Secondary Isotope Effects on the Deslipping Reaction of Rotaxanes: High‐Precision Measurement of Steric Size

Abstract

Small structural changes can cause unexpectedly large effects on the deslipping reaction [1] of rotaxanes [2] (Scheme 1). A particularly striking example [3] is the replacement of the isophthalaldehyde unit in 6 a (Scheme 2) by a 2,6-pyridinedicarboxamide, that is, the simple exchange of a CH group by an isoelectronic N atom to yield 6 b. Without any other changes in the rotaxane structure, the barrier for the passage of the wheel over one of the stopper groups increases from approximately 80 to 135 kJ mol À1 as a result of the formation of intramolecular hydrogen bonds. This increase is reflected in a more than 10 4 -fold increase in the half-life. Probably one of the smallest steric changes which can be made in a molecule is the replacement of hydrogen atoms by deuterium atoms.[4] According to electron-diffraction studies and calculations, the time-averaged length of the C À D bond is approximately 0.005 shorter than that of the CÀH bond. [5] In addition, the CÀD bond has a lower vibrational frequency and vibrational amplitude than the CÀH bond.[6] This difference is reflected in a smaller van der Waals radius. Further experimental evidence comes from the isomerization reactions of suitably substituted biaryls [7] and [2.2]metacyclophanes, [8] as well as from the reactions of 2,6-dimethylated pyridines with Lewis acids and electrophiles. [9] The smaller steric size of the CÀD bond is reflected in a tighter crystal packing of deuterated molecules [10] and in the 13 C NMR chemical shifts of groups located in the proximity of deuterated substituents.[11] Herein, we describe the results of deslipping experiments performed with unlabeled rotaxanes 9 a and 10 a and their isotopologues 9 b and 10 b that have deuterated stopper groups (Scheme 2).The deuterated di- [D 9 ]tert-butylphenol stopper 5 b was synthesized according to established literature procedures [12] as depicted in Scheme 2. Threefold Friedel-Crafts alkylation of benzene 1 is followed by replacement of one of the tertbutyl groups in 2 b by an acetyl substituent in a Friedel-Crafts acylation which deactivates the aromatic system and prevents it from further reaction. A Baeyer-Villiger oxidation of intermediate 3 b with m-chloroperbenzoic acid (MCPBA) followed by hydrolysis of ester 4 b yields stopper 5 b. The rotaxanes 9 a,b and 10 a,b can be prepared from the corresponding stoppers 5 a,b, one of the axle center pieces 7 or 8, and wheel 6 by an anion template effect in a yield of 25-30 %.[13] The degree of isotopic labeling could be determined to be greater than 95 % from the 1 H NMR spectra as well as from the isotope patterns of the MALDI mass spectra (see Supporting Information).The deslipping reaction is unimolecular and follows a first-order rate law. It is easily monitored by 1 H NMR spectroscopy because of the sizeable chemical shift differences (Dd) between the rotaxane signals and those of the free axle (Figure 1). The largest values for Dd are found for the Scheme 1. The deslipping reaction of rotaxanes: the mechanical bond is broken and ...

Deslipping of Rotaxanes with Axle Center Pieces of Different Lengths

“…The rate-determining step of the deslipping reaction is the passage of the wheel over the stopper. The deslipping reaction has been used for determining steric size, and even the difference between CH 3 and CD 3 groups located at the stopper periphery affected the deslipping rate indicating the precision with which even small effects can be measured [9,10]. This result obtained through isotopic substitution, which affects the ratedetermining step of the deslipping reaction confirms the results presented here.…”

“…The aromatic rings in the wheel exert an anisotropy effect on the protons of the axle center pieces, and upfield shifts are usually observed that are also taken as evidence for rotaxane formation. They can be quite significant, and values of up to Dd ¼ 1:7 ppm have been observed [9,10]. Longer axles normally result in smaller values for SCHEME 1 Schematic illustration of the deslipping process.…”

Improved Pseudorotaxane and Catenane Formation from a Derivative of Bis(m‐phenylene)‐32‐crown‐10

“…Fujita et al [4] observed that the distortion of a cyclodextrin cavity could significantly restrict the guest rotation. Schalley et al [5] found that small structural changes such as the simple exchange of a CH group for an isoelectronic N atom could have unexpectedly large effects on the deslipping reactions of rotaxanes. We have previously reported a paraquat (N,NЈ-dimethyl-4,4Ј-bipyridium) substituent effect on complexation with a dibenzo-24-crown-8-based cryptand [6] and control of the complex-plexes" between BMP32C10 and paraquat derivatives.…”

Dendritic EDA-Schiff Bases of the Salen-Type

“…21,22 These results certainly stimulate the synthesis of optically active salen compounds with dendritic substituents at the EDA carbon positions and with various types of dendritic wedges, 23 starting from enantiopure D-and L-1,2-bis(2-hydroxyphenyl)ethylenediamine. 14b,c Scheme 1 Synthesis of N,N¢-disalicylidene-meso-1,2-bis(3,5-benzyloxydiphenyl)ethylenediamine (6) and N,N¢-disalicylidene-meso-1,2-bis(3,5-benzoyloxydiphenyl)ethylenediamine (7) …”