Dehydrogenation hydrogen acceptor

The aniline then reacts with the ap-unsaturated aldehyde by 1 4-addition the addition product, under the influence of strong acid, cyclises to form 1 2-dihydroquinaldine. The latter is dehydrogenated by the condensation products of aniline with acetaldehyde and with crotonaldehyde simultaneously produced ( .c., ethylideneaniline and crotonylideneaniline) these anils act as hydrogen acceptors and are thereby converted into ethylaniline and n-butyl-aniline respectively. 

Dehydrogenation of A -imidazolines (294 Z = NR) gives imidazoles, but requires quite high temperatures and a catalyst such as nickel or platinum. Alternatively, hydrogen acceptors such as sulfur or selenium can be used (70AHC(12)103). 

Flavin Adenine Dinucleotide (FAD) (C27 H33 N9 O15P2) is a coenzyme that acts as a hydrogen acceptor in dehydrogenation reactions in an oxidized or reduced form. FAD is one of the primary cofactors in biological redox reactions. 

A technique used to overcome the unfavorable thermodynamics of one reaction is to couple that reaction with another process that is thermodynamically favored. For instance, the dehydrogenation of cyclohexane to form benzene and hydrogen gas is not spontaneous. Show that, if another molecule such as ethene is present to act as a hydrogen acceptor (that is, the ethene reacts with the hydrogen produced to form ethane), then the process can be made spontaneous. 

Ruthenium NHC dihydride complex 26 was found to exhibit interesting reversible hydrogenation/dehydrogenation activity (Scheme 10.11) [35,36]. When excess acetone was used as a hydrogen acceptor, dehydrogenation of several alcohols was achieved (Table 10.5). 

Finally, whilst rhenium hydride complexes have not been reported to hydrogenate alkenes, there are several reports of the dehydrogenation of alkanes in the presence of tBuCH=CH2 as an hydrogen acceptor (Scheme 6.14) [136-142]. For example, cycloalkanes are transformed catalytically into the corresponding cyclic alkene, which shows that, in principle, a Re-based catalyst could be designed. 

Remarkably, the same Shvo complex can be used for the catalytic transfer dehydrogenation of aromatic amines to give imines (Scheme 7.14) [80]. This reaction produces high yields when carried out for 2-6 h in refluxing toluene with 2 mol.% catalyst. A quinone is used as the hydrogen acceptor, giving the corresponding hydroquinone. 

Falck has recently reported dehydrogenative silylation of heteroarenes with triethylsilane (18) [97]. Coupling with the Si-H bond of triethylsilane, rather than the disilane Si-Si bond, in conjunction with the use of norbomene that presumably acts as a hydrogen acceptor, gives good yields with indoles, thiophenes, and furans, under relatively mild condition (80°C). Unlike the reaction shown in Scheme 7, silylation of indole did not require protection of the N-H group. 

The iridium(l) PCP pincer complexes 1 exhibit remarkable activity in the catalytic dehydrogenation of unfunctionalized alkanes (Scheme 12.1). The H2, which is formally produced during this process, may be transferred to either tert-butyleth-ylene (TBE) or norbomene (NBE) as a sacrificial hydrogen acceptor. For example, complex la converts cyclooctane (COA) to cyclooctene (COE) in the presence of TBE, which in turn is reduced to tert-butylethane (TBA ueo-hexane) [6]. 

Palladium-charcoal has also been used in the presence of cinnamic acid as a hydrogen acceptor. Raney nickel alone and in the presence of hydrogen acceptors, the best being cyclohexanone, has also been utilized, although dehydrogenation did not occur with 6-chloro- and 6-nitro-l,2,3,4-tetrahydrocarbazoles." Selenium has also been used. ... 

A related dehydrogenation of primary and secondary alcohols to the corresponding aldehydes/ketones has been achieved using the dihydride iridium compound [IrH2(C6H3-2,6 CH2P-t-Bu2 2)j as the precursor s catalyst and t-butylethylene as hydrogen acceptor (Eq. 13). The reactions are carried out at 200 °C with a 99% yield in 18 h (alcohol/Rh = 10/1) [55]. 

SOLVENT-FREE DEHYDROGENATION OF SECONDARY ALCOHOLS IN THE ABSENCE OF HYDROGEN ACCEPTORS USING ROBINSON S CATALYST... 

Many biological oxidation reactions are dehydrogenations in which one or two hydrogen atoms (IT + e ) are transferred from a substrate to a hydrogen acceptor. Oxidation-reduction reactions in living cells involve specialized electron carriers. 

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... 

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