Heteroatoms oxidative reactions

Several types of flavoprotein monooxygenases exist. One group catalyzes electrophilic aromatic substitution or heteroatom oxidation reactions, whereas the other group catalyzes Baeyer-Villiger-type oxidations of ketones (Fig. 2) (13, 17). 

With this brief preamble on the more important current theoretical results for the general structural and electronic characteristics of dioxiranes, we shall now examine the computed transition structures of the oxygen transfer in epoxidations, heteroatom oxidations and CFI insertions. Since each reaction type exhibits its individual mechanistic features, these oxyfunctionalizations shall be presented separately. 

Niobia-supported MTO has been prepared either by the deposition of sublimed MTO onto the support, or by the impregnation of the support by a solution of MTO, and has been well characterised [54]. A large variety of oxidation reactions were efficiently performed with niobia-supported MTO, such as olefin metathesis catalysis [53,54], reactions of ethyl diazoacetate, heteroatom oxidation (amine and phosphine oxidations) and olefin epoxidation with hydrogen peroxide [55] (Scheme 13). 

The mono-oxygenases which catalyse a series of oxidations such as hydroxylation, epoxidation, heteroatom oxidation and Baeyer-Villiger oxidation (Figure 2.24), depend on NADH or NADPH and cofactors usually Fe or Cu. A particularly important reaction is the direct incorporation of molecular oxygen into non-activated carbon centres, such as in synthesis of important steroidal drags by microbial 11 dr-hydroxylation of... 

Figure 2.24 Reactions catalysed by mono-oxygenases, hydroxylation of carbon centres, aromatic hydroxylation, epoxidation of alkenes, heteroatom oxidation and Baeyer-Villiger oxidation of a ketone.

Reaction of nucleophiles with the polarized N=C bond of azines proceeds via dearomatization and formation of the corresponding 1,2-adduct. With alkyllithiums, for example, it is possible to isolate the dihydro products by careful neutralization of the reaction mixtures these are, in general, rather unstable, however, and can easily be reoxidized to the fully aromatic compounds (Scheme 4). The dihydro adducts formed in these direct nucleophilic addition reactions can also be utilized for the introduction of substituent groups /3 to the heteroatom. Thus, reaction of (35) with one of a number of electrophiles, followed by oxidation of the intermediate dihydro product, constitutes a simple and, in many cases, effective method for the introduction of substituent groups at both the 2- and 5-positions of the pyridine ring (Scheme 4). Use of LAH in this sequence, of course, results in the formation of 3-substituted pyridines. 

Reaction of tricoordinated phosphorus compounds with heteroatomic oxidizing agents... 

The chemoselectivity, regioselectivity, diastereoselectivity, and enantioselectivity of heteroatom oxidations, epoxidations and CH insertions by dioxiranes have been reviewed.26 The selective ring-opening reactions of epoxides at high pressures have been reviewed (in Japanese).27... 

Comparison of the stereochemical outcomes in entries 2 and 3 of Table 1 implies that the heteroatom functionality directs the oxidation reaction more effectively from pseu-doequatorial orientation (the O—C—C=C dihedral angle is 140°). Selective formation of yyw-epoxides is also known for olefins having carbamate18, acetal19, ether20 and halogen groups21 in allylic positions. There are many cases in the literature where the epoxidation... 

As our last examples of oxidation reactions of heteroatoms, we consider the reactions depicted in Figure 17.40. These reactions are oxidations and, in contrast to the reactions in Figures 17.34-17.38, these oxidations also are condensation reactions, since the oxidizing reagent remains in the product. The mechanistic details outlined in Figure 17.40 are so familiar by now that no further explanation is needed. 

FAD ready for a subsequent catalytic cycle. This enzyme catalyzed Baeyer-Villiger oxidation bears great resemblance to the analogous chemical reaction performed by peroxides or peracids, which act as nucleophiles. Globally these flavin-enzymes can perform the same reactions as peracids, i.e. epoxidations, Baeyer-Villiger-reactions and nucleophilic heteroatom oxidation [26-28]. 

Scheme 1 Typical reactions catalyzed by cytochromes P450 (a) hydroxylation of saturated hydrocarbons (b) heteroatom oxidation and dealkylation (c) olefin epoxidation (d) aromatic hydroxylation (e) desaturation (f) aldehyde deformylation.

---