Early transition metal catalyzed-hydroboration of alkenes

A variety of well-defined early transition metal complexes were tested as catalyst for the hydroboration of 1-hexene using catecholborane as boration agent. The variations applied concerned both metal (Y, La, Ti, Zr) and spectator ligand system (bis(cyclopentadienyl), bis(pentamethylcyclopentadienyl), bis(benzamidinate), cyclopentadienyl-benzamidinate, cyclopentadienyl-aryloxy, and cylopentadienyl-amido). Most systems proved to be catalytically active (anti-Markovnikov regioselectivity, side reactions negligible), but the observed activity was in all cases lower than reported for Cp*2LaCH(SiMe3)2 [1]. Inactivation of the catalyst by catecholborane or Lewis base-metal complex induced disproportionation of catecholborane appeared to compete effectively with the catalytic conversion, thus leading to inefficient catalytic performance and complicated product mixtures. Systems with bidentate, cyclopentadienyl ligands with a pending anionic (aryloxy, amido) function showed a moderate catalytic activity but excellent catalyst stability and open up interesting perspectives for further exploration