Oxygen arylation/oxidation

Further insight into the carbon-oxygen reductive elimination from Pt(IV) and the involvement of five-coordinate Pt(IV) intermediates has been provided recently. The first direct observation of high-yield C-0 reductive elimination from Pt(IV) was described and studied in detail (50,51). Carbon-oxygen coupling to form methyl carboxylates and methyl aryl ethers was observed upon thermolysis of the Pt(IV) complexes ( P2 )PtMe3(OR) ( P2 =bis(diphenylphosphino)ethane or o-bis(diphenyl-phosphino)benzene OR=carboxylate, aryl oxide). As shown in Scheme 47, competitive C-C reductive elimination to form ethane was also observed. 

The charges on aryl oxide ion ( p) and phenol (c,) oxygen atoms are defined as -1 and zero respectively. The change in effective charge on the oxygen of the phenol is computed to be -1.36 from Equations (25) and (26). 

The C—H activation of a alkyl group by early transition metals (M = Ti, Zr, Hf) occurs with aryl oxide in which the aryl rings are orthosubstituted by Bu-t. In this case, however, the oxygen atom is covalently bonded to the metal . 

Molecular oxygen is used in a variety of oxidative detoxification reactions aromatic hydroxylation aryl oxidation O- and M-deaUcylation deamination and sulfoxidation. These reactions either modify functional groups or add oxygen to the foreign compound, forming metabolites that lack pharmacological activity and which are more rapidly eliminated in the urine because of increased water solubility. 

Catalytic amounts of Co(OAc)2 or Mn(OAc)2 in combination with ferf-butyl hydrogen peroxide (1.2 equiv.) as oxidant proved to be effective for amination of benzoxazoles, benzothiazoles, and 2-phenyl-1,3,4-oxadiazole [95]. It is worth noting that in amination of benzoxazoles with secondary aliphatic amines the cobalt catalyst provides better results, while the manganese catalyst is more effective for a similar reaction with primary alkylamines and ammonia. Also the reactions of benzoxazoles with secondary aliphatic amines or formamides have been performed successfully with Cu(OAc)2 (20 mol.%) and oxygen as oxidant [96]. Other azoles do not react with primary alkylamines in the presence of this oxidant. In contrast, when FeCls was used in 0.25-1 equiv. amounts, amination of benzoxazoles and 2-aryl-1,3,4-oxadiazoles (with the exception of their nitro derivatives) proved to... 

Ultraviolet radiation causes cleavage of the aryl ether linkage (23). DMPPO undergoes oxidation when exposed to ultraviolet light and oxygen by direct attack on the aromatic ring to produce a variety of ring-cleaved and quinoidal stmctures (24). 

This class was first reported in 1924 and was formed 62HC(17)l) by cyclization of a-bromo-/3-aryl-y-nitroketones. The direct synthesis by oxygenation of 2-isoxazolines has not been reported. To date only 3-substituted derivatives have been prepared. Aryl-nitromethanes react with nitrostilbene to form isoxazoline A-oxide by a nitrile ion displacement (Scheme 138) <62HC(17)1, 68TL3375). 

In the oxaziridines (1) ring positions 1, 2 and 3 are attributed to oxygen, nitrogen and carbon respectively. The latter almost always is in the oxidation state of a carbonyl compound and only in rare cases that of a carboxylic acid. Oxaziridinones are not known. The nitrogen can be substituted by aryl, alkyl, H or acyl the substituent causes large differences in chemical behavior. Fused derivatives (4), accessible from cyclic starting materials (Section 5.08.4.1), do not differ from monocyclic oxaziridines. 

There is evidence from a detailed study of the photolyses of 2-alkyl-substituted aryl azides 40 in diethylamine that A3,7V-diethyl-1 //-azepin-2-amines are formed as oxygen-sensitive, meta-stablc intermediates that can give rise to a variety of byproducts, including 5-acyl- A%V-diethyl-pyridin-2-amines and 6-alkyl-7-(diethylamino)-2//-azepin-2-ones 11 however, formation of these oxidation products can be avoided by refluxing the photolysate mixture with methanol prior to exposure to oxygen, in which case practicable yields of the /V,/V-diethyl-3W-azepin-2-amines 41 result. 

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

Oxidative Heck arylation of enamides with arylboronic acids, using oxygen gas as a reoxidant for Pd(0) and 2,9-dimethyl-1,10-phenanthroline as a chelating regiocontrolling ligand, yielded a (= internally) arylated reaction product as the major compound with a very good a//3 selectivity [92]. Microwave irradiation with prepressurized sealed vials proved useful in reducing the reaction time (Scheme 82). 

More recently, Curran and Keller found that the (TMSlsSiH-mediated addition of aryl iodides to arenes are facilitated by oxidative rearomatization with oxygen (Reaction 69). Here, AIBN is not necessary for good performance of the reaction. The reaction proceeds well in both inter- and intra-molecular (see above) versions. 

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