Synthesis and Characterization of Alkyllanthanum Biphenolate Complexes as Catalysts for Hydroamination/Cyclization and Hydrosilylation

Abstract

AbstractThe homochiral, dimeric biphenolate alkyllanthanum complex [La{(R)‐Biphen}{CH(SiMe3)2}]2 can be prepared by facile alkane elimination starting from [La{CH(SiMe3)2}3] and enantiopure (R)‐3,3′‐di‐tert‐butyl‐5,5′,6,6′‐tetramethyl‐1,1′‐biphenyl‐2,2′‐diol [H2{(R)‐Biphen}]. Single‐crystal X‐ray diffraction revealed that the two La{(R)‐Biphen}{CH(SiMe3)2} fragments are connected through bridging phenolate groups of the biphenolate ligands. The two different phenolate groups undergo an intramolecular exchange process in solution leading to their equivalence on the NMR timescale. The biphenolate alkyl complex shows high catalytic activity for hydroamination/cyclization of aminoalkenes, similar to previously known lanthanocene catalysts, but only low enantioselectivity. Addition of THF to [La{(R)‐Biphen}{CH(SiMe3)2}]2 leads to a monomeric tris‐THF adduct [La{(R)‐Biphen}{CH(SiMe3)2}(THF)3] with higher catalytic activity than the THF‐free homochiral dimer in the cyclization of 2,2‐dimethylpent‐4‐enylamine, suggesting that the dimeric structure of the catalyst system prevails under catalytic conditions in the absence of THF. Addition of HN(SiHMe2)2 to [La{(R)‐Biphen}{CH(SiMe3)2}(THF)3] results in the formation of [La{(R)‐Biphen}{N(SiHMe2)2}(THF)3] which is in equilibrium with its homochiral dimer [La{(R)‐Biphen}{N(SiHMe2)2}(THF)]2 at elevated temperatures. The biphenolate alkyl complexes exhibit good catalytic activity and diastereoselectivity in the hydrosilylation of styrene. Hydrosilylation of 1‐hexene and norbornene also proceeds with high diastereoselectivity but rather low activity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)