Reverse Anomeric Effect in Large‐Amplitude Pyridinium Amide‐Containing Mannosyl [2]Rotaxane Molecular Shuttles

Abstract

The reverse anomeric effect (RAE) was investigated in different mannosyl [2]rotaxane molecular shuttle isomers that contain dibenzo-24-crown-8 (DB24C8) as the macrocycle, and anilinium and pyridinium amide as molecular stations. The switching on or off of the RAE was possible depending on both the pyridinium amide motif and the localization of the DB24C8 along the thread. The (1) C4 mannopyranosyl chair-like conformation was observed in all the non-interlocked molecules because the anomeric carbon of the mannose is linked to the positively charged nitrogen of the pyridinium unit. In the protonated rotaxanes, the (1) C4 chair conformation of the mannose end remains because the DB24C8 resides around the best anilinium station, which is located at the other end of the axle. Upon deprotonation of the anilinium, the DB24C8 shuttles with a large-amplitude motion toward the pyridinium amide stations, where it interacts in a different fashion depending on the pyridinium motif. In one molecular shuttle, the RAE could be switched on or off with control at one end of the encircled thread upon protonation/deprotonation of the other end, through shuttling of the DB24C8.

How Secondary and Tertiary Amide Moieties are Molecular Stations for Dibenzo-24-crown-8 in [2]Rotaxane Molecular Shuttles?

“…Several secondary stations for the DB24C8 have been reported in the literature to date. As a non-comprehensive list of them, we can cite the following positively charged stations: bipyridinium [ 4 , 5 , 6 , 7 , 8 , 9 , 10 ], 1,2-bis-(pyridinium)ethane [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ], triazolium [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ], pyridinium [ 31 , 32 , 33 ], pyridinium amide [ 34 , 35 , 36 , 37 ], imidazolium [ 38 , 39 , 40 , 41 ] moieties, oxidized ferrocene [ 42 ], etc. Some neutral molecular stations are also important to be mentioned, such as urea [ 43 ], thiourea [ 44 ], carbamate [ 45 , 46 , 47 , 48 , 49 , 50 ], isoxazole [ 51 ], and benzylic ester [ 52 ], that usually interact either through π-π stacking with the aromatic rings of the crown ether, or through hy...…”