A class of chiral tridentate ligands was explored to improve the titanium(IV) catalyzed asymmetric intramolecular hydroamination of aminoallenes. Alkylation of D- and L-amino acid esters with Grignard reagents, followed by condensation with salicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, or 5-fluorosalicylaldehyde yielded dianionic Schiff base ligands (X2L). The modular synthesis allows for substantial steric and electronic differentiation of the ligands. Retention of stereochemistry was confirmed by NMR studies using chiral shift reagents. Four ligands were structurally characterized by X-ray crystallography. Subsequent coordination to titanium(IV) metal precursors was examined. The X-ray structure of Ti(X2L)Cl(NMe2) was obtained and the complex exhibits distorted square pyramidal geometry relative to the titanium center. Metalation with Ti(NMe2)4 yielded Ti(X2L)(NMe2)2 complexes, which were active catalysts for the ring closing hydroamination of aminoallenes. The reaction of hepta-4,5-dienylamine with 5 mol% catalyst shows poor regioselectivity between the pyridine and pyrrolidine products. However, the reaction with 6-methyl-hepta-4,5-dienylamine with 5 mol% catalyst gives exclusively the 2-(2-methyl-propenyl)-pyrrolidine product with up to 20% enantiomeric excess.

Catalytic intramolecular hydroamination of aminoallenes using titanium complexes of chiral, tridentate, dianionic imine-diol ligands

2019 Spring National Meeting

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