Aromatization transfer-hydrogenation

In disproportionation, rosin is heated over a catalyst to transfer hydrogen, yielding dehydro (5) and dihydro (8) resin acids. The dehydro acids are stabilized by the aromatic ring the dihydro acids contain only an isolated double bond in place of the less stable conjugated double bonds. 

Several methods ean be employed to eonvert eoal into liquids, with or without the addition of a solvent or vehiele. Those methods which rely on simple pyrolysis or carbonization produce some liquids, but the mam produet is eoke or char Extraction yields can be dramatically increased by heating the coal over 350°C in heavy solvents sueh as anthraeene or eoal-tar oils, sometimes with applied hydrogen pressure, or the addition of a eatalyst Solvent eomponents whieh are espeeially benefieial to the dissolution and stability of the produets eontain saturated aromatic structures, for example, as found in 1,2,3,4 tctrahydronaphthalene Ilydroaromatie eompounds are known to transfer hydrogen atoms to the coal molecules and, thus, prevent polymerization... 

On the other hand, one of the first chiral sulfur-containing ligands employed in the asymmetric transfer hydrogenation of ketones was introduced by Noyori el al Thus, the use of A-tosyl-l,2-diphenylethylenediamine (TsDPEN) in combination with ruthenium for the reduction of various aromatic ketones in the presence of i-PrOH as the hydrogen donor, allowed the corresponding alcohols to be obtained in both excellent yields and enantioselectivities, as... 

The above results show that the dihydro-aromatics do not directly contribute to rearrangements. Secondly the dihydroaromatics rapidly aromatize by hydrogen transfer or dispropositionation. This implies that the rearrangement... 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

PfefFer, de Vries and coworkers developed the use of ruthenacycles, based on chiral aromatic amines as enantioselective transfer hydrogenation catalysts. These authors were able to develop an automated protocol to produce these catalysts by reacting ligand and metal precursor in the presence of base, KPFS in CH3CN. After removal of the solvent, isopropanol was added followed by the substrate, acetophenone, and KOtBu. In this way, a library of eight chiral... 

I. S. 3/ >.4/ >j-2-AZANORBORNYLMETIIANOL, AN EFFICIENT LIGAND FORRUTHENIUM-CATALYSED ASYMMETRIC TRANSFER HYDROGENATION OF AROMATIC KETONES... 

The procedure is very easy to reproduce and the asymmetric transfer hydrogenation may be applied to a wide range of aromatic ketones. Table 9.3 gives different substrates that can be reduced with the Ru(II)-(2-azanorbornylmetha-nol) complex in Ao-propanol... 

The transfer hydrogenation of aromatic ketones, which is typically catalyzed by ruthenium half-sandwich complexes using, e.g., formic acid as hydrogen source, was chosen as another model system. After applying an appropriate TSA molecule as template, i. e., a ruthenium phosphinato complex, the resulting MIP catalyzed the hydrogenation of benzophenone approximately twice as effectively as the CP [116]. 

Table 4.8 Asymmetric transfer hydrogenation of aromatic ketones, catalyzed by [lrH(CO)(PPh3)3]/83 under base-free conditions.

Mashima and Tani et al., and employed in the asymmetric transfer hydrogenation of aromatic ketones [39, 40],... 

The treatment of [Cp MCl2]2 (M = Rh and Ir) with (S,S)-TsDPEN gave chiral Cp Rh and Cp Ir complexes (12a and 12b Scheme 5.9). An asymmetric transfer hydrogenation of aromatic ketones using complex 12 was carried out in 2-propanol in the presence of aqueous KOH (1 equiv.) the results obtained are summarized in Table 5.4. In all of the reactions, the (S)-alcohols were obtained with more than 80% enantiomeric excess (ee) and in moderate to excellent yields. The rhodium catalyst 12a was shown to be considerably more active than the iridium catalyst... 

Ikariya and Noyori et al. also reported the synthesis of new chiral Cp Rh and Cp Ir complexes (13 and 14) bearing chiral diamine ligands [(R,R)-TsCYDN and (R,R)-TsDPEN] (Scheme 5.10) these are isoelectronic with the chiral Ru complex mentioned above, and may be used as effective catalysts in the asymmetric transfer hydrogenation of aromatic ketones [42], The Cp Ir hydride complex [Cp IrH(R,R)-Tscydn] (14c) and 5-coordinated amide complex (14d), both of which would have an important role as catalytic intermediates, were also successfully prepared. 

Table 5.5 Asymmetric transfer hydrogenation of aromatic ketones catalyzed by preformed chiral catalysts and KO Bu system in 2-propanol. ...

Analogous water-soluble Cp Rh and Cp lr complexes were prepared by Williams et al., and used in the asymmetric transfer hydrogenation of aromatic ketones under aqueous conditions [43]. These catalyst complexes contain water-soluble chiral diamine ligands (Scheme 5.11), and were prepared in situ by reacting [Cp MCl2]2 (M = Rh, Ir) with ligands 15a or 15b in the presence of a base, and used immediately. The results of the asymmetric transfer hydrogenation of... 

The highly efficient catalytic system for the chemoselective transfer hydrogenation of aldehydes was reported by Xiao et al. [52]. This system consisted of [Cp IrCl2]2 (1), a diamine and HCOONa, and worked on water and in air. A wide range of aromatic aldehydes were reduced to the corresponding primary alcohols in a highly chemoselective manner some representative examples are summarized in Table 5.9. 

Carbon-Nitrogen Bond Formation Based on Hydrogen Transfer 123 Table 5.9 Transfer hydrogenation of aromatic aldehydes with HCOONa in water. ... 

In 2007, AntiUa and coworkers disclosed the first asymmetric organocatalytic reduction of acyclic a-imino esters (Scheme 23) [39], Chiral VAPOL phosphate (5)-16 (5 mol%) served as a catalyst for the transfer hydrogenation of the latter (62) employing commercially available dihydropyridine 44a to give both aromatic and aliphatic a-amino esters 63 in very high yields (85-98%) and enantioselectivities (94-99% ee). 

The reduction of aromatic nitro compQunds (see p. 98) is also due to the action of an enzyme system in which a dehydrogenase transfers hydrogen to a diphosphopyridine nucleotide-flavoprotein, which in turn reduces the nitro group (Westfall, as well as Bueding and Jolliffe ). 

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