Study On Dynamic Configurations Of Hydrogen-bonded Hydrazones And Acylhydrazones

Dynamic covalent chemistry(DCv C)deals with reversible covalent reactions that allow the free exchange of molecular components to achieve thermodynamic minimum of the system at equilibrium.The reversible nature of the reaction enables the intelligent properties of proot-reading and self-repairing in the synthesis of dynamic covalent chemistry.Therefore,the smart functions of the self-assemble systems can be endowed by introducing specific structures and changing equilibrium conditions,such as temperature,p H.With the recent development of dynamic covalent chemistry,it has been utilized in various fields ranging from organic synthesis and medicinal chemistry to supramolecular chemistry,and the fabrication of constitutionally dynamic materials,and so on.The core concept of dynamic covalent chemistry is dynamic covalent bonds.Hydrazones and acylhydrazones based on dynamic covalent bond(C=N)have attracted more attention for their modularity,functional diversity,straightforward synthesis,and stability towards hydrolysis.We have synthesized some compounds with such functional groups and studied their dynamic covalent bonding properties incorporating hydrogen bonds.First of all,we prepared a series of acylhydrazone-based switches,of which C=N double bond rotation can be stimulated by a p H response.The switches prefer Z rather than E configuration in the ground state due to the intramolecular hydrogen bonding,which was analyzed both thermodynamically and kinetically.Solvation studies show a linear relationship between the isomerization energy in E/Z states(?G^o_E/Z)of the switches and the solvent's hydrogen bond basicity scales(?and DN).This solvent effect was also found to be relevant to the skeleton of compound,the less shield of hydrogen bond domain the more sensitive the compound is to the solvent environment.By introducing multiple intramolecular hydrogen bonds,the Z isomer can completely be predominated,offering an advantage over a full E/Z isomerization switching.The protonation and deprotonation processes were confirmed by nuclear resonance magnetic and UV-visible spectroscopy.To further explore the effect of hydrogen bonding on the isomerization of these molecular switches,we designed and synthesized a series of hydrazone compounds with different substituents,and studied the relationships among substituent effect,hydrogen bond and E/Z isomerization,whose positive correlations were revealled and supported by the results of single crystallography,nuclear magnetic resonance and the studies of Hammettlinear free energy relationships.In order to examine whether this hydrogen-bonded dynamic covalent bond could be applied to larger molecular systems,we tried to use this kind of dynamic covalent bond in the construction of foldamers.A series of molecular fragments with aldehyde group and hydrazine/hydrazide group were synthesized,and thus the corresponding oligomers with specific length were constructed based on the dynamic C=N bonds.Nuclear magnetic resonance studies showed that oligomers were first connected with E-configuration of C=N bond.Then the C=N bond gradually turned into Z-configuration due to the intramolecular hydrogen bond,and the oligomers finally adopted helical structure as main conformation.