Transfer hydrogenation olefin substrates

Yet another possibility is illustrated by the propene (or ethylene) dimerization catalyzed by 7r-l,l,3,3-tetraphenylallylnickel bromide (26) activated with ethylaluminum dichloride the isolation of considerable amounts of 1,1,3,3-tetraphenylpropene (27) from the reaction mixture suggests that a hydrogen atom has been transferred from the substrate olefin to the sterically hindered 1,1,3,3-tetraphenylallyl system under formation of 3 [Eq. (7)] (81). The subsequent formation of the HNiY species from 3 can then take place by insertion of a second propene molecule and /3-hydrogen elimination, as discussed above. 

First, solvent molecules, referred to as S in the catalyst precursor, are displaced by the olefinic substrate to form a chelated Rh complex in which the olefinic bond and the amide carbonyl oxygen interact with the Rh(I) center (rate constant k ). Hydrogen then oxidatively adds to the metal, forming the Rh(III) dihydride intermediate (rate constant kj). This is the rate-limiting step under normal conditions. One hydride on the metal is then transferred to the coordinated olefinic bond to form a five-membered chelated alkyl-Rh(III) intermediate (rate constant k3). Finally, reductive elimination of the product from the complex (rate constant k4) completes the catalytic cycle. 

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). 

Wilkinson s catalyst undergoes the hydrogenation of alkenes without isotopic scrambling between Hj and and without isotopic scrambling of Dj and protons in the solvent or on the olefin substrate. Under carefully controlled conditions, remarkably clean cis addition of hydrogen (or deuterium) is achieved. This lack of scrambling implies that the mechanism involves a dihydride intermediate in which both hydrides are transferred to the same alkene and that the mechanism involves a migratory insertion in which the metal and the hydride add in a cis fashion across the alkene, as discussed in Chapter 9. 

Under transfer hydrogenation conditions with formic acid, the reduction of alkyne 35 gave cts-olefin 36 in moderate yield (Scheme 10). ° Overhydrogenation is a problem under these reaction conditions with many substrates. 

Primary amines can be used as substrates for C-C bond activation reactions that consist of four independent transformations [29]. This process is exemplified by reaction of 3-phenylpropan-l-amine (49) with 3,3-dimethylbut-l-ene (47) in the presence of 16 and 21, which produces both the symmetric dialkyl ketone 51 and tmsymmetric ketone 50 (Scheme 10a). The route followed in this reaction (Scheme 10b) begins with rhodium mediated transfer hydrogenation between amine 49 and alkene 47 to generate phenethylimine 52, which then undergoes transimination with 21 to yield the aminopicoline derived imine 53. Chelation-assisted hydroimination of 53 with the olefin then forms ketimine 54, which upon acid promoted hydrolysis produces ketone 50. In a competing pathway, Rh(I)-catalyzed C-C bond activation of ketimine 54, followed by subsequent addition of 47, affords the symmetric dialkyl ketimine 55, which is converted to symmetric dialkyl ketone 51 upon hydrolysis. 

Hydrogenation of the olefinic system in MA and esters can also be achieved by transfer hydrogenation. Braude et and Bandi et have studied the Hantzsch ester 4 as a hydrogen donor. Nearly all the hydrogen was transferred to the substrate. The results in Table 3.1 were obtained by Braude et when using equimolar quantities of MA and Hantzsch ester. 

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