Rhenium-Containing Phosphorus Donor Ligands for Palladium-Catalyzed Suzuki Cross-Coupling Reactions:  A New Strategy for High-Activity Systems

The chiral racemic methyl complex (η5-C5H5)Re(NO)(PPh3)(CH3) is converted to the rhenium-containing phosphorus donor ligands (η5-C5H5)Re(NO)(PPh3)((CH2)nPR2) (n/R = 3a, 0/Ph; 3b, 0/t-Bu; 3c, 0/Me; 5a, 1/Ph; 5b, 1/t-Bu) and (η5-C5H4PR2)Re(NO)(PPh3)(CH3) (7; R = a, Ph; b, t-Bu) via standard reactions (3, TfOH/CH2Cl2 or HBF4/chlorobenzene, then PR2H, then t-BuOK; 5, Ph3C+X-, then PR2H, then t-BuOK; 7, n-BuLi, then PR2Cl). (η5-C5H4PR2)Re(CO)3 (R = Ph, t-Bu) is prepared from (η5-C5H5)Re(CO)3 analogously to 7. Most of these species are effective ligands for palladium-catalyzed Suzuki couplings. Typical conditions involve toluene solvent, an aryl bromide (1.0 equiv), phenylboronic acid (1.5 equiv), K3PO4 (2.0 equiv), Pd(OAc)2 (1 mol %), the rhenium/PR2 species (4 mol %), and 60−100 °C. In the cases of 3 and 5, the rhenium/PR2 species are generated in situ from indefinitely stable conjugate acids [rhenium/PR2H]+ and t-BuOK (2 equiv or 8 mol %). The bulkier and more electron-rich rhenium/P(t-Bu)2 systems generally give more active catalysts than the rhenium/PPh2 analogues. Under many conditions, the activities of 3a and 3b approach (but do not exceed) those of the corresponding organophosphines PPh3 and P(t-Bu)3, the latter being a benchmark ligand for Suzuki couplings. Turnover numbers of >1000 are easily realized. Chloroarenes can be coupled, but at much slower rates and in lower yields. The crystal structures of 5b and 7b are determined. The trigonal phosphorus atoms become increasingly pyramidalized in the series 5b < 5a < 7b.