Conformational analysis and stereoelectronic effects in trans-1,2-dihalocyclohexanes: 1H NMR and theoretical investigation.
The conformational equilibrium of trans-1,2-difluoro- (1), trans-1,2-dichloro- (2) and trans-1,2-dibromo-cyclohexane (3) was studied through a combined method of NMR, theoretical calculations and solvation theory. The solvent dependence of the 3JH1,H2 NMR coupling constants together with theoretical calculations allow the direct determination of the conformational equilibria without recourse to model compounds. The coupling constants were obtained with the aid of spectrum simulation, since these symmetric molecules present complex coupling systems. The observed couplings, when analysed by solvation theory and utilising DFT geometries (B3LYP/6-311+G**), gave energy values of E(ee) -E(aa) of 0.10, 0.95 and 1.40 kcalmol(-1) in the vapour phase for 1, 2 and 3, respectively, decreasing to -0.63, 0.36 and 0.93 kcalmol(-1) in CCl4 and to -1.91, -0.80 and -0.05 kcalmol(-1) in DMSO solution. The diaxial preference for all compounds is explained by natural bond orbital (NBO) analysis, which shows important hyperconjugative effects in this conformation. The "gauche effect" for compounds with more electronegative substituents, which are in gauche arrangement in the ee conformation, also plays a relevant role in more polar solvents.
Freitas
Matheus P
MP
Physical Organic Chemistry Laboratory, Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, 13084-971 Campinas, SP, Brazil.
Rittner
Roberto
R
Tormena
Cláudio F
CF
Abraham
Raymond J
RJ
D003510:Cyclohexanes / Q000737:chemistry*; D006846:Hydrocarbons, Halogenated / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008968:Molecular Conformation; D015394:Molecular Structure; D013237:Stereoisomerism; D013816:Thermodynamics
D003510:Cyclohexanes; D006846:Hydrocarbons, Halogenated