Synthesis of New Chiral Bidentate (Phosphinophenyl)benzoxazine P,N-Ligands

Abstract

Two new chiral bidentate (phosphinophenyl)benzoxazine P,N-ligands 2a and 2b were synthesized from highly enantiomer-enriched 2-(1-aminoalkyl)phenols 4. Ligand rac-2a was obtained on refluxing the t-Bu-substituted (aminomethyl)phenol 4a with 2-(diphenylphosphino)benzonitrile in chlorobenzene in the presence of anhydrous ZnCl2 followed by decomplexation (Scheme 2). This reaction, when carried out with (+)-(S)-4a, was accompanied by racemization at the stereogenic center of the alkyl side chain. The enantiomerically pure ligands (+)-(R)-2a and (-)-(S)-2a were obtained using a stepwise procedure via the amides (-)-(R)- and (+)-(S)-5b, respectively, followed by cyclization to benzoxazines (+)-(R)- and (-)-(S)-7b, respectively, with triflic anhydride and by F-atom substitution by diphenylphosphide (Schemes 3 and 5). In the case of the i-Pr analogue 2b, this last step resulted in racemization (Scheme 6). This was overcome by preparing the bromo derivative and introducing the diphenylphosphine group via Br/Li exchange and reaction with chlorodiphenylphosphine (Scheme 7). The first application of (+)-(R)-2a in an asymmetric Heck reaction showed high enantioselectivity (91%) (Scheme 8).