Palladium® complexes organopalladium derivatives

Palladium-catalyzed coupling reactions of the Heck type have in many instances involved indole and pyrrole derivatives. Although the mechanisms are complex, organopalladium species are implicated (84H(22)1493). Vinylation of A-substituted-3 -iodoindoles with amidoacrylate groups provides a useful functionalization of indoles (Scheme 81) (90JOM(39l)C23). Yields are improved in intramolecular reactions, e.g. (406 — 407) and (408 — 409) <92H(34)219,91CPB2830). 

Three methods are commonly employed for the in situ preparation of organopalladium derivatives (i) direct metallation of an arene or heterocyclic compound with a palladium(II) salt (ii) exchange of the organic group from a main group organometallic to a palladium(II) compound and (iii) oxidative addition of an organic halide, triflate or aryldiazonium salt to palladium(O) or a palladium(O) complex. 

Palladium-catalyzed coupling reactions of the Heck type have in many instances involved indole and pyrrole derivatives. Although the mechanisms are complex, organopalladium species are impli-... 

To access useful arylamines derivatives, the palladium catalyst is usually prepared as a cationic aqua or diaqua palladium(ll) complex or other organopalladium complex. In the Scheme 6.23, a putative catalytic cycle for the Pd-catalyzed asymmetric addition reaction is shown. The hydroxyl-Pd species (A) is formed in the presence of a base or MS that undergo transmettJlation with an organometallic reagent such as ArM to produce the corresponding aryl palladium complex (B). Insertion of an imine substrate into the Ar-Pd bond, followed by hydrolysis, affords the desired product [29b]. 

Several examples of the cyclization of indole derivatives with alkenic side chains in the 3-position have been reported.6 In these examples, palladium chloride in combination with silver tetrafluoroborate is the cyclizing agent. The palladium tetrafluoroborate, presumably formed, should be a very reactive palladating species and probably is the reason why these reactions proceed at room temperature, although the mechanism is not yet completely clear. These reactions were worked up reductively (by addition of sodium borohydride) in order to reduce the expected alkenic product or any relatively stable organopalladium complexes that may have been formed (equation 4).6... 

One of the most synthetically useful reactions involving organopalladium intermediates is that between rr-allyl complexes and relatively stable carbanions such as those derived from malonate esters and jS-ketoesters. The tt- allyl complexes can be synthesized separately and used in stoichiometric amount or they can be generated in situ by reaction of allylic acetates with a catalytic amount of tetrakis (triphenylphosphine)palladium. In the catalytic version of the reaction, the tt-allyl complex is formed by reaction of the allylic acetate and the Pd(0) species which is regenerated in the elimination step. 

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