Involvement of Pd IV in Catalytic Cycles

Formation of Pd(IV) intermediates by oxidative addition of alkyl halides to Pd(II) complexes [197, 198] and in other oxidative processes [199] is a well-known process. However, C(sp )-X electrophiles, such as aryl halides, are much less reactive in the oxidative addition to Pd(II) complexes, and therefore, the formation of Pd(IV) species from these electrophiles is less likely. The hypothetical mechanism for the Heck reaction based on Pd(II)/Pd(IV) has been analyzed computationally [200]. 

Reaction of [Ph2l]OTf with Pd(II) and Pt(II) gives metal(IV) species by formal transfer of Ph+ to the metal center [201, 202]. Intramolecular oxidative additions of C-C bonds to Pt(II) to form hexacoordinated Pt(IV) complexes is also known [203]. 

A Pd(IV) intermediate has also been proposed in the Negishi coupling using the pincer-Pd(II) complex 25 as catalyst [205]. Experimental and computational results suggested that a T-shaped 14e pincer complex 26 was the key intermediate, which undergoes oxidative addition with aryl bromides to yield the pentacoordinated 

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