Coupling reactions, suitable for use in organic or aqueous media, are performed by contacting a 1,2,4,5-tetrazine with a dienophile. The dienophile may be covalently bonded to a protein, and the coupling reaction may be performed in biological media such as those containing cells or cell lysates. The reactions may be performed in the presence of primary amines, thiols, acetylenes, azides, phosphines, and products of Staudinger and/or Sharpless-Huisgen reactions Novel 3-substituted cyclopropene compounds and trans-cyclooctenes are exemplary dienophiles for these reactions.