Cycloglycolurils: Hybrid Glycoluril-Cyclobenzil Macrocycles.

Two novel glycoluril macrocycles have been synthesized from cyclotetrabenzil and cyclotribenzoin precursors using solvent-free condensations with urea. The crystal structure of the cyclotetra(p-phenylene)glycoluril macrocycle shows a twisted ring conformation, while that of the cyclotri(m-phenylene)glycoluril hybrid exhibits a distinct tubular supramolecular packing. These structures establish a potentially broad new class of macrocycles with intriguing guest binding properties owing to their available N-H motifs.

Topoisomers and aromaticity of a redox active 50π core-modified isophlorinoid.

A 50π decathiophene expanded isophlorin adopts a unique [6+4] conformation and a near-planar conformation depending on the solvent of crystallization and undergoes a reversible two electron oxidation to yield the largest planar antiaromatic dication bearing 48π-electrons. Cyclic voltammetry studies revealed the redox active nature of this macrocycle with multiple oxidation and reduction potentials. Spectro-electrochemical measurements confirmed a facile reversible two-electron oxidation and the unstable radical cation intermediate in these systems. Quantum chemical calculations that were employed to estimate NICS(0) values revealed a weak diatropic ring current for the 50π macrocycle and an intense paratropic ring current for the 48π dicationic species.

Three dimensional isophlorinoid tetrapodal molecular cage.

Steric hindrance induced by thiophene molecules in predesigned precursors favors the exclusive formation of a three dimensional (3D) π-conjugated cage and quasi-cage like molecules instead of a porphyrinoid macrocycle. Herein we report the synthesis of a tetrapod 3D fully π-conjugated molecular cage using a simple acid catalysed reaction. The X-Ray crystallography analysis confirmed the tetrapod cage structure and intermediates, which resemble three-fourths or half of the cage structures.

A wide-range of redox states of core-modified expanded porphyrinoids.

Core-modified hexaphyrin and octaphyrin display four- and six-electron reversible redox reactions respectively, to alternate between aromatic and anti-aromatic states of a given macrocycle. We have identified and isolated a hexaphyrin in three discrete states with 26π, 28π and 30π electrons that are inter-convertible with each other. Its higher congener, octaphyrin, can exist as four discrete species with 34π, 36π, 38π and 40π electrons. A difference of two-electrons between each stable redox state is reflected by the significant variation in their electronic and structural properties as characterized in both solution and solid states. The observed redox inter-conversions were achieved by a combination of both proton coupled electron-transfer (PCET) and electron-transfer (ET) processes respectively.

Isophlorinoids: The Antiaromatic Congeners of Porphyrinoids.

Ever since the discovery of the porphyrin ring in "pigments of life", such as chlorophyll and hemoglobin, it has become a prime synthetic target for optoelectronic properties and in the design of metal complexes. During one such early expedition on the synthesis of porphyrin, Woodward proposed that condensing pyrrole with an aldehyde under acidic conditions yields the "precursor" porphyrinogen macrocycle. Its four-electron oxidation leads to the "transitory" 20π isophlorin, which undergoes subsequent two-electron oxidation to form the 18π "porphyrin". Due to its fleeting lifetime, it has been a synthetic challenge to stabilize the tetrapyrrolic isophlorin. This macrocycle symbolizes the antiaromatic character of a porphyrin-like macrocycle. In addition, the pyrrole NH also plays a key role in the proton-coupled, two-electron oxidation of isophlorin to the aromatic porphyrin. However, a major aspect of its unstable nature was attributed to its antiaromatic character, which is understood to destabilize the macrocycle upon conjugation. Antiaromaticity in general has not gained significant attention mainly due to the lack of stable 4nπ systems. In this regard, a stable isophlorin and its derivatives provide a glimmering hope to peek into the world of antiaromatic systems. This review will focus on the attempted synthesis of antiaromatic isophlorin ever since its conception. Based on recent synthetic advances, the chemistry of isophlorins can be expected to blossom into expanded derivatives of this antiaromatic macrocycle. Along with the synthetic details, the structural, electronic, and redox properties of isophlorin and its expanded derivatives will be elaborated.