Dehydrogenation, oxidative transfer

Dihydropyrimidines are normally readily oxidized to the corresponding pyrimidines by dehydrogenation, hydrogen transfer, or disproportionation reactions <1994HC(52)1, 1996CHEC-II(6)93>. For example, the oxidation of a series of trifluoromethyl ketones 522 with DDQ occurred readily at room temperature <1997H(44)349>. Facile room temperature oxidation with ceric ammonium nitrate (CAN) has also been achieved <2003ARK(xv)22>. 

Lattice silver also can perform a dehydrogenative oxidation of alcohols with O2. For example, fert-butyl alcohol can be oxidized to isobutylene oxide on an O2 covered Ag(l 10) surface at elevated temperatures (85). However, other oxidation products also were produced. Experiments using 02 labeling revealed that the oxygen in the product is from the original alcohol and they believe the hydrogen atom from the methyl C—H bond is directly transferred to either O2 or another molecule of tcrf-butyl alcohol. Lattice silver is still widely used in industry and further studies hold promise for other industrially suitable methods (Fig. 14). 

Dehydrogenative oxidation of secondary alcohols in the presence of acetone is the reverse process of transfer hydrogenation of ketones with 2-propanol [87b, 95b]. Kinetic resolution of racemic secondary alcohols is possible using this process with an appropriate chiral catalyst and suitable reaction conditions. As exemplified in Scheme 45, a variety of racemic aromatic or unsaturated alcohols can be effectively resolved in acetone with a diamine-based Ru(II) complex 42 or 50 [129]. Chiral alcohols with an excellent optical purity are recovered at about... 

The most important reaction of dihydropyrimidines is their oxidation to the corresponding pyrimidines (via dehydrogenation, hydrogen transfer, or disproportionation). However, although many such oxidations have been carried out, they were aimed at enabling identification of dihydropyrimidines from the pyrimidine formed, rather than a study of the kinetics and mechanism of the oxidation reactions themselves. Thus besides spontaneous oxidation in air, l,4(l,6)-dihydropyrimidines were oxidized by KMn04,152 15S 156,171-173 DMSO,147-151153 or potassium hexacyanoferrate(III)187 1,2-dihydropyrimidines were oxidized by KMn04,175 176 178 183 184 DMSO,163 or 2,3-dichloro-5,6-dicyanobenzo-quinone (DDQ).199... 

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). 

An alternative method for the oxidation of alcohols is dehydrogenative oxidation via a hydrogen transfer reaction [2a]. The process of dehydrogenation of alcohols by... 

The dehydrogenation of 2-butanol is conducted in a multitube vapor-phase reactor over a zinc oxide (20—23), copper (24—27), or brass (28) catalyst, at temperatures of 250—400°C, and pressures slightly above atmospheric. The reaction is endothermic and heat is suppHed from a heat-transfer fluid on the shell side of the reactor. A typical process flow sheet is shown in Figure 1 (29). Catalyst life is three to five years operating in three to six month cycles between oxidative reactivations (30). Catalyst life is impaired by exposure to water, butene oligomers, and di-j -butyl ether (27). 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

Dehydrogenation, Ammoxidation, and Other Heterogeneous Catalysts. Cerium has minor uses in other commercial catalysts (41) where the element s role is probably related to Ce(III)/Ce(IV) chemistry. Styrene is made from ethylbenzene by an alkah-promoted iron oxide-based catalyst. The addition of a few percent of cerium oxide improves this catalyst s activity for styrene formation presumably because of a beneficial interaction between the Fe(II)/Fe(III) and Ce(III)/Ce(IV) redox couples. The ammoxidation of propjiene to produce acrylonitrile is carried out over catalyticaHy active complex molybdates. Cerium, a component of several patented compositions (42), functions as an oxygen and electron transfer through its redox couple. 

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. 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

All conventional reactors, tested before using the micro reactor (simply since micro reactors were hardly available at that time), only fulfilled the demands of one measure, at the expense of the other measures. For instance, a single-tube reactor can be operated nearly isothermally, but the performance of the oxidative dehydrogenation suffers from a too long residence time. A short shell-and-tube reactor provides much shorter residence times at improved heat transfer, which however is still not as good as in the micro reactor. 

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