Olefin insertions into dihydride complexes

A preliminary investigation concerning the mechanism of the Rh-catalyzed hydrogenation of acrylamide substrates with model bisphosphine ligands has been reported [67]. Acrylamide was used as the model substrate and [Rh(PH3)2] as the model catalyst (Fig. 8). The mechanism tmder investigation consists of hydrogen coordination to a [Rh(acrylamide)(PH3)2] complex based on the four possible approaches of H2, oxidative addition to form dihydrides, and olefin insertion into the Rh-H bond to form alkyl hydride complexes. Reductive elimination was not examined because it is facile in comparison to oxidative addition and olefin insertion steps. 

Mechanisms Occurring by Insertions of Olefins into Dihydride Complexes... 

The mechanism begins with the dihydride Ir(III) complex 127, and in the first two steps, the olefin coordinates in the trans position with respect to the phosphoms and molecular hydrogen coordinates in the axial position. Simultaneously, the olefin next undergoes a migratory insertion into the axial Ir-H bond, and H2 undergoes oxidative addition. The resulting species 130 is extremely labile with... 

Figure First, an active Rh(I) catalyst (A) (16 electron species) is generated via a loss of one CO molecule from coordinatively saturated HRh(CO)2(PPh3)2 (B). Then, coordination of an alkene substrate 4 to the active catalyst A occurs to form n-olefin-Rh complex C (18 electron species). This leads to the formation of alkyl-Rh complex D (16 electron species) followed by CO coordination to give saturated alkyl-Rh complex E. Migratory insertion of CO into the alkyl-Rh bond gives unsaturated acyl-Rh species F. Next, oxidative addition of molecular hydrogen to F gives acyl-Rh dihydride complex G (18 electron species). The final product, aldehyde 5, is formed via reductive elimination and the active catalyst species A is regenerated.

The alkyl hydride complex resulting from the migratory insertion of the olefin into the rhodium hydride is the dominant species in solution at lower temperatures (-40 °C). Characterization of this complex by multinuclear NMR spectroscopy showed the legiochem-istry of the migratory insertion, which generates a five-membered metallacycle involving the partially reduced substrate. To form this ring, the hydride has become attached to the p-carbon, and the rhodium to the a-carbon of the olefin. The olefin dihydride complex in Scheme 15.7 has not been directly characterized, but has been characterized in related rhodium systems. 

The next step is the oxidative addition of hydrogen, converting the square planar diastereomers of line 2 into the octahedral dihydrides of line 3 [93]. In the present system this reaction is the rate-determining step. The fast step following is the insertion of the coordinated olefin into one of the Rh-H bonds, giving rise to the two diastereomeric (T-alkyl complexes of line 4. By reductive elimination they generate the enantiomeric forms of the product, regenerating the catalytically active square planar species, which reenters the catalytic cycle. 

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