π-Extension of Strained Benzenoid Macrocycles Using the Scholl Reaction.

A series of bent p-terphenyl-containing macrocycles have been synthesized and then regioselectively brominated, arylated, and subsequently subjected to a Scholl-based cyclodehydrogenation reaction. Shortening the alkyloxy bridging unit of these macrocycles increases the bend in the p-terphenyl unit, as well as the strain energy (SE) of the central para-phenylene ring system. For the first time, incremental increases in SE of the macrocyclic structure of this class of benzenoid compounds have been investigated in the context of π-extension to strained polycyclic aromatic hydrocarbon systems using the Scholl reaction.

A Macrocyclic 1,4-Diketone Enables the Synthesis of a p-Phenylene Ring That Is More Strained than a Monomer Unit of [4]Cycloparaphenylene.

The synthesis of a p-terphenyl-based macrocycle, containing a p-phenylene unit with 42.6 kcal/mol of strain energy (SE), is reported. The conversion of a macrocyclic 1,4-diketone to a highly strained arene system takes place over five synthetic steps, featuring iterative dehydrative reactions in the aromatization protocol. Spectroscopic data of the deformed benzenoid macrocycle are in excellent agreement with other homologues that have been reported, indicating that the central p-phenylene ring of 9 is aromatic.

Overcoming Strain-Induced Rearrangement Reactions: A Mild Dehydrative Aromatization Protocol for Synthesis of Highly Distorted p-Phenylenes.

A series of p-terphenyl-based macrocycles, containing highly distorted p-phenylene units, have been synthesized. Biaryl bonds of the nonplanar p-terphenyl nuclei were constructed in the absence of Pd-catalyzed or Ni-mediated cross-coupling reactions, using 1,4-diketones as surrogates to strained arene units. A streamlined synthetic protocol for the synthesis of 1,4-diketo macrocycles has been developed, using only 2.5 mol % of the Hoveyda-Grubbs second-generation catalyst in both metathesis and transfer hydrogenation reactions. Under protic acid-mediated dehydrative aromatization conditions, the central and most strained benzene ring of the p-terphenyl systems was susceptible to rearrangement reactions. To overcome this, a dehydrative aromatization protocol using the Burgess reagent was developed. Under these conditions, no strain-induced rearrangement reactions occur, delivering p-phenylene units with up to 28.4 kcal/mol strain energy and deformation angles that sum up to 40°.

A non-cross-coupling approach to arene-bridged macrocycles: synthesis, structure, and direct, regioselective functionalization of a cycloparaphenylene fragment.

A new synthetic strategy that employs a relatively unstrained, 1,4-diketo-bridged macrocycle as a precursor to a strained, 1,4-arene-bridged (bent para-phenylene) macrocycle has been developed. The distorted p-terphenyl nucleus (CPP fragment) of the macrocycle has been characterized by X-ray crystallography, and a direct, regioselective bromination protocol of the macrocyclic system is reported.