Oxidations hydrogen-transfer type

Rhodium- and ruthenium-catalyzed hydrogen-transfer type oxidations of primary and secondary alcohols have recently been reported by Matsubara and coworkers (Scheme 6.99) [202], Thus, secondary alcohols were converted into the correspond-... 

Hydrogen-Transfer Type Oxidations Catalyzed by Rhodium(I) and Ruthenium(II) Complexes... 

The MW irradiation can also be applied for the hydrogen-transfer-type oxidation [18] of alcohols in the presence of the rhodium(I) or ruthenium(II) complexes with phosphine ligands [RhCl(CO)(PPh3)2] (15) and [RuCl2(PPh3)3] (16),... 

Ri - H or aliphatic, R2 - aliphatic or ammatic gmup Scheme 18.7 Hydrogen-transfer-type oxidation of primary and secondary alcohols [19]. 

TABLE 18.5 Hydrogen-Transfer-Type Oxidation of Alcohols Under MW Irradiation Using the Rhodiuma) [RhCKCOXPPhj) ] (15) and Ruthenium(II) [RuCl2(PPh3)3] (16) Complexes as Catalyst Precursors"... 

Scheme 18.9 Selective hydrogen-transfer-type oxidation of secondary ahphatic alcohols [19].

TakahashiM, OshimaK, Matsubara S. Hydrogen transfer type oxidation of alcohols by rhodium and ruthenium catalyst under microwave irradiation. Tetrahedron Lett. 2003 44 9201-9203. 

Hydrogen Transfer Oxidation of Alcohols (Oppenauer-Type Oxidation)... 

Hydrogen transfer reactions from an alcohol to a ketone (typically acetone) to produce a carbonyl compound (the so-caUed Oppenauer-type oxidation ) can be performed under mild and low-toxicity conditions, and with high selectivity when compared to conventional methods for oxidation using chromium and manganese reagents. While the traditional Oppenauer oxidation using aluminum alkoxide is accompanied by various side reactions, several transition-metal-catalyzed Oppenauer-type oxidations have been reported recently [27-29]. However, most of these are limited to the oxidation of secondary alcohols to ketones. 

Electron or hydrogen transfer between a substrate and sensitizer is often responsible for initiation of a type II/ivRH photo-oxidation. Consequently, the type II/ivRH reaction can often be suppressed by physically separating (isolating)... 

The catalytic coke produced by the activity of the catalyst and simultaneous reactions of cracking, isomerization, hydrogen transfer, polymerization, and condensation of complex aromatic structures of high molecular weight. This type of coke is more abundant and constitutes around 35-65% of the total deposited coke on the catalyst surface. This coke determines the shape of temperature programmed oxidation (TPO) spectra. The higher the catalyst activity the higher will be the production of such coke [1],... 

Metal-catalyzed oxidation of alcohols to aldehydes and ketones is a subject that has received significant recent attention [21,56,57]. One such method that utilizes NHC ligands is an Oppenauer-type oxidation with an Ir or Ru catalyst [58-62]. These alcohol oxidation reactions consist of an equilibrium process involving hydrogen transfer from the alcohol substrate to a ketone, such as acetone (Eq. 5), or an alkene. Because these reactions avoid the use of a strong oxidant, the potential oxidative instability of NHC ligands is less problematic. Consequently, these reactions represent an important target for future research into the utility of NHCs. 

Catalyst coking may involve carbonaceous species such as partially hydrogenated fragments (QHy) and may be initiated on metal or than acidic-oxide sites [1]. Three types of carbonaceous deposits may be formed on say Pt [2], which may be differentiated by temperature-programmed oxidation. SnO,-promoted Pt catalysts are important in reforming of alkanes [3] and low temperature CO oxidation [4]. Of course Sn02 is an n-type semiconductor and certainly in photoelectrolysis one expects metal-oxide electron transfers across the junction [51, but the nature of the Pt-SnOt interaction in catalytic systems remains unclear. 

Hydride elimination reactions are characterized by the transfer of a hydrogen atom from a ligand to a metal. Eifectively, this may be considered an oxidative addition, with both the coordination number and the formal oxidation state of the metal being increased (the hydrogen transferred is formally considered as hydride, H ). The most common type is p elimination, with a proton in a p position on an alkyl ligand being transferred to the metal by way of an intermediate in which the metal, the a and P carbons, and the hydride are coplanar. An example is shown in Figure 14-11. p Elimination is the reverse of 1,2-insertion. 

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