The Reactivity of Isomeric Nitrenium Lewis Acids with Phosphines, Carbenes, and Phosphide

Alkylation of spiro[fluorene-9,3'-indazole] at N(1) and N(2) with tBuCl affords the nitrenium cations [C₆ H₄ N₂ (tBu)C(C₁₂ H₈ )][BF₄ ], 1 and 2, respectively. Compound 1 converts to 2 over the temperature range 303-323 K with a free energy barrier of 28±5 kcal mol^-1 . Reaction of 1 with PMe₃ afforded the N-bound phosphine adduct [C₆ H₄ N(tBu)N(PMe₃ )C(C₁₂ H₈ )]BF₄ ] 3. However, phosphines attack 2 at the para-carbon atom of the aryl group with concurrent cleavage of N(2)-C(1) bond and proton migration to C(1) affording [(R₃ P)C₆ H₃ NN(tBu)CH(C₁₂ H₈ )][BF₄ ] (R=Me 4, nBu 5). Analogous reactions of 1 and 2 with the carbene SIMes prompt attack at the para-carbon with concurrent loss of H^. affording the radical cation salts [(SIMes)C₆ H₃ N(tBu)NC(C₁₂ H₈ )^. ][BF₄ ] 6 and [(SIMes)C₆ H₃ NN(tBu)C(C₁₂ H₈ )^. ][BF₄ ] 7, whereas reaction of 2 with BAC gives the Lewis acid-base adduct, [C₆ H₄ N(BAC)N(tBu)C(C₁₂ H₈ )][BF₄ ] 8. Finally, reactions of 1 and 2 with KPPh₂ result in electron transfer affording (PPh₂ )₂ and the persistent radicals C₆ H₄ N(tBu)NC(C₁₂ H₈ )^. and C₆ H₄ NN(tBu)C(C₁₂ H₈ )^. . The detailed reaction mechanisms are also explored by extensive DFT calculations.