Reaction migratory olefin insertion

The catalytic cycle proposed for the cyclization-hydrosilylation with the cationic palladium catalyst is classified into the type D in Scheme 2. The reaction consists of an olefin insertion into palladium-silicon bond and the metathesis between palladium-carbon and hydrogen-silicon bond, regenerating the silylpalladium intermediate and releasing the product where migratory insertion of the pendant olefin into the alkylpalladium is involved before the metathesis (Scheme 26).83a... 

Figure 1.. The two proposed reaction pathways based on experimental results for hydroboration reactions of olefins catalyzed by the Wilkinson catalyst. (O.A. Oxidative Addition Olefin Migratory Insertion R.E. Reductive Elimination)...

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Hydroformylation (the oxo process) involves the addition of H2 and CO to an olefin to form aldehydes (eq. 2.8), which have a number of important industrial applications. Extensive mechanistic studies have shown that this reaction involves migratory insertion of a bound alkyl group (formed by insertion of an olefin into a metal hydride) into a bound CO, followed by reductive elimination of the aldehyde. The rate-limiting step for the hydroformylation in liquids is either the reaction of olefin and HCo(CO)4 or the reaction of the acyl complex with H2 to liberate the product aldehyde. The high miscibility of CO in sc C02 is therefore not necessarily a major factor in determining the rate of the hydroformylation. Typically, for a-olefins, linear aldehydes are preferred to branched products, and considerable effort has gone into controlling the selectivity of this reaction. 

The only isolated metal-hydroxo complex that has been shown to react with olefins to form products from transfer of hydroxide is Cp Ir(PMe3)(Ph)(OH). However, this formal insertion process does not occur by a migratory insertion mechanism. Instead, the reaction with olefin is catalyzed by trace amounts of Cp Ir(PMe3)(Ph)(OTf) and appears to involve replacement of tiiflate with ethylene to generate the cationic [Cp Ir(PMe3)(Ph)(CjH )], which undergoes attack by the separate iridium hydroxido complex, as shown in Scheme 9.11. 

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. ... 

Migratory insertion reactions involving olefins, that is, reactions of H/D exchange, isomerization, cis-trans isomerization, hydroformylation and other 0x0 syntheses. 

The Mizoroki-Heck reaction is a metal catalysed transformation that involves the reaction of a non-functionalised olefin with an aryl or alkenyl group to yield a more substituted aUcene [11,12]. The reaction mechanism is described as a sequence of oxidative addition of the catalytic active species to an aryl halide, coordination of the alkene and migratory insertion, P-hydride elimination, and final reductive elimination of the hydride, facilitated by a base, to regenerate the active species and complete the catalytic cycle (Scheme 6.5). 

A plausible mechanism proposed for this reaction involves migratory insertion of an olefin into the Pd-Si bond of a paUadium-silyl intermediate I followed by migratory insertion of the pendant olefin into the resulting Pd-C bond of II forming palladium-alkyl intermediate III. Reaction of Iff with hydrosilane releases the carbocy-cle to regenerate the palladium-silyl complex I (Scheme 3-21) [61]. 

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

Vinylation or arylation of alkenes with the aid of a palladium catalysts is known as the Heck reaction. The reaction is thought to proceed through the oxidative addition of an organic halide, RX onto a zero-valent [PdL2] species followed by coordination of the olefin, migratory insertion of R, reductive elimination of the coupled product and dehydrohalogenation of the intermediate [HPdXL2] (Scheme 6.1). 

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