Insights into Rare-Earth Metal Complex-Mediated Hydroamination

Abstract

Since the precise understanding of the underlying mechanism is important for the rational design of catalysts, much attention has been paid for hydroamination reactions catalyzed by organolanthanides during the past three decades. Distinct mechanisms were proposed on the basis of some key experimental features. It is a challenge to distinguish these mechanisms. The present study focuses on this controversial topic by synthesizing an almost entire range of rare-earth metal bisalkyl complexes (Py-CH2-Flu)Ln(CH2SiMe3)2(THF)x (Ln = Sc (1), Lu (2), Tm (3), Er (4), Ho (5), Y (6), Dy (7), Tb (8), Gd (9), and Pr (10)) stablized by a pyridine methylene fluorenyl ligand to catalyze intramolecular hydroamination (IAHA) of prototypical 1-amino-2,2-diphenyl-4-pentene and intermolecular hydroamination (IEHA) of styrene and pyrrolidine. Among them, thulium-, terbium-, and gadolinium-based catalysts were investigated for the first time. As observed for all the reported catalytic systems, the rate of IAHA increased almost linearly with increasing metal ion radius, whereas the reactivity pattern in IEHA with respect to the ionic radius size follows the opposite trend. Kinetic studies were carried out to obtain the empirical rate law v = k[Ln]1.0[S]0.0 for IAHA and v = k[Ln]1.0[St]1.0[pyrrolidine]0.0 for IEHA. The zero-order dependence on amine was attributed to the population ratio between the rare-earth metal amido adduct and its precursor being larger than 104. Density functional theory studies on scandium, yttrium, and praseodymium complex-mediated IAHA and IEHA were investigated using a stepwise σ-insertive mechanism and a concerted noninsertive mechanism, respectively. The results suggested the prevailing mechanism to be a stepwise σ-insertive pathway that involves the insertion of C═C into the Ln–N bond and protonolysis of the Ln–C bond. The protonolysis reaction is the turnover-limiting step. The reason behind the influence of the metal ion radius on the catalytic activity was elucidated.