Carbopalladation, norbornene

When /J-hydride elimination in the carbopalladation relay is completely blocked as in norbornene 116 and norbornadiene 117, the coupling with 6 -bromostyrene 113 furnished the respective cyclopentannelation products 118 and 119 exclusively and in good yields. 

Under palladium catalysis, the < -iodoallenylbenzene 146 first undergoes an intermolecular carbopalladation of the double bond in the added norbornene 116, and only then follows an intramolecular carbopalladation of the allene moiety in 146 ensuing /3-hydride elimination finally provides the tricyclic compound 147 (Scheme 37)7 ... 

Difunctionalization of Unactivated Alkenes and Acetylenes 8.2.1.1 Carbopalladation of Norbornene and its Analogues... 

As indicated in the previous example, carbopalladation does not necessarily culminate in /i-hydride elimination as the expected outcome of reactions under Heck conditions. In those cases where the initial carbopalladation can be reversed at a later stage in the sequence fascinating options for catalytic processes may evolve. In particular, Catellani (for an overview see [67]) has established that norbornene, a strained olefin, which is reversibly introduced and eliminated, might efficiently serve as a relay to open new pathways for Pd-mediated processes. Indeed, most of the processes were explored and conducted both in a stoichiometric and in a catalytic fashion. 

In this novel domino sequence, norbornene (or norbomadiene) acts as a promoter as well as a reaction partner. It enters and exits the catalytic cycle and is eventually incorporated in the product, and therefore the reaction requires an excess of this reagent. The standard Pd-catalyzed/norbomene-mediated ortho alkylation of the aryl iodide with the alkyne would form the arylpalladium species 146 (Scheme 3.36). Intramolecular carbopalladation onto the tethered alkyne unit gives vinylpalladiuml47. Intermolecularcarbopalladationwitha moleculeofnorbornene... 

The carbopalladation of a double bond in a Cj-bridge of a bicyclic alkene leads to an intermediate, which does not contain a syn-oriented P-hydrogen and thus cannot undergo a syn-P-hydride eHmination to recreate a double bond. Such carbopalladation intermediates of, for example, norbornene with arylpalladium hahdes therefore find other modes of reaction (Section 8.3.7). In the presence of formic acid or potassium formate as a hydride source, norbornene (65), norbornene derivatives, and heteroanalogs such as 247 and 248, as well as bicyclooctene[2.2.2]octene (251), under typical Heck conditions undergo hydroarylation and hydroalkenyla-tion reactions (Scheme 8.52) [374]. A number of hydroarylation products of type... 

The complex catalytic cycle commences with oxidative palladium insertion on the aryl iodide bond. The following carbopalladation with norbornene creates a bulky intermediate that is not prone to p-hydride elimination and allows C-H activation of the ortho position of the aryl moiety. Subsequent oxidative addition of the alkyl iodide allows cross-coupling and elimination of the norbornene by retro-carbopalladation. The remaining intermediate 146 then undergoes aMizoroki-Heck reaction with the acrylate, which completes the catalytic process and furnishes the desired cinnamate 148 derivative. 

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