Zirconium‐Catalyzed Enantioselective Hydroamination/Cyclization.

Abstract

Ring closure reactionsRing closure reactions O 0130 Zirconium-Catalyzed Enantioselective Hydroamination/Cyclization. -A chiral zirconium alkyl cation catalyzes the cyclization of certain aminoalkenes with enantioselectivities up to 82%, the highest thus observed so far for such a process. -(KNIGHT, P. D.; MUNSLOW, I.; O'SHAUGHNESSY, P. N.; SCOTT*, P.; Chem. Commun. (Cambridge) 2004, 7, 894-895; Dep. Chem., Univ. Warwick, Coventry CV4 7AL, UK; Eng.) -M. Paetzel 33-046

Transition‐Metal‐Catalyzed Hydroamination Reactions

“…This enhanced reactivity may be attributable to the formally cationic metal center in this zwitterionic species, thereby realizing a catalytic species that is isoelectronic with the more reactive rare earth elements. Interestingly, while previously reported cationic Zr species have had a substrate scope limited to secondary amines [64], Sadow's zwitterionic catalysts are limited in most cases to primary amines. Further evidence consistent with the proposed mechanism shows that secondary amine substrates can indeed undergo cyclohydroamination in the presence of 10-30 mol% of n-propylamine as a primary amine additive.…”

“…This focused topic has been comprehensively reviewed by Hultzsch [280] and is briefly summarized in the comprehensive review of 2008 [10]. Most published contributions have focused on the use of rare earth and early transition metal (Section 15.2.5) catalysts and indeed these have been some of the most successful examples reported to date [43,63,64,66,67,[74][75][76][77][78][79][80][81][82][83][84][85][281][282][283][284][285][286][287][288], likely because of the clear mechanistic requirement of a reactive M-N bond (single or double, depending on the mechanistic proposal) in the catalytic cycle. In late transition metals, where many reactions have been shown to proceed via outer-sphere amine addition to coordinated alkene, it is challenging to control such additions distal to the metal.…”

“…To realize expanded reactivity with unactivated alkenes, protected nitrogen substrates have been used to advantage and this will be summarized later in this section. Interestingly, while intramolecular enantioselective hydroamination of unactivated alkenes with alkylamines has been extensively reported for rare earth metals [85,[284][285][286][287] and early transition metals [43,63,64,66,67,[74][75][76][77][78][79][80][81][82][83][84]288], it was not until 2010 that late transition metals were reported to give comparable results. Buchwald [291] disclosed asymmetric intramolecular hydroamination of unprotected secondary aminoalkenes to give chiral pyrrolidine products with ee's up to 91%.…”

Modular N,O‐Chelating Ligands: Group‐4 Amidate Complexes for Catalytic Hydroamination

“…There have been a few reports of cationic group-4 complexes that can catalyze the hydroamination of secondary aminoalkenes. [113,114] These reactions are thought to proceed through a σ-bond insertion mechanism, analogous to the established mechanism for the isovalent lanthanide catalysts. [115,116] Furthermore, these cationic group-4 complexes are unable to catalyze the cyclization of primary aminoalkenes, an observation that was attributed to the formation of catalytically inactive imido complexes.…”

“…[128][129][130][131][132][133][134] The cationic group-4 complexes that were mentioned in the preceding section are capable of asymmetric hydroamination of secondary aminoalkenes, albeit with modest enantioselectivities. [114] Very recently, Bergman and co-workers reported a number of neutral bis(amido)zirconium precatalysts for the asymmetric hydroamination of aminoalkenes which displayed ee values of up to 80 %. [135] Our efforts in this area include the synthesis of tethered bis(amidate) ligands using 2,2Ј-diamino-6,6Ј-dimethylbiphenyl as the diamine backbone to control both the coordination isomerism and the chirality.…”