Modular ligand variation in calcium bisimidazoline complexes: effects on ligand redistribution and hydroamination catalysis

Abstract

A series of calcium complexes supported by chiral bisimidazoline ligands have been studied in the catalytic intramolecular hydroamination/cyclisation of amino-olefins. The complexes [Ca(R-BIM){N(SiMe(3))(2)}(THF)] (R = 4-C(6)H(4)Me, 5a, 4-C(6)H(4)F, 5b and (t)Bu, 5c) have given competitive enantioselectivities (up to 12%) when compared to current literature studies involving calcium. Bisimidazolines offer a significant advance over similar bisoxazoline ligands, by allowing a greater structural variance through a modular synthetic pathway.

Enantioselective Main Group Catalysis: Modern Catalysts for Organic Transformations

“…Recent attempts to use the chiral calcium-BOX system observed in hydrosilylation reactions (see Scheme 12, section 4) have shown promise at inducing stereogenic centers in hydroamination reactions producing almost complete conversion, however, the scope of the reaction was limited to intramolecular cyclization as before, with poor enantioselectivities being reported (ca 10% ee). It can be hypothesized that similarly poor enantiomeric excess is seen for the same reasons as observed in hydrosilylation reactions [127][128][129]. As stated previously, one of the issues that sometimes opposes the widespread utilization of group 2 centered catalysts, in particular calcium, is their propensity for rapid ligand redistribution.…”

“…The minutia of why this unexpectedly high selectivity has yet to be detailed, however, it is expected that -stacking of the phenyl rings on the diamine catalyst and substrate may have a stabilizing effect through the intermediate steps of the catalytic cycle. Further work by this group looked at the adaptation of these ethylene diamine ligands to form bisimidazoline ligands through the condensation reaction of the diamine 87 and Pinner salt 89 (Scheme 21) [128]. This adaptation proved detrimental to the function of the catalysts with selectivity of the cyclic hydroamination product being reduced to ca 10%.…”

Highly Fluorinated Tris(indazolyl)borate Silylamido Complexes of the Heavier Alkaline Earth Metals: Synthesis, Characterization, and Efficient Catalytic Intramolecular Hydroamination

“…[16][17][18] Asymmetric versions of these reactions have also been reported. [19][20][21] Some of us have recently described new Ca, Sr, and Ba heteroleptic [(LO)Ae{N(SiMe 2 R) 2 }(thf) x ] (R= H, Me) complexes bearing different types of N-and O-based amino-phenolate ligands (LO À ). The rates for the cyclohydroamination reactions decreased with increasing atomic number of the metal.…”

Chiral Imidazoline Ligands and Their Applications in Metal‐Catalyzed Asymmetric Synthesis†

“…In 2011, Ward and co‐workers synthesized calcium complex Ca(Bim)(py)[N(TMS) 2 ] 22 using isopropyl group on the stereogenic center and varying N ‐substituents. [ 48 ] Ca(Bim) 2 was also observed then treating 22 with THF‐d 8 , indicating ligand redistribution. The THF adduct Ca(Bim)[N(TMS) 2 ](THF) 23 was used in asymmetric intramolecular hydroamination.…”