Synthesis of Overloaded Cyclopentadienyl Rhodium(III) Complexes via Cyclotetramerization of tert-Butylacetylene
Herein we describe the synthesis and reactivity of rhodium catalysts with the very bulky cyclopentadienyl ligand C8H3tBu4 (designated as tBu4Cp). The reaction of [Rh(cod)Cl]2 with tert-butylacetylene in the presence of Et3N gives the complex (tBu4Cp)Rh(cod) (60–65% yield), in which the cyclopentadienyl ligand tBu4Cp is assembled from four alkyne molecules. The oxidation of (tBu4Cp)Rh(cod) with chlorine or bromine gives the corresponding halide complexes (tBu4Cp)RhX2 (X = Cl (85%), Br (95%)), which have unusual 16-electron monomeric structures due to the steric shielding provided by tBu groups. A similar reaction with iodine gives the ionic dinuclear complex [(tBu4Cp)RhI3Rh(tBu4Cp)]I (99%) with halide bridges. The bromide complex (tBu4Cp)RhBr2 reacts with phosphorus ligands such as P(OMe)3, P(OPh)3, PMe2Ph, and PMePh2 to give the 18-electron adducts (tBu4Cp)RhBr2(PR3), but no reaction occurs with larger phosphines such as PPh3. The racemic chloride (tBu4Cp)RhCl2 can be separated into enantiomers by preparative TLC of its diastereomeric adducts with (R)-phenylglycinol. The complex (tBu4Cp)RhBr2 catalyzes C–H activation and annulation of O-pivaloyl-hydroxamate as well as insertion of phenyldiazoacetate into E–H bonds, although the reaction rates and the substrate scope are limited by the bulky tBu4Cp ligand.