Transfer hydrogenation of alkenes

Fig. 35.8 Optical activities achieved by enantioselective transfer hydrogenation of alkenes.

Brunner, Leitner and others have reported the enantioselective transfer hydrogenation of alpha-, beta-unsaturated alkenes of the acrylate type [50]. The catalysts are usually rhodium phosphine-based and the reductant is formic acid or salts. The rates of reduction of alkenes using rhodium and iridium diamine complexes is modest [87]. An example of this reaction is shown in Figure 35.8. Williams has shown the transfer hydrogenation of alkenes such as indene and styrene using IPA [88]. 

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... 

Ruthenium complexes of chiral phosphines notably BINAP, 2,2 -bis(diphenyl-phosphino)-l,l -binaphthyl are very useful for industrial hydrogenations and H-transfer hydrogenation of alkenes and ketones. This is due to high turnover numbers and enantiomeric excesses (ee) of the products. A specific example41 of an... 

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

Scheme 17 Transfer hydrogenation of alkenes catalyzed by 16-18 using amine borane as a hydrogen source...

In extension to the dehydrogenation studies of amine boranes and the transfer hydrogenations of alkenes catalyzed by the Re(I) complexes 16 and 17, highly efficient hydrogenations of alkenes were established based on the co-catalytic systems of 16/Me2NH-BH3 or 17/Lewis acid, which exhibited catalytic activities comparable to those of Wilkinson- or Schrock-Osbom-type hydrogenations accomplished with platinum group metal catalysts [24]. 

Scheme 9.7 Transfer hydrogenation of alkenes in the presence of an in situ generated nickel catalyst derived from Binapine.

Negishi reported the hydrogen transfer hydroalumination of alkenes with (/-Bu AKTIBA) and catalytic amounts of palladium and other late transition metal complexes.125 Although uncatalyzed hydroaluminations of alkenes with di-and trialkylalanes at elevated temperatures have long been known, their scope and limitations as well as their synthetic utility have not been extensively explored. 

A recent development is the transfer hydrogenation of heterocyclic systems such as pyrrole, pyridinium and quinoline systems. Whilst at present the yields and enantioselectivities are modest, further development may improve this situation. For example, 1-methyl-isoquinoline has been reduced to the tetrahydro species and 1-picoline has been reduced to 1-methylpiperidine [86]. Interestingly, these reductions involve alkene as well as imine reduction. 

A unique example,80 the reduction via transfer hydrogenation of aryl-substituted alkenes, is facilitated by A1C13 ... 

Until now, hydrogen sources other than formates have been rarely reported in microwave-assisted transfer hydrogenations of carbon-carbon multiple bonds. An exception is a transfer hydrogenation of electron-deficient alkenes where a series of 1,4-dihydropyridines supported on silica gel were used as the hydrogen source (Scheme 4.6). The influences of electronic effects of the alkene, steric effects of the dihydropyridine and type and power of the microwave irradiation were studied24. 

Aliphatic amines can be readily oxidized by Pd(II) to imines or iminium salts and hydrido complexes. The latter can transfer hydrogen to alkenes, leading to the formation of alkanes as byproducts of the Heck reaction (last example, Scheme 8.18). Such reactions can be avoided by using alkali carbonates as base instead of aliphatic amines [148]. Treatment of stannanes or organoboron derivatives with electron-deficient alkenes under acidic reaction conditions can also lead to formal products of Michael addition instead of the products of a Heck-type reaction [149, 150] (Scheme8.19). 

The catalytic, asymmetric hydrogenations of alkenes, ketones and imines are important transformations for the synthesis of chiral substrates. Organic dihydropyridine cofactors such as dihydronicotinamide adenine dinucleotide (NADH) are responsible for the enzyme-mediated asymmetric reductions of imines in living systems [86]. A biomimetic alternative to NADH is the Hantzsch dihydropyridine, 97. This simple compound has been an effective hydrogen source for the reductions of ketones and alkenes. A suitable catalyst is required to activate the substrate to hydride addition [87-89]. Recently, two groups have reported, independently, the use of 97 in the presence of a chiral phosphoric acid (68 or 98) catalyst for the asymmetric transfer hydrogenation of imines. 

The ionic hydrogenation of alkenes is a process where a proton is transferred before a hydride, through the combined use of a strong acid (CF3CO2H) and a silane (HSiR.3) [9]. This reaction is different from a C=C hydrosilylation, which... 

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]. 

A wide variety of hydrides act as H atom donors for example, the key step in the hydrogenation of alkenes by [HCo (CN)5] is known to go in this way (equation 19). A number of metal hydrides react with CCI4 to give CHCI3 and the metal halo complex. The mechanism may involve electron transfer, followed by H atom abstraction from the metal by CCI3. 

Table 2 Enantioselective transfer-hydrogenations of prochiral alkenes catalyzed by a transition-metal-NORPHOS complex ...

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