Reductive Elimination from Palladium IV Metallacycles

The alkylpalladium(IV) complexes obtained by oxidative addition of alkyl (methyl, allyl, and benzyl) halides to the palladium(ll) metallacycle spontaneously undergo reductive elimination. This implies migration of the alkyl group R onto the aromatic site of the palladacycle (Eq. 19, R=CH2Ph, 72% yield). 

The reductive elimination process is likely to require that the coupling groups are placed in axial-equatorial rather than in equatorial-equatorial positions with respect to the plane defined by phenanthroline and palladium, and halide dissociation could favor this rearrangement [28bj. 

In the absence of phenanthroline as ligand the new palladium(II) complex thus obtained reiterates ring closure, oxidative addition, and reductive elimination (Eq. 20). 

A selective double alkylation at the two ortho positions of the aryl group is thus achieved. 

At this point the insertion equilibrium of norbornene into the o,o -disubsti-tuted aryl group is no longer favorable due to the steric effects exerted by substituents. As a consequence, norbornene spontaneously deinserts affording a new o,o -disubstituted arylpalladium halide. The entire reductive elimination sequence has been proved unequivocally by NMR monitoring and isolation of the organometallic intermediates [2]. 

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