(A) Rapid decomposition of b-Tz in CuTC. (B) Proposed Liebeskind-Srogl transmetallation mechanism. (C) Optimized Pd-catalyzed cross-coupling of tetrazines b-Tz and 1b with various nucleophiles (yields determined by GC w/dodecane as a standard). Conditions: (a) Pd2dba3 (12.5 mol%), Cs2CO3 (3.0 eq.), dioxane, 70 °C, 90 min. (b) [Pd(allyl)Cl]2 (10 mol%), THF, 70 °C, 2 h. (c) Pd(OAc)2 (10 mol%), TBAF (1.0 eq.), dioxane, 70 °C, 2.5 h. (d) Pd2dba3 (15 mol%), DMF, 60 °C, 20 h. (e) DMF, 60 °C, 20 h. (D) Screening of silver(I) and copper(I) additives for condition e. (E) Screening of tetrazines 1a-g under condition e.