'Click'-BINOLs: A New Class of Tunable Ligands for Asymmetric Catalysis

Abstract

A new class of easily tunable 1,2,3-triazole-BINOL ligands, 'click'-BINOLs, have been synthesized from readily available alkynes and several azido-BINOL derivatives using the powerful Huisgen [3+2] cycloaddition 'click' approach. The activity of these ligands in asymmetric Lewis acid catalysis has been explored for the first time in the diethylzinc addition to aldehydes. The C ₂-symmetric ligand 1d showed an interesting catalytic behavior, which suggests the non-innocent participation of the triazole units. Thus, good enantioselectivities (up to 86% ee) were obtained by both the right selection of the substitution pattern at the triazole ring and the fine tuning of the reaction conditions.

Key words

Lewis acids - asymmetric catalysis - 'click' chemistry - BINOL ligands - alkynes - aldehydes - diethylzinc additions

• References


For some recent reviews on BINOL and BINOL derivatives, in asymmetric catalysis, see:
• 1a Pu L. Chem. Rev. 1998; 98: 2405
• 1b Chen Y, Yekta S, Yudin A. Chem. Rev. 2003; 103: 3155
• 1c Kocovsky P, Vyskocil S, Smrcina M. Chem. Rev. 2003; 103: 3213
• 1d Brunel JM. Chem. Rev. 2005; 105: 857
• 1e Shibasaki M, Matsunaga S. Chem. Soc. Rev. 2006; 35: 269
• 1f Nájera C, Sansano JM, Saá JM. Eur. J. Org. Chem. 2009; 2385

For some recent reviews on Huisgen [3+2] cycloaddition:
• 2a Diez-Gonzalez S. Curr. Org. Chem. 2011; 15: 2830
• 2b Agalave SG, Maujan SR, Pore VS. Chem.–Asian J. 2011; 6: 2696
• 2c Majumdar KC, Ray K. Synthesis 2011; 3767 ; and references cited therein
• 2d See also: Huisgen R. In 1,3-Dipolar Cycloaddition Chemistry . Padwa A. Wiley; New York: 1984: 1

For the first contributions on copper-catalyzed alkyne-azide cycloaddition (CuAAC), see:
• 3a Tornoe CW, Christensen C, Meldal M. J. Org. Chem. 2002; 67: 3057
• 3b Rostovtsev VV, Green LG, Fokin VV, Sharpless KB. Angew. Chem. Int. Ed. 2002; 41: 2596
• 4 For the introduction and definition of the term 'click' chemistry, see: Kolb H, Finn MG, Sharpless KB. Angew. Chem. Int. Ed. 2001; 40: 2004
• 5 For the synthesis and use of 1,2,4-triazole-BINOLs in asymmetric catalysis, see: Liu B, Jiang F.-Y, Song H.-B, Li J.-S. Tetrahedron: Asymmetry 2006; 17: 2149

For some reviews on the addition of Et₂Zn to aldehydes, see:
• 6a Noyori R, Kitamura M. Angew. Chem. Int. Ed. 1991; 30: 49
• 6b Soai K, Niwa S. Chem. Rev. 1992; 92: 833
• 6c Knochel P, Singer RD. Chem. Rev. 1993; 93: 2117
• 6d Noyori R. Asymmetric Catalysis in Organic Synthesis . Wiley; New York: 1994. Chap. 5
• 6e Pu L, Yu H.-B. Chem. Rev. 2001; 101: 757

For examples of the non-innocent action of triazoles in asymmetric catalysis, see:
• 7a Bastero A, Font D, Pericàs MA. J. Org. Chem. 2007; 72: 2460
• 7b Alza E, Cambeiro XC, Jimeno C, Pericàs MA. Org. Lett. 2007; 9: 3717
• 8 For classical metallation of the C–H bonds in 1,2,3-triazoles with organolithium reagents, see, for example: Gschwend HW, Rodriguez HR. In Organic Reactions . Heteroatom-Facilitated Lithiations . Wiley; New York: 2005. 1.
• 9 The 5-substituted triazole ligand 1d′ was synthesized by the reaction of potassium phenylacetylide with the corresponding azide: Kwok SW, Fotsing JR, Fraser RJ, Rodionov VO, Fokin VV. Org. Lett. 2010; 12: 4217
• 10 Alternatively, Ru-catalyzed 1,3-cycloadditions can be used to form 1,5-triazoles, see for example: Zhang L, Chen X, Xue P, Sun HH. Y, Williams ID, Sharpless KB, Fokin VV, Jia G. J. Am Chem. Soc. 2005; 127: 15998
• 11 Li H, Da C.-S, Xiao Y.-H, Li X, Su Y.-N. J. Org. Chem. 2008; 73: 7398
• 12 Lian J.-J, Chen P.-C, Lin Y.-P, Ting H.-C, Liu R.-S. J. Am. Chem. Soc. 2006; 128: 11372
• 13 Shi M, Wang CJ. Tetrahedron: Asymmetry 2002; 13: 2161
• 14 Hamashima Y, Sawada D, Kanai M, Shibasaki M. J. Am. Chem. Soc. 1999; 121: 2641