Palladacycle Formation and Reactivity

Palladacycles are amply reported in the literature [18]. We shall limit our review to those prepared by norbornene insertion and subsequent cyclization, which are relevant to the synthetic methods treated here. 

The intermediacy of palladacycles in the reaction of bromobenzene with norbornene in the presence of Pd(PPh3)4 as catalyst and KOAc as a base in anisole as solvent was initially suggested by the isolation, among other products, of two compounds (Eq. 9), the former resulting from norbornene insertion into an alkylpalladium bond, the latter clearly deriving from palladium migration from the alkyl to the aryl site and double norbornene insertion. In both cases the termination step involved p,y-C-C bond cleavage followed by P-H elimination. The stereochemistry of the norbornane unit invariably was exo [19]. 

This pointed to the intermediacy of a palladacycle which was actually isolated from the reaction of phenylnorbornylpalladium chloride dimer with sodium phenoxide at room temperature by trapping it with phenanthroline (62% yield) (Eq.lO) [20]. 

NMR spectroscopy showed that the phenanthroline ligand lies approximately on the palladacycle plane. 

A direct way of preparing a palladacycle was found for the special case of m-bromocyanobenzene which was reacted with Pd(PPh3)4 and norbornene in anisole at 105 °C. The palladacycle precipitated in an oligomeric form from the reaction solution in good yields (67% on Pd). The complex contains one molecule of triphenylphosphine per palladium and a coordination site is occupied by the cyano group of another palladacycle unit. (Eq. 11) [21]. 

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