This PDF file includes:

• Synthesis and cycloaddition reactions of 1,2,4,5-tetrazines
  
• Hyperpolarization experiments
  
• ¹H, ¹³C, and ¹⁵N spectra
  
• fig. S1. ¹H NMR comparison between tetrazine and cycloaddition product.
  
• fig. S2. Experimental setup for hyperpolarization and hyperpolarized reaction experiments.
  
• fig. S3. Hyperpolarized signal decay of magnetization and singlet at variable concentrations.
  
• fig. S4. Hyperpolarization of magnetization and singlet as a function of magnetic field.
  
• fig. S5. Comparison of the originally hyperpolarized singlet and diluted signal.
  
• fig. S6. Small-tip-angle spectra of the hyperpolarized cycloaddition product 3a.
  
• fig. S7. SABRE-SHEATH experiment using methanol-d₄/D₂O mixture as solvent.
  
• table S1. Magnetization and singlet enhancements and lifetimes at variable concentrations.
  
• References (44, 45)

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