Furthermore, the reactions of azides 1b-d to give rotaxanes 4b-d (entries 3-5) proceeded with selectivities that paralleled the polarization of the N-H unit; trifluoro acetamide 1d produced rotaxane 4d in the highest selectivity (70% de), followed by trichloroacetamide 1c (48% de) then acetamide 1b (36% de). The SCXRD structure of the major isomer of 4d (Figure ) revealed the same (Rma,Rco-c) configuration as 4a with an NH•••O H-bond observed between the amide NH and sulfoxide units. Methylated trifluoroacetamide rotaxane 4e was produced in 10% de (entry 6), which suggests that there is some inherent facial bias between the azide and alkyne half-axles in the AT-CuAAC reaction of 2. The effect of temperature on the stereoselectivity of the reactions of 1a and 1d was more complicated. Whereas reducing the reaction temperature in the synthesis of 4a from rt (entry 1), to -40 °C (entry 8), to -78 °C (entry 9) increased the observed selectivity, that for 4d was higher at -40 °C (entry 10) and then fell at -78 °C (entry 11). We suggest that this slightly counterintuitive observation can be rationalized in broad terms by considering that the AT-CuAAC reaction takes place over several steps, 20 which include an equilibrium between diastereomeric azide/acetylide complexes I followed by irreversible formation of the corresponding triazolides II (Scheme 2). The observed stereoselectivity is thus a composite function of the pre-equilibrium step (Keq) and the relative rates (kRR, kSR) at which intermediates I progress to triazolides II. The effect of temperature on the reaction to produce 4d suggests the pre-equilibrium and kinetic resolution steps respond differently to changes in temperature, resulting in the observed behavior.