Acetophenone, hydrogen transfer

In the same study, these authors have prepared another series of amino-sulf(ox)ide ligands based on the (Nor)ephedrine and 2-aminodiphenylethanol skeletons, bearing two chiral centres in the carbon backbone.Their application to the iridium-catalysed hydrogen-transfer reduction of acetophenone generally gave better yields, but the enantioselectivity never exceeded 65% ee (Scheme 9.4). 

Finally, the use of S/P ligands derived from (i )-binaphthol has been considered by Gladiali et al. in the asymmetric rhodium-catalysed hydrogen-transfer reduction of acetophenone performed in the presence of i-PrOH as the hydrogen donor.It was noted that racemisation occurred when the reaction time increased and consequently the corresponding alcohol was obtained in only low enantioselectivities (< 5% ee) as shown in Scheme 9.21. Similar results were more recently reported by these authors by using iridium combined with the same ligands. ... 

Ir(cod)Cl]2 reacts with Q-diimines LL (derived from glyoxal and biacetyl) to yield cationic [Ir(cod)LL]+.523 If the reaction is carried out in the presence of SnCl2, then the pentacoordinate Ir(SnCl3)(cod)LL species results. The compounds are active catalysts in the homogeneous hydrogen transfer from isopropanol to cyclohexanone or to acetophenone followed by hydrogenation... 

More recently a Ru-catalyzed hydrogen transfer of acetophenone under microwave conditions with monotosylated (S,S)-diphenylethylenamine as a chiral source, has... 

Sinou and coworkers evaluated a range of enantiopure amino alcohols derived from tartaric acid for the ATH reduction of prochiral ketones. Various (2R,iR)-i-amino- and (alkylamino)-l,4-bis(benzyloxy)butan-2-ol were obtained from readily available (-I-)-diethyl tartrate. These enantiopure amino alcohols have been used with Ru(p-cymene)Cl2 or Ir(l) precursors as ligands in the hydrogen transfer reduction of various aryl alkyl ketones ee-values of up to 80% have been obtained using the ruthenium complex [93]. Using (2R,3R)-3-amino-l,4-bis(benzyloxy)butan-2-ol and (2R,3R)-3-(benzylamino)-l,4-bis(benzyloxy)butan-2-ol with [lr(cod)Cl]2 as precursor, the ATH of acetophenone resulted in a maximum yield of 72%, 30% ee, 3h, 25 °C in PrOH/KOH with the former, and 88% yield, 28% ee, 120 h with the latter. 

Enantioselective reduction of acetophenone was achieved in a ruthenium-catalysed hydrogen transfer reaction using isopropanol as the hydrogen source in the presence of mono-tosylated (R, R)-diphenylethylenediamine, ephedrine or norephedrine as chiral auxiliary ligands. Under optimised conditions, ( R)-l-phenylethanol was obtained in 90% yield and 82% enantiomeric excess (ee) within 9 min. f-Butylphenylketone was reduced under similar conditions in almost quantitative yield but in moderate ee... 

Asymmetric hydrogen transfer from 2-propanol to aromatic ketones such as acetophenone (99) has been achieved by using the same chiral Ru complex in 2-propanol containing KOH at room temperature, and (S)-1 -phenylethanol (100) with 98% ee was obtained [68,69]. Similarly, efficient Ru-catalysed transfer hydrogenation of aromatic ketones using the cyclic amino alcohol [(I. S, 3R,4i )-2-azanorbomylmetha-nol] (110) [70] and bis(oxazolinylmethyl) amine (111) [71] was reported. 

Merck reported the synthesis and isolation of (7 )-3,5-bistrifluoromethylphenylethanol (170) in high yields and enantiomeric excess by asymmetric hydrogen transfer. Reduction of 3,5-bistrifluoro-acetophenone (128, Ar = 3,5-(CF3)2C6H3, R = Me) with catalyst 169, prepared in situ from [RuCl2(p-cymene)]2 and (I S,2R)-cA-l-aminoindan-2-ol, produced the chiral alcohol 170 in 91-93% ee (Scheme 12.67).213... 

Scheme 4.65 Acetophenone reduction under hydrogen transfer conditions.

Scheme 5 Enantioselective hydrogen -transfer reduction of acetophenone.

Homer and Klaus examined the same photopinacolization reaction in the presence of chiral lactates [16]. Radical 6 is formed by hydrogen transfer from the lactic acid hydroxy group to the triplet excited acetophenone. At room temperature, 4% ee were observed in the presence of an equimolar amount of /-menthyl /-lactate. With /-menthyl dj-lactate no asymmetric induction was obtained. This shows that the stereocenter of the lactic acid influences the enantiodifferentiating dimerization step. 

In contrast to polymerisates, polycondensates can not be depolymerized under inert conditions. Decomposition usually leads to the destruction of the chemical structure and the monomers. The thermal decomposition of PET starts at about 300°C in an inert atmosphere [25]. Between 320 and 380°C the main products are acetaldehyde, terephthalic acid, and carbon oxides under liquefaction conditions. The amounts of benzene, benzoic acid, acetophenone, C1-C4 hydrocarbons, and carbon oxides increase with the temperature. This led to the conclusion that a P-CH hydrogen transfer takes place as shown in Eigure 25.8 [26]. Today the P-CH-hydrogen transfer is replaced as a main reaction in PET degradation by several analytic methods to be described in the following sections. The most important are thermogravimetry (TG) and differential scanning calorimetry (DSC) coupled with mass spectroscopy and infrared spectroscopy. 

In the hydrogen transfer between propan-2-ol and acetophenone catalyzed by ruthenium catalyst L 2Ru(methallyl)2 (L 2 = chiral diphosphine ligand), Genet et al. observed racemization of a-methylbenzyl alcohol 63 formed as a final product (Scheme 12.8) [28]. 

Hydrogen Transfer - Irradiation of valerophenone (29) in aqueous solution has been studied. The reaction follows the same path as that in hydrocarbon solution and yields acetophenone and cyclobutanols. The reaction in water arises from the triplet state. Interestingly, the formation of the cyclobutanols cis. trans ratio is 2.4 1) is more efficient in the aqueous system than in hydrocarbons. Cyclobutanols are also formed on irradiation of the butanoate derivatives (30). Hydrogen abstraction by the triplet excited state carbonyl group occurs from the alkyl groups on C2 of the butanoate chain. 

The hydrogen-transfer reduction of acetophenone under FBS conditions was also readily achieved in the presence of fluorous chiral diamines, diimines and (3-amino alcohols derived from tartaric acid (e.g. 18-20 Scheme 5.5) in combination with [Ir(COD)Cl]2 or Rii(p-cyrricri(jCJJ, [42], but much lower enantioselectivities (up to 31% ee in the case of 18/[Ir(COD)Cl]2) were obtained. 

Although there is no evidence for this process taking place in the reduction of ketones with hydrogen transfer from formate [251], in a related system the rate of hydrogenation of acetophenone, catalyzed by the same catalyst in the presence of NEt3 was substantially increased upon mixing the... 

Another McLafferty-type hydrogen transfer was reported for the cation radical 247 from which the enol of ionized acetophenone (249) is formed, and it was suggested that the neutral may correspond to the silene 248 (reaction 97)61. 

Lopez, J Valente, JS Clacens. JM Figueras. F. Hydrogen transfer reduction of 4-tert-biitylcyclohexanone and aldol condensation of benzaldehyde with acetophenone on basic solids. Journal of Catalysis. 2002 208. 30-37. 

Chaudret and coworkers have studied the activity of chiral aminoalcohol- and oxazoline-stabilized Ru colloids in the hydrogen transfer reaction (Scheme 11.8) [44]. The reduction of acetophenone has been studied using isopropanol as hydrogen source and preformed nanocatalysts under basic conditions and at room temperature (Table 11.10). 

Table 11.10 Hydrogen transfer of acetophenone catalyzed by amlnoalcohol- and oxazoline-stabilized Ru colloids effect of added ligand in the reaction mixturei i.

Table 11.11 Hydrogen transfer of acetophenone catalyzed by Ru nanocatalysts (Ru) in comparison with molecular systems (RuM)i"l. Adapted from Ref [44],...

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