Oxidative addition of a 8-bromotheobromine derivative to d 10 metals

Reaction of 8-bromo-7-ethyl-3-methylxanthine 1 with zerovalent group 10 metal complexes gave via an oxidative addition of the C–Br bond the neutral complexes trans-[2] (M = Pd) and trans-[3] (M = Pt) bearing a theobromine-derived azolato ligand. While the oxidative addition to [Pd⁰(PPh₃)₄] gave exclusively trans-[2], the reaction with the more substitution-inert [Pt⁰(PPh₃)₄] yielded after 1 day the kinetic product cis-[3], which was converted under heating for a total of 3 days completely into the thermodynamically more stable complex trans-[3]. Treatment of trans-[2], trans-[3] or the mixture of cis-/trans-[3] with the proton source HBF₄⋅Et₂O led to complexes trans-[4]BF₄, trans-[5]BF₄ and a mixture of cis/trans-[5]BF₄ with retention of the original ratio of cis to trans, respectively. The molecular structures of the azolato complexes trans-[2] and trans-[3] and of the theobromine derived pNHC complexes trans-[4]BF₄ and trans-[5]BF₄ have been determined by X-ray diffraction studies.