UNUSUALLY STRONG DEPENDENCE OF CONFORMATION ON SOLVENT

Relative partition coefficients between aqueous methanol and pentane o f the two stereoisomers of a series of 4-tert-butylcyclohexylamines we re measured by NMR. The cis isomer shows a larger partition coefficien t, with a Delta Delta G(org-->aq)degrees up to 1.4 kcal/mol. A thermod ynamic cycle relates these values to a solvent dependence of the A val ue for conformational equilibrium of an amino substituent. The variati on with stereoisomer is attributed to a greater hydrophilicity of a cy clohexylamine with an equatorial amino group, or equivalently to a gre ater steric bulk of an amino group whose lone pair is hydrogen bonded to water. The strong dependence on solvent inferred for A(NMe2) was co nfirmed by direct measurement of conformer populations at low temperat ure.