Jt-allylpalladium intermediate

The vinylcyclopropane 144, bearing two electron-withdrawing groups, undergoes the intermolecular palladium-catalyzed [3 + 2]cycloaddition reaction of the Jt-allylpalladium intermediate 145 with a,/ -unsaturated esters or ketones to provide a useful method for forming the cyclopentane ring of 146 [74], (Scheme 51)... 

In the absence of a nucleophile, the reaction of allenes with aryl bromides provides 1,3-dienes in good yield (Scheme 16.27) [32], The reaction is very sensitive to the reaction temperature, solvent, base and amount of phosphine used. The formation of a 1,3-diene may be explained by either /3-hydrogen elimination or deprotonation at the a-carbon of the Jt-allylpalladium intermediate. 

The stereoselectivity of the allene attack has been demonstrated by the catalytic reaction of cis and trans seven-membered allenic compounds, in which cis- and trans-fused 5,7-ring systems are provided in 76 and 40% yield, respectively, via Jt-allylpalladium intermediates (Scheme 16.91). 

The hydrocarbonation of 3,3-dihexylcyclopropene proceeds via the formation of palladacyclobutene intermediate 28 (Scheme 6). The pallada-ene type reaction between 28 and 1 produces the Jt-allylpalladium intermediate 29. Reductive elimination gives the allylated products 12 and/or 13. 

The Pd-catalyzed hydrocarbonations of methyleneaziridines 14 do not proceed through the formation of a Jt-allylpalladium intermediates, instead via a chelation effect. The hydropalladation of methyleneaziridines with the Pd(II) hydride species 16, followed by reductive elimination gives the non-ring-opened products 15. It is noteworthy to mention that the palladium-catalyzed intermolecular or intramolecular addition of nitriles to carbon-carbon multiple bonds can be explained by the hydropalladation mechanism, except for the intramolecular addition to the C=C triple bond of alkynes (vide infra). 

In the allylic substitution of racemic 2-propenyl acetates or related substrates with the same substituents at 1 and 3 positions, the jt-allylpalladium intermediate containing a meso type 7r-allyl group is formed from both enantiomers of the allylic substrate. Two jt-allyl carbons at the 1- and 3-positions are diastereotopic on coordination of a chiral phosphine ligand to palladium. The asymmetric induction arises from preferential attack by the nucleophile on either of the two diastereotopic TT-allyl carbon atoms (Scheme 2-28). 

In the total synthesis of desethylibogamine [88a], Trost and GenCt transformed an allylacetate with a pendant secondary amine (243) into the desired isoquinuclidine 244 (Scheme 44). The cyclization took place via nucleophilic attack on a Jt-allylpalladium intermediate by the secondary amine. An asymmetric version of the above transformation was accomplished by Trost, Godleski, and GenSt [88b]. Such an operation involving the n-allylpalladium intermediate appears to have proceeded with complete stereocontrol, since the ee (60%) remained constant throughout the transformation. Moreover, Trost, Godleski, and Belletire [88c] utilized the n-allylpalladium method to achieve a formal synthesis of ( )-catharanthine. Under the same reaction conditions as... 

Reactions Involving Jt-Allylpalladium Intermediates Generated via Alkene Carbopalladation... 

This reaction constitutes a special type of process in which a hydrogen and a nucleophile are added across the diene, with formation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20], hydroamination [21, 22], and addition of active methylene compounds [21a, 23, 24]. These reactions are initiated by an oxidative addition of H-Nu to the palladium(O) catalyst, which produces a palladium hydride species 1 in which the nucleophile is coordinated to the metal (Scheme 11.1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a JT-allylpalladium intermediate. Now, depending... 

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