Recent Advances in Group 14 and 15 Lewis Acids for Frustrated Lewis Pair Chemistry

Frustrated Lewis pairs (FLP) which rely on the cooperative action of Lewis acids and Lewis bases, played a prominent role in the advancement of main-group catalysis. While the early days of FLP chemistry witnessed the dominance of boranes, there is a growing body of reports on alternative Lewis acids derived from groups 14 and 15. This short review focuses on the discovery of such non-boron candidates reported since 2015.

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.