Stoddart Mechanostereochemistry Group

 

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Pretzelanes Go Nano

Through its exploitation of noncovalent bonding interactions and self-assembly processes, supramolecular assistance to covalent synthesis has established itself as an efficient means of creating molecules with nanoscale dimensions. For two decades, researchers have harnessed the power of post-assembly covalent modification to produce an array of mechanically interlocked molecular compounds, some of which have been shown to behave as molecular machines and switches on surfaces and at interfaces, respectively. A template-directed protocol has been developed for the construction of [2]catenanes composed of a crown ether containing π-electron-rich aromatic ring systems and a tetracationic cyclophane comprised of two π-electron-deficient bipyridinium units. If the crown ether is covalently tethered to this second component, then the resulting cyclization(s) could occur either intramolecularly and generate a pretzelane or intermolecularly and generate cyclic or linear oligo/polycatenanes. Introduction of a stereogenic center by a stereospecific synthesis into an optically active, donor–acceptor pretzelane, that exhibits helicity as well as fixed chirality, leads to a marked preference for one conformational diastereoisomer over the other in both acetone and dimethylsulfoxide that can be understood from computational models.

The Stoddart and Houk groups have reported (Chem. Commun 2005, 3927– 3929) the synthesis of the chiral pretzelane (S)–(P/MP4+ (Box). In the pretzelane P4+ helical chirality arises from the location of the crown ether on either one of the two bipyridinium units on the tetracationic cyclophane. Therefore, a pair of (P) and (M) enantiomers have been observed with a free energy of activation of 17.5 kcal/mol for their inversion in CD3COCD3 solution. Force-field modeling has been employed to provide insight into the diastereoisomeric conformational preference, namely that the (M)-isomer is preferred over the (P)-isomer in (S)–P4+. The calculation, based on the MM2 force field, matches closely that of the experimental ΔG° value of 1.3 kcal/mol. The energy minimized structures associated with both conformations retain the expected noncovalent bonding interactions, including the [π…π], [C– H…π] and [C–H…O] interactions that are operative when the 1,5-dioxynaphthalene unit is located inside the cavity of the tetracationic cyclophane. The methyl group on the linker in the (S)–(PP4+ isomer points in towards (Box a and c) the cavity formed by one of the tetraethylene glycol loops of the crown ether component and one of the bipyridinium units in the tetracationic cyclophane while, in the case of the (S)–(MP4+ isomer, the methyl group points away (Box b and d) from this cavity. The crowding that the methyl group experiences in the (S)–(PP4+ isomer is presumably the origin of the preference for the (S)–(MP4+ isomer.

 

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