Palladium complexes purine

The first purine-8-ylidene palladium complex was synthesised shortly afterwards by the Herrmann group [104]. Cleavage of a dimeric palladacycle with the azolium salt in the presence of sodium acetate (to preserve the acetate ligand on palladium) resulted in the respective monomeric palladium(II) carbene adducts (see Figure 6.45). 

Benzy 1-6-chloropurine reacts with tetrakis(triphenylphosphine)palladium in DCE to give, not only the 6-purinylpalladium(Il) complex 102, but a dinuclear complex 103. Using Stille coupling (RSnBus) only the 6-substituted purine is obtained <96ACS462>. 

As an alternative to addition of anionic nucleophiles followed by reoxidation, rhodium(l)-catalyzed C-H activation allowed the nucleophilic addition of alkenes to the intermediate Rh(i) carbene complex <2002JA13964, 2004JOC7329>. Purine behaved anomalously compared to other heterocycles, for which selective monoalkylation was observed, and underwent sequential substitution first at C-8 and then at C-6 (Equation 8). Caffeine was monoalkylated at C-8 in low yield (15%). Selectivity for C-8-arylation was also observed in the palladium-catalyzed C-H activation of 6-phenyl-9-benzylpurine (aryl iodides, 0.05 equiv Pd(OAc)2, 3 equiv Cul, 2.5 equiv CS2CO3, DMF, 160 °C, 60 h, 48-95% yields) <2006OL5389>. 

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