Equilibrium between η2-(o-ethynylbenzoyl)rhenium complexes and rhenium isobenzofuryl carbene complexes and subsequent reactions of isobenzofuryl carbene complexes

Thermal rearrangements of eta(2)-(o-ethynylbenzoyl)-Re complexes 2 produced isobenzofuryl-Re carbene complexes 3. This reaction might proceed via nucleophilic attack of the carbonyl oxygen on the Re-bound alkyne or might proceed through a [1.5,1] Re shift followed by an electrocyclic ring closure. The two species, alkyne complex 2 and carbene complex 3, were observed at equilibrium. Isobenzofuryl carbene complexes 3 were characterized by H-1 and C-13 NMR spectroscopy and by trapping via Diels-Alder reactions with DMAD. Air oxidation of the equilibrium mixtures of 2 and 3 led to oxidation of the isobenzofuryl carbene complexes 3 to unexpected Re eta(2)-ketone complexes 5. Studies with O-17-labeled dioxygen are consistent with oxygen addition to the isobenzofuran followed by rearrangement of the resulting endoperoxide that involves movement of the furan oxygen to the eta(2)-ketone site in 5.