Synthesis and reactivity of the cyclohexadienyl rhodium complexes

In a search for an active catalyst for CH-activation reactions we have synthesized the half-sandwich rhodium complexes with a cyclohexadienyl ligand. In particular, the reaction of [(cyclooctene)2RhCl]2 with 1,3,5,5-tetramethylcyclohexadiene followed by addition of bromine gave the cyclohexadienyl polybromide complex [(η5-C6H3Me4)RhBr3]n (1) in 73% yield. Treatment of 1 with ethylene converted it into the Rh(III) bromide complex [(η5-C6H3Me4)RhBr2]2 (2), which is analogous to the classical catalyst [Cp*RhCl2]2. The reaction of 2 with 2 equiv. of 2-electron ligands L produced the expected adducts (η5-C6H3Me4)RhBr2L (L = tBuNC, pyridine, P(OMe)3, PPh3) in 80–90% yields. On the other hand, the reactions of 1 or 2 with excess of tBuNC led to complete replacement of the cyclohexadienyl ligand. The complex 2 was found to be inactive as catalyst (at 10 mol-% loading) for reactions of diphenylacetylene with benzoic acid, acetanilide, O-Boc-phenylhydroxamic acid, or benzophenone oxime. The absence of catalytic activity was attributed to the displacement of the cyclohexadienyl ligand.