Acetophenones ionic hydrogenation

A competition between stoichiometric hydrogenation of acetone and acetophenone resulted in hydrogenation of the acetone [42]. Competitions of this type could be influenced by both the basicity of the ketone, as well as by the kinetics of hydride transfer to the protonated ketone. An intramolecular competition between an aliphatic and aromatic ketone resulted in preferential hydrogenation of the aliphatic ketone, with the product shown in Eq. (24) being isolated and fully characterized by spectroscopy and crystallography. Selective ionic hydrogenation of an aldehyde over a ketone was also found with HOTf and [Cp(CO)3WH],... 

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. 

Fig. 41.11 R u-complex and task-specific ionic liquid for the hydrogenation of acetophenone derivatives.

A ruthenium complex containing a novel imidazolium salt moiety catalyses the asymmetric transfer hydrogenation of acetophenone derivatives, with a formic acid- triethylamine azeotropic mixture in an ionic liquid, [bmim][PF6]. The yields and ee are excellent.308... 

The reports in the organic sections of this review are now considered. Irradiation of valerophenone is well known to yield both acetophenone and cyclobutanols by a Norrish Type II process but Zepp et al. report that the latter product (cis trans ratio 2.4 1) is more efficient in aqueous systems than hydrocarbons. Such ketones as 1 readily undergo the Type II process in the solid phase and from a detailed study involving the use of chiral auxiliaries as counter ions of its carboxylate derivative, Leibovitch et al. conclude that the ionic chiral auxiliary approach is a viable general method for asymmetric synthesis. Crystals of the ketone 2 are apparently photostable at room temperature but when finely ground or at elevated temperatures intramolecular hydrogen abstraction and formation of the benzocyclobutene 3 occurs (Ito et al), and the same workers also note that irradiation of S-4 at 4 °C in the solid state and at 34% conversion gives the SS product 5 with a diastereoselectivity of 99%. 

Zhu and coworkers [22] reported an asymmetric hydrogenation of aromatic ketones in the presence of a rhodacarborane-based chiral catalyst 17, which was derived from rhodacarborane precursor 16 and (R)-BINAP, in ionic liquids (Scheme 7.5). The hydrogenations of acetophenone in the presence ofthe catalyst precursor 16 and (R)-BINAP (0.001 0.0015 for acetophenone) were performed in the ionic liquid medium such as [omim][BF4], [brnim lh7,, or a new hquid salt comprised of 1-carbododecaborate ions and N-n-butylpyridinium (BP) ions, [BP][CB1XioHi2], and tetrahydrofuran (THF) at 50°C under H2 (12atm) for 12h. As shown in Table 7.5, the catalytic activities and enantioselectivities in ionic liquids (entries 1-3) were higher in comparison with those obtained in TH F (entry... 

Enantioselective Catalysis in Ionic Liquids 245 Table 7.6 Asymmetric transfer hydrogenation of acetophenone in ionic liquids. 

Table 5.3-6 Recycling of 1 and 2-Ru (Fig. 5.3-10) in the asymmetric transfer hydrogenation of acetophenone using the formic acid-triethylamine azeotropic mixture in the ionic liquids [BMIM][PF6].

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