On the role of guests in enforcing the mechanism of action of gated baskets.

ABSTRACT

We designed and prepared a spacious and gated basket of type 2 (V = 318 Å(3)) in ten synthetic steps. With the assistance of (1)H NMR spectroscopy, we found that the pyridine gates at the rim of 2 form a seam of N-H∙∙∙N hydrogen bonds, thereby adopting right- (P) and left-handed (M) helical arrangements. The recognition characteristics of the smaller basket 1 (V = 226 Å(3)) and the larger 2 for various solvents as guests were quantified by (1)H NMR spectroscopy in CD2Cl2 (61 Å(3)), CDCl3 (75 Å(3)), CFCl3 (81 Å(3)) and CCl4 (89 Å(3)); the apparent guest binding equilibria Ka were found to be inversely proportional to the affinity of bulk solvents KS for populating each host. The rate of the P/M racemization (krac, s(-1)) was, for both 1 and 2, studied in all four solvents using dynamic NMR spectroscopy. From these experiments, two isokinetic relationships (ΔS++P/M vs. ΔH++P/M) were identified with each one corresponding to a different mechanism of P/M racemization. A computational study (B3LYP/6-31+G**//PM6) of 1 and 2 in the gas phase indicates two competing racemization pathways (a) RM1-2 describes a pivoting of a single gate followed by the rotation of the remaining two gates, while (b) RM3 depicts simultaneous (geared) rotation of all three gates. The racemization of the larger basket 2, in all four solvents (packing coefficient, PC = 0.19-0.28), conformed to one isokinetic relationship, which also coincided with the operation of the smaller basket 1 in CD2Cl2 (PC = 0.27). However, in CDCl3, CFCl3 and CCl4 (PC = 0.33-0.39), the mode of action of 1 appears to correlate with a different isokinetic relationship. Thus, we propose that the population of the basket's inner space (PC) determines the mechanism of P/M racemization. When PC < 0.3, the mechanism of operation is RM1-2, whereas, a greater packing, represented when PC > 0.3, enforces the geared RM3 mechanistic alternative.