Olefin hydrogenation cationic

The catalytic activity of cationic rhodium precursors of formula [Rh(diene)(di-phosphine)]+ was also explored by Schrock and Osborn [28]. Halpern and coworkers made very detailed mechanistic studies of olefin hydrogenation by [RhS2(diphos)]+ species (diphos = l,2-bis(diphenylphosphino)ethane S = solvent) [31]. Significant differences have been observed in the reaction of the catalyst precursors [Rh(NBD)(PPh3)2]+ and [Rh(NBD)(diphos)]+ in methanol, as shown in Eqs. (8) and (9) ... 

Intermediates which are involved in heterogeneous catalysis could have ionic character, which require an extention of the general treatment of complex reactions. As an example we can consider the catalytic hydrogenation over oxides and sulphides, where intermediates of cationic character were proposed. Ionic intermediates are also possible in catalysis over metals, for instance in the case of neopentane transformations over electron deficient palladium, which occur via formation of carbocations. If we consider olefin hydrogenation over oxides or sulphide with a heterolytic dissociation of hydrogen, the mechanismn of this reaction can be presented in the following form... 

More recently, Marks et al. have used sulfated metal oxide supports by using the acidic protons of these supports in order to generate cationic surface species (Tables 6 and 7). While NMR spectroscopy is consistent with the presence of cationic surface species, their exact nature is still unknown. However, they display very good activity in olefin hydrogenation and/or polymerization. 

C—C bridge. This phosphine is a remarkable ligand for asymmetric hydrogenation of olefins with cationic rhodium catalysts and ketones with ruthenium (11) catalysts. 

The synthesized zwitterionic iridium complexes containing various chiral P, N ligands with imidazoline or oxazoline were used as precatalysts for the asymmetric hydrogenation of unfunctionalized olefins. The cationic complexes with fluorinated borates as anions were superior catalysts in dichloromethane, whereas the iridium betaines were good catalysts in pure hydrocarbons. ... 

The possible reaction mechanism for a cascade olefination-hydrogenation reaction is illustrated in Scheme 1.21. First, the reaction of proline with ciis-isomer 67 generates the iminium cation 68, which reacts with electrophile 64 via a Mannich-type reaction to generate Mannich product 69. A retro-Mannich or base-induced elimination reaction of amine 69 would furnish active olefin 70. The subsequent hydrogen-transfer reaction is dependent on the electronic nature of the in situ-generated conjugated system or, more precisely, the HOMO-LUMO gap of reactants 65 and 70. 

Thus, the mechanisms of olefin hydrogenation are different for the catalyst systems. The Wilkinson complex accepts H2 prior to olefin, whereas the cationic Rh(I) catalyst forms the olefin complex first, which reacts with H2 subsequently. In either case the dihydride complex is the key intermediate, and two hydride ligands add successively to the olefin molecule situated at the adjacent coordination site. 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Studies on the dimerization and hydrogenation of olefins with transition metal catalysts in acidic chloroaluminate(III) ionic liquids report the formation of higher molecular weight fractions consistent with cationic initiation [L7, 20, 27, 28]. These... 

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