Synthesis of N-Alkylated and N-Arylated Derivatives of 2-Amino-2‘-hydroxy-1,1‘-binaphthyl (NOBIN) and 2,2‘-Diamino-1,1‘-binaphthyl and Their Application in the Enantioselective Addition of Diethylzinc to Aromatic Aldehydes

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Abstract

High-yield reductive alkylation of (R)-(+)-1 and (R)-(+)-2 has been accomplished with a series of ketones (even as bulky as 2-adamantanone) and NaBH4/H2SO4 in THF at room temperature to give the respective N-alkylated binaphthyls 6a−c, 9a−c, and 10a−c. Related N-phenyl derivatives 14, 16, and 17 were obtained via the Pd(0)-catalyzed coupling of (R)-(+)-1 and (R)-(+)-2, respectively, with PhBr; no racemization was observed. Subsequent reductive methylation with CH2O and NaBH4/H2SO4 afforded the bidentate ligands 8a−c, 12a−c, 13a−c, 15, 18, and 19, which comprise a new class of binaphthyls. Their utility as chiral ligands has been demonstrated for the addition of Et2Zn to benzaldehyde and its congeners; the highest level of asymmetric induction was observed for N,N-dimethyl NOBIN (R)-(+)-3 (3 mol %) in conjunction with n-BuLi (5.4 mol %), which gave (R)-(+)-21a in 88% ee. Derivatives of the amino phenol 1 (NOBIN) proved more efficient than the corresponding diamines derived from 2. The stereochemical outcome and the enhancement of asymmetric induction by Li+ are discussed in terms of the chelated transition state 26.

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