Reductive Amination on Highly Strained Trans-Cyclooctene: Developments in Reaction Optimization and Monitoring

• Benjamin Brenner, Biochemistry, University of Delaware

• Colette Makara, Chemistry and Biochemistry, University of Delaware

Tetrazine trans-Cyclooctene (TCO) ligation, developed in 2008 by the Fox Group, is the fastest known reaction in bioorthogonal chemistry with a rate constant of 10^4 to 10^6 (M*s)^-1. During this reaction, electron-rich tetrazine undergoes a 3+2 cycloaddition (Diels Alder) with TCO, an electron-poor dienophile. This addition is followed by a retro Diels Alder reaction, releasing inert nitrogen gas. Synthesis of TCO from cis starting material has previously been achieved with a flow photoisomerization apparatus, allowing non-functionalized TCOs such as (E)-cyclooct-4-enone (Keto-TCO) to be synthesized effectively. This work explores the reductive amination of Keto-TCO as a possible means of functionalized TCO synthesis, using n-butylamine as a model for prospective amines. Reductive amination converts a carbonyl group to an imine and then reduces it to an amine. Since many of its mechanical steps revolve around the carbonyl carbon, reductive amination of molecules with ring strain and steric bulk proves challenging, thus typical reaction conditions yield unsatisfactory results when performed on Keto-TCO. When monitored by thin-layer chromatography, imine formation appears to be the most significant bottleneck to chemical yield. In attempts to drive imine formation, several drying reagents were tested, yielding indistinguishable results. Furthermore, concentrated reactions seemed to decrease yield. Following this, reduction was attempted in several solvents. It was found that methanol aids in imine formation kinetics but encourages an unfavorable imine-to-ketone equilibrium. Additionally, dichloromethane facilitates near-full consumption of starting material, though the product formed is ambiguous, and it seems to be unstable in this solvent for long periods. Lastly, weaker reducing agents were found to minimize unintended reductions, but they failed to drive full conversion of imine.