Crystal structure of (+)-isomenthyl p-bromophenylcarbamate and absolute configuration and conformation of isomenthol by X-ray diffraction and nuclear magnetic resonance studies

Abstract

Crystals of (+)-isomenthyl p-bromophenylcarbamate, C₁₀H₁₉O·CO·NH·C₆H₄Br are orthorhombic, space group P2₁2₁2₁, and unit cell dimensions a= 25.58 (5)Å, b= 13.18(2)Å. c= 16.70(3)Å. There are 12 molecules in the unit cell, 3 in each asymmetric unit. The structure was solved by the heavy-atom method, and refined by block-diagonal least-squares to R 0.087 for 2927 reflections. In the crystalline state, the isomenthol of the three molecules in the asymmetric unit exist in two different types of chair conformations. Two have the methyl substituent at C(1) axial, while the p-bromophenylcarbamate and the isopropyl substituents at C(3) and C(4) are both equatorial. The third molecule has the alternate chair conformation, i.e., methyl equatorial at C(1), p-bromophenylcarbamate axial at C(3), and isopropyl axial at C(4). ¹H and ¹³C n.m.r. spectra of isomenthol in carbon disulphide and dimethyl sulphoxide solutions show it exhibits conformational mobility involving the two chair forms. The isomenthol conformer with the hydroxy- and the isopropyl-groups equatorial is the major component, that with methyl equatorial and the other two substituents axial the minor. The absolute configurations at the asymmetric carbons as derived from anomalous dispersion method are 1R,3S,4R. There is no flattening observed in the cyclohexane ring as a whole, only some distortions at the substituted positions. The p-bromophenylcarbamate chain is almost planar forming angles of 53° 43′, 53° 13′, and 81° 10′ with the planes of the isomenthol for molecules (A), (B), and (C).