Cationic transfer hydrogenation systems

The role of alkali metal cations in the [ RuCL(p-cymene) 2l-pseudo-dipepLide-catalysed enantioselective transfer hydrogenation of ketones with propan-2-ol has been examined. Lithium salts were shown to increase the enantioselectivity of the reaction when 2-PrONa or 2-PrOK was used as the base. An alternative reaction mechanism for the pseudo-dipeptide-based systems, in which the alkali metal cation is an important player in the ligand-assisted hydrogen-transfer step, has been proposed.370... 

The radical cations of diene systems in cyclic molecules are also capable of reaction as demonstrated by Demnth, Roth and their coworkers. They have studied the influence of phase on the photochemical reactivity of some naturally occurring dienes. Thns the irradiation of the diene 10 in homogeneous solution (acetonitrile/water) in the presence of an electron-accepting sensitizer such as cyanonaphthalene (CN) or DCB brings about fraws,cw-isomerization only. However, when the electron transfer reaction is carried out in the presence of sodinm dodecyl sulphate, transannular hydrogen abstraction reactions yield the two prodncts 11 and 12. Similar reactivity is observed with frans-geranyl acetate 13 and all-trawi-famesyl acetate 14. The authors report that these cyclizations are the flrst examples of biomimetic processes brought about under SET conditions. 

Reductions can also be performed in water. Systems for reduction of ketones in water can be water-compatible sodium and lithium borohydrides, amino acid-based cationic surfactants to reduce aryl ketones [19], iridium hydrides used in transfer hydrogenations, such as [Cp Irm(bpy)H]+ (Cp — q5-C5Mes, bpy = 2,2 - bipyridine) [20], and IrHCI2(cod) 2 with a chiral diaminodiphosphine ligand to form secondary alcohols in high enantioselectivity and almost quantitative yield (Equation 4.12) [21]. 

The asymmetric counteranion directed organocatalysis has been also applied to the enantioselective transfer hydrogenation of a,f)-unsaturated ketones employing catalyst 11, which involves a chiral cation such as a valine ester anunonium salt and a chiral binaphthol derived phosphate [23]. This combination, in the presence of the Hantzsch ester 4, is a very active and enantioselective system for the transfer hydrogenation of a variety of cyclic a,P-unsaturated ketones (Scheme 2.5). Acyclic ketones are also reduced but with slightly lower enantioselectivities. 

A series of picolyl-functionalized NHC ligands attached to a cationic halfsandwich Ru core showed transfer hydrogenation chemistry that was dependent on both the N- and backbone substituents [21]. Thus, in the presence of PrOH/KOH at 82°C, conversion of acetophenone (C B S loading of 1 10 100) followed the order 6 > 9 > 7 8. The most active system (6) with R = Me/R = H proved capable of reducing a range of 4-halo-substituted acetophenones, alkyl ketones, as well as the imine, N-benzylideneaniline. Related efforts with an oxazoline-NHC Ru(arene) system [22], [Ru(p-cymene)(picolyl-NHC) Cl]", and the pyrimidine-NHC complex 10 produced less active catalysts [23,24]. 

To test and train the ReaxFF parameters for reactive events we considered a number of hydrogen and proton transfer reactions for neutral, cation, and anion systems. Figure 6.9 and Table 6.1 compare the QM and ReaxFF barriers... 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

---