Alkynes, transfer hydrogenation

A few remarkable, but rather uncommon, transfer hydrogenations also deserve mention within the context of this chapter namely, the reduction of alkynes to alkenes using a chromium catalyst, and the reduction of double bonds using diimines. 

Transfer Hydrogenation including the Meerwein-PonndotfVeriey Reduction Table 20.4 Reduction of alkynes to trans-alkenes by chromous sulfate. 

Scheme 17 Carbonyl arylallylation from the alcohol oxidation level via iridium-catalyzed transfer hydrogenation employing alkynes as allyl donors...

These reactions are most common for polar double bond as reactants (carbonyls and imines) than for non-polar substrates (alkene and alkyne). Hence, hydrogen-transfer processes are a very interesting option in order to perform polar double bond hydrogenation since they allow mild conditions, high selectivity,... 

The hydrogenation step is stereospecific and transfers hydrogen in the suprafacial manner. For example, alkynes are converted to m-alkenes ... 

A further step towards optimised conditions in the catalytic transfer hydrogenation of alkenes was achieved with the introduction of the ionic liquid N-butyl-N -methylimidazolium hexafluorophosphate (BMIMPFg) as a solvent. The reduction of alkenes with formates and Pd/C in BMIMPF6 leads to saturated hydrocarbons in high yields. With an alkyne, a mixture of cis/trans alkenes and the saturated alkane was obtained (Scheme 4.5). Sufficiently pure products were isolated by a simple liquid-liquid... 

A primary alcohol and amines can be used as an aldehyde precursor, because it can be oxidized by transfer hydrogenation. For example, the reaction of benzyl alcohol with excess olefin afforded the corresponding ketone in good yield in the presence of Rh complex and 2-amino-4-picoline [18]. Similarly, primary amines, which were transformed into imines by dehydrogenation, were also employed as a substrate instead of aldehydes [19]. Although various terminal olefins, alkynes [20], and even dienes [21] have been commonly used as a reaction partner in hydroiminoacylation reactions, internal olefins were ineffective. Recently, methyl sulfide-substituted aldehydes were successfully applied to the intermolecu-lar hydroacylation reaction [22], Also in the intramolecular hydroacylation, extension of substrates such as cyclopropane-substituted 4-enal [23], 4-alkynal [24], and 4,6-dienal [25] has been developed (Table 1). 

Furthermore, in the presence of Pd or Rh catalysts, the Cp2TiCl/H20 couple can also be used for the hydrogenation of alkenes and alkynes by hydrogen transfer from water (Scheme 33) [83]. 

As illustrated in Equation (149), 2-alkyne-substituted THFs could be ring-opened via intramolecular transfer hydrogenation using TpRuPPh3(MeCN)2PE5 (Tp = tris(l-pyrazolyl)borate) as a catalyst to provide dienyl ketones <2004JOC4692>. 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

Radical Addition to Fluorinated Alkynes with Hydrogen Transfer... 

The enantioselective version of the relay transformation by organic and metallic catalyses was successfully demonstrated by Gong and coworkers (Scheme 3.39) [83]. They accomplished the direct transformation of o propargylaniline derivatives into tetrahydroquinolines in a highly enantioselective manner through the hydroamina tion of alkynes/isomerization/enantioselective transfer hydrogenation (see Sec tion 3.3 for details) sequence under the relay catalysis of an achiral Au complex/ chhal phosphoric acid binary system. 

Soon after these initial reports, the groups of Antilla [92] and You [93] indepen dently applied the chiral phosphoric acid catalysis to the enantioselective hydro genation of a imino esters. The method provides an alternative route to the enantioselective synthesis of a amino esters. Antilla and coworkers employed a new type of axially chiral phosphoric acid (9) derived from VAPOL originally developed by his research group (Scheme 3.42), whereas lg was used in You s case. In both cases, excellent enantioselectivities were achieved. You and coworkers further applied the method to the enantioselective reduction of a imino esters having an alkynyl substituent at the a position (Scheme 3.43) [94]. Both alkyne and imine moieties were reduced under transfer hydrogenation conditions with an excess amount of... 

Transfer hydrogenation. Saturation of double bonds in the presence of Pd/C shows selectivity correlating with steric hindrance. Semihydrogenation of alkynes with this system is reported. 

Scheme 13 Transfer hydrogenations of alkenes and alkynes in the flow-through mode using palladium(O) particles...

In addition to transfer hydrogenation reactions, arene ruthenium complexes also display excellent activity in the catalytic hydrogenation of olefins and alkynes including asymmetric reduction [40]. Remarkably, this process occurs under milder conditions, than required for catalysis with the dissociation of arene-metal bond. Lately, arene iridium complexes have also been found to be effective hydrogenation catalysts [41 ]. It is noteworthy that iridium can also promotes addition to the carbon-nitrogen double bond. 

For this purpose, nano dispersed palladium 15 was introduced into the composite material locating the Pd next to the polymer-bound ammonium cations. This was achieved after ionic attachment of Pd as palladate followed by reduction to Pd(0) and then pumping a solution of borohydride or hydrazine through the reactor (Scheme 12). Besides benzyl ether cleavage transfer hydrogenations were utilized for the reduction of alkenes, alkynes, and aromatic nitro groups (Scheme 13). 

Activated carbon containing 10% paUaditun has also been used to catalyze transfer hydrogenations of various organic substrates with nitro, alkene, and alkyne groups... 

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