Experimental and theoretical studies on the asymmetric cyanosilylation of C2-symmetric hydrazones
作者:Abel Ros、Elena Díez、Eugenia Marqués-López、Eloísa Martín-Zamora、Juan Vázquez、Javier Iglesias-Sigüenza、Rafael R. Pappalardo、Eleuterio Álvarez、José M. Lassaletta、Rosario Fernández
DOI:10.1016/j.tetasy.2008.03.020
日期:2008.5
The Et2AlCl-promoted asymmetric cyanosilylation of (2S,5S)-1-amino-2,5-diphenylpyrrolidine-derived aliphatic hydrazones affords the corresponding hydrazino nitriles with high diastereoselectivity (dr 91:9 to >99:1). The resolving properties of the auxiliary allowed the isolation of the adducts as single diastereoisomers (dr >99:1) in good yields (80-84%) after chromatography. Ab initio MO calculations indicated that the formation of the hydrazone-promoter complex inhibits n ->pi conjugation and increases the nucleophilicity of the dialkylamino nitrogen, enabling the basic activation of TMSCN. The calculated geometries for these complexes show the shielding of the Si face of the C=N bond by one of the phenyl groups in the auxiliary, providing an explanation for the observed absolute configuration. (C) 2008 Elsevier Ltd. All rights reserved.
Asymmetric Mannich-Type Addition of Ketene Silyl Acetals and Thioacetals to<i>N,N</i>-Dialkylhydrazones
作者:Rosario Fernández、José María Lassaletta、Elena Díez、Auxiliadora Prieto、Monika Simon、Juan Vázquez、Eleuterio Álvarez
DOI:10.1055/s-2006-926283
日期:——
The choice of the 2,6-diphenylpiperidine moiety as the N,N-dialkylamino auxiliary in simple aliphatic dialkylhydrazones and the use of scandium triflate as the catalyst in aqueous media appear as the key strategies that enable the highly diastereoselective nucleophilic addition of ketene silyl acetals and thioacetals. The reaction proceeds to afford the expected adducts in high yields (88-98%) and diastereomeric ratios of up to 99:1. N-N bond cleavage of adducts affords enantiomerically pure β-amino esters.