Reaction mechanism allene carbopalladation

Ring expansion occurs by the reaction of allenylclobutanols with halides to cyclopentanones [7], hitermolecular reaction of the allenylcyclobutanol 44 with iodobenzene afforded the cyclopentanone 45. Larock and Reddy explained the reaction by the following mechanism. The carbopalladation of the allene with Ph-Pd-I generates the jr-allylpalladium 46, and the concerted rearrangement and ring expansion as shown by 47 provide 48, which isomerizes to 45 [8]. 

In this section, Pd(0)-catalyzed reactions of allenes with nucleophiles are treated, which are clearly different mechanistically from the reactions explained in the above. Attack of nucleophiles may occur at C-1, C-2, and C-3 carbons of the allenes 63. Among them, attack at C-3 to give 64 is predominant. Most importantly, reactions of allenes with pronucleophiles start by the oxidative addition of pronucleophiles to Pd(0) to generate H-Pd-Nu 65. The formation of 64 by hydro-carbonation can be explained in two ways in the case where Nu-H is the carbon pronucleophile. As one possibility, hydropalladation of one of the two double bonds occurs to afford the terminal palladium intermediate 66, which is stabilized by the formation of 7r-allyl complex 67, and reductive elimination provides the C-3 adduct 68. Another possibility is carbopalladation to generate 69, and subsequent reductive elimination provides 68. Of these two possibilities, the hydropalladation mechanism is preferable. 

Based upon the studies on the mechanism of the Cl sequence we rationalized that the elusive allenol intermediate 19 (Chap. 2.2) could participate in intramolecular trapping reactions as an allenyl ether. Furthermore, vinyl allenes are perfectly suited as dienes in Diels-Alder reactions. Considering both reactive functionalities, allenyl ethers and vinyl allenes, which are perfectly suited for domino processes, we designed an insertion sequence based upon cyclizing carbopalladation [76], where the vinyl aUene results from an isomerization of an alkynylation of a vinyl... 

Palladium-catalyzed annulation reactions of conjugate acceptors and allenyl boronic ester provide substituted cyclopentenes in high yields and diastereose-lectivities (Scheme 6.24). These reactions are hypothesized to commence by the conjugate addition of a nucleophilic propargyl-palladium complex. Transmetalation of allenylboronic acid pinacol ester with a Pd(II) catalyst proceeds via an SE2 mechanism to provide the propargyl-palladium complex, which on conjugate attack on the electrophile furnishes an allene intermediate. Finally, endo carbopalladation of the pendant allene and protodepalladation generates the cyclopentene [28]. 

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