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

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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 CH2O and NaBH4 (5 → 6), Pd(0)-catalyzed coupling of the corresponding triflate with Ph2P(O)H (7 → 8), and reduction of the resulting phosphine oxide with Cl3SiH (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 Ph2P(O)H, and the resulting phosphine oxide 18 was reduced with Cl3SiH to give the aminophosphine 19, which was then subjected to reductive alkylation with individual ketones and NaBH4. 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 Cs2CO3 in CH2Cl2 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.

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