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Partial core-modification of a porphyrin can be employed to synthesize the 20π antiaromatic isophlorin. Unlike the tetra-, tri-, and dipyrrole derivatives of a porphyrin, a monopyrrole porphyrin exhibits antiaromatic characteristics. It undergoes a two-electron reversible ring oxidation to yield the 18π aromatic dication. (1) H NMR analysis provides distinct evidence of the altered electronic characteristics through typical paratropic and diatropic ring current effects for the 4n and the (4n+2) π-electron systems, respectively.

Reactive intermediates are key to halting and promoting chemical transformations; however, due to their elusive nature, they are not straightforwardly harnessed for reaction design. Herein, we describe studies aimed at stabilizing reactive intermediates in the N-heterocyclic carbene (NHC) catalytic cycle, which enabled the full shutdown of the known benzoin coupling pathway, while rerouting its intermediates toward deuteration. The reversible nature of NHC catalysis and the selective stabilization of reaction intermediates facilitated clean hydrogen–deuterium exchange reactions of aromatic aldehydes by D2O, even for challenging electron-withdrawing substrates. In several cases, the addition of catalytic amounts of phenyl boronic acid was used to further stabilize highly reactive intermediates and mitigate the formation of benzoin coupling byproducts. The mechanistic understanding at the foundation of this work resulted in unprecedented mild conditions with base and catalyst loadings as low as 0.1 mol %, and a scalable deuteration reaction applicable to a broad substrate scope with outstanding functional group tolerance. More importantly, adopting this approach enabled the construction of a guideline for identifying the appropriate catalyst and conditions for different substrates. Experimental studies combined with machine learning and computational methods shed light on the nontrivial mechanistic underpinnings of this reaction.

Amongst the various porphyrinoids, octaphyrin has attracted significant attention owing to its diverse syntheses, conformations, and metal-ligationp roperties. Octaphyrin is ah igher homologue of porphyrin and is formed by linking together heterocycles such as pyrrole, furan, thiophene, ands elenophene through a-a or a-meso carbon bonds. The planar conformation is mainly achievedt hrough inversion of the heterocyclic units from the center of macrocycle;a voiding meso-bridges;introducing a para-quinodimethane bridge;e mployinganeo-confusion approach; protonation;a nd by generating dianionics pecies. In this Focus Review,r ecent synthetic advancements in the field of octaphyrins are summarized.T he twisted conformation of the octaphyrinb inds to two metal ions in at etracoordinate geometry.T he diphosphorus complex of octaphyrin represents the first example of as table expanded isophlorin.

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