Enantioselective hydroamination/cyclization of aminoalkenes by (bis)-C2 symmetric and (mono)-C2 symmetric anionic tetraamide complexes of La(III)

Abstract

a b s t r a c tEnantioselective hydroamination/cyclizations of several monosubstituted and disubstituted aminoalkenes have been achieved, catalyzed by tetracoordinate anionic La(III) complexes.Ó 2015 Published by Elsevier Ltd.The intramolecular hydroamination of alkenes constitutes a powerful and atom-economical method for the synthesis of nitrogen-containing heterocycles. 1,2 Both neutral and anionic complexes of Y(III) have shown utility in the asymmetric variation of this process. 3a,b We recently disclosed that neutral chelating diamide complexes of several group 3 metals prepared from simple N-(diaryl)methyl substituted diamide motifs derived from (R)-1,1 0 -binapthyl-2-2 0 -diamine are effective catalysts for enantioselective hydroamination/cyclization. 4 We subsequently demonstrated that (R)-N,N 0 -dibenzosuberyl-1,1 0 -binaphthyl-2,2 0 -diamine (1) is a vastly superior proligand for this process and showed that the chelating diamide complexes derived from La(III), the largest lanthanide metal, provided higher enantioselectivities than the more sterically congested Y(III) congeners. 5 In this communication, we describe the reactivity and enantioselectivity for intramolecular hydroamination catalyzed by a new family of Y(III)and La(III) ate-complexes originating from enantiopure 1 and achiral amide ligands. Catalyst generationThe catalytic complexes, 8a,b, 9a,b, and 10a-d utilized in this study were prepared by the following procedure. Sequential treatment of YCl 3 (THF) 3.5 or LaCl 3 (THF) 1.5 (1 equiv each) with Me 3 SiCH 2 Li (4 equiv) in THF generated solutions of the corresponding anionic homoleptic alkyl (Me 3 SiCH 2 ) 4 M À Li + complexes (4a: M = Y, 4b: M = La). Immediate addition of 1 (1 equiv, 20 min, 23°C) delivered the preformed solutions of 4a,b. Addition of supporting ligands 6, 7, or 1 (1 equiv each) subsequently afforded solutions of the anionic complexes 8a,b, 9a,b, and 10a,b. The anionic complex 10c (M 1 = La, M 2 = Na) was generated by the sequential treatment of 1 (2 equiv) with La[N(t-Bu)(TMS)] 3 5 (1 equiv) followed by NaN(TMS) 2 (1 equiv), whereas the neutral complex 10d was prepared by the treatment of 1 (2 equiv) with La[N(t-Bu)(TMS)] 3 (1 equiv). The tetra(amide) complexes 5a and 5b were directly prepared by amine elimination involving the sequential exposure of YCl 3 (THF) 3.5 or LaCl 3 (THF) 1.5 to LiN(t-Bu)(SiHMe 2 ) (4 equiv) followed by 1 (1 equiv). The selection of the -N(t-Bu)(SiHMe 2 ) ligand was based on the documented propensity of this substituent to engage in agostic bonding of the Si-H moiety to group 3 metals. 6 After complex formation was completed, THF was removed in vacuo and replaced by C 6 D 6 prior to the addition of the aminoalkenes (Scheme 1). Enantioselective hydroamination studiesWe initiated this investigation by examining the enantioselective hydroamination/cyclization of aminoalkene 11a catalyzed by anionic ate-complexes 4a and 4b. Cyclization of 11a in the http://dx.

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