Migratory ethene insertion reaction

The hydride cycle starts with palladium hydride and a fast migratory insertion of ethene. Insertion of CO is also a last reaction, enhanced by the preferential coordination of CO with respect to ethene. The product is formed by reaction of propanoylpalladium with methanol which coordinates to palladium cis to the acyl group. A palladium zero complex may form that regenerates palladium hydride via an oxidative addition with a proton. 

Kinetic studies of migratory insertion reactions of the ligands that are involved as (P-P)Pd" fragments in either the propagation cycle of ethene/CO copolymerisation or ethene dimerisation to butenes have been reported by Brookhart [28] and Bian-chini [5e, fj. 

In contrast to theoretical results reported by Morokuma [29] and Ziegler [30], as well as previous studies with Pd"-phen model compounds [26], the lowest experimental energy barrier was found for the migratory insertion of the acyl (ethene) complex (Eq. (10)). The relative rates of alkyl to CO and alkyl to ethene migratory insertion reactions allowed one to estimate that sequential ethene insertions occur once for every ca. 10 insertions of CO into the Pd-alkyl bond [18]. 

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

The carbomethoxy cycle starts with the attack of a methoxy group at a coordinated carbonyl group or a migratory insertion of CO in a palladium methoxy bond. Any type of methoxy species will have a low concentration in the acidic medium of the reaction. In Figure 12.20 many details of these reactions, discussed above in section 12.2, have been omitted and only a shorthand notation is presented. Subsequently insertion of ethene takes place. It is known from stoichiometric experiments that both reactions are relatively slow. In the final step a formal protonation takes place, which as we saw before, may actually involve enolate species. 

Evidence for the migratory insertion of ethylene [46] and vinylsilane [47] into the Ru-Si bond yielding vinylsilane and two bis(silyl)ethene regioisomers [E-l,2-bis(silyl)ethene and l,l-bis(silyl)ethene],respectively,has proved that in the reaction referred to as the metathesis of vinylsilanes and their cometathesis with olefins, instead of the C=C bond cleavage formally characterizing alkene metathesis (Eq. 24a), a new type of olefin conversion that is a silylative coupling of olefins with vinylsilanes occurs (Eq. 24b). 

Extensive mechanistic studies have been performed on reactions catalyzed by rhodium and platinum complexes containing enantiopure C2-symmetric diphosphine ligands.As discussed above, (1) the formation of the Tr-olefin-Rh(H) complex 19, (2) stereospecific cis addition of the hydridorhodium to the coordinated olefin to form the alkyl-Rh complex 20 (and then 2, and (3) the migratory insertion of a carbonyl ligand giving the acyl-Rh complex 17 with retention of configuration, have been established in the hydroformylation of 1-alkenes or substituted ethenes. Thus, it is reasonable to assume that the enantioselectivity of the reaction giving a branched aldehyde is determined at the diastereomeric (1) TT-olefin-Rh complex 19 formation step, (2) alkyl-Rh complex 20 formation step, or (3) acyl-Rh complex 17 formation step. 

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