EARLY TRANSITION-METAL CATALYZED-HYDROBORATION OF ALKENES

A variety of well-defined early transition metal complexes were tested as catalyst for the hydroboration of l-hexene using catecholborane as boration agent. The variations applied concerned both metal (Y, La, T i, Zr) and spectator ligand system (bis(cyclopentadienyl), bis(pentame thylcyclopentadienyl), bis(benzamidinate), cyclopentadienyl-benzamidin ate, cyclopentadienyl-aryloxy, and cylopentadienyl-amido). Most system s proved to be catalytically active (anti-Markovnikov regioselectivity , side reactions negligible), but the observed activity was in all cas es lower than reported for Cp2LaCH(SiMe(3))(2) [1]. Inactivation of t he catalyst by catecholborane or Lewis base-metal complex induced disp roportionation of catecholborane appeared to compete effectively with the catalytic conversion, thus leading to inefficient catalytic perfor mance and complicated product mixtures. Systems with bidentate, cyclop entadienyl ligands with a pending anionic (aryloxy, amido) function sh owed a moderate catalytic activity but excellent catalyst stability an d open up interesting perspectives for further exploration.