pH‐Triggered Dethreading–Rethreading and Switching of Cucurbit[6]uril on Bistable [3]Pseudorotaxanes and [3]Rotaxanes

Abstract

The CH bond activation of small alkanes by the gaseous MgO+. cation is probed by mass spectrometric means. In addition to H‐atom abstraction from methane, the MgO+. cation reacts with ethane, propane, n‐ and iso‐butane through several pathways, which can all be assigned to the occurrence of initial CH bond activations. Specifically, the formal CC bond cleavages observed are assigned to CH bond activation as the first step, followed by cleavage of a β‐CC bond concomitant with release of the corresponding alkyl radical. Kinetic modeling of the observed product distributions reveals a high preference of MgO+. for the attack of primary CH bonds. This feature represents a notable distinction of the main‐group metal oxide MgO+. from various transition‐metal oxide cations, which show a clear preference for the attack of secondary CH bonds. The results of complementary theoretical calculations indicate that the CH bond activation of larger alkanes by the MgO+. cation is subject to pronounced kinetic control.

Sequence‐Specific Self‐Sorting of the Binding Sites of a Ditopic Guest by Cucurbituril Homologues and Subsequent Formation of a Hetero[4]pseudorotaxane

“…CB6 prefers to bind the diaminotriazole sites rather than the dodecyl spacer as previously reported. [8] The driving force for this type of complexation is mainly due to size selectivity as triazole has an appropriate size to fill the cavity of the CB6 as well as ion-dipole interactions between ammonium ions and the oxygen atoms of the carbonyl portals. Figure 1 b shows the 1 H NMR spectrum of [3]pseudorotaxane of CB6 with Axle A.…”

Kinetics Inside, Outside and Through Cucurbiturils

“…We propose that the second pathway is much more likely, as it would eliminate the large desolvation penalty imposed on the ammonium group if it were to transit through CB [6] as a charged species (up to 60 kcal/mol for complete desolvation vs. only 4 kcal/mol for a neutral amine). [77] More than ten years ago, Isaacs and coworkers presented the kinetic self-sorting of guests 38 and 39 in the presence of a mixture of CB [6] and CB [7], and its exceedingly slow reshuffling (56 days!) This conclusion is corroborated by the observation that shuttling rates were pH-independent within a pH range of 0 to 12.…”

“…[115] The inhibition of the corresponding oxidation of cysteamine (84) with Fe(III) chloride or chloropicrin may be attributed to pathway 2, but pathway 1 may be at play when oxygen is the oxidizing agent, [114] when CB [6] prevents hydrogen peroxide oxidation of selenocystamine (85) to selenous acid 87, [115] or when CB [7] encapsulation of the alkanolamine form of sanguinarin 76 prevents its photo oxidation to 6-oxysanguinarine (88) by oxygen. [35,116] Since the seminal work by Mock and coworkers in 1989 describing [3 + 2] alkyne-azide cycloadditions catalyzed with CB [6] [117] and subsequent applications, [77,118,119] a large number of studies have shown the potential of the CB[n] family members to promote reactions within their cavity or at their portal, and a majority of those propose solid rationales for the observed rate changes. We take the advantage here to highlight the remarkable and very recent functionalization of proteins by Francis and coworkers, who managed to prepare protein À DNA, protein À peptide, protein À polymer, and protein À drug conjugates using the CB[6]-catalyzed alkyne-azide cycloaddition.…”

Assembly of [2]Rotaxanes in Water

“…The released water molecules participate in the hydrogen‐bonding network of the water bulk, which results in an enthalpy gain. Most known water‐soluble rotaxane systems that rely on the hydrophobic effect comprise cyclodextrin, pillar[ n ]arene, or cucurbit[ n ]uril as host molecules, and for some of these systems acidic media or organic co‐solvents are required , . Prominent examples of synthetically modifiable water‐soluble macrocycles are the quaternary ammonium‐functionalized dicationic spiro‐cyclophanes developed and extensively studied by Diederich and co‐workers , .…”