Sequential dehydrogenation-oxidative

Scheme 3.8 The one-pot process of sequential dehydrogenation-oxidative Heck cyclization.

It was clearly important to drive adduct formation to completion any unreacted starting material 32 was not rejected in isolation of the product 33 and the result was an impurity in the finasteride product. Excess DDQ was required to ensure complete conversion of 32, but any DDQ remaining after adduct formation produced over-oxidation products in the thermolysis. The three over-oxidation products shown in Figure 3.4 were identified. Sequential dehydrogenation in the B-ring... 

This is a multistep reaction, which probably proceeds through a sequential dehydrogenation that can produce several intermediates, as indicated in the general scheme proposed by Bagotzky and coworkers [86]. Details of the dehydrogenation of methanol on Pt have not been established. From kinetic isotope measurements of CD3OH and CH3OH oxidation, it was shown that the first removal of H in the reaction... 

In 2013, Hong and coworkers reported a one-pot protocol to achieve similar structures by a sequential Pd(II)-catalyzed dehydrogenation-oxidative Heck-cyclization process [23]. Using 20 mol% Pd(TFA)2, 1.0 equiv. of Cu(OAc)2, the cyclohexanones could easily react with electron-deficient olefins to afford coumarins in PivOH under an O2 atmosphere with modest to high yields (Scheme 3.8). It is noted that the product scope is somewhat extended to the coumarins with electron-withdrawing groups on the aromatic rings. In addition, the obtained coumarin products could be further alkenylated to provide an extended it-system. 

NADH and FADHg are produced as a result of substrate level dehydrogenations. Oxidation of these reduced coenzymes by oxygen is accomplished by the intervention of a series of electron carriers between the primary reductant and the terminal oxidant (Fig. 2). The electron-transport components represent redox couples of increasing redox potential and are therefore favored thermodynamically. The respiratory chain can be separated into four multienzyme complexes NADH-Q reductase (complex I), succinate-Q reductase (complex II), QH2"Cytochrome c reductase (complex III), and cytochrome c oxidase (complex IV). At each of these successive oxidation-reduction steps, a certain amount of free energy is available, the amount being determined by the difference in the oxidation-reduction potential of the two sequential components. The difference in the redox potential between... 

Dimethylhydrazine is metabolized by a sequence of oxidation steps, first dehydrogenation to azomethane, A -oxidation of this to azoxymethane and finally a C-oxidation to methylazoxymethanol (Fiala, 1975, 1977). This last metabolite decomposes to give the highly reactive methyldiazonium ion to which the carcinogenicity of the compound has been attributed. The sequential nature of these oxidation steps has been shown in the isolated perfused rat liver (Wolter Frank, 1982). Fiala (1977) showed that the C-oxidation of azoxymethane to methylazoxymethanol is catalysed by hepatic microsomes, while Schoental (1973) found that methylazoxymethanol was converted to the corresponding aldehyde by an NAD-dependent dehydrogenase. 

In some cases a whole series of dehydrogenation reactions may proceed sequentially to yield aromatic or highly conjugated products. An example of this is seen in the aerial oxidation of the nickel(n) complex of the macrocycle formed by the template condensation of biacetyl bishydrazone with formaldehyde. The product of the oxidation is the fully aromatic dianionic macrocyclic complex (Fig. 9-24). 

An alternative approach is to combine the two steps, that is, the oxidative dehydrogenation of propane to propene and the epoxidation of propene to PO, in a single reactor, with two sequential catalytic beds, but with similar reaction conditions for the two steps [45b]. The problem is that most of the active and selective catalysts in propane ODH operate at temperatures that are too high for propene epoxidation. 

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