Abstract
The 13 C NMR chemical shifts of the C2 carbon atom in the heteroaromatic nuclei of seventeen 5(3- and 4-substituted arylazo)-4,6-dimethyl-3-cyano-2-pyridones were determined in deuterated DMSO solution. For quantitative assessment of the substituent effects on the 13 C NMR chemical shifts simple and extended Hammett equations and Swain-Lupton equation were used. The mode of transmission of substituent effects have been discussed in a relation to the geometry of investigated molecules. The geometry data were obtained using semiempirical MNDO-PM6 energy calculations. The observed 13 C NMR substituent chemical shifts were correlated with literature 13 C NMR data for the corresponding 3- and 4-substituted-2’,6’-dimethylazobenzenes and 3- and 4-substituted azobenzenes. A good linear dependence obtained in this way, provided a basis to discuss the influence of the ortho-methyl groups on the degree of coplanarity of azobenzene and pyridone systems, and on the efficiency of transmission of electronic substituent effect from one ring to the other one.
Highlights
It has been known for many years that azo compounds are the most widely used compounds in various fields such as the dying of textile fibers, the coloring of different materials, colored plastic, and polymer, in biological-medical studies, and advanced application in organic synthesis.[1,2,3,4] The success of azo colorants is due to the simplicity of their synthesis by diazotization and azo coupling, to the almost innumerable possibilities presented by variation on the diazo compounds and coupling components, to the generaly high molar extinction coefficient, and to the medium to high light and wet fastness properties.[5]
The intention of the present investigation was to study the effects of substituents at the azobenzene ring on the 13C NMR chemical shifts of the C2 carbon atom in the heteroaromatic ring of investigated 5-(3- and 4-substituted arylazo)-4,6dimethyl-3-cyano-2-pyridones to get an insight into the factors determining chemical shifts
Since the quantum chemical calculations suggest that the investigated dyes prefer trans-2pyridone tautomeric form, the mode of transmission of substituent effects in 1 were discussed in relation to such molecular geometry
Summary
It has been known for many years that azo compounds are the most widely used compounds in various fields such as the dying of textile fibers, the coloring of different materials, colored plastic, and polymer, in biological-medical studies, and advanced application in organic synthesis.[1,2,3,4] The success of azo colorants is due to the simplicity of their synthesis by diazotization and azo coupling, to the almost innumerable possibilities presented by variation on the diazo compounds and coupling components, to the generaly high molar extinction coefficient, and to the medium to high light and wet fastness properties.[5]. The observed SCS(C2) for 5-(3- and 4-substituted arylazo)-4,6-dimethyl-3-cyano-2pyridones are compared with previously reported literature data[16] for the corresponding 3- and 4substituted-2’,6’-dimethyazobenzenes (2-Me) and 3- and 4-substituted azobenzenes (2-H) as a probe for exploring the influence of the ortho-methyl groups on the degree of coplanarity of azobenzene and pyridone systems and the efficiency of transmission of electronic substituent effects from one ring to the other.
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