The Importance of the Rotor in Hydrazone-Based Molecular Switches

The pH-activated E/Z isomerization of a series of hydrazone-based systems having different functional groups as part of the rotor (R = COMe, CN, Me, H), was studied. The switching efficiency of these systems was compared to that of a hydrazone-based molecular switch (R = COOEt) whose E/Z isomerization is fully reversible. It was found that the nature of the R group is critical for efficient switching to occur; the R group should be a moderate H-bond acceptor in order to (i) provide enough driving force for the rotor to move upon protonation, and (ii) stabilize the obtained Z configuration, to achieve full conversion

The importance of the rotor in hydrazone-based molecular switches

Abstract The pH-activated E/Z isomerization of a series of hydrazone-based systems having different functional groups as part of the rotor (R = COMe, CN, Me, H), was studied. The switching efficiency of these systems was compared to that of a hydrazone-based molecular switch (R = COOEt) whose E/Z isomerization is fully reversible. It was found that the nature of the R group is critical for efficient switching to occur; the R group should be a moderate H-bond acceptor in order to (i) provide enough driving force for the rotor to move upon protonation, and (ii) stabilize the obtained Z configuration, to achieve full conversion. Findings Nature is full of elegant examples of perfectly designed biological motors and machines Previously, we have shown that hydrazone-based rotary switches can change their configuration (i.e., E/Z isomerization) as a function of pH Scheme 2: The hydrazone-based molecular systems that were analysed in this paper, each having different rotors. The stable isomer(s) in solution and their protonation products are shown. PPH-1-Z-H + , which is the more stable isomer. During this process, an E/Z isomerization takes place, which can be fully reversed by the addition of base to the solution. In order to fine tune the properties of the hydrazone switches, we studied the effect of different R groups in the rotor part (Scheme 2) on the switching cycle. The target hydrazones were Beilstein J. Org. Chem. 2012, 8, 872-876. 874 synthesized either by the direct condensation of phenylhydrazine with the corresponding aldehyde (PPH-2) or ketone (PPH-3), or by Japp-Klingemann reaction (PPH-4, PPH-5) In certain cases it has been shown that intramolecularly H-bonded hydrazones exist predominantly as the kinetically stable Z isomer in solution On the other hand, PPH-4, in which R is a strong electron-withdrawing group (-CN) shows two sets of signals in the 1 H NMR spectrum (CD 3 CN), indicating that two isomers, having a 4:1 ratio, coexist in solution. The major isomer shows a hydrazone N-H signal at 9.60 ppm, indicating that it is the Z isomer, in which an intramolecular H-bond is not present. On the other hand, the hydrazone N-H signal of the minor isomer resonates at 15.12 ppm, which is characteristic of H-bonded N-H signals, suggesting that the minor isomer is actually the E configuration. Such an unusual E/Z isomer ratio was reported before for similar systems, and it was attributed to kinetic stability of the Z isomer, in addition to solvent effects Structurally, PPH-5 is the closest to PPH-1, that is, instead of an acyl ester group, PPH-5 has an acetyl residue as the R group. The 1 H NMR spectrum of PPH-5 in CD 3 CN shows only one set of signals, and a sharp singlet at 14.54 ppm for the hydrazone N-H proton, indicating that it is H-bonded to the pyridyl nitrogen. Since the acetyl group is a less effective H-bond acceptor than ethyl ester, it is reasonable that PPH-5 exists exclusively in the E form in solution. When TFA is added to the solution, a second set of signals arises, which grows as the amount of acid increases. The protonation of the pyridyl ring results in the downfield shift of the aromatic signals, except for proton H1, which shifts from 8.92 to 8.70 ppm as it is no longer affected by the H-bond In order to rationalize the different behaviour of the structurally similar switches, PPH-1 and PPH-5, a quantitative evaluation of the thermodynamic process is necessary. Taking a look at the acid-activated switching process of PPH-1 (Scheme 1), we can formulate the following equations for the acid-induced E/Z isomerization: (1) Conclusion In summary, we have synthesized four hydrazone-based systems having different R groups as part of the rotor section. The role of the R group was assessed vis-à-vis the switching of the system, and it was found that for the switch to operate effectively it is crucial that (1) the R group be able to offer a second H-bond-accepting site in order to provide enough driving force for the rotor to move; and (2) the R group be a moderate H-bond acceptor, otherwise the isomer generated will not be stable enough to enable full conversion (isomerization)

Модернизация технологического процесса механической обработки детали ступица КЗК 12-0602605 с разработкой проекта участка цеха, средств технологического оснащения и исследованием электрохимического шлифования

Bimetallic Pd/Cu and Pd/Ag catalytic systems were used for borylation of propargylic alcohol derivatives. The substrate scope includes even terminal alkynes. The reactions proceed stererospecifically with formal S(N)2' pathways to give allenyl boronates. Opening of propargyl epoxides leads to 1,2-diborylated butadienes probably via en allenylboronate intermediate.AuthorCount:4;</p

Sequentially Palladium-Catalyzed Processes in One-Pot Syntheses of Heterocycles

Sequentially Pd-catalyzed processes are excellent entries to heterocycle synthesis. The broad mechanistic variety combined with often very mild reaction conditions allow the concatenation of elementary organic and organometallic steps to novel sequences in the sense of one-pot domino and multicomponent reactions. Given the numerous opportunities of alkyne coordination and their Pd-mediated transformations, alkynylation and carbometallation play a key role, both for purely organometallic sequences as well as in those processes that are intercepted by cyclocondensation. Pd-catalyzed aminations also find more and more entry into novel heterocycle syntheses based upon this theme