Acetophenone transfer hydrogenation

In addition, the related complexes 13 and 14 act as catalysts in enantioselective transfer hydrogenations (Table 5). The reactivity of acetophenone derivatives... 

The scope and limitations for transfer hydrogenation employing either the iron porphyrin system or the combination of iron compound/terpy/PPhs are listed in Table 8. In most cases, the FeCVterpy/PPhs system displays a higher activity. Except for chloromethyl- and cyclopropyl-acetophenone, the desired products were obtained in good to excellent yields. It should be noted that a ring opened product was not observed when cyclopropyl acetophenone was employed. Hence, a radical-type reduction pathway was excluded and a hydride mechanism appeared to be reasonable. 

Application of this system in the continuous transfer-hydrogenation reaction of acetophenone gave a stable conversion of about 87%, an ee of 94%, and a space-time yield of 255 g L"1 d"1. A continuous dosage of isopropoxide was necessary in order to compensate for deactivation caused by traces of water in the feed stream. Under these circumstances a TTON of 2360 was reached. Comparison of this system with an enzymatic process showed that both approaches offer different advantages and are therefore complementary. 

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... 

Ionic liquids have also been applied in transfer hydrogenation. Ohta et al. [110] examined the transfer hydrogenation of acetophenone derivatives with a formic acid-triethylamine azeotropic mixture in the ionic liquids [BMIM][PF6] and [BMIM][BF4]. These authors compared the TsDPEN-coordinated Ru(II) complexes (9, Fig. 41.11) with the ionic catalyst synthesized with the task-specific ionic liquid (10, Fig. 41.11) as ligand in the presence of [RuCl2(benzene)]2. The enantioselectivities of the catalyst immobilized by the task-specific ionic liquid 10 in [BMIM][PF6] were comparable with those of the TsDPEN-coordinated Ru(II) catalyst 9, and the loss of activities occurred one cycle later than with catalyst 9. 

Table 41.15 Recycling of 9- and 10-Ru in the asymmetric transfer hydrogenation of acetophenone with the azeotrope in [BMIM][PF6] [110].

Scheme 6-51. Asymmetric transfer hydrogenation of acetophenone in the presence of 119. Reprinted with permission by Am. Chem. Soc., Ref. 112.

Ruthenium-catalysed asymmetric transfer hydrogenation of acetophenone 133... 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

The highest ttn published to our knowledge so far for chemzymes (in the sense of polymer-enlarged chemical catalysts) is found in the transfer hydrogenation process catalyzed by Gao-Noyori s catalyst bound to a siloxane polymer (Fig. 3.1.3, 4) [13, 14]. In this transfer hydrogenation acetophenone is reduced to (S)-phenyl-ethanol using isopropanol as hydrogen donor. The product is produced in a CMR with 91% ee at a space-time yield of 578 g L d the ttn for the catalyst is 2633. 

To specify the position and the nature of the transferred hydride, the reaction was performed with 2-propanol-dj as solvent/donor, sodium 2-propylate as base and Fe3(CO)12/PPh3/TerPy as catalyst under optimized conditions. In the transfer hydrogenation of acetophenone a mixture of two deuterated 1-phenylethanols was obtained (Scheme 4.7, 9a and 9b). The ratio between 9a and 9b (85 15) indicated a specific migration of the hydride, albeit some scrambling was detected. However, the incorporation is in agreement with the monohydride mechanism, implying the formation of metal monohydride species in the catalytic cycle. 

Adam W, Arnold MA, Nau WM, Pischel U, Saha-Moller CR (2001b) Structure-dependent reactivity of oxyfunctionalized acetophenones in the photooxidation of DNA base oxidation and strand breaks through photolytical radical formation (spin trapping, EPR spectroscopy, transient kinetics) versus photosensitization (electron transfer, hydrogen-atom-abstraction). Nucleic Acids... 

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