Transfer hydrogen catalysts

Heating with cuprous chloride in aqueous hydrochloric acid isomerizes 2-butene-l,4-diol to 3-butene-l,2-diol (98)] Various hydrogen-transfer catalysts isomerize it to 4-hydroxybutyraldehyde [25714-71-0] (99), acetals of which are found as impurities in commercial butanediol and... 

Figure 6.3 Racemization of a secondary alcohol in the presence of a ruthenium hydrogen-transfer catalyst.

Hydrogen transfer reactions are reversible, and recently this has been exploited extensively in racemization reactions in combination with kinetic resolutions of racemic alcohols. This resulted in dynamic kinetic resolutions, kinetic resolutions of 100% yield of the desired enantiopure compound [30]. The kinetic resolution is typically performed with an enzyme that converts one of the enantiomers of the racemic substrate and a hydrogen transfer catalyst that racemizes the remaining substrate (see also Scheme 20.31). Some 80 years after the first reports on transfer hydrogenations, these processes are well established in synthesis and are employed in ever-new fields of chemistry. 

Scheme 20.4 Possible pathways of hydrogen transfer during the racemizations of alcohols using the corresponding carbonyl compound and a hydrogen transfer catalyst.

Complexation of [Cp IrCl2]2 with iV-heterocyclic carbenes has led to complexes such as 25, developed by Peris and coworkers [107, 108], and 133, developed by Crabtree and coworkers [12]. Complex 24 is activated by the addition of silver triflate and is effective for the iV-alkylation of amines with alcohols and for the iV-alkylation of anilines with primary amines. Complex 25 has also been shown to couple benzyl alcohol 15 with a range of alcohols, including ethanol 134, to give ether products such as ether 135 (Scheme 31). Complex 133 was an active hydrogen transfer catalyst for the reduction of ketones and imines, using 2-propanol as the hydrogen source. It was also an effective catalyst for the iV-alkylation of amines... 

The Shvo catalyst is one of the most paradigmatic hydrogen-transfer catalysts due to its great versatility. It is able to hydrogenate polar (ketones, imines) and non-polar bonds (alkenes,... 

Moreover, MPVO reactions are traditionally performed with stoichiometric amounts of Al(III) alkoxides. Some improvements came from the use of dinuclear AI(III) complexes that can be used in catalytic amount [6, 7]. This is why there has been an ever-increasing interest in catalytic MPVO reactions promoted by lanthanides and transition-metal systems [8]. In these cases, it is believed that reaction proceeds via formation of a metal hydride, in contrast with the mechanism accepted for traditional aluminum alkoxide systems, which involves direct hydrogen transfer by means of a cyclic intermediate [9]. As well as La, Sm, Rh and Ir complexes, Ru complexes have been found to be excellent hydrogen transfer catalysts. The high flexibility of these systems makes them very useful not only for MPVO-type reactions, but also for isomerization processes [10]. 

The N-tosyl derivative 5, a possible precursor for the synthesis of ruthenium hydrogen transfer catalysts of type 6, is obtained in good yield by slow addition of TsCl in dichloromethane at 0 °C (eq 4). 

Palladium in the form of Pd black or Pd/C is the most effective catalyst. Although Raney nickel has been used, there is doubt that it serves only as a hydrogen transfer catalyst because it contains a considerable amount of adsorbed hydrogen. Platinum and rhodium have been found to be ineffective. Both alkenes and alkynes have been hydrogenated and syn addition to 1,2-diphenylacetylene has been demonstrated. ... 

Iridium and osmium are little used for ketone to carbinol reduction, Ir mainly as a hydrogen transfer catalyst, and Os in order to minimize ring saturation in keto-com-pounds having an aromatic nucleus. ... 

The classical oxidative, intramolecular formation of aryl-aryl bonds by thermal cyclodehydrogenation and photocyclization reactions have already been discussed in Sects. 1 and 2 respectively. Compared to the thermal approach, higher yields have often been obtained using classical hydrogen transfer catalysts such as highly dispersed platinum or palladium on suitable supports or Friedel-Crafts type catalysts like the classical AlCl3/NaCl melt (Scholl reaction, see Scheme 26 and 27 [7,38 d,e,108]). 

Decomposition of ( /2- 2 2)( )( )2 in methanol produced OsH2(H2)(CO)(P P3)2,167 which despite its wide use as a hydrogen transfer catalyst was not determined to have rf-H2 until nearly 10 years later,57 yet another example that illustrates how difficult it can be to prove the presence of H2 ligands Another unusual synthesis involves hydrogenation of an ethylene complex either in solution or even in the solid state at 60°C [Eq. (3.14)]113... 

Polysiloxane-bound ruthenium-based hydrogen-transfer catalysts were reported by liese et al. [45] [Eq. (20)]. The ruthenium-bound system was used for the asymmetric reduction of acetophenone in a continuously operated membrane reactor for the reduction of acetophenone to (S)-phenylethanol. With a constant dosage of base and an initial catalyst concentration of 0.5%, a stable operation was possible for more than 150 h (residence time 1 h). The maximum space-time yield was 580 g d" for the membrane reactor for more than 200 residence times, with a maximum ee of 92% (er = 24). 

BackvaU et al. also reported what is described as the fastest hydrogen transfer catalyst [67, 68] that has been widely used in the DKR of secondary alcohols, P-hydroxy nitriles [69] and other important intermediates (Schemes 4.29 and 4.30) [70-74]. 

In 1986, a peculiar hydrogen transfer catalyst was reported by Shvo and co-workers (272). Shvo s catalyst is a dinuclear ruthenium complex (I) that dissociates in solution into compounds II and III (Fig. 68), which interconvert in the presence of a hydrogen acceptor D or a donor DH2. Both hydrogens, the metallic hydride and that of the hydroxocyclopentadienyl substituent, can be transferred... 

Baratta and coworkers reported highly active and enantioselective ruthenium and osmium hydrogen transfer catalysts based on a combination of an amino P3rridine and a diphosphine ligands (263,264,266,267,324,325). The amino pyridines employed are l-substituted-l-(pyridine-2-yl)methanamines (I) or or-thometallated CNN tridentate ligands derived from II and III (Fig. 74). The diphosphine were PPh2(CH2)4PPh2, (S,iS)-Chiraphos, (SyS)-Skewphos, (S)-MeO-Biphep, (i2,i )-Diop, (S,K)-Josiphos (i ,S)-Josiphos, or (i ,S)- Bu-Josiphos. The source of chirality is the amino pyridine, the diphosphine, or both. Acetophenone and aryl-substituted acetophenones are reduced with TOF up to 1.4 x 10 h at 50% conversion and up to 99% ee. 

Exchange reactions may be carried out using tritiated solvent or tritiated gas, as in the Wilzbach method [19]. In the former procedure the sample is heated in a sealed tube with a hydrogen transfer catalyst and a tritiated solvent. In the Wilzbach procedure to which recourse is made only when... 

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