Catalytic "active-metal" template synthesis of [2]rotaxanes, [3]rotaxanes, and molecular shuttles, and some observations on the mechanism of the Cu(I)-catalyzed azide-alkyne 1,3-cycloaddition

Journal of the American Chemical Society

Volumn 129, Issue 39, 2007, Pages 11950-11963

  Aucagne, Vincent ^a   Berná, José ^a   Crowley, James D ^a   Goldup, Stephen M ^a   Hänni, Kevin D ^a   Leigh, David A ^a   Lusby, Paul J ^a   Ronaldson, Vicki E ^a   Slawin, Alexandra M Z ^b   Viterisi, Aurélien ^a   Walker, D Barney ^a  

^a UNIVERSITY OF EDINBURGH   (United Kingdom)

^b UNIVERSITY OF ST ANDREWS   (United Kingdom)

Author keywords

[No Author keywords available]

Indexed keywords

CATALYST ACTIVITY; CYCLOADDITION; LIGANDS; METAL IONS; SYNTHESIS (CHEMICAL);

ROTAXANE ARCHITECTURES; TEMPLATE REACTION;

PARAFFINS;

ALKYNE; AZIDE; COPPER; MACROCYCLIC COMPOUND; PYRIDINE DERIVATIVE; ROTAXANE;

ARTICLE; CATALYSIS; COVALENT BOND; CYCLOADDITION; CYTOARCHITECTURE; HUISGEN MELDAL FOKIN REACTION; ION TRANSPORT; METAL BINDING; MOLECULAR MECHANICS; PROTON NUCLEAR MAGNETIC RESONANCE; REACTION ANALYSIS; STOICHIOMETRY; SYNTHESIS;

EID: 34848870567     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja073513f     Document Type: Article

References (120)

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83. Note: Copper(I) acetylides are generally complex extended multiatom aggregates, at least in the solid state and in the absence of good nitrogen ligands (see ref 24). The types of reactive intermediates shown in Scheme 1 are not meant to be precise or definitive structures - indeed, some of them (e.g., A(ii) and B(ii)) are very closely related - but rather are meant to illustrate different (possible) features of the putative reactive intermediate: How many copper atoms are needed to play significant structural or electronic roles during the catalysis? Is the copper σ-acetylide π-activated by an additional Cu atom? If the intermediate has two or more copper atoms, is it doubly bridged - as envisaged for a Glaser coupling, which can be ruled out with bidentate ligands for Cu if azide is also coordinated - or singly bridged? Are the reacting azide and alkyne attached to the same or different Cu atoms?
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89. We also examined the effect of different alkyne substrates on the rotaxane formation. The alkynes (see the Supporting Information) were submitted to the standard click reaction conditions (Scheme 2). Using a longer, more flexible, alkylalkyne or an arylalkyne in the place of 2 led to similar yields of the corresponding [2]rotaxanes (and similar overall conversions of the substrates into triazole products), indicating the active-metal template rotaxane-forming reaction is rather insensitive to substrate modifications.
90. The standard reaction conditions use 1 equiv of each reagent and low temperatures to minimize the background uncatalyzed thermal cycloaddition. These conditions were chosen to allow the relative efficacy of the different macrocycles in promoting rotaxane formation to be assessed. No attempt was made to optimize the reaction conditions to improve the rotaxane yields reported in Figure 4. For macrocycles 1a, 1i, 1j, 1f, 11, 1m, and 1n - all of which produce significant rotaxane and are not degraded under the reaction conditions - virtually all the macrocyclic ligand can be converted to rotaxane by using extended reaction times and an excess of the azide and alkyne building blocks.
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102. 6), indicating that copper(I) ions interact with the macrocycles in every case.
103. Goshe, A. J.; Crowley, J. D.; Bosnich, B. Helv. Chim. Acta 2001, 84, 2971-2985.
104. 6) which catalyzes the formation of the thread 5.
105. The multidentate ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl) methyl]amine (TBTA) is a highly effective copper ligand for the CuAAC reaction [see refs 6b and 17a,b and Gupta, S. S.; Kuzelka, J.; Singh, P.; Lewis, W. G.; Manchester, M.; Finn, M. G. Bioconjugate Chem. 2005, 16, 1572-1579]. However, the individual triazole-functionalized "arms" of TBTA are weakly coordinating ligands, so TBTA probably functions by the alkyne and azide displacing two of the triazole groups to form the reactive intermediate.
106. Minimum energy conformations were calculated for each of the metal-free macrocycles using the Spartan molecular modeling program (Semi-Empirical, AM1). The results are provided in the Supporting Information.
107. In the case of 1e no hydrogen bonds are present, thus allowing the Cu(I) ion better access to the macrocycle's pyridine nitrogen atom. However, in each of macrocycles 1e and 1f the nitrogen atom of the pyridine is more hindered, which may reduce the binding ability of the ligand.
108. Internal alkynes have been found to react under CuAAC conditions; see: Díez-González, S.; Correa, A.; Cavallo, L.; Nolan, S. P. Chem. - Eur. J. 2006, 12, 7558-7564.
109. Macrocycle 1k is also photosensitive and decomposes slowly in ambient light.
110. Bidentate bipyridine and phenanthroline ligands have been shown to significantly enhance the kinetics of the CuAAC reaction; see ref 17b.
111. 23 Cu(I) present in solution than many of the other macrocycles shown in Figure 4 - is most likely the reason for the modest yield of rotaxane using 1 equiv of this macrocycle. We reasoned the yield should be improved by increasing the macrocycle:copper ratio.
112. For examples of 1,2,3-triazoles as ligands see refs 3, 6s, and 17a,b and (a) Liu, D.; Gao, W.; Dai, Q.; Zhang, X. Org. Lett. 2005, 7, 4907-4910.
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117. 6) sufficiently, nor isolate that motion from other conformational processes at low temperatures, to determine an accurate shuttling rate.
118. Durola, F.; Sauvage, J.-P. Angew. Chem., Int. Ed. 2007, 46, 3537-3540.
119. For an evaluation of ligand exchange rates at Cu(I) centers, see: Riesgo, E.; Hu, Y.-Z.; Bouvier, F.; Thummel, R. P. Inorg. Chem. 2001, 40, 2541-2546.
120. Note added in proof: Very recent calculations (Straub, B. F. Chem. Commun. published online 12th July 2007 DOI: 10.1039/b706926j) suggest that the metallacycle formed in Scheme 1 consists of a strain-free Cu-C(Cu)=C unit rather than the depicted Cu=C=C unit. The second copper atom in the type B(ii) and A(ii) reactive intermediates shown in Scheme 1 means they are perfectly set up to form such a Cu-C(Cu)=C metallacycle.