Crystal and molecular structure of cis-2,trans-3-dibromo-cis-4-t-butyl-cyclohexyl p-nitrobenzoate: a case of non-chair conformation of a saturated monocyclic cyclohexane derivative
Abstract
The complete crystal structure of the title compound(I) was determined by X-ray diffraction. Crystals are monoclinic, space group P21/c, with Z= 4 in a unit cell of dimensions a= 7.697(5), b= 12.506(7), c= 19.520(9)Å, β= 99.43(4)°. The structure was solved by Patterson and Fourier methods from diffractometer data and refined by least-squares techniques to R 0.078 for 1 356 observed reflections. The molecule exists in a twist conformation, with the staggered t-butyl group and the ester group in pseudo-equatorial positons and the bromines in a pseudoaxial and an isoclinal position. This conformation is that in which interactions between substituents are minimized, and is slightly biased towards a boat form. This is the first reported structure of a saturated monocyclic cyclohexane derivative in a non-chair conformation, and confirms the strong repulsive interaction between t-butyl and a trans-2-substituent on cyclohexane, which had been deduced from spectroscopic and thermodynamic data. In solution at room temperature the title compound exists, according to its 1H n.m.r. spectrum, as an equilibrium probably also involving a chair conformation.