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Two new atropisomeric 4-aminopyridine-based nucleophilic catalysts containing terphenyl 'blocking groups' have been prepared and evaluated for kinetic resolution (KR) of aryl alkyl sec-alcohols. One of these biaryls is shown to be the most selective atropisomeric catalyst yet prepared for several sec-alcohols but its low reactivity makes it non-optimal for use at room temperature (rt). Optimisation of the conditions for conducting KRs at rt using a previously described catalyst (containing a phenyl blocking group) at the 1 mol% level indicates that PPh3 (1 equiv) is beneficial for enantioselectivity and allows KR of (G)-1-(naphthyl)ethanol in less than 30 min with sO15 (i.e., w40% recovered alcohol with O95% ee). These conditions constitute a convenient and practical method for rapid KR of sec-alcohols and are anticipated to facilitate a detailed kinetic study of this catalytic manifold by calorimetry. q 2005 Elsevier Ltd. All rights reserved. Scheme 1. Synthesis of catalysts 2a-d from triflates 1a and 1b. Reagents and conditions: For 2a and 2b see Refs. 11 and 12, respectively. For 2c and 1b: (a) i. 3,5-diPh(C6H3)Br, Mg, THF; ii. PdCl2 (dppp) (23%); (b) 3,5-diPh(C6H3)B(OH)2, Pd(OAc)2, P(biph)Cy2, LiCl, K3PO4, toluene (50%). For 2d from 1b: (a) 3,5-di-(3,5-diMeC6H3)C6H3B(OH)2 (3), P(biph)Cy2, LiCl, K3PO4, toluene (39%). |
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