Cyclohexene hydrogen donor

In the same study, several ligands variously functional on both the nitrogen and the sulfur atoms have been developed, providing a new class of cyclo-hexylamino sulfide ligands derived from cyclohexene oxide. All the ligands depicted in Scheme 9.7 were evaluated for the Ir-catalysed hydride-transfer reduction of acetophenone in the presence of i-PrOH as the hydrogen donor, providing enantioselectivities of up to 70% ee. 

A number of hydrogen donors such as cyclohexene, 1,4-cyclohexadiene, and formic acid have been successfully used in the presence of different heterogeneous Pd catalysts (Scheme 4.53). 

Usually the benzyl ether is dissolved in EtOH and cyclohexene, and 20% Pd(OH)2/C is added (1 10 catalyst-substrate by weight) and stirred under reflux for the required period (thin layer chromotography monitoring).246 Without cyclohexene, the benzyl ether was usually recovered, signifying that the EtOH solvent is not a good hydrogen donor under these conditions. 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

Hydrogen donors that function poorly with homogenous catalysts include hydrazine hydrate, alkenes (e.g., cyclohexene), and ascorbic acid. This is somewhat surprising as they can be very effective in heterogeneous transfer hydrogenation. 

Benzene and its homologs can be converted to the corresponding cyclo-hexadienes and cyclohexenes, and even cyclohexanes, by treatment with dissolving metals lithium, sodium, potassium or calcium in liquid ammonia or amines. Conversions are not complete, and the ratio of cyclohexadienes to cyclohexenes depends on the metal used, on the solvent, and on the presence of hydrogen donors (alcohols) added to the ammonia or amine [392, 393, 394]. 

These composite data strongly suggest that the presence of adventitious water or other hydrogen donors can markedly affect the observed rate of epoxidation. For example, Murray and Gu have reported AH = 5.0 kcalmol" for the DMDO epoxidation of cyclohexene in CDCI3 and 7.4 kcalmol" in acetone as solvent . Curci and coworkers also reported an a value of 9.3 kcalmol" for the DMDO epoxidation of isobutylene in acetone . These barriers are significantly lower than the 13-18 kcalmoD gas-phase barriers reported " at the B3LYP level of theory (Tables 3 and 4). Activation barriers of 12.6,... 

Hydrogenolysis of Z Amino Adds and Z Peptides with Cyclohexene as Hydrogen Donor General Procedure ... 

Selective reduction to hydroxylamine can be achieved in a variety of ways the most widely applicable systems utilize zinc and ammonium chloride in an aqueous or alcoholic medium. The overreduction to amines can be prevented by using a two-phase solvent system. Hydroxylamines have also been obtained from nitro compounds using molecular hydrogen and iridium catalysts. A rapid metal-catalyzed transfer reduction of aromatic nitroarenes to N-substituted hydroxylamines has also been developed the method employs palladium and rhodium on charcoal as catalyst and a variety of hydrogen donors such as cyclohexene, hydrazine, formic acid and phosphinic acid. The reduction of nitroarenes to arylhydroxyl-amines can also be achieved using hydrazine in the presence of Raney nickel or iron(III) oxide. ... 

The reductive cyclization of the 2-nitrodihydrocinnamoyl group has found application for the protection of alcohols and amines using sodium phosphinate or cyclohexene as the hydrogen donor. - The hydrogen transfer reduction of both esters and amides regenerates the alcohols or amines under mild conditions (equation 12). 

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