Abstract
Previously, an “additivity” equation relating experimental 13C chemical shift data for two monosubstituted diphenyl-1,3-thiazolidin-4-one series was developed to predict chemical shifts for a similarly substituted bis-disubstituted thiazolidinone series. The sites of interest in the 1,3-thiazolidin-4-one are at the C-2, C-4, and C-5 carbons. The empirically derived equation for predicting the chemical shifts is dXY = dH + (dX-dH) + (dY-dH) where dXY is the predicted chemical shift for the disubstituted thiazolidinone: dH is the experimental chemical shift for the unsubstituted thiazolidinone, dX is the experimental chemical shift for substituent in the 2-phenyl ring, and dY is the experimental chemical shift for substituent in the N-(3)-phenyl ring. This article discusses the application of the aforementioned equation with respect to a new series of 2-(p-chlorophenyl)-phenyl-substituted-3-phenyl-1,3-thiazolidin-4-oneswith a comparison of both experimental and predicted 13C chemical shifts for the C-2, C4 and C-5 sites in the thaizolidinone ring. Utilization of the equation showed a level of chemical shift predictability with a degree of accuracy in concert with a previously reported series. The degree of predictability again showed a dependency on the particular substituent and the chemical shift for the site being predicted. This was ±0.06 ppm for C-2, ±0.2 ppm for C-4 and ±0.09 ppm for C5. Finally, there was a correlation between Hammett s values and the substituent chemical shifts using 13C values at C-2. The r value was -0.86 indicating that the C-2 carbon preferred a positive charge.
Highlights
Previous studies have reported Hammett correlations between the 13C experimental chemical shift values for two series of monosubstituted 2,3-diphenyl-1,3-thiazolidin-4-ones (Woolston, Lee, Swinbourne, & Thomas, 1992; Woolston, Lee, & Swinbourne, 1993; Tierney et al, 1996a)
Equation 1, which was previously formulated from the linear combination of two monosubstituted series of thiazolidinone chemical shift values, appears to have a reasonable sensitivity toward predicting the 13C chemical shifts at C-2, C-4 and C-5 for the present series of substituted thiazolidin-4-ones, Series 5
There was a reasonable correlation for the transmission of substituent effects from the groups on the N-3 through the nitrogen to the C-2 site if a correction is made for the presence of the para-chloro group situated on the phenyl group at C-2
Summary
Previous studies have reported Hammett correlations between the 13C experimental chemical shift values (or substituent chemical shifts–SCS) for two series of monosubstituted 2,3-diphenyl-1,3-thiazolidin-4-ones (Woolston, Lee, Swinbourne, & Thomas, 1992; Woolston, Lee, & Swinbourne, 1993; Tierney et al, 1996a). These two groups used slightly different substituents on the two phenyl rings; the resulting correlations were very similar. Correlations relating electronic effects from substituents in the phenyl rings to the observed chemical shifts for the monosubstituted and bis-disubstituted series utilized a form of the Hammett Equation, shown by Equation 1. A comparison of the correlation determined here with the previous studies will be further discussed
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