Stereoelectronic and inductive effects on 1H and 13C NMR chemical shifts of some cis-1,3-disubstituted cyclohexanes

✍ Scribed by Paulo R. de Oliveira; Ljubica Tasic; Silvana A. Rocco; Roberto Rittner

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 144 KB
• Volume: 44
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1850

✦ Synopsis

Abstract

This work presents the substituent effects on the ^1^H and ^13^C NMR chemical shifts in the cis‐isomer of 3‐Y‐cyclohexanols (Y = Cl, Br, I, CH~3~, N(CH~3~)~2~ and OCH~3~) and 3‐Y‐1‐methoxycyclohexanes (Y = F, Cl, Br, I, CH~3~, N(CH~3~)~2~ and OCH~3~). It was observed that the H‐3 chemical shift, due to the substituent α‐effect, increases with the increase of substituent electronegativity when Y is from the second row of the periodic table of elements, (CH~3~ 3)~2~ 3 n~Y~ → *σ~C3H3a~ interaction energy. This interaction energy, for the halogenated compounds, decreases with an increase in size of the halogen, and this is a possible reason for the largest measured chemical shift for H‐3 of the iodo‐derivatives. The β‐effect of the analyzed compounds showed that the chemical shift of hydrogens at C‐2 and C‐4 increases with the decrease of n~Y~ → *σ~C2C3~ and n~Y~ → *σ~C3C4~ interaction energies, respectively, showing a behavior similar to H‐3. The α‐effect on ^13^C chemical shifts correlates well with substituent electronegativity, while the β‐effect is inversely related to electronegativity in halogenated compounds. NBO analysis indicated that the substituent inductive effect is the predominant effect on ^13^C NMR chemical shift changes for the α‐carbon. It was also observed that C‐2 and C‐4 chemical shifts for compounds with N(CH~3~)~2~, OCH~3~ and F are more shielded in comparison to the compounds having a halogen, most probably because of the larger interaction of the lone pair of more electronegative atoms (n~N~ > n~O~ > n~F~) with *σ~C2C3~, *σ~C3C4~ and *σ~C3H3a~ in comparison with the same type of interaction with the lone pair of the other halogens. Copyright © 2006 John Wiley & Sons, Ltd.