Migratory insertions of olefin

Polymerization occurs by repeated migratory insertion of olefin into the (Tv-oriented metal-carbon bond by the generally accepted Cossee mechanism [5, 60]. This mechanism is believed to be shared by all transition metal coordination polymerization... 

However, the mechanism of this practically important catalytic reaction remains unclear. The mechanism proposed in early papers [70, 71] for the Wilkinson catalyst involves oxidative addition of a B-H bond to the metal center, followed by olefin coordination to the metal center accompanied with dissociation of one of the two PPh3 s, further followed by migratory insertion of olefin into the M-H bond and subsequent reductive elimination of the B-C bond. However, still unresolved are several important questions (1) whether the reaction occurs with phosphine dissociation or not, (2) which of M-B and M-H bond insertions of olefin is energetically more favorable, and (3) how competitive is the a-bond metathesis pathway involving coordination of the HBcat and olefin to the complex followed by simulta-... 

Several examples in which the migratory insertion of olefins into metal- lkyl complexes has been observed directly are shown in Equations 9.60-9.65. Eqiiations 9.60 and 9.61 show the insertions of propene into lutetium-alkyl bonds reported by Watson. Equation 9.62 depicts the labeling experiment reported by Flood, which revealed the insertion of an olefo into a cationic latemetal-alkyl complex. Equations 9.63 and 9.64 depict low-temperature studies by... 

Contrary to the previously reported reactions with the M-H and M-Si initial complexes the proposed mechanism of catalysis by [(cod)M(OSiMe3)]2 (where M= Rh, Ir) does not involve highly activated migratory insertion of olefin into the Rh-Si bond (the associative mechanism) since the final step of the product formation occurs via a lower activated step of reductive elimination of product (the dissociative mechanism) (Scheme 4). The reaction under study is conceptually related to dehydrogenative silylation since the basic reaction involves the silylation of a substrate such as styrene by vinylsilane instead in the hydrosilane, equations 17a and 17b. by hydrosilanes... 

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. 

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... 

With Pd(0) generated in situ, the oxidative addition of aryl bromide 102 to Pd(0) proceeds to form Pd(II) intermediate 104. Migratory insertion of 104 then occurs to furnish the cyclized indoline intermediate 105. Subsequent reductive elimination of 105 takes place in a cis fashion, giving rise to exo-cyclic olefin 107, which then tautomerizes spontaneously to the thermodynamically more stable indole 103. The reductive elimination by-product as a palladium hydride species 106 reacts with base, regenerating Pd(0) to close the catalytic cycle. 

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). 

The general mechanistic features of the ethene/CO copolymerisation cycle (Scheme 7.2) are substantially valid also for styrene. In particular, the propagation steps are similar for both alkenes and consist of subsequent alternated migratory insertions of alkyl to CO and of acyl to olefin, with P-chelate and y-chelate resting states. The structures of the first intermediates in the syndiotactic copolymerisation of styrene derivatives with CO have been determined by an in situ NMR study using [(Pr DAB)Pd(Me)(NCMe)]BAr4 as precursor (Scheme 7.21) [38]. 

The alkyne-cobalt carbonyl complex 3 formed from the alkyne 1 and dicobalt octacarbonyl 2 should lose at least one of the GOs on the metal to provide the vacancy for the incoming olefins. Subsequently, an olefin-bound complex 5 rearranged oxidatively to yield a metallacyclic intermediate 6. Migratory insertion of GO of 6 would provide the homologated ring intermediate 7, and the following two successive reductive eliminations afford the cyclopentenone... 

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

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. 

The pentacyclic framework of natural product maxonine was prepared in an intramolecular 1-exo Heck cyclization. The migratory insertion of the pendant olefin into the arylpalladium complex could have led either to the formation of an eight or a seven membered ring, of which only the latter was observed (5.3.), 3... 

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. 

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). 

A mechanistic scheme of this new type of silyl olefin conversion involves the migratory insertion of the olefin into the Ru-Si bond and vinylsilane into the M-H bond followed by /3-hydrogen and /3-silicon elimination to give l,2-bis(silyl)ethenes, l,l-bis(silyl)ethenes and ethylene (Scheme 1) [46,47]. 

Alkyl iridium compounds are also accessible via insertion (see Migratory Insertion) of alkenes into Ir H bonds. Analogously, alkenyl iridium compounds may be formed via insertion of alkynes into Ir-H bonds. These types of reactions have been studied to shed tight on the mechanism of alkene and alkyne hydrogenation processes. For example, HIr(CO)(PPh3)2 (65) will react with ethylene and higher olefins to produce the alkyl iridium compounds (equation 17). 

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