Alleno-Acetylenic Cage (AAC) Receptors: Chiroptical Switching and Enantioselective Complexation of trans-1,2-Dimethylcyclohexane in a Diaxial Conformation.

Four enantiopure 1,3-diethynylallenes (DEAs) with OH termini were attached to the rim of a resorcin[4]arene cavitand. The system undergoes conformational switching between a cage form, closed by a circular H-bonding array, and an open form, with the tertiary alcohol groups reaching outwards. The cage form is predominant in apolar solvents, and the open conformation in small, polar solvents. Both states were confirmed in solution and in X-ray co-crystal structures. ECD spectra of the alleno-acetylenic cages (AACs) are highly conformation sensitive, the longest wavelength Cotton effect at 304 nm switches from Δϵ=+191 m-1  cm-1 for open (P)4 -AAC⊂acetonitrile to Δϵ=-691 m-1  cm-1 (ΔΔϵ=882 m-1  cm-1 ) for closed (P)4 -AAC⊂cyclohexane. Complete chiral resolution of (±)-trans-1,2-dimethylcyclohexane was found in the X-ray structures, with (P)4 -AAC exclusively bound to the (R,R)- and (M)4 -AAC to the (S,S)-guest. Guest inclusion occurs in a higher energy diaxial conformation.