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

Abstract

Alkylation of spiro[fluorene-9,3’-indazole] at N(1) and N(2) with tBuCl affords the nitrenium cations [C6H4N2(tBu)C(C12H8)][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 [C6H4N(tBu)N(PMe3)C(C12H8)]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 [(R3P)C6H3NN(tBu)CH(C12H8)][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)C6H3N(tBu)NC(C12H8).][BF4] 6 and [(SIMes)C6H3NN(tBu)C(C12H8).][BF4] 7, whereas reaction of 2 with BAC gives the Lewis acid-base adduct, [C6H4N(BAC)N(tBu)C(C12H8)][BF4] 8. Finally, reactions of 1 and 2 with KPPh2 result in electron transfer affording (PPh2)2 and the persistent radicals C6H4N(tBu)NC(C12H8). and C6H4NN(tBu)C(C12H8).. The detailed reaction mechanisms are also explored by extensive DFT calculations.