Potassium Arylethynyltrifluoroborate as an Unstrained Dienophile for Ultrafast and On Demand Activation of Bioorthogonal IEDDA Reactions

<p>Strained alkenes and alkynes are the predominant dienophiles used in inverse electron-demand Diels-Alder (IEDDA) reactions, however, their instability, cross-reactivity and accessibility are problematic. Unstrained dienophiles, although physiologically stable and synthetically accessible, react with tetrazines significantly slower relative to strained variants. Here we report the development of potassium arylethynyltrifluoroborates as unstrained dienophiles for ultrafast, chemically triggered IEDDA reactions. By varying the substituents on the tetrazine (e.g. pyridyl- to benzyl-substituents), cycloaddition rates can vary from nearly spontaneous (<i>t</i>_1/2≈ 9 s) to no reaction with the unstrained alkyne-BF3 dienophile. The reported system was applied to protein modification and enabled mutually orthogonal labelling of two distinct proteins.</p>

Potassium Arylethynyltrifluoroborate as an Unstrained Dienophile for Ultrafast and On Demand Activation of Bioorthogonal IEDDA Reactions

<p>Strained alkenes and alkynes are the predominant dienophiles used in inverse electron-demand Diels-Alder (IEDDA) reactions, however, their instability, cross-reactivity and accessibility are problematic. Unstrained dienophiles, although physiologically stable and synthetically accessible, react with tetrazines significantly slower relative to strained variants. Here we report the development of potassium arylethynyltrifluoroborates as unstrained dienophiles for ultrafast, chemically triggered IEDDA reactions. By varying the substituents on the tetrazine (e.g. pyridyl- to benzyl-substituents), cycloaddition rates can vary from nearly spontaneous (<i>t</i>_1/2≈ 9 s) to no reaction with the unstrained alkyne-BF3 dienophile. The reported system was applied to protein modification and enabled mutually orthogonal labelling of two distinct proteins.</p>

Isomeric triazines exhibit unique profiles of bioorthogonal reactivity

Isomeric triazines can be tuned to exhibit unique reaction profiles with biocompatible strained alkenes and alkynes.

Reversible alkene binding and allylic C–H activation with an aluminium(i) complex

The monomeric molecular aluminium(i) complex 1 [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl) reacts with a series of terminal and strained alkenes including ethylene, propylene, allylbenzene and norbornene to form alkene bound products.

Reversible Alkene Binding and Allylic C–H Activation with an Aluminum(I) Complex

Reversible alkene binding to a low-valent main group complex is documented. The reaction involves an aluminium(I) reagent and includes both terminal and strained alkenes. This reversible binding event is just a forerunner to non-reversible C–H activation of the allylic position of the alkene. Mechanistic analysis shows that in contrast to common transition metal systems, the C–H activation does not proceed from a metal bound alkene complex. Dissociation of the alkene and reformation of the aluminium(I) fragment is required to liberate the active site and frontier molecular orbitals involved in C–H activation.

Reversible Alkene Binding and Allylic C–H Activation with an Aluminum(I) Complex

Reversible alkene binding to a low-valent main group complex is documented. The reaction involves an aluminium(I) reagent and includes both terminal and strained alkenes. This reversible binding event is just a forerunner to non-reversible C–H activation of the allylic position of the alkene. Mechanistic analysis shows that in contrast to common transition metal systems, the C–H activation does not proceed from a metal bound alkene complex. Dissociation of the alkene and reformation of the aluminium(I) fragment is required to liberate the active site and frontier molecular orbitals involved in C–H activation.

Reversible Alkene Binding and Allylic C–H Activation with an Aluminum(I) Complex

Reversible alkene binding to a low-valent main group complex is documented. The reaction involves an aluminium(I) reagent and includes both terminal and strained alkenes. This reversible binding event is just a forerunner to non-reversible C–H activation of the allylic position of the alkene. Mechanistic analysis shows that in contact to common transition metal systems, the C–H activation does not proceed from a metal bound alkene complex. Dissociation of the alkene and reformation of the aluminium(I) fragment is required to liberate the active site and frontier molecular orbitals involved in C–H activation.

Acylation-Mediated ‘Kinetic Turn-On’ of 3-Amino-1,2,4,5-tetrazines

The fast and biocompatible ligation of 1,2,4,5-tetrazines with strained alkenes has found numerous applications in biomedical sciences. The reactivity of a 1,2,4,5-tetrazine can generally be tuned by changing its electronic properties by varying the substituents in the 3- and/or 6-position. An increased reactivity of such bioorthogonal probes upon conjugation or attachment to a target molecule has not previously been described. Such an approach would be beneficial, as it would minimize the impact of residual tetrazine reagents and/or impurities. Herein, we describe such a ‘kinetic turn-on’ of 1,2,4,5-tetrazines upon conjugation. On the basis of the significant increase in reactivity following N-acylation predicted by quantum chemical calculations, we prepared 3-aminotetrazines and their corresponding acetylated derivatives. An investigation of the reaction kinetics indeed revealed a remarkable increase in reactivity upon acylation.

Orthogonal, dual protein labelling by tandem cycloaddition of strained alkenes and alkynes to ortho-quinones and azides

Novel click chemistry using SPAAC and SPOCQ in tandem efficiently provides dual-labelled antibody–drug–dye conjugates.