The identification of vicinally substituted cyclohexane isomers in their mixtures by H-1 and C-13 NMR spectroscopy

The radical addition reactions of organobromine compounds, XBr (X = CH2COOM e, PhCH2, CHBr2 and CCl3) wi th cyclohexene afforded mixtures of cis/trans isomer pairs of 1-X-2-Br-cyclohexanes. In addition to benzyl benzoyloxy der ivatives are formed also, when benzoyl peroxide is used as an initiator. Ow ing to the great difficulties in separating these cis/trans isomer pairs, t hey are identified directly in their mixtures by NMR spectroscopy. In addit ion to one-dimensional (1D) H-1, proton decoupled C-13 and DEPT-135, also t wo-dimensional (2D) C-13-C-13 INADEQUATE as well as H-1-C-13 HMQC experimen ts have been used in assigning the signals of each compound in their mixtur es. The identification of each isomer was based on comparison of experiment al (3)J(H,H) coupling constants with theoretical ones based on the well-kno wn Karplus type relationship. The more stable conformation for each isomer was estimated using the semiempirical AMI molecular orbital method. The cal culations support the isomer pair elucidations. (C) 2000 Elsevier Science B .V. All rights reserved.