Abstract
AbstractStereoelectronic effects in cyclohexanones, methylenecyclohexanes, spiro, and epoxy compounds of cyclohexanes and further derivatives were investigated by measuring1JC,Hcoupling constants and by identification of Perlin effects, that is, of differences in the coupling constants for equatorial and axial CH bonds in the methylene groups of six‐membered rings. The Perlin effects were correlated with results from natural bond orbital analyses. NMR experiments and calculations were performed with conformationally restricted 4‐tert‐butyl‐substituted derivatives. It turned out that the coupling constants are strongly influenced not only by stereoelectronic interactions with CC, CO, and CN π bonds, or with the π‐type CC or CO bonds of the three‐membered rings, but also by the s character of the respective CH bonds' carbon orbital. Reliable correlations of measured and calculated coupling constants were achieved with B3LYP/6‐311++G(d,p) and BP86/aug‐cc‐pVTZ‐J functionals.
Highlights
Stereoelectronic effects have a significant influence on the stability, structure, and reactivity of chemical compounds and on their physical and spectroscopic properties.[1]
Stereoelectronic effects in cyclohexanones, methylenecyclohexanes, spiro, and epoxy compounds of cyclohexanes and further derivatives were investigated by measuring 1JC,H coupling constants and by identification of Perlin effects, that is, of differences in the coupling constants for equatorial and axial C H bonds in the methylene groups of six-membered rings
It has already been mentioned by Perlin and Casu that those equatorial hydrogens in tetrahydropyranes, which are next to the ring oxygen, show a larger 1JC,H coupling than the respective axial hydrogens.[4]
Summary
Stereoelectronic effects have a significant influence on the stability, structure, and reactivity of chemical compounds and on their physical and spectroscopic properties.[1]. Significant (normal) Perlin effects are observed for the α positions of the cyclohexanone derivatives 1–5, that is, the 1J coupling constants of equatorial C H bonds are SCHEME 2 Synthesis of alkenes 2 and 3 and of spiro compounds 6 and 7. We performed quantum chemical investigations with tert-butyl-substituted substrates 1–9, but MartínezMayorga et al have already performed similar calculations (calculation of coupling constants and NBO analyses) with the parent cyclohexanone and methylenecyclohexane.[6f] In full agreement with the experimental data obtained by us, they observed a larger Perlin effect for cyclohexanone They argued that π* of the carbonyl group is a better acceptor as compared with that of the C C bond in methylenecyclohexane. Together with the data collected in the presented study of cyclohexanone derivatives, we can provide a sufficiently large dataset to TABLE 5 Correlation of calculated and measured coupling constants
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