When instead formamide-containing azide (R)-1g was reacted with 2 and 3, even at rt rotaxane 16 30 was obtained in reasonable stereopurity (57% ee, entry 3), which was improved further at -40 °C (67% ee, entry 4). Conducting this reaction at -78 °C reduced the observed stereoselectivity (59% ee, entry 5), suggesting that, as with azide 1d, the preequilibrium and kinetic resolution steps result in an unusual temperature dependence. CSP-HPLC analysis of a sample of rotaxane 16 produced by formylation of a sample of rotaxane (Rma)-15 of known stereopurity and comparison with the same compound produced from (R)-1g confirmed that the latter produces (Rma)-16 as the major stereoisomer. When (S)-1g was reacted instead (Sma)-16 was produced (70% ee, entry 6). The solid-state structure of 6 obtained by SCXRD (Figure ) did not display the expected intermolecular NH•••O H-bond; instead, the same interaction was found to occur in an intermolecular fashion within the unit cell. Scheme 6. Direct synthesis of enantioenriched mechanically axially chiral rotaxanes 15 and 16 a a Reagents and conditions: i. 1 (1.1 equiv.), 2 (1 equiv.), 3 (1.1 equiv.), [Cu(CH3CN)]PF6 (0.96 equiv.), i Pr2NEt (2 equiv.), CH2Cl2, 16 h. b Determined by analytical CSP-HPLC. The different co-conformational behaviors of 4a, 15 and 16 are clear from analysis of their respective 1 H NMR spectra. Diastereomers (Rma,Rco-c)-4a and (Sma,Rco-c)-4a are separable species; heating a mixture diastereomers 4a resulted in no change in their ratio (Figure ), confirming that the macrocycle cannot shuttle between the two compartments due to the large NHBoc unit. In contrast, the diastereotopic triazole resonances Hd of amine rotaxane 15 appear as two sharp singlets at 298 K, indicating that diastereomeric coconformations (Rma,Rco-c)-5 and (Sma,Rco-c)-5 are in fast exchange on the 1 H NMR timescale through rapid shuttling of the macrocycle between the two triazole-containing compartments (Figure ). The same resonances for formylated rotaxane 16 are broad at 298 K, although once again only two signals are observed (Figure ). This observation is consistent with (Rma,Rco-c)-16 and (Sma,Rco-c)-16 exchanging on the 1 H NMR timescale, albeit more slowly than (Rma,Rco-c)-15 and (Sma,Rco-c)-15, in keeping with the larger steric bulk of the formamide group of 6. Accordingly, increasing the temperature resulted in sharpening of the two resonances corresponding to Hd (Figure ).