Bridge‐Clamp Bis(tetrazine)s with [N]₈ π‐Stacking Interactions and Azido‐s‐Aryl Tetrazines: Two Classes of Doubly Clickable Tetrazines

Abstract

Click chemistry at a tetrazine core is useful for bioorthogonal labeling and crosslinking. Introduced here are two new classes of doubly clickable s‐aryl tetrazines synthesized by Cu‐catalyzed cross‐coupling. Homocoupling of o‐brominated s‐aryl tetrazines leads to bis(tetrazine)s structurally characterized by tetrazine cores arranged face‐to‐face. [N]8 π‐stacking interactions are essential to the conformation. Upon inverse electron demand Diels–Alder (iEDDA) cycloaddition, the bis(tetrazine)s produce a unique staple structure. The o‐azidation of s‐aryl tetrazines introduces a second proximal intermolecular clickable function that leads to double click chemistry opportunities. The stepwise introduction of fluorophores and then iEDDA cycloaddition, including bioconjugation to antibodies, was achieved on this class of tetrazines. This method extends to (thio)etherification, phosphination, trifluoromethylation and the introduction of various bioactive nitrogen‐based heterocycles.

Coordination Chemistry of a Bis(Tetrazine) Tweezer: A Case of Host-Guest Behavior with Silver Salts

“…Bis(tetrazine) 3 and its functionalized derivatives display in the solid state a centroid-to-centroid separation of the central rings ranging between 3.3 and 3.5 Å ( Figure 1 ) [ 3 ]. Given this structure and the presence of eight nitrogen atoms as potential coordinating donors, we were intrigued about the potential coordination properties of this new bridge clamp structure with transition metals.…”

“…To obtain a first insight into the stoichiometry of the formed species, an additional titration following the continuous variation method was performed [ 33 , 34 , 35 , 36 , 37 , 38 ]. According to the Job plot shown in Figure 11 [ 27 , 28 , 29 , 30 , 31 , 32 ], the maximum of the variation of the chemical shift of H 9 protons is reached for x = [ 3 ] tot /([Ag] tot + [ 3 ] tot ) = 0.5. This suggests a 1:1 Ag/ 3 stoichiometry for the prevailing complex in solution.…”

“…Moreover, the fact that the optimized δ 12 value (6.98 ppm) for [Ag( 3 ) 2 ] + is ranging in-between those obtained for the free tweezer 3 (7.56 ppm) and the corresponding 1:1 silver complex (6.88 ppm) provides further support. Accordingly, an intermediate conformation of both tweezer molecules in [Ag( 3 ) 2 ] + , located somewhere between the closed form of 3 stabilized by [C 2 N 4 ]···[C 2 N 4 ] intramolecular π – π interactions [ 3 ], and the more open structure found in [Ag( 3 )] + , might be postulated.…”

“…The s -tetrazine (Tz) unit ( Figure 1 , 1 ) is an object of high interest in photophysics and for biomedical applications, owing to its electron-poor poly(hetero)aromatic nature that is also amenable to click chemistry [ 1 , 2 , 3 ]. The diversification of synthetic routes to conceive new tetrazine structures has become very useful to expand current applications [ 3 , 4 , 5 , 6 , 7 , 8 , 9 ]. We recently reported a copper-catalyzed homocoupling protocol, which allowed the efficient synthesis of constrained bis(tetrazines) with a unique bridge clamp structure ( Figure 1 , 3 ) [ 3 ].…”

“…The diversification of synthetic routes to conceive new tetrazine structures has become very useful to expand current applications [ 3 , 4 , 5 , 6 , 7 , 8 , 9 ]. We recently reported a copper-catalyzed homocoupling protocol, which allowed the efficient synthesis of constrained bis(tetrazines) with a unique bridge clamp structure ( Figure 1 , 3 ) [ 3 ]. These compounds have been studied in the solid state by X-ray diffraction (XRD, Figure 1 a) and analyzed by DFT.…”