Catalysis alkane oxidation

Catalysis of oxidation reactions will continue to be of enormous importance in the future. Areas that continue to be of active interest are the development of efficient methods for the direct epoxidation of olefins, hydroxylation and substitution of aromatics as well as the selective oxidation of alkanes. The application of methods developed for industrial chemicals to the synthesis of more complex molecules is worthy of more attention. A few examples have been discussed in the text. On the whole, however, synthetic chemists have not exploited these methods. 

Early hydrocarboxylation-hydroesterification literature deals largely with Ni and Co as activating metals, but during the last three decades the noble group VIII metals, especially Pd, Pt, Rh and Ir, have been studied. Similarly, the use of pyridine promoted Co catalysts has been optimized. This section will not include references to metals of lesser or more specialized activity, such as Fe, Ru and Cu(I), nor strong acid catalysis, nor oxidative carbonylation of alkanes. 

Besides being very active catalysts for alkane oxidation by Oj, the iron perhalopoiphyiins are also most active in the decomposition of alkyl hydroperoxides. The nature of products formed depends on the stmcture of the aliphatic substrate and can be rationalized by a catalytic pathway very efficiently generating alkyl and alkoxy radicals at low temperatures. Obviously, similar considerations apply also to the mechanism of oxidation catalysis by CoCTPPFioP-Clg) (Figure 34). 

As shown above. Fig. 7.11, many systems that activate CH bonds are now known and it is possible that if these systems can be made stable that some could be used as the basis for development of catalysts for alkane oxidation. In many cases, the CH activation rates of these reported systems are quite rapid when rates are extrapolated to temperatures of -200 °C. On the basis of these observations and assuming that stable motifs can be identified that would facilitate CH activation and functionalization, it might be assumed that acceptable catalysis rates can be readily obtained by simply basing catalyst designs on these CH activation systems. However, this is not likely to be the case and other considerations need to be taken into account... 

Reaction conditions depend on the reactants and usually involve acid or base catalysis. Examples of X include sulfate, acid sulfate, alkane- or arenesulfonate, chloride, bromide, hydroxyl, alkoxide, perchlorate, etc. RX can also be an alkyl orthoformate or alkyl carboxylate. The reaction of cycHc alkylating agents, eg, epoxides and a2iridines, with sodium or potassium salts of alkyl hydroperoxides also promotes formation of dialkyl peroxides (44,66). Olefinic alkylating agents include acycHc and cycHc olefinic hydrocarbons, vinyl and isopropenyl ethers, enamines, A[-vinylamides, vinyl sulfonates, divinyl sulfone, and a, P-unsaturated compounds, eg, methyl acrylate, mesityl oxide, acrylamide, and acrylonitrile (44,66). 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

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