Regioselectivity in nonsymmetric methyl pentyl Pillar[5]arene bound to non-symmetric axles.

The present work illustrates regioselective binding of nonsymmetric axle BuX (X = F, Cl, Br, CN) and 5-bromovaleronitrile (BVN) to the non-symmetric methyl pentyl pillar[5]arene (MPP5). Theoretical calculations reveal that the guest encapsulation within MPP5 is spontaneous and the conformer showing X weakly bound to pentyl rim of MPP5 is favoured over its other conformer wherein it interacts with methyl rim of the host. The noncovalent interactions namely C-H---π, C-H---X and H-H prevail over C-H⋯O hydrogen bonding in the complexes of MPP5. The manifestations of these to vibrational spectra obtained from the present theory are discussed. The strength of host-guest binding further is shown to correlate well with weakening of the C-X bond through natural bond orbital analyses.