Chirality Transfer in a Calixarene-Based Directional Pseudorotaxane Complex

Hexamethoxycalix[6]arene 3 forms a directional pseudorotaxane complex with the chiral axle (S)-(alpha-methyl-benzyl)benzylammonium 2(+). Between the two (endo-chiral)-2(+)@3 and (exo-chiral)-2(+)@3 pseudorotaxane stereoisomers, the former is preferentially formed. This result confirms the validity of the "endo-alpha-methyl-benzyl rule", previously reported by us. DFT calculations suggest that C-H horizontal ellipsis pi interactions between the methyl group of 2(+) and the calixarene aromatic rings, determine the stereoselectivity of the threading process toward the "endo-alpha-methyl-benzyl preference". An amplification of optical rotation is observed upon formation of the pseudorotaxane complex (endo-chiral)-2(+)@3 with respect to free axle 2(+). Thus, the specifical rotation of the 1:1 mixture of chiral 2(+)center dot B(ArF)(4)(-) salt and achiral 3 was augmented upon formation of the pseudorotaxane and DFT calculations were used to rationalize this result.