Computational investigation of host–guest chiral recognition in incommensurate 2-bromoalkane/urea inclusion compounds

Abstract

In conventional urea inclusion compounds, the urea molecules form a crystalline solid host structure (space group P6₁22 or P6₅22) within which there are linear, parallel tunnels. The walls of these tunnels are formed by a spiral arrangement of urea molecules; appropriate guest molecules may be located within these tunnels. In order to investigate the extent and nature of the chiral recognition between the chiral urea tunnel structure and chiral guest molecules, computational investigations of host–guest interaction in 2-bromoalkane/urea inclusion compounds have been carried out. As the 2-bromoalkane/urea inclusion compounds have an incommensurate relationship between the periodicities of the host and guest structures along the tunnel axis, it is important to consider the way in which the chiral recognition varies as a function of the position of the 2-bromoalkane guest molecule along the tunnel. All 2-bromoalkanes from 2-bromoheptane to 2-bromohexadecane were studied; in each case, four different ‘types’ of 2-bromoalkane guest molecule were considered, representing the R and S enantiomers, both for the conformation with the Br atom trans (CH₃ group gauche) and for the conformation with the Br atom gauche (CH₃ group trans).

For each enantiomer of the 2-bromoalkane guest molecule, it is found that the Br trans/CH₃gauche conformation is preferred over the Br gauche/CH₃trans conformation at all positions along the tunnel (in contrast, the Br gauche/CH₃trans conformation is preferred for isolated 2-bromoalkane molecules). For the preferred Br trans/CH₃gauche conformation within the P6₁22 host structure, the R enantiomer is preferred over the S enantiomer at all positions along the tunnel for all guest molecules studied, with the exception of 2-bromoundecane (for 2-bromoundecane, there are restricted regions within which there is a slight preference for the S enantiomer). On taking into account the incommensurate nature of the 2-bromoalkane/urea inclusion compounds, an overall excess of the R enantiomer within the P6₁22 host structure is predicted in all cases. Structural reasons underlying the host–guest chiral recognition in the 2-bromoalkane/urea inclusion compounds are discussed.