Nucleophilic oxygen

The structural features especially the very polar nature of the carbonyl group point clearly to the kind of chemistry we will see for aldehydes and ketones in this chapter The partially positive carbon of C=0 has carbocation character and is electrophilic The planar arrangement of its bonds make this carbon relatively uncrowded and susceptible to attack by nucleophiles Oxygen is partially negative and weakly basic... 

SECTION 8.8. ACYLATION OF NUCLEOPHILIC OXYGEN AND NITROGEN GROUPS... 

The synthesis of a large number of y-pyrones and y-pyranols from enamines has been brought about through the use of a wide variety of bifunctional molecules. These molecules include phenolic aldehydes (126,127), phenolic Mannich bases (128), ketal esters (129), and diketene (120-132). All of these molecules have an electrophilic carbonyl group and a nucleophilic oxygen center in relative 1,4 positions. This is illustrated by the reaction between salicylaldehyde (101) and the morpholine enamine of cyclohexanone to give pyranol 102 in a quantitative yield (127). 

The intramolecular Michael addition11 of a nucleophilic oxygen to an a,/ -unsaturated ester constitutes an attractive alternative strategy for the synthesis of the pyran nucleus, a strategy that could conceivably be applied to the brevetoxin problem (see Scheme 2). For example, treatment of hydroxy a,/ -unsaturated ester 9 with sodium hydride furnishes an alkoxide ion that induces ring formation by attacking the electrophilic //-carbon of the unsaturated ester moiety. This base-induced intramolecular Michael addition reaction is a reversible process, and it ultimately affords the thermodynamically most stable product 10 (92% yield). 

In the following transformations, the nucleophilic oxygen of the sulfoxide group plays a... 

A few of the reactions carried out by the monooxygenase system of methanotrophic bacteria are summarized in Figure 2.9, and it is on account of this that methylotrophs have received attention for their technological potential (Lidstrom and Stirling 1990). An equally wide metabolic potential has also been demonstrated for cyclohexane monooxygenase, which has been shown to accomplish two broad types of reaction one in which formally nucleophilic oxygen reacts with the substrate, and... 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

The iodocyclization products have a potentially nucleophilic oxygen substituent (3 to the iodide, which makes them useful in stereospecific syntheses of epoxides and diols. 

Second, metabolism of 6-fluoroBP by rat liver microsomes yields the same BP quinones obtained in the metabolism of BP (23). This suggests that these products are formed by an initial attack of a nucleophilic oxygen atom at C-6 in the 6-fluoroBP radical cation with displacement of the fluoro atom. In fact, when 6-fluoroBP is treated with the one-electron oxidant Mn(0Ac)3, the major products obtained are 6-acetoxyBP and a mixture of 1,6- and 3,6-diacetoxyBP (15), indicating that reaction occurs via an initial attack of acetate ion at C-6 of the 6-fluoroBP radical cation. On the other hand electrophilic substitution of 6-fluoroBP with bromine or deuterium ion shows no displacement of fluorine at C-6, although in both cases substitution occurs at C-l and/or C-3. These results indicate that... 

We propose that the first step in the formation of quinones, as shown in Scheme 3 for BP, involves an electron transfer from the hydrocarbon to the activated cytochrome P-450-iron-oxygen complex. The generate nucleophilic oxygen atom of this complex would react at C-6 of BP in which the positive charge is appreciably localized. The 6-oxy-BP radical formed would then dissociate to leave the iron of cytochrome P-450 in the normal ferric state. Autoxidation of the 6-oxy-BP radical in which the spin density is localized mainly on the oxygen, C-l, C-3 and C-12 (19,20) would produce the three BP diones. 

Sulfation in most aspects is very similar to phosphorylation, except that sulfation is not involved in intracellular signal transduction, but in other forms of signaling. The mechanism of sulfation is similar to that of phosphorylation as a general base from the enzyme active site that deprotonates the hydroxyl groups of tyrosine residues. The nucleophilic oxygen then attacks the /3-position, in contrast to the 7-position in phosphorylation, and releases adenosine 3, 5 -diphosphate. 

Like the trichloromethyl peroxide radical, peroxothio compounds can perform even nucleophilic oxygenation of substrates that are inert to Oj" in aprotic solvents. For example, stilbene is not changed in dry benzene containing 18-crown 6-ether and KOj. In the presence of diphenylsul-fide, however, the interaction takes place and results in the formation of stilbene epoxide. According to Oae et al. (1981), stilbene initially gives PhCH(00 )CH Ph anion-radical adduct. Abstraction of O from the adduct leads to stilbene epoxide with 40% yield (Oae et al. 1981). 

In Studying asymmetric oxidation of methyl p-tolyl sulfide, employing Ti(OPr-/)4 as catalyst and optically active alkyl hydroperoxides as oxidants, Adam and coworkers collected experimental evidence on the occurrence of the coordination of the sulfoxide to the metal center. Therefore, also in this case the incursion of the nucleophilic oxygen transfer as a mechanism can be invoked. The authors also used thianthrene 5-oxide as a mechanistic probe to prove the nucleophilic character of the oxidant. 

---