ON THE MECHANISM OF C-H BOND ACTIVATION IN THE PHOTOCHEMICAL ARYLATION OF RHENIUM(V) OXOIODIDE COMPLEXES

Photolysis of the Re(V) oxo-iodide compound (HBpz(3))ReO(I)Cl (1) in a rene solvents gives the aryl complexes (HBpz(3))ReO(Ar)Cl. Substituted arenes react with electrophilic selectivity exclusively at aromatic C -H bonds, and a variety of functional groups are tolerated. Yields are improved when photolysis is carried out in the presence of pyridine. Photolysis of the diiodide complex, (HBpz(3))ReOI2 (2), gives a mixtur e of mono- and disubstituted Re-aryl complexes, and the diphenyl deriv ative (HBpz(3))ReOPh2 has been structurally characterized. The isotope effect found when 1 is photolyzed with a 1:1 mixture of C6H6 and C6D6 was found to depend on the concentration of 1. Lower concentrations ( similar to 0.5 mM) show a 1.2(2):1 ratio of do to dg products, while h igher concentrations (similar to 20 mM) show higher d(0):d(5) ratios o f 1.8(2):1. The apparent isotope effects increase with increasing conv ersion and are also affected by the presence of additives such as pyri dine or iodine. Photolysis of 1 with 1,3,5-trideuteriobenzene shows a 4.0(4):1 ratio of C-H vs C-D activation, which Is independent of react ion conditions. A mechanism for arene activation is proposed that invo lves initial arene binding, which discriminates intermolecularly betwe en arenes with a low isotope effect, followed by C-H bond cleavage, wh ich discriminates intramolecularly within an arene with a higher isoto pe effect. The reactive species appears to be a rhenium(IV) oxo comple x which can add aromatic C-H bonds across the Re=O linkage.