Insertion, migratory involving dienes

The proposed mechanism involves the usual oxidative addition of the aryl halide to the Pd(0) complex affording a Pd(II) intermediate (Ar-Pd-Hal), subsequent coordination of allene 8 and migratory insertion of the allene into the Pd-C bond to form the jt-allylpalladium(II) species 123. A remarkable C-C bond cleavage of 123 leads by decarbopalladation to 1,3-diene 120 and a-hydroxyalkylpalladium species 124. /8-H elimination of 124 affords aldehyde 121 and the H-Pd-Hal species, which delivers Pd(0) again by reaction with base (Scheme 14.29). The originally expected cyclization of intermediate 123 by employment of the internal nucleophilic hydroxyl group to form a pyran derivative 122 was observed in a single case only (Scheme 14.29). 

Pd complex-catalyzed copolymerization of alkene and CO affords the polyketones via alternating insertion of the two monomers [166, 167]. The polymer growth involves migratory insertion of CO into the metal-carbon bond as a crucial step, which is unique to the late transition metal complexes such as Ni, Pd, Rh, and Co. The copolymerization of allenes and methylenecydopropanes with CO has attracted much less attention than the alkene-CO copolymerization, although it would provide further functionalized polyketones due to the dual functionality of the dienes and the derivatives. 

Diene-allenes are also effective substrates in the [2+2+1] cycloaddition (160) Substrates containing ether, amine, and diester tethers were converted to the PK-type adducts in good to excellent yields. A significant rate enhancement was observed when the reactions were performed in trifiuoroethanol (TFE) or in the presence of certain Br0nsted acids. The authors suggest that possible cocatalysis involving protonation of CO in the migratory insertion step may explain the role of TFE. 

The catalytic cycle for simple olefins hydrogenation was also postulated in these early works of Osborn et al. It involves oxidative addition of H2 to a precatalyst 4, hydrogenation of the coordinated diene by the activated hydrogen in 5 that leads to a mixed cyclooctene-olefin complex 6, fast oxidative addition of another molecule of H2 yielding 3, slow migratory insertion affording alkylhydrido complex 7, and reductive elimination followed by coordination of olefin that recovers the catalyst-substrate complex 6 (Figure 1.26). 

Nolan and co-workers also showed that alkenes could react with NHCs in what appeared to be a net migratory insertion, although in this case an inter-molecular attack was proposed. Reaction of diene adduct 17 with an excess of IPr led to 18 (Equation (2.3)). Calculations (R = H) showed that 18 formed via initial addition of an NHC ligand to the Pt centre followed by intermolecular attack of a second NHC moiety. The intermolecular attack occurred with an activation barrier of +7.6kcalmol whereas intramolecular migratory insertion involving the Pt-bound NHC would entail a barrier of -f 40kcalmoH ... 

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