Transfer hydrogenation, ionic

The concept of performing microwave synthesis in room temperature ionic liquids (RTIL) as reaction media has been applied to several different organic transformations (Scheme 4.18), such as 1,3-dipolar cycloaddition reactions [54], catalytic transfer hydrogenations [55], ring-closing metathesis [56], the conversion of alcohols to alkyl halides [57, 58], and several others [59-61],... 

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. 

As mentioned earlier (see Section 11.1.4), cycloalkenes (primarily cyclohexene) can be used to carry out transfer hydrogenations. Similar processes during which the alkene itself serves as hydride ion donor may be observed under ionic hydrogenation conditions 223... 

A further step towards optimised conditions in the catalytic transfer hydrogenation of alkenes was achieved with the introduction of the ionic liquid N-butyl-N -methylimidazolium hexafluorophosphate (BMIMPFg) as a solvent. The reduction of alkenes with formates and Pd/C in BMIMPF6 leads to saturated hydrocarbons in high yields. With an alkyne, a mixture of cis/trans alkenes and the saturated alkane was obtained (Scheme 4.5). Sufficiently pure products were isolated by a simple liquid-liquid... 

Berthold, H., Schotten, T. and HOnig, H., Transfer hydrogenation in ionic liquids under microwave irradiation, Synthesis, 2002, 1607-1610. 

The ionic transfer hydrogenation in superacidic media has also been studied in the... 

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... 

One of the earliest reports on hydrogenation, in, what can possibly be considered as ionic liquid, dates back to 1982, when Buchanan et al. reported on the transfer-hydrogenation of polycyclic aromatic hydrocarbons in molten SbCl3 (mp. 73°C) which acts as both reaction medium and... 

Table 3.5 Transfer-hydrogenation reactions in ionic liquids...

OH + e (aq) + n + HzO + H + H2O2 + H2 The first 14 water molecules per nucleotide in the hydration layer surrounding DNA have approximately the same mass as DNA [84] and, therefore, the same number of ionizations are expected to occur in the primary hydration layer as in the DNA strand. However, it is unknown how the water molecules in the primary hydration layer are affected by radiation. One possibility is that water cations and electrons are formed, which transfer their ionic character to the DNA strand (quasi-direct effects). Water cations can also transfer protons to neighboring water molecules resulting in hydroxyl radicals. The products formed in the hydration layer (hydroxyl radicals, hydrogen atoms or aqueous electrons) can subsequently react with DNA (indirect effects). Quasi-direct and indirect effects are expected to yield very different radicals. 

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

Room temperatnre ionic liquid, A-butyl-A-methylimidazolium hexafluoro-phosphate (Figure 25.3 Ib [BMIM]+[PF6]) has been used as a solvent for catalytic transfer hydrogenations of different... 

The same steps involved in ionic hydrogenation — and H transfer to the substrate ketone — are involved in catalysis of hydrogen transfer from isopropyl alcohol to a prochiral ketone (eq 3). Traditionally, ketone coordination to the catalyst was invoked to rationalise the achievement of enantioselectivity. While transfer hydrogenation is usually done under basic conditions, the base is required more for the generation of the active catalyst than for the reaction itself the dihydride HjRuLj can catalyse the transfer of hydrogen from isopropyl alcohol to various ketones by itself [88, 89]. 

Scheme 7.5. Ionic liquids and microwave heating in transfer hydrogenation reactions.

For this purpose, nano dispersed palladium 15 was introduced into the composite material locating the Pd next to the polymer-bound ammonium cations. This was achieved after ionic attachment of Pd as palladate followed by reduction to Pd(0) and then pumping a solution of borohydride or hydrazine through the reactor (Scheme 12). Besides benzyl ether cleavage transfer hydrogenations were utilized for the reduction of aUcenes, aUcynes, and aromatic nitro groups (Scheme 13). 

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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