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. 2012 Jun 6;134(22):9199-208.
doi: 10.1021/ja3000936. Epub 2012 May 24.

Reactivity of biarylazacyclooctynones in copper-free click chemistry

Chelsea G Gordon  1 , Joel L Mackey, John C Jewett, Ellen M Sletten, K N Houk, Carolyn R Bertozzi
Affiliations
Free PMC article

Reactivity of biarylazacyclooctynones in copper-free click chemistry

Chelsea G Gordon et al. J Am Chem Soc. .
Free PMC article

Abstract

The 1,3-dipolar cycloaddition of cyclooctynes with azides, also called "copper-free click chemistry", is a bioorthogonal reaction with widespread applications in biological discovery. The kinetics of this reaction are of paramount importance for studies of dynamic processes, particularly in living subjects. Here we performed a systematic analysis of the effects of strain and electronics on the reactivity of cyclooctynes with azides through both experimental measurements and computational studies using a density functional theory (DFT) distortion/interaction transition state model. In particular, we focused on biarylazacyclooctynone (BARAC) because it reacts with azides faster than any other reported cyclooctyne and its modular synthesis facilitated rapid access to analogues. We found that substituents on BARAC's aryl rings can alter the calculated transition state interaction energy of the cycloaddition through electronic effects or the calculated distortion energy through steric effects. Experimental data confirmed that electronic perturbation of BARAC's aryl rings has a modest effect on reaction rate, whereas steric hindrance in the transition state can significantly retard the reaction. Drawing on these results, we analyzed the relationship between alkyne bond angles, which we determined using X-ray crystallography, and reactivity, quantified by experimental second-order rate constants, for a range of cyclooctynes. Our results suggest a correlation between decreased alkyne bond angle and increased cyclooctyne reactivity. Finally, we obtained structural and computational data that revealed the relationship between the conformation of BARAC's central lactam and compound reactivity. Collectively, these results indicate that the distortion/interaction model combined with bond angle analysis will enable predictions of cyclooctyne reactivity and the rational design of new reagents for copper-free click chemistry.

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Figures

Figure 1
Figure 1
Reagent development for copper-free click chemistry. (a) The 1,3-dipolar cycloaddition between azides and cyclooctynes. (b) Oct, the first cyclooctyne developed as a bioorthogonal reagent, and its corresponding second-order rate constant for the reaction with benzyl azide., The reactivity of a cyclooctyne can be altered through (c) electronic, and (d) strain modulation. All rate constants are second order (M–1 s–1) and were measured at room temperature in CD3CN except for values noted with an asterisk (*) which were measured in CD3OD.
Figure 2
Figure 2
Distortion/interaction model. (a) Activation energy (ΔE⧧) for the reaction between 2-butyne and methyl azide is the sum of distortion energy (ΔE⧧d) and interaction energy (ΔE⧧i). (b) Perfluorination of the alkyne reduces ΔE⧧ of the reaction by increasing the magnitude of stabilizing interactions in the transition state and decreasing distortion energy. (c) Constraining the alkyne into an eight-membered ring reduces ΔE⧧ by decreasing the distortion energy required to bend the starting materials into their preferred transition state conformations. For a–c, calculated values are electronic energies, the potential energy of the molecule on a vibrationless potential energy surface. As all reactions are represented on separate energy diagrams, the depictions are only intended to facilitate comparisons of ΔE⧧, ΔE⧧d, and ΔE⧧i values and not the overall energies of starting materials or triazole products. Calculations were performed using B3LYP/6-31G(d). See Supporting Information for details.
Figure 3
Figure 3
Bond angles and reactivities of BARAC analogues. (a) BARAC analogues targeted for our initial study of distortion/interaction modulation. (b) Reactivity was probed empirically by measuring the second-order rate constant for the reaction of each analogue with benzyl azide in CD3CN at rt by 1H NMR spectroscopy. (c) Table shows both calculated and measured (X-ray crystallography data shown in parentheses) alkyne bond angles for compounds 6–16 as well as measured second-order rate constants and activation free energies (ΔG⧧exp) for the model reaction with benzyl azide. Also shown are calculated interaction (ΔE⧧i,calc) and total distortion energies (ΔE⧧d,calc = ΔE⧧d,calc, azide + ΔE⧧d,calc, alkyne) as well as overall electronic energies of activation (ΔE⧧calc) and free energies of activation (ΔG⧧calc) for the reaction of each analogue with methyl azide. All computational data provided for compounds 6–16 are for the trans-BARAC isomer. *Free energies were calculated for the reaction in acetonitrile. **The second-order rate constants shown for 15 and 16 were measured in CDCl3 due to the limited solubility of 15 in CD3CN.
Figure 4
Figure 4
Structural analysis of BARAC. (a) DFT calculations (B3LYP/6-31G(d)) of cis- and trans-BARAC. (b) Front and side view of BARAC obtained via X-ray crystallography. Crystalline BARAC exists as the trans conformer. Thermal ellipsoid plots are shown at 50% probability.
Figure 5
Figure 5
Flagpole methyl substituents sterically hinder the transition state. (a) Front and side views of the transition state of the reaction of 6 with methyl azide. (b) Front and side views of the transition state of the reaction of 15 with methyl azide. Transition states were modeled using B3LYP/6-31G(d).
Figure 6
Figure 6
Strain modulation through rehybridization. (a) Ring strain in cyclooctynes increases with increased unsaturation. We hypothesize that BARAC’s fused aryl rings and central lactam contribute significantly to the compound’s ring strain. (b) X-ray crystal structures show that the nitrogen atom of BARAC’s central lactam is sp2.2 hybridized, whereas DIMAC’s amide nitrogen atom is sp2 hybridized. Thermal ellipsoid plots are shown at 50% probability.
Figure 7
Figure 7
Under Curtin-Hammett conditions, BARAC’s amide conformation influences reactivity and regioselectivity. (a) The reaction coordinate diagram displays calculated activation free energies for reaction of the parent BARAC compound 6 with methyl azide in acetonitrile. Also shown are the relative energies of cis-6 and trans-6 and the barrier to cis/trans interconversion. Transition state images show only the lowest-energy regioisomers. (b) Calculated values for the interconversion and reaction of analogue 15 with methyl azide in acetonitrile. Transition state images show only the lowest-energy regioisomers.
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