CONFORMATIONAL PREFERENCES AND INTERCONVERSION BARRIERS IN CYCLOHEXENE AND DERIVATIVES

The conformational preferences (E4 values) for 4-fluoro-, 4-chloro-, 4-bromo-, and 4-iodocyclohexenes have been determined by low-temperature nmr spectroscopy and are 0.014, 0.200, 0.077, and –0.016 kcal/mol. Although smaller, these values show the same trend in magnitude as A values, except for iodo which actually shows a preference for the axial position. Assuming the boat as the transition state, the chair-chair interconversion barriers (Delta;G‡) in kilocalories/mole are 5.37 for cyclohexene-2,3,3,4,4,5,6,6-d8, 5.3 for 4-fluorocyclohexene, 6.3 for 4-chlorocyclohexene, 6.3 for 4-bromocyclohexene, 6.5 for 4-iodocyclohexene, and 7.4 for cis.-4,5-dicarbomethoxy-cyclohexene. The chemical-shift difference between the axial and equatorial C-4 protons in cyclohexene-2,3,3,4,4,-5,6,6-d8 is 19.0 Hz (60 MHz). For cyclohexene-d8, assuming the only contribution to Delta;S‡ is the reaction pathway multiplicity, Δ‡ ~ R In 2 and ΔH‡ is greater than ΔG‡* by TΔS‡, i.e., AH ~ 5.5 kcal/mol. © 1969, American Chemical Society. All rights reserved.