Conformations methylene cyclohexanes

Viewed from the side, the chair conformation of cyclohexane appears to have one methylene group puckered upward and another puckered downward. Viewed from the Newman projection, the chair has no eclipsing of the carbon-carbon bonds. The bond angles are 109.5°. 

The boat conformation of cyclohexane (Figure 3-20) also has bond angles of 109.5° and avoids angle strain. The boat conformation resembles the chair conformation except that the footrest methylene group is folded upward. The boat conformation suffers from torsional strain, however, because there is eclipsing of bonds. 

For methylene cyclohexanes and C-X o-acceptors (Figure 6.86), the dominant interaction pattern is reminiscent of the anomeric effect. Experimental NMR conformational measurements and detailed dissection of NBO interactions in these systems has been provided by Tormena and coworkers. The 1,3 allylic strain can serve as an additional destabilizing effect that disfavors equatorial conformers for substituted alkenes. 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Booth and Little40 observed a chemical shift difference (Aae) of about 0.4 ppm for the C-2 and C-6 methylene protons in 4-methylpiperidine, which was assumed to exist as an equilibrium containing 95% of the C-methyl equatorial conformation. N-Methylation of 4-methylpiperidine caused an upfield shift of 0.22 ppm for the C-2 equatorial proton and of 0.68 ppm for the C-2 axial proton. Comparison of these values with those observed for the proton adjacent to methyl groups in cyclohexane led these authors to the... 

More recently, Usuki etal. studied, using NMR techniques, the conformation of spirocyclic oxaziridines 3 derived from substituted cyclohexanones <1995JOC8028>. Depending on substitution and on the stereochemistry, these compounds exhibit a substantial upfield shift of the cyclohexyl methylene or methane protons with a 1,3-diaxial relationship to the oxaziridine N-substituent. This effect is ascribed to a conformation that places an aromatic group over the plane of the cyclohexane ring. This conformation has also been observed in the solid state by X-ray crystallography and is further supported by molecular mechanics calculations. 

If all six methylenes rock, the chair is converted to another chair in which all six former equatorial hydrogens have become axial and the six axial hydrogens have become equatorial If only three methylenes rock, the cyclohexane ring will be in the boat conformation. It would then have four pairs of eclipsed hydrogens in addition to a stern-bow interaction. 

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