Reactions with electrophilic oxygen

In the propane ammoxidation a lower selectivity for acrolein plus acrylonitrile is observed. The formation of partial (amm)oxidation products from propane requires more elemental steps than their formation from propene. All these intermediates can undergo a side reaction with electrophilic oxygen species yielding degradation products. 

These compounds generally exist in carbonyl forms. The oxygen function can be converted into halogen by phosphorus halides. Reactions with electrophiles are quite complex. Thus urazole (511) reacts with diazomethane quickly to yield (512), which is more slowly converted into (513). 1-Phenylurazole gives (514) however, 4-phenylurazole yields (515). Oxadiazolinones of type (516) can be alkylated at both O- and N-atoms. 

This section will cover aspects of monohydride terminal surface reactions that were carried out under free-radical conditions. The description will be circumscribed to the reactions with molecular oxygen and monounsaturated compounds. Mechanistic information for these reactions is scarce mainly due to the complexity of the system, and mechanistic schemes are often proposed in analogy with radical chemistry of organosilane molecules. H—Si(lll) has a band gap of about 1.1 eV while the HOMO LUMO gap in (Me3Si)3SiH is within 8-11 eV and, therefore, has very important consequences for the reactions with nucleophilic and electrophilic species where frontier orbital inter-... 

Reactions with electrophiles occur more readily at the carbonyl oxygen atom than is the case for pyran-2-ones. Hydrogen chloride, for example, yields aromatic 4-hydroxypyrylium chlorides (4.2) when reacted with 2,6-disubstituted pyran-4-ones. 

Reaction of Z-a./j-unsaturated iron-acyl complexes with bases under conditions similar to those above results in exclusive 1,4-addition, rather than deprotonation, to form the extended enolate species. However, it has been demonstrated that in the presence of the highly donating solvent hexamethylphosphoramide, y-deprotonation of the -complex 6 occurs. Subsequent reaction with electrophiles provides a-alkylated products such as 736 this procedure, demonstrated only in this case, in principle allows access to the a-alkylatcd products from both Z- and it-isomers of a,/j-unsaturated iron-acyl complexes. The hexamethylphosphoramide presumably coordinates to the base and thus prevents precoordination of the base to the acyl carbonyl oxygen, which has been suggested to direct the regioselective 1,4-addition of nucleophiles to -complexes as shown (see Section 1.1.1.3.4.1.2.). These results are also consistent with preference for the cisoid conformations depicted. 

Reaction with Electrophiles, including Oxidizing Agents 5.13.3.3.1 Electrophilic interactions at the ring oxygen atom... 

The reaction of thietanes with electrophilic oxygen will be covered in Section 5.14.4.3 (Synthesis of Derivatives of the Ring Systems). 

A behavior similar to that of pyridine and pyrazine N-oxides is exhibited by complexes of pyridines with hexafluoroacetone. Such complexes are expected to enhance the acidity of the pyridine hydrogens by oxygen-lithium chelating effects and therefore direct 2-deprotonation. In fact, treatment of f-butylpyridine (495) with LiTMP/THF-Et20/- 107°C generated, via the known complex 507, the stable 2-lithio species 508 which, upon reaction with electrophiles, furnished 2-substituted pyridines 509 (Scheme 154) (83JOC4156). 

Although the methyl groups in acetylacetonate complexes retard some reactions by steric hindrance, they provide some electronic enhancement in reactions with electrophiles and furthermore protect the donor oxygen atoms from electrophilic attack. These properties have been discerned by a comparison of the numerous reactions of acetylacetone complexes with the relatively few successful reactions of complexes of formylacetone and malondialdehyde. 

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