Carbopalladation allene termination

The possibility of Jt-allylpalladium complex formation through carbopalladation is excluded from the observation that no four- and/or six-membered rings are produced. The reaction apparently proceeds via an alternative pathway which involves a sequence of Jt-coordination of PhPdl to an allenic terminal double bond, oxypallada-tion and ensuing reductive elimination (Scheme 16.9). 

Although attempts to use allenes in the cycUc carbopalladation-carbonylative termination process have not been successful, various types of dienes, oUgoenes, enynes, and other rr-compounds may prove to be worth exploring. 

A one-pot synthesis of 3,3-disubstituted indolines was achieved by taking advantage of a sequential carbopalladation of allene, nucleophile attack, intramolecular insertion of an olefm and termination with NaBPh4 (Scheme 16.6) [10]. First, a Pd(0) species reacts with iodothiophene selectively to afford ArPdl, probably because the oxidative addition step is facilitated by coordination with the adjacent sulfur atom. Second, the ArPdl adds to allene, giving a Jt-allylpalladium complex, which is captured by a 2-iodoaniline derivative to afford an isolable allylic compound. Under more severe conditions, the oxidative addition of iodide to Pd(0) followed by the insertion of an internal olefm takes place to give an alkylpalladium complex, which is transmetallated with NaBPh4 to release the product. 

Whereas alkene insertion is followed by facile dehydropalladation whenever there is a /S-hydrogen to afford alkenes and Pd(0) catalytic species, the alkyne insertion produces the thermally stable alkenylpalladium species 2 and 7, which can not be terminated by themselves and further transformations are required in order to terminate the reactions and to regenerate Pd(0) species for catalytic recycling. In other words, it is generally believed that the reaction of generated alkenylpalladium species 2 and 7 can not be terminated, because the f-R elimination (formation of alkynes or allenes) even in the presence of a /1-hydrogen is not possible. Therefore the carbopalladation of alkynes is a living process, in which alkynes play a role of relay to pass the ability of carbon-carbon bond formation to other reactants. 

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. 

F.i.a. Soft Nucleophiles. As mentioned before, dialkyl malonate enolates are useful terminators for cascade carbopalladations, leading eventually to rr-allylpalladium intermediates. This has been demonstrated by Ma and Negishi in their studies regarding allenes as relays for medium- to large-ring synthesis (Scheme... 

The Pd(OAc)2/dppb-catalyzed cascade reaction of o-iodophenol with 1,2-nonadiene and CO starts with a CO insertion which is followed by carbopalladation of the allene and subsequently terminated by an intramolecular nucleophilic substitution to afford O-con-taining six-membered a-methylenebenzo-y-dihydropyrones (Scheme 34). ... 

Domino or cascade reactions provide valuable approaches, especially to various carbo- and heterocyclic systems with three, four, or even more annelated rings. The Heck reaction has successfully been employed in various inter-inter-, intra-inter-, inter-intra-, as well as all-intramolecular reaction cascades, hi this section, the termination of these processes by alkenes, arenes, and related ir-bond systems such as alkynes and allenes will be described. A cascade Heck reaction is considered to consist of an oxidative addition of a heteroatom-carbon bond to palladium (starter), carbopalladation of a nonaromatic carbon-carbon double or triple bond without immediate dehydropalladation (relay), one, two, or more fimher car-bopalladation(s) of a carbon-carbon double or triple bond, and eventually ensuing dehydropalladation. Crucial for a cascade reaction of this kind to occur is the blockage or retardation of a dehydropalladation at one of the intermediate stages by using 1,1-disubsti-tuted alkenes and appropriately substimted cycloalkenes, bicycloalkenes, or alkynes as relays since they give kinetically stable alkyl- or alkenylpalladium intermediates, respectively. 

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