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.