Paul N. O'Shaughnessy

Biaryl amine ligands for lanthanide catalysed enantioselective hydroamination/cyclisation of aminoalkenes

Abstract Arylations of (R)-2,2′-diamino-6,6′-dimethylbiphenyl and the corresponding racemic compound under palladium catalysis gives a range of C2-symmetric secondary amine proligands H2L. The 3,5-di-tert-butylphenyl substituted compound reacts with the silylamides [M{N(SiMe2H)2}3(THF)2] (M=Y, Sm, La) to produce (±)-[ML{N(SiMe2H)2}(THF)2]. However, deprotonation of more sterically hindered proligands H2L proceeded very slowly, but the reaction with the more basic akylamide [Y(NiPr2)3(THF)2] gave [YL1-4(NiPr2)(THF)2] readily. Such catalysts formed in situ from the amine proligand H2L and metal amide, cyclised 2,2′-dimethylaminopent-4-ene to the corresponding pyrrolidine with enantiomeric excesses up to 50%. The alkylamide catalysts gave significantly faster turnover than the silylamides.

Biaryl-bridged Schiff base complexes of zirconium alkyls: synthesis structure and stability

Abstract Three substituted salicylaldimine derivatives H2L1–3 of 2,2′-diamino-6,6′-dimethylbiphenyl give, under appropriate conditions, isolable alkyls of zirconium [ZrL1–3R2] (R=CH2Ph, CH2But). Two molecular structures confirm their cis-α geometry (C2-symmetric with cis alkyl ligands). They decompose via 1,2-migratory insertion of an alkyl group to imine, followed in some instances by a second similar reaction. The dimeric molecular structure of one such doubly-inserted product is presented. The kinetics of decomposition by this process are studied briefly, and it is noted that the rate increases with increased steric demand of the salicylaldimine unit.

Non-planar manganese Schiff-base complexes; synthesis and molecular structures

Abstract The reactions of anhydrous sodium salts of three chiral biaryl-bridged salicylaldimine tetradentate proligands H2L with manganese(II) chloride give the corresponding solvated complexes [MnIIL]. The molecular structure of one chiral nonracemic example with a relatively low steric demand ligand set is shown to be composed of a homochiral dimer with bridging phenoxy groups. Both LMn units adopt the cis-β structure with Δ helicity as predetermined by the (R)-configuration of the biaryls. Oxidation of these compounds with halogens gives manganese(III) complexes. A complex [MnIIILI] has a trigonal bipyramidal structure with similar cis-β structure to that above. In contrast, the complex [MnL(OH2)2]Cl has the rather rare C2-symmetric cis-α structure. It is thus apparent then that while the biaryl unit in these and similar compounds is able to predetermine the chirality-at-metal very efficiently, it is quite possible for conversions between diastereomeric forms cis-β and cis-α, albeit with the same helicity, to occur in response to the nature of the co-ligands.

Zirconium catalysed enantioselective hydroamination/cyclisation

A chiral zirconium alkyl cation catalyses the cyclisation of certain aminoalkenes with enantioselectivity up to 82%, the highest thus far observed for such a process.

Chiral-at-metal organolanthanides: enantioselective aminoalkene hydroamination/cyclisation with non-cyclopentadienyls

Chiral non-racemic complexes [ML{N(SiMe2H)2}(thf)] (M = Y, La, H2L = salicylaldimine ligands derived from 2,2′-diamino-6,6′-dimethylbiphenyl) are found not to be effective catalysts for the intramolecular hydroamination of aminoalkenes, but new amino/phenoxide ligand designs without reducible functional groups led to long-lived and enantioselective catalysts.