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Correlation between the reactivity and spectroscopic properties of N ‐substituted secondary thioamides. New intramolecular N···H + ···N binding approach and proton complexes based on thioamide ligation
Author(s) -
Spychała Jarosław
Publication year - 2003
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1157
Subject(s) - chemistry , intramolecular force , hydrogen bond , reactivity (psychology) , thio , thioamide , amide , steric effects , medicinal chemistry , stereochemistry , crystallography , molecule , organic chemistry , medicine , alternative medicine , pathology
On the basis of a comparison of chemical shifts and wavenumbers of several secondary thioamides and amides having monocationic substituents attached to thiocarbamoyl or carbamoyl groups by a polymethylene chain, new intramolecular unconventional N···H + ···N hydrogen bonding effects were discovered. It is argued that the CH 2 —N rotation is hindered and two + H ···N H CH 3 non‐equivalent protons occur in a proton spectrum of hydrochloride 1a (at 10.68 and 2.77 ppm, respectively) instead of two + N H 2 CH 3 protons. Presumably, the above steric factors inhibit the acidic hydrolysis of 1a (stabilized by strong intramolecular N···H + ···N hydrogen bonds) to an amide and prevent intramolecular cyclization of 2a (stabilized by strong intramolecular neutral–neutral N···HN hydrogen bonds) to a cyclic amidine. Postulation of additional dihydrogen bond formation is helpful in understanding the spectroscopic differences of 4 and 5 . The above new bonding is also compared with intramolecular N···H—N + hydrogen bonds in primary amine salts 7 and 8 . In contrast to 3 , a cooperative hydrogen bonded system is observed in 9 and 10 . The weak hydrogen bonds in 7 – 10 facilitate the hydrolysis and cyclization reactions of secondary thioamides. The spectroscopic data for secondary (thio)amides are especially useful for characterizing the electronic situation at the (thio)carbamoyl nitrogen atoms and they are perfectly correlated with the reactivity. Examples of chelation of protons by thioamides ( 11 and 12 ), which contain strongly electron‐donating pyrimidine groups, are presented to show the contribution of dihydrogen bonding in the protonation reaction similar to 1 and 4 . Copyright © 2003 John Wiley & Sons, Ltd.