Organopalladium Addition with Hydride Elimination

Oxidative addition [1, 38] of 1-alkenyl, i-alkynyl, allyl, benzyl, and aiyl halides to a palladium(O) complex affords a stable rra .s-<7-palladium(II) complex (11). The reaction proceeds with complete retention of configuration for alkenyl halides and with inversion for allylic and benzylic halides. Alkyl halides having /3-hydrogens are rarely useful because the oxidative addition step is very slow and may compete with /3-hydride elimination from the a-organopalladium(II) species. However, it has been recently shown that iodoalkanes undergo the cross-coupling reaction with organoboron compounds (Section 2.4.5). 

Oxidative addition of palladium(O) species into unsaturated halides or triflates provides a popular method for the formation of the a-bound organopalladium(II) species. It is important to use an unsaturated (e.g. aryl or alkenyl) halide or tri-flate, as (3-hydride elimination of alkyl palladium species can take place readily. Oxidative addition of palladium(0) into alkenyl halides (or triflates) occurs stere-ospecifically with retention of configuration. The palladium is typically derived from tetrakis(triphenylphosphine)palladium(0), [Pd(PPh3)4], or tris(dibenzylidene-acetone)dipalladium(O), [Pd2(dba)3], or by in situ reduction of a palladium(II) species such as [Pd(OAc)2] or pd(PPh3)2Cl2]. 

The reaction is thought to proceed by co-ordination of the alkene with the organopalladium(II) species, followed by carbopalladation. Subsequent p-hydride elimination regenerates an alkene and releases palladium(II). This is reduced (reductive elimination) to palladium(O) in the presence of a base, to allow further oxidative addition and continuation of the cycle (1.211). The carbopalladation and p-hydride elimination steps occur syn selectively. Excellent regioselectivity, even for intermolecular reactions, is often observed, with the palladium normally adding to the internal position of terminal alkenes (except when the alkene substituent is electron-rich as in enamines or enol derivatives), thereby leading to linear substitution products. 

The first step in catalytic reactions of the Heck type is the oxidative addition of the organic halide to Pd(0) species to form an intermediate organopalladium halide constituting the Pd(II) species. This is followed by insertion of the olefinic bond and subsequent /khydrogen elimination [scheme (30)]. The catalyst is recycled by the reaction of the Pd(II)-hydride species with a base [scheme (31)]. It is worth noting here that palladium species, L2(X)Pd—ArCH=CH2, do not propagate the chain growth polymerisation of the CH2=CHArX monomer via its olefinic bond in the discussed process. 

The addition of an organopalladium intermediate to the double bond with subsequent cleavage of palladium hydride and restoration of unsaturation is the basis of another important process involving organopalladium compounds, the Heck reaction. In a catalytic version, the organopalladium intermediate is formed by oxidative addition to carbon-halogen or other carbon-heteroatom bonds, and the hydridopalladium complex is believed to be recycled by base-promoted reductive elimination (Scheme 5.7). 

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