Produção Nacional
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Stereoelectronic and inductive effects on 1 H and 13 C NMR chemical shifts of some cis ‐1,3‐disubstituted cyclohexanes
2006; Wiley; Volume: 44; Issue: 8 Linguagem: Inglês
10.1002/mrc.1850
ISSN1097-458X
AutoresPaulo Roberto de Oliveira, Ljubica Tasić, Silvana A. Rocco, Roberto Rittner,
Tópico(s)Inorganic and Organometallic Chemistry
ResumoAbstract 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 <N(CH 3 ) 2 <OCH 3 <F) and decreases with the increase of electronegativity when Y is a halogen (Cl < Br < I). The natural bond orbital (NBO) analysis showed that the α‐effect on the H‐3 chemical shift increases with the decrease of n Y → *σ C3H3a 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 C2C3 and n Y → *σ C3C4 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 C2C3 , *σ C3C4 and *σ C3H3a in comparison with the same type of interaction with the lone pair of the other halogens. Copyright © 2006 John Wiley & Sons, Ltd.
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