Cyclohexanone-Driven Discriminatory Behavior Change of Host Compound (+)-(2<i>R</i>,3<i>R</i>)-TETROL for Isomeric Methylcyclohexanone Guests

Abstract

Using appropriate competition experiments, host compound TETROL [(+)-(2 R,3 R )-1,1–4,4-tetraphenylbutane-1,2,3,4-tetraol] was revealed to discriminate between the isomeric methylcyclohexanone guests; the host selectivity was in the order 2- ≫ 3- > 4- methylcyclohexanone. Surprisingly, addition of unsubstituted cyclohexanone to these competitions instigated a complete reversal in the host’s preference for the alkylcyclohexanones: a selectivity order of cyclohexanone >4- > 3- > 2-methylcyclohexanone was attained. It is proposed that it is the significant preference of TETROL for cyclohexanone that is responsible for the host behavior change in the presence of this guest. As evidence, the crystal packing of a mixed complex, TETROL·(72%)cyclohexanone·(28%)4-methylcyclohexanone, was shown to be isostructural with pure TETROL·cyclohexanone (which differs from TETROL·4-methylcyclohexanone). The addition of cyclohexanone to the competition experiments therefore steers crystallization toward crystals with similar packing to that of the preferred guest complex accounting for TETROL’s change in selectivity in the presence of this guest. Hirshfeld surface analyses were used to demonstrate that TETROL·cyclohexanone experiences a significantly larger number of stabilizing O···H/H···O interactions (15.2%) compared with the other complexes (11.5–14.4%), accounting for the host’s overwhelming preference for this guest. Thermogravimetric analysis further confirmed that cyclohexanone was more strongly bound in the crystal than the other guests.