Nickel hydrogen transfer

The reaction of olefins with dihalogenomethanes to form homologs with one additional carbon atom has been included in the list (no. 37) of reactions in Table I because CH2X2 should form a carbenoid methylene-nickel bond able to give a metallacycle with the olefin, followed by hydrogen transfer. 

Catalytic reactions involving addition of two molecules of butadiene and hydrogen transfer from an acidic carbon atom (adjacent to a ir-allyl group) to the nickel oxygen bond were recently described (example 12, Table IV). 

Transition-metal catalysts are, in general, more active than the MPVO catalysts in the reduction of ketones via hydrogen transfer. Especially, upon the introduction of a small amount of base into the reaction mixture, TOFs of transition-metal catalysts are typically five- to 10-fold higher than those of MPVO catalysts (see Table 20.7, MPVO catalysts entries 1-20, transition-metal catalysts entries 21-53). The transition-metal catalysts are less sensitive to moisture than MPVO catalysts. Transition metal-catalyzed reactions are frequently carried out in 2-propanol/water mixtures. Successful transition-metal catalysts for transfer hydrogenations are based not only on iridium, rhodium or ruthenium ions but also on nickel [93], rhenium [94] and osmium [95]. It has been reported that... 

Formic acid, anhydrous (M.W. 46.03, m.p. 8.5°, b.p. 100.8°, density 1.22), or a 90% aqueous solution, is an excellent hydrogen donor in catalytic hydrogen transfer carried out by heating in the presence of copper [77] or nickel [77]. Also its salt with triethylamine is used for the same purpose in the presence of palladium [72, 73], Conjugated double bonds, triple bonds, aromatic rings and nitro compounds are hydrogenated in this way. 

According to a possible mechanism, transfer hydrogenation requires a catalyst-mediated formation of a donor-acceptor complex, followed by a direct hydrogen transfer. An alternative possibility is a simple consecutive dehydrogenation-hydrogenation process. While the former mechanism on palladium is supported by numerous experimental evidence,78 direct hydrogen transfer on nickel was disproved.79... 

Nickel,40 41 like almost all metal catalysts (e.g., Ti and Zr) used for alkene dimerization, effects the reaction by a three-step mechanism.12 Initiation yields an organometallic intermediate via insertion of the alkene into the metal-hydrogen bond followed by propagation via insertion into the metal-carbon bond [Eq. (13.8)]. Intermediate 11 either reacts further by repeated insertion [Eq. (13.9)] or undergoes chain transfer to yield the product and regenerate the metal hydride catalyst through p-hydrogen transfer [Eq. (13.10)] ... 

One of the earliest studies of the reaction of C2H4 with D2, in which a full mass spectrometric analysis of the products was performed, used a nickel wire as catalyst [115,116]. Some typical results are shown in Fig. 11. These results showed that ethylene exchange was rapid and the deutero-ethylenes are probably formed in a stepwise process in which only one deuterium atom is introduced during each residence of the ethylene molecule on the surface, that is there is a high probability of ethylene desorption from the surface. From Fig. 11(a) it can also be seen that the major initial products are ethane-d0 and ethane-d,. This is consistent with a mechanism in which hydrogen transfer occurs by the reaction... 

The results show that these high-surface MgO catalysts are inferior to the carbon-based sorbents. Even with added nickel to aid in hydrogen transfer reactions, the conversions are lower than those obtained with the carbon catalysts. Although some condensation of the aldehyde intermediates to C3 compounds must have occurred, the condensation reaction does not proceed as well to the C4 compounds. 

We have already mentioned that the co-oligomerization of butadiene with ethylene leads to the formation of decatriene (DT) by a hydrogen-transfer process. The ratio of cyclized to open-chain product depends on the temperature and the nature of the ligand bonded to the nickel. An additional factor which affects the product distribution is the presence and nature of substituents on the olefin. Aryl and ester groups are particularly effective in promoting a hydrogen-transfer reaction, and are treated in detail below. 

A parallel may be drawn between the formation of CDD and DT from ethylene and butadiene using a naked-nickel catalyst and the formation of trialkylamine and octatriene. In both cases low temperature favors the formation of a C—C or C—N bond (i.e., formation of CDD or R3N), whereas at higher temperatures (< 60°) the hydrogen-transfer reaction becomes predominant (i.e., formation of DT and w-octatriene). 

Yang and Burton studied reductive radical additions of iododifluoroacetate 37 to olefins 38 and dienes catalyzed by 6-17 mol% of a catalyst generated from NiCl2 and stoichiometric amounts of zinc in the presence of water (Fig. 8) [90, 91]. Olefins gave the reductive addition products 40a in 60-83% yield, while 1,5-hexadiene or 1,8-nonadiene provided double addition products exclusively in 55% and 73% yield. 1,7-Hexadiene gave an inseparable mixture of the expected acyclic double addition product and a tandem addition/cyclization product, in which the former dominated. The radical nature of the addition is supported by inhibition of the reaction by para-dinitrobenzene. The reaction proceeds probably via initially formed atom transfer product 39, which is subsequently reduced by nickel(0) and zinc. This is supported by deuterium incorporation, when D20 was used instead of water. No deuterium incorporation was observed with THF-dg, thus ruling out hydrogen transfer from the solvent. 

Besides feed properties and operational variables, the type of catalyst has a profound effect on hnal olehns level in the gasoline prodnct. Catalysis with better metal tolerance, especially to nickel and vanadium, are most suitable for olehn reduction. Catalyst capacity to saturate olehns and to form corresponding paraffins depend upon the hydrogen transfer index (HTI). 

Palladium in the form of Pd black or Pd/C is the most effective catalyst. Although Raney nickel has been used, there is doubt that it serves only as a hydrogen transfer catalyst because it contains a considerable amount of adsorbed hydrogen. Platinum and rhodium have been found to be ineffective. Both alkenes and alkynes have been hydrogenated and syn addition to 1,2-diphenylacetylene has been demonstrated. ... 

Combinations of TT-allyl nickel chloride and bis(Tr-allyl) nickel produce macrocyclic oligomers from butadiene containing from four to above eight monomer units [138] in the absence of the chloride dimers and trimers only are produced. This is of interest in that termination occurs by ring closure rather than by hydrogen transfer. 

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