Stereochemistry migratory insertions

The migratory insertion reactions of Pd(acyl) (alkyl acrylate) and Pd(acyl) (vinyl acetate) have been investigated by several authors and valuable mechanistic information on this important elementary step of the chain growth process has been obtained by NMR spectroscopy [li, jj. Of particular relevance are recent NMR studies by Braunstein dealing with the stereochemistry of insertion of methyl acrylate into Pd-acyl bonds derived from the reaction of CO into Pd-Me or... 

Selectivity and Stereochemistry. An important property of transition-metal complexes is that they coordinate groups in a specific manner permitting high regio-and stereoselectivity in the catalytic reaction. The migratory insertion step is a highly stereospecific transformation. The four-center transition state 16 illustrated for the Wilkinson catalyst requires a coplanar arrangement of metal, hydride, and alkene n bond ... 

The stereochemical outcome was in agreement with a mechanism for the palladium-catalyzed cyclization/carboalkoxylation of a substituted alkene (Scheme 47) that involves outer-sphere attack of the indole on the palladium-olefin complex I which, coupled with loss of HCI, would form the alkylpalladium intermediate II. 1,1-Migratory insertion of CO into the Pd-C bond of II with retention of stereochemistry would form the acyl-palladium complex III, which could undergo methanolysis to release c/.v-product and form a palladium(0) complex. Oxidation with Cu(II) would then regenerate the active Pd(II) catalyst. 

This fundamental experiment has strong implications on related catalyst-controlled Mizoroki-Heck cyclizations of precursors of this type. As axial chirality in 113 sets the stereochemistry in 114, enantioinduction was rationalized to arise from a dynamic kinetic resolution of (at elevated temperature) rapidly interconverting enanhomers of 113 in the oxidative addition step, rather than in the alkene coordination-migratory insertion event. Such a dynamic kinetic resolution process has been previously proposed by Stephenson et al. within their mechanistic study regarding the conformations of helically chiral 2-iodoanilides in intramolecular asymmetric Mizoroki-Heck reactions [72],... 

Examples of the insertions of alkenes or alk5mes into metal-amido bonds are also rare. Examples of the insertions of alkenes into tihe M-N bonds of isolated amido complexes include the reaction of a rhodium anilide complex with alkenes to form imines witii kinetic behavior that is consistent with migratory insertion,and the formal insertion of the strongly electrophilic acrylonitrile into a platinum anilide. Additional examples include reactions of a lanthanide-amido complex generated in situ, a catalytic carboamination process in which the stereochemistry implies insertions of olefins into amides, and a catalytic hydroamination that appears to occur through an aminoalkyl complex generated by S3m addition of the iridium and amido groups across the C=C bond of norbomene. 

Equation 9.84 depicts the reaction of aniline, norbomene, and [Ir(PEt3)2(CjH )jCl] to form an iridium-aminoalkyl complex. This process is thought to occur by oxidative addition of the aniline to form an amido hydride complex, followed by insertion of the strained alkene into the iridium-nitrogen bond. Products from oxidative addition of aniline to the same iridium species were shown to form in the absence of the olefin. The syn stereochemistry of the aminoalkyl product indicated that a migratory insertion pathway was followed. 

Oxidative aminations of olefins with sulfonamides also appear to occur by insertions of alkenes into Pd-N bonds in some cases. An example of the use of stereochemistry to probe for syn or anti addition of palladium and nitrogen aaoss an alkene during oxidative amination is shown in Equation 9.89b. The stereochemistry of the alkene unit indicates that the reactions occur by insertions of olefins into Pd-N bonds. As shown in Equation 9.89b, frie E-olefin containing a pendant sulfonamide would form the E-product if syn addition of the palladium and sulfonamido group occurs during the catalytic process, but it would form the Z-product if anti addition occurred. The product from syn addition is formed, and such syn stereochemistry is consistent with migratory insertion of the olefin into a palladium-sulfonamido intermediate. ... 

Prochiral olefins such as the (Z)-a-acetamidocinnamic acid derivatives, MAC and EAC, bind to the rhodium-DIPAMP catalyst to form the diastereomeric olefin complexes A and A shown in Scheme 15.18. Oxidative addition of dihydrogen to each of the diastereom-ers forms diastereomeric hydride complexes B and B, each of which imdergoes migratory insertion to form alkyl hydride complexes C and C. These complexes then undergo reductive elimination to form the amino acid ester products. The stereochemistry of the organic products can be predicted for the two parallel paths, assuming that the addition of occms to the face of the olefin coordinated to Rh. The N-acetyl-(R)-phenylalanine ester would be produced from the olefin adduct A and the (S)-product from A. ... 

Pathways to form trans addition products by Qialk-Harrod and modified Qialk-Harrod mechanisms are also shown in Scheme 16.10. The formation of trans addition products is rationalized by the dynamics of ri -vinyl complexes or by a zwitterionic intermediate." AVinyl complexes are known to exchange stereochemistry and are thought to do so by one of the mechanisms shown at the bottom left of tire two cycles in Scheme 16.10. One pathway involves rotation of the C-C bond upon reopening of the -r)--vinyl complex and a second involves formation of a zwitterionic intermediate. The steric interactions in the initial cis T -vinyl intermediate make it less stable than the trans T -vinyl complex. Such trans insertions of alkynes were discussed in detail in Chapter 9 (migratory insertions). 

The C-0 and C-N bond in the product of the oxidations with oxygen and nitrogen donors forms by either nucleophilic attack on the coordinated olefin or by insertion of the olefin into a palladium-oxygen or palladium-nitrogen bond. More detailed descriptions of the nucleophilic attack on coordinated ligands were provided in Chapter 11 and a more detailed description of migratory insertions was provided in Chapter 9. These reactions are discussed in the context of the effect of additives on the stereochemistry of the catalytic processes in several earlier sections on the Wacker process. Henry conducted the same stereochemical study for reactions of alcohols with the resolved allylic alcohol in Scheme 16.24 as was conducted for reactions of water. The results of these experiments were similar to those on the reactions of water. ... 

Other experiments have corroborated these results. It is appropriate to reiterate the results of Hayashi s study on the stereochemistry of the cychzation of the ortho-allyl phenol that was presented in Chapter 9. As shown in Scheme 16.32, the reaction with t7-ans-3-deuterio cyclohexenylphenol forms deuterated products, and the formation of these products demonstrates that the C-0 bond formation occurs by migratory insertion. As noted in Chapter 9, migratory insertion to form a cis ring juncture places the palladium trans to deuterium, and this disposition of the deuterium and the palladium leads to P-hydrogen elimination instead of p-deuterium elimination. 

Different mechanistic pathways followed for the reaction of Pd(PPha)2(Et)2 with CO gas, depending upon the stereochemistry of the reactant. The products support movement of the ethyl group in the migratory insertion step for both reactants. 

The stereochemistry for migratory insertion of alkynes has led to effective methods for creating each of the different stereoisomers of deuterated terminal alkenes. Eq. 12.51 shows a sequence of reactions involving f-butylacetylene. Due to the 100% stereoselective syn addition of the Zr-D bond, only one product is obtained. 

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