Oxidation nitro complexes

The 4-hydroxy-1-alkene (homoallylic alcohol) 81 is oxidized to the hetni-acetal 82 of the aldehyde by the participation of the OH group when there is a substituent at C3. In the absence of the substituent, a ketone is obtained. The hemiacetal is converted into butyrolactone 83[117], When Pd nitro complex is used as a catalyst in /-BuOH under oxygen, acetals are obtained from homoallylic alcohols even in the absence of a substituent at C-3[l 18], /-Allylamine is oxidized to the acetal 84 of the aldehyde selectively by participation of the amino group[l 19],... 

It has been proposed [2] (Figure 24.1) that after binding to cytochrome, the substrates such as epoxides, A-oxides, nitro compounds, and lipid hydroperoxides accept two electrons and are reduced to the compounds RH(H)2. In contrast, the oxidizable substrates react with the oxygenated P-450 complex (RH)Fe2+02 (RH)Fe3+ 02 -. After transfer the second electron substrate RH is hydroxylated to ROH and cytochrome P-450 is oxidized to the starting Fe3+ state, completing the catalytic cycle. It is possible that hydroxylation proceeds through the formation of hydroxyl and carbon radicals [3], but a true role of free radicals at the final stages of hydroxylation is still obscure. 

Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25).

Oxidation of Alkenes by Palladium(II)- and Rhodium(III)-Nitro Complexes 373... 

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

Schiff base-cobalt-nitro complexes are too mild as oxidants to react as such with alkenes. However, the addition of Lewis acids (e.g. BF3 Et20, LiPF6) to these complexes activates the nitro ligand and produces a variety of both stoichiometric and catalytic oxidations. Stoichiometric transformations involve the oxidation of sulfides to sulfoxides and 1,3-cyclohexadiene to benzene.467 Alcohols such as benzyl alcohol and cycloheptanol are catalytically transformed into the corresponding carbonyl compounds.467,474... 

Oxygen-transfer reactions have been shown to occur from cobalt(III)-nitro complexes to alkenes coordinated to palladium.472 Thus ethylene and propene have been oxidized stoichiometrically in quantitative yields to acetaldehyde and acetone respectively, with the concomitant reduction of the nitro- to the nitrosyl-cobalt analog. A catalytic transformation with turnover numbers of 4-12 can be achieved at 70 °C in diglyme. The mechanism shown in Scheme 11 has been suggested. 

The cationic rhodium(III)-nitro complex [(MeCN)4RhN02]2+(BF4)2 also oxidizes ethylene and 1 -octene to acetaldehyde and 2-octanone, respectively. However, no catalytic oxidation takes place in the presence of dioxygen.477... 

Recent work has been reported on the nitro complexes [Os(N02)(terpy)(bipy)]+, made from [Os(terpy)(bipy)Cl]+ and nitrite ion in ethylene glycol, and on 0s(N02)bipy2Cl, prepared from [Os(NO)bipy2Cl]2+ and sodium hydroxide. Oxidation of [Os(N02)(terpy)(bipy)]+ with cerium(IV) to give [0s(N02)(terpy)(bipy)]2+ is followed by its disproportionation (equation 8). 

Since the nitro complexes can be reduced to MNO, for example, by alkenes, there is the possibility of oxidation reactions that are catalytic. 

The acid sensitive 2-methyl-3-buten-2-ol (10) was oxidized to the corresponding methyl ketone (11) with a palladium-nitro complex (90% conversion, 90% selectivity) or PdCl2-benzoquinone (98% conversion, 90% selectivity) (equation 10). ... 

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