Strain-Induced Double Carbon-Carbon Bond Activations of Cycloparaphenylenes by a Platinum Complex: Application to the Synthesis of Cyclic Diketones

Allosteric Binding-Induced Intramolecular Mechanical-Strain Engineering

Recently, polymer mechanochemistry has attracted much scientific interest due to its potential to develop degradable polymers. When the two ends of a polymer chain experience a linear pulling stress, molecular strain builds up, at sufficiently strong force, a bond scission of the weakest covalent bond results. In contrast, bond-breaking events triggered by conformational stress are much less explored. Here, we discovered that a Zn salen complex would undergo conformational switching upon allosteric complexation with alkanediammonium guests. By controlling the guest chain length, the torsional strain experienced by Zn complex can be modulated to induce bond cleavage with chemical stimulus, and reactivity trend is predicted by conformational analysis derived by DFT calculation. Such strain-release reactivity by a Zn(salen) complex initiated by guest binding is reminiscent of conformation-induced reactivity of enzymes to enable chemical events that are otherwise inhibited.

Non-Planar Perylene Bisimide Analogues with Inserted Carbonyl and Methylene Subunits

The synthesis and properties of dinaphthotropone bisimide (DNTrBI) and dinaphthocycloheptatriene bisimide (DNCHepBI) are described. Their molecular design is conceptually based on the insertion of a carbon atom into a perylene bisimide (PBI) core. These molecules adopt non-planar structures due to the presence of a seven-membered ring. The PBI derivative into which a carbonyl group was inserted (DNTrBI) immediately underwent nonradiative decay and/or intersystem crossing in its excited state. The PBI derivative into which a methylene group was inserted (DNCHepBI) was susceptible to deprotonation on account of the two electron-withdrawing naphthalene monoimide units. Subsequent aerobic oxidation resulted in the formation of a C-C bond at the central methylene unit, thus affording a σ-dimer. The formation of this C-C bond is dynamically redox-active, i.e., electron injection into the σ-dimer almost quantitatively regenerated the deprotonated DNCHepBI.

Confined Spaces in [n]Cyclo-2,7-pyrenylenes

A set of strained aromatic macrocycles based on [n]cyclo-2,7-(4,5,9,10-tetrahydro)pyrenylenes is presented with size-dependent photophysical properties. The K-region of pyrene was functionalized with ethylene glycol groups to decorate the outer rim and thereby confine the space inside the macrocycle. This confined space is especially pronounced for n=5, which leads to an internal binding of up to 8.0×104  m-1 between the ether-decorated [5]cyclo-2,7-pyrenylene and shape-complementary crown ether-cation complexes. Both the ether-decorated [n]cyclo-pyrenylenes as well as one of their host-guest complexes have been structurally characterized by single-crystal X-ray analysis. In combination with computational methods the structural and thermodynamic reasons for the exceptionally strong binding have been elucidated. The presented rim confinement strategy makes cycloparaphenylenes an attractive supramolecular host family with a favorable, size-independent read-out signature and binding capabilities extending beyond fullerene guests.

A BN-Doped Cycloparaphenylene Debuts

The first example of a BN-doped cycloparaphenylene BN-[10]CPP was synthesized and characterized. Its reactivity and photophysical properties were evaluated in direct comparison to its carbonaceous analogues Mes-[10]CPP and [10]CPP. While the photophysical properties of BN-[10]CPP remains similar to its carbonaceous analogues, the electronic structure changes associated with the introduction of a 1,2-azaborine BN heterocycle into a CPP scaffold enables facile and selective late-stage functionalizations that cannot be accomplished with carbonaceous CPPs. Specifically, Ir-catalyzed hydrogenation of BN-[10]CPP selectively reduces the BN heterocyclic ring, which upon hydrolysis produces a rare example of a macrocyclic paraphenylene 6 incorporating the versatile ketone functionality within the macrocyclic ring.