In the first system, metallic palladium was found to catalyze the oxidation of methane and ethane by dioxygen in aqueous medium at 70-110 °C in the presence of carbon monoxide [40]. In aqueous medium, formic acid was the observed oxidation product from methane while acetic acid, together with some formic acid, was formed from ethane [40 a]. No alkane oxidation was observed in the absence of added carbon monoxide. The essential role of carbon monoxide in achieving difficult alkane oxidation was shown by a competition experiment between ethane and ethanol, both in the presence and absence of carbon monoxide. In the absence of added carbon monoxide, only ethanol was oxidized. When carbon monoxide was added, almost half of the products were derived from ethane. Thus, the more inert ethane was oxidized only in the presence of added carbon monoxide. 

Experimental verification of the CMD mechanism was pubUshed in 2012 by Ozawa and coworkers, who isolated a single crystal of complex 42 suitable for X-ray analysis and studied the catalytic activity towards C H activation. In solution, this dimeric species disproportionates into two molecules of 43, which is the active catalytic species. They used this species under catalytic conditions to compare the rates of C-H activation of 2-methylthiophene in a competition experiment with half an equivalent of bromobenzene and 2-bromo-l,3-dime-thylbenzene (Scheme 19.7). Oxidative addition, not C H activation, was determined to be the rate-limiting step since coupling with bromobenzene took place preferentially. ... 

Typical evidence that Ebonex functions as a non-active anode is that when Ebonex was compared with different anodes for oxidations of acetaminophen (AP) and p-benzoquinone (BQ), Ebonex behaved much more like BDD (non-active) than like Ti/Ir02-Ta205 (active) in terms of the low yield of BQ formed from AP, and in the rates of transformation versus complete mineralization [5, 6]. The oxidation of coumarin to the fluorescent product, 7-hydroxycoumarin, is also a useful qualitative probe for the presence of hydroxyl species in these oxidations [7, 8]. The activity of Ebonex is consistently less than that of BDD, suggesting a lower concentration of hydroxyl species. In competition experiments... 

The observation that the C-O cleavage reaction of ethyl aryl ethers occurs without the need to block ortho C-H activation suggests that the 1,2-dehydroaryloxylation is a more facile reaction than the ether C-O oxidative addition. Accordingly a competition experiment, in which a mixture of 4-methoxy-2,3,5,6-tetrafluorotoluene and 4-ethoxy-2,3,5,6-tetrafluorotoluene was added to (PCP)Ir(TBE), resulted in exclusive formation of (PCP)Ir(H)(OAr) and (PCP)Ir(ethylene) (the products obtained from C-O cleavage of the ethyl ether), and no evidence of any reaction of the methyl ether (Scheme 4.12). 

Arylation with bromobenzene of the activated pyridine is then carried out in the presence of palladium acetate (5 mol %), tri-r-butylphosphine (15 mol%), and potassium carbonate under anhydrous conditions. The arylated. V-iminopyridinium ylide is afforded in 80% yield as a sole isomer and it can be reduced efficiently to the corresponding pyridine under known conditions. Interestingly, the competition experiment to compare the difference of reactivity between pyridine iV-oxide and the V-iminopyridinium ylide shows that pyridine iV-oxide is not arylated, while 56% of arylated V-iminopyridinium ylide is obtained. In addition, bis-arylation is observed in less than 10% yield. These... 

The aerobic dehydrogenative annulation of 2-aryl-substituted pyrroles and indoles for a variety of alkynes, using the system ruthenium(Il) catalyst with oxidant Cu(0Ac)2.H20, was then reported. The reaction was now performed under ambient air as the ideal sacrificial oxidant, thus only 10 mol% of Cu (0Ac)2.H20 could be used for efficient transformations of indoles [(Eq. 89)] [178]. This method could also be applied to synthesize pyrrolo[2,l-a]isoquinolines from 2-arylpyrroles with dialkyl-, diaryl-, or alkylarylacetylenes with an excellent regioselectivity. The competition experiments showed that an electron-deficient alkyne favours this reaction and that the more acidic C-H bond activation is favoured [(Eq. 89)] [178]. 

A range of studies have now appeared modeling complete catalytic cycles based on an initial C-H bond activation and the subsequent functionalization steps. The functionalization processes involve reactions with an insertion partner (an alkyne, an alkene, or a carbene/nitrene source) and lead to the formation of new heterocychc rings or direct replacement of the C—H bond with a new C-R group. In many cases, the focus of the computational studies is on these functionalization processes as well as the reoxidation steps (particularly with internal oxidants). The C-H activation in these catalytic processes is typically of the intramolecular AML A-6 type, although exceptions do exist, as will be detailed later. Input from parallel experimental studies, including the determination of kfj/ko KIEs, the observation of H/D exchange, competition experiments, and the isolation of key intermediates both complement and provide benchmark data... 

Soon after our initial experiments, we realized that we had something unusual, with potentially important opportunities. We were able to obtain some foreign patents on the biimidazoles and were concerned that these might tip off others in this field, for there was considerable activity, as can be seen from a listing of biimidazole work as reported in Chemical Abstracts. By 1964, we filed our first application on the leucodye oxidation. By then, we had a sufficient head start on this chemistry that we became less concerned about competitive activities. 

---