The Proton Complex of a Diaza-macropentacycle: Structure, Slow Formation, and Chirality Induction by Ion Pairing with the Optically Active 1,1′-Binaphthyl-2,2′-diyl Phosphate Anion

The protonation of a sterically crowded [N2S6] macropentacycle (1) with 1 equiv of CF3SO3H in CDCl3 is slow and gives the singly (oo+ [1·H]+) and doubly (o+o+ [1·2H]2+) protonated forms as kinetic products, the i+o form of [1·H]+ being the thermodynamic product. i+o [1·H]+ is C3 helically chiral in the solid state and in solution. The barrier to racemization (ΔG‡) of the [1·H]+ propeller is >71 kJ mol−1. The ammonium proton is encapsulated in the tetrahedral coordination sphere provided by the endo (i) nitrogen bridgehead atom and the three proximal thioether sulfurs, which makes [1·H]+ a proton complex. Use of the optically active acid (R)-(−)- or (S)-(+)-1,1′-binaphthyl-2,2′-diyl hydrogen phosphate (BNPH) in chloroform allowed us to induce a significant diastereomeric excess (24% de), which produced a detectable ICD. The de was decreased in acetone-d6 (10%), suggesting that the sense of chirality of [1·H]+ is controlled by ion-pair interactions. Detailed NMR studies allowed us to locate the chiral anion on the endo side of [1·H]+, in the cavity lined by endo t-Bu groups, and to establish that the rate of anion exchange in [1·H][(S,R)-(±)-BNP] was higher than the rate of propeller inversion of [1·H]+.