Dehydrogenative couplings with oxygen

Nickel peroxide, an undefined black oxide of nickel, is prepared from nickel sulfate hexahydrate by oxidation in alkaline medium with an ozone-oxygen mixture [929] or with sodium hypochlorite [930, 931, 932, 933]. Its main applications are the oxidation of aromatic side chains to carboxyls [933], of allylic and benzylic alcohols to aldehydes in organic solvents [929, 932] or to acids in aqueous alkaline solutions [929, 930, 932], and of aldehydes to acids [934, the conversion of aldehyde or ketone hydrazones into diazo compounds [935] the dehydrogenative coupling of ketones in the a positions with respect to carbonyl groups [931] and the dehydrogenation of primary amines to nitriles or azo compounds [936]. 

Oxidation the loss of electrons. Classically, O. was defined as combination with oxygen or removal of hydrogen. The electrons are transferred to the oxidizing agent, which becomes reduced. Therefore O. is always coupled to Reduction (see), so that any O. or reduction is part of an oxidoreduction process. In metabolism there are different mechanisms of enzyme catalysed O., i.e. dehydrogenation, electron transfer, introduction of oxygen, and hydroxylation (Table). 

Another interesting feature of Pd complexes is their ability to use oxygen as terminal oxidant for oxidative functionalizations. Several examples of oxidative C yi—H olefinations (oxidative Heck reactions) proceed with O2 as a reagent. Furthermore, aerobic dehydrogenative aryl couplings with high site selectivity have been developed. ... 

Cross-Dehydrogenative-Coupling Reactions with Molecular Oxygen as the Terminal Oxidant... 

It was reported that iron salts catalyzed the oxidative a-phosphonation of A,A-dimethylanilines with (R0)2P(0)H in the presence of tert-butylhydroperoxide [196]. In the study electrochemical C-H bond activation was utilized to generate iminium species, followed by C-P bond formation. An efficient dehydrogenative coupling reaction involving iminium salt as a possible intermediate was also reported with copper salt as catalyst and molecular oxygen as oxidant [197]. 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

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