A Focus on Triazolium as a Multipurpose Molecular Station for pH-Sensitive Interlocked Crown-Ether-Based Molecular Machines

Abstract

The control of motion of one element with respect to others in an interlocked architecture allows for different co-conformational states of a molecule. This can result in variations of physical or chemical properties. The increase of knowledge in the field of molecular interactions led to the design, the synthesis, and the study of various systems of molecular machinery in a wide range of interlocked architectures. In this field, the discovery of new molecular stations for macrocycles is an attractive way to conceive original molecular machines. In the very recent past, the triazolium moiety proved to interact with crown ethers in interlocked molecules, so that it could be used as an ideal molecular station. It also served as a molecular barrier in order to lock interlaced structures or to compartmentalize interlocked molecular machines. This review describes the recently reported examples of pH-sensitive triazolium-containing molecular machines and their peculiar features.

Keywords: lasso structures; macrocycles; molecular machines; molecular muscles; rotaxane; triazolium.

Figure 1

General chemical route to triazolium-containing molecular machines.

Figure 2

A diverted route to triazolium-containing rotaxanes.

Figure 3

Usual main influence of the DB24C8 on the ¹H NMR chemical shifts of the ammonium and N-methyltriazolium stations.

Scheme 1

Preparation of a mannosyl pH-sensitive [2]rotaxane molecular machine. Reagents and conditions: a) DB24C8 (2 equiv), CH₂Cl₂, RT; b) 2,6-lutidine, Cu(MeCN)₄PF₆, CH₂Cl₂, RT, 24 h, 72 %; c) 1) CH₃I, 2) NH₄PF₆, quantitative; d) DIEA; e) 1) HCl/Et₂O, 2) NH₄PF₆.

Scheme 2

Actuation of a triazolium-containing double-leg molecular elevator.

Scheme 3

Mimicking the wing-flapping motion of a butterfly using a triazolium-containing pentiptycene bis(crown ether)-based [2](2)rotaxane.

Scheme 4

Mimicking a molecular pulley in a triply interlocked triazolium-containing rotaxane molecular machine.

Scheme 5

Switching “on” or “off” a triazolium-containing [2]rotaxane catalyst for Michael-type addition.

Scheme 6

Utilization of a switchable triazolium-containing rotaxane catalyst for the introduction of an electrophile at the α position of an aldehyde.

Scheme 7

A chiral switchable triazolium-containing rotaxane catalyst for stereoselective conjugated addition. Reagents and conditions: a) NaOH; b) 1) HCl, 2) KPF₆.

Scheme 8

A triazolium-containing [2]rotaxane with fluorescent output.

Scheme 9

A triazolium-containing [2]rotaxane with two fluorescent outputs.

Scheme 10

A triazolium-containing [3]rotaxane with fluorescent output.

Scheme 11

A triazolium-containing [3]rotaxane with fluorescent output.

Scheme 12

A triazolium-containing [1]rotaxane with fluorescent output.

Scheme 13. Scheme

A triazolium-containing bis-branched [1]rotaxane with reversible fluorescence, stretching/contraction motion, and reversible aggregation.

Scheme 14

An ammonium/triazolium-containing pH-sensitive molecular muscle. Reagents and conditions: a) 1) mannosyl azide (pictured), Cu(CH₃CN)₄PF₆, lutidine, 2) CH₃I then NH₄PF₆; b) NaOH (aq)/CH₂Cl₂; c) 1) HCl/Et₂O, 2) NH₄PF₆.

Scheme 15

An ammonium/triazolium-containing pH-sensitive muscle-like monomer for the synthesis of a metallosupramolecular polymer. Reagents and conditions: a) NaOH (aq)/CH₂Cl₂; b) 1) HCl/Et₂O, 2) NH₄PF₆.

Scheme 16

Triazolium-based building block for microfiber-induced formation.

Scheme 17

N-methyltriazolium as a molecular station for the DB24C8 in a three-station-based [2]rotaxane. Reagents and conditions: a) 1) HCl/Et₂O, 2) NH₄PF₆, CH₂Cl₂/H₂O, 30 min; b) DIEA; c) CD₂Cl₂, 223 K.

Scheme 18

N-methyltriazolium as a molecular station for the DB24C8 in a three-station based molecular muscle. Reagents and conditions: a) 1) HCl/Et₂O, 2) NH₄PF₆, H₂O/CH₂Cl₂, RT, 30 min; b) DIEA, Boc₂O, CH₃CN, RT, 5 d; c) DIEA, Boc₂O, CH₃CN.

Scheme 19

N-benzyltriazolium as both a molecular station and a barrier for the DB24C8 in a [2]rotaxane molecular machine. Reagents and conditions: a) 1) HCl, 2) NH₄PF₆; b) DIEA; c) 1) BnBr, CH₂Cl₂, 4 d, RT, 2) NH₄PF₆, CH₂Cl₂/H₂O, 30 min, 79 %; d) Boc₂O, DIEA, 15 h, RT, 93 %; e) 1) BnBr, CH₂Cl₂, 5 d, RT, 2) NH₄PF₆, CH₂Cl₂/H₂O, 30 min, 65 %; f) 1) HCl, 1 h, RT, 2) NH₄PF₆, CH₂Cl₂/H₂O, 30 min, 60 %.

Scheme 20

N-benzyltriazolium as both a molecular station and a barrier in a lasso molecular architecture. Reagents and conditions: a) 1) BnBr, CH₂Cl₂, <5×10⁻⁴m, 3 d, RT, 2) NH₄PF₆, H₂O/CH₂Cl₂, RT, 30 min, 27 %; b) NaOH/H₂O (1 m); c) 1) HCl/Et₂O, 2) NH₄PF₆, CH₂Cl₂/H₂O.

Scheme 21

N-benzyltriazolium as both a molecular station and a barrier in a peptide-containing lasso molecular architecture. Reagents and conditions: a) DIEA; b) 1) HCl/Et₂O, 2) NH₄PF₆, CH₂Cl₂/H₂O.

Scheme 22

Triazolium-based double-lasso molecular architecture and loosening–tightening motion. Reagents and conditions: a) 1) H₂N(CH₂)₁₂NH₂ (1 equiv), CH₂Cl₂ (0.5 mm), 25 °C, 3 d, 2) CH₃I, 4 d, 3) NH₄PF₆, H₂O/CH₂Cl₂, 30 min; b) NaOH/H₂O/CH₂Cl₂; c) 1) HCl/Et₂O, 2) NH₄PF₆/H₂O/CH₂Cl₂, quanti 61/61’.

Scheme 23

Molecular jump-rope motion in a triazolium-based double-lasso spherical molecular muscle.