Cycloheptatrienyl Cyclopentadienyl Titanium Phosphane Ligands in Palladium‐Catalyzed Suzuki–Miyaura Cross‐Coupling Reactions

Abstract

The reaction of the cycloheptatrienyl cyclopentadienyl (Cht‐Cp) titanium sandwich complexes [(η7‐C7H7)Ti(η5‐C5R5)] (R = H, Me) with nBuLi followed by treatment with the chlorophosphanes ClPR′2 (R′ = Cy, tBu) afforded the Cht‐substituted phosphanes [(η7‐C7H6PR′2)Ti(η5‐C5R5)] (R = H, Me; R′ = Cy, tBu). Together with their previously reported Cp‐substituted analogues [(η7‐C7H7)Ti(η5‐C5H4PR2)] (R = Cy, tBu), these complexes were employed as ancillary phosphane ligands in palladium‐catalyzed Suzuki–Miyaura coupling between sterically encumbered aryl bromides and aryl boronic acids. The unusually short reaction times indicated rapid generation of catalytically active Pd0 species, and the reaction of Pd(OAc)2 with an excess of the phosphorus ligands afforded linear bis‐phosphane palladium(0) complexes. These reactions gave no indication of phosphane oxide formation, and it is supposed that the Cht‐Cp titanium sandwich moiety serves as an integrated reducing agent. Accordingly, the reaction of [(η7‐C7H7)Ti(η5‐C5M5)] with Pd(OAc)2 or Ag(OAc) resulted in a three‐electron redox reaction with formation of the half‐sandwich complex [(η5‐C5Me5)Ti(OAc)3] together with 7,7′‐bi‐1,3,5‐cycloheptatriene (ditropyl) and elemental Pd or Ag metal. On reaction with silver(I) trifluoromethanesulfonate, a four‐electron redox process furnished the tropylium titanate complex [C7H7][(η5‐C5Me5)Ti(OTf)4].