Transfer hydrogenation active catalyst species

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... 

The main features of the chemiluminescence mechanism are exemplarily illustrated in Scheme 11 for the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of imidazole (IMI-H) as base catalyst and the chemiluminescent activators (ACT) anthracene, 9,10-diphenylanthracene, 2,5-diphenyloxazole, perylene and rubrene. In this mechanism, the replacement of the phenolic substituents in TCPO by IMI-H constitutes the slow step, whereas the nucleophilic attack of hydrogen peroxide on the intermediary l,l -oxalyl diimidazole (ODI) is fast. This rate difference is manifested by a two-exponential behavior of the chemiluminescence kinetics. The observed dependence of the chemiexcitation yield on the electrochemical characteristics of the activator has been rationalized in terms of the intermolecular CIEEL mechanism (Scheme 12), in which the free-energy balance for the electron back-transfer (BET) determines whether the singlet-excited activator, the species responsible for the light emission, is formed ... 

The catalyst precursors 112 and 114, the true catalysts 113 and 115, and the reactive intermediate 116 in the transfer hydrogenation were isolated and the mechanism of the transfer hydrogenation has been clearly established [69]. The catalyst precursor 114, the 18-electron complex, was prepared by reacting [RuCbO/Vvcymcnc), (S,S)-TsDPEN and KOH (1 1 1) as orange crystals. Elimination of HC1 from 114 by treatment with one equivalent of KOH produces the true catalyst 115 as the 16-electron, neutral Ru(II) complex. The complex 115 shows distinct dehydrogenative activity for 2-propanol. Rapid formation of acetone occurs to produce the Ru-hydride species 116 as yellow needles when 115 is treated with 2-propanol at room... 

Study of these new catalysts is intensive. Small molecular-weight distribution was demonstrated by Petrova (112) and by Baulin et al. (113). In addition, polymer substrates have been used (114-116) in order to increase lifetime and activity. As shown by Suzuki (36), stabilization is caused by inhibition of reduction by polymeric ligands. Karol (117, 118) described the reaction of chromocene with silica to form highly active catalysts sensitive to hydrogen. An unknown role is played by the structure mt—CH2—CH2—mt which is formed with ethylene and reduced forms of titanium (119). For soluble systems, it has been shown that the mt—CH2—CH2—mt structure is formed in a biomolecular reaction with /3-hydrogen transfer (120). It was considered that this slow, but unavoidable, reaction is the reason for changes in activity during reaction and that the only way to avoid it is to prevent bimolecular reaction of two alkylated species. 

Figure 9 shows a proposed mechanism (24) to account for the product distribution data given in Fig. 7. In very dilute base, the homoannular enolate (XX) is formed first and, as it forms, is preferentially adsorbed on the catalyst in the cis configuration. Hydrogenation of this species with proton transfer from the solution will give predominantly the trans product. In these very dilute basic solutions, competitive hydrogenation of the neutral molecule also occurs. As the base concentration increases more of the homoannular enolate is formed, adsorbed, and hydrogenated. At the base concentration corresponding to the first breakpoint in Fig. 7, the number of enolate ions formed equals the number of active sites on the catalyst and, hence, there is a dependency on the quantity of catalyst at this point. In more concentrated base solutions, more homoan-...

The catalytic cycle involving [RuCbCPPhsjs], one of the more active transfer-hydrogenation catalysts, is shown in Scheme 10. The catalyst first forms an alkoxido complex, with elimination of HCl, when allowed to react with the secondary alcohol. This pentacoordinate complex forms an 18-electron species by coordinating a molecule of alkene. The alkoxido ligand transfers its Q -deuterium atom to the metal, after which the ketone oxidation product of the secondary alcohol is eliminated. The steps are believed to occur in... 

The most striking product result is the extensive formation of propane over very active catalysts. Venuto et al. (99) reported analogously that dealkylation of rf-butylbenzene over rare earth-exchanged X zeolite at 260° gave isobutane as the major gaseous product. Such paraffin formation is presumably the result of hydride transfer reactions to the car-bonium ions formed by initial electrophilic cleavage of the alkylbenzene 100) or by protonation of the olefin. Reasonable hydride donors are cumene and propylene the resultant hydrogen-deficient species are then precursors of residue formation (32, 89). Parafiin formation by treatment of alkylbenzenes with aluminum halides in the presence of cyclohexane or decalin has been known for 30 years 47), and there is ample evidence for hydride transfer between carbonium ions and hydrocarbons 10, 22, 27,53). 

---