Derivatives of 2-Amino-2‘-diphenylphosphino-1,1‘-binaphthyl (MAP) and Their Application in Asymmetric Palladium(0)-Catalyzed Allylic Substitution

Abstract

( R )-(+)-2-Amino-2‘-hydroxy-1,1‘-binaphthyl (NOBIN, 5 ) can be readily converted into a series of novel N,N -disubstituted aminophosphines 9 and 23 − 25 . The N,N -dimethyl derivative ( R )- 9 (MAP) was prepared via a sequence involving reductive alkylation with CH 2 O and NaBH 4 ( 5 → 6 ), Pd(0)-catalyzed coupling of the corresponding triflate with Ph 2 P(O)H ( 7 → 8 ), and reduction of the resulting phosphine oxide with Cl 3 SiH ( 8 → 9 ). Variation of this scheme was required for the preparation of 23 − 25 as the phosphinylation failed in the presence of bulky N substituents; the N -protected triflate 17 was first coupled with Ph 2 P(O)H, and the resulting phosphine oxide 18 was reduced with Cl 3 SiH to give the aminophosphine 19, which was then subjected to reductive alkylation with individual ketones and NaBH 4 . The new P,N -binaphthyls thus obtained ( 23 − 25 and 9 ) were utilized as chiral ligands in Pd(0)-catalyzed allylic substitution. The enantioselectivites obtained for racemic 1,3-diphenylprop-2-en-1-yl acetate (±)- 26 and malonate nucleophiles, which gave ( S )-(−)- 28, ( R )-(+)- 29, and ( R )-(+)- 30 as the respective products (in up to 71−73% ee at room temperature with Cs 2 CO 3 in CH 2 Cl 2 and 9 or 23 as a ligand), are interpreted in terms of the chelated transition state 37 and preferential attack at the allylic terminus that is trans with respect to the phosphorus acceptor atom.