Complexes with strong symmetric hydrogen bonds and their solvation in the trifluoroacetic acid—N,N-dimethylformamide system studied by IR spectroscopy

Abstract

Complex formation in the trifluoroacetic acid (TFA)--DMF system containing TFA from 0 to 100 mol.% was studied by IR multiple attenuated total reflectance (MATR) spectroscopy at 30 ~ The formation of uncharged I : I TFA--DMF complexes with a quasi-ion structure and partial proton transfer to the O atom of the DMF molecule (quasi-ion pairs) were observed in the TFA--DMF system with the TFA content from 0 to 90 tool%. Depending on the ratio of the components, the quasi-ion pairs are solvated by the I.)MF or TFA molecules. The solvation of the quasi-ion pairs slightly changes the parameters of the central strong H bond. When the acid concentration reaches 80 tool.%, the proton adds to the DMF molecule to form the ion pair {DMF)H+-(A...H..A) -. whose anion contains a strong s.~mmetric H bond.

“On water” synthesis of N-unsubstituted pyrazoles: semicarbazide hydrochloride as an alternative to hydrazine for preparation of pyrazole-3-carboxylate derivatives and 3,5-disubstituted pyrazoles

“…The crude ester was used for the next step immediately after synthesis without further purification because prolonged time of exposure to air contributed to contaminated pyrazole products. The reaction of all prepared diketoesters with SC•HCl proceeded smoothly to give pyrazole products in excellent isolated yields 35 (85-98%), as presented in Scheme 2.…”

“…3 Very recently, kinetic and 25 thermodynamic investigations have established a crucial role of hydrogen-bonding ability of the surface water molecules indicating the presence and the nature of the water substrate interface as an essential criteria for "on water" mechanism during a heterogenous reaction. 4 Some examples of such organic 30 reactions include preparation of phenylselenoesters, 5 synthesis of heterocyclic ortho-quinones, 6 35 derivatives, 10 preparation of 1,2-diazetidines 2 and 1,3-dipolar cycloadditions. 11 As an important class of heterocyclic pharmacophores, pyrazole-3(5)-carboxylic acid derivatives constitute the core structure of clinically interesting compounds.…”

“…30 Both compounds, prepared by different methods show very similar values of the chemical shift in CDCl 3 and correct assignment of the tautoisomer using simple comparison with the literature chemical shifts is not possible. To solve this indecision, we obtained suitable single crystal of 9 by slow evaporation of 35 EtOAc-hexane solution and 3-carboxylate tautomeric structure was confirmed by x-ray diffraction technique 29 (Fig. 1).…”

“…The above mentioned species differ from one another in the 35 degree of proton transfer which is very dependent on reaction conditions such as ratio of starting diketone and water, temperature and standing time of the mixture after reaction. Repeating the reaction by slight varying these parameters, we detected a presence of deprotonated pyrazole compound as a 40 result of incomplete proton transfer from HCl to pyrazole molecule.…”

In Situ Generation and Intramolecular Schmidt Reaction of Keto Azides in a Microwave‐Assisted Flow Format

“…The replacement of DMF by CH 3 CN (AN) afforded complete conversion of the intermediate azide to the lactam, even at reduced power (125W), presumably because AN does not attenuate the acidity of TFA. [8] The reaction mixture was loaded into a sample loop made of PEEK tubing that was introduced into the beginning of the flow path. The first syringe pump delivered only solvent, removing any possibility that reactants would remain in the system uncollected.…”