Unravelling mechanistic features of organocatalysis with in situ modifications at the secondary sphere.

Secondary-sphere interactions serve a fundamental role in controlling the reactivity and selectivity of organometallic and enzymatic catalysts. However, there is a dearth of studies that explicitly incorporate secondary-sphere modifiers into organocatalytic systems. In this work, we introduce an approach for the in situ systematic modification of organocatalysts in their secondary sphere through dynamic covalent binding under the reaction conditions. As a proof-of-concept, we applied boronic acids as secondary-sphere modifiers of N-heterocyclic carbenes that contained a hydroxy handle. The bound system formed in the reaction mixture catalysed the enantioselective benzoin condensations of a challenging substrate class that contains electron-withdrawing groups. Linear regression coupled with data visualization served to pinpoint the divergent origins of enantioselectivity for different substrates and decision tree algorithms served to formulate selection criteria for the appropriate secondary-sphere modifiers. The combination of this highly modular catalytic approach with machine-learning techniques provided mechanistic insights and guided the streamlined optimization process of a gram-scale reaction at low organocatalyst loading.

A naphthalene-fused dimer of an anti-aromatic expanded isophlorin.

We report the first synthesis of a covalent expanded isophlorin dimer from two 24-π doubly S-confused sapphyrin-like pentathiaisophlorins. It exhibits marginal peripheral aromaticity rather than strong global diatropicity or paratropicity and weak intermacrocycle electronic communication. Quantum chemical methods discern that cross-conjugation is responsible for these unusual electronic features.

"To Twist or Not to Twist": Figure-of-Eight and Planar Structures of Octaphyrins.

Amongst the various porphyrinoids, octaphyrin has attracted significant attention owing to its diverse syntheses, conformations, and metal-ligation properties. Octaphyrin is a higher homologue of porphyrin and is formed by linking together heterocycles such as pyrrole, furan, thiophene, and selenophene through α-α or α-meso carbon bonds. The planar conformation is mainly achieved through inversion of the heterocyclic units from the center of macrocycle; avoiding meso-bridges; introducing a para-quinodimethane bridge; employing a neo-confusion approach; protonation; and by generating dianionic species. In this Focus Review, recent synthetic advancements in the field of octaphyrins are summarized. The twisted conformation of the octaphyrin binds to two metal ions in a tetracoordinate geometry. The diphosphorus complex of octaphyrin represents the first example of a stable expanded isophlorin.

Controlled Core-Modification of a Porphyrin into an Antiaromatic Isophlorin.

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.

Metal assisted cyclomerization of benzodipyrrins into expanded norroles, aza-heptalene and acyclic dimers.

Benzofused dipyrrins react with metal salt copper(II) acetate to predominantly yield a cyclodimer along with a cyclotrimer. N-confused monobenzo-dipyrrin cyclomerized to trioxo-expanded norrole 13a and an acyclic dimer 14 whereas doubly N-confused monobenzo and dibenzo-dipyrrins yielded aza-heptalene 15 and an acyclic dimer 16.

The synthesis and characterization of the meso-meso linked antiaromatic tetraoxa isophlorin dimer.

The first synthesis of meso-meso linked antiaromatic isophlorin has been described. The synthetic pathway is different from the routinely employed oxidative coupling of meso free porphyrin into its dimer and oligomers. Spectroscopic and structural analyses revealed paratropic ring current effects and each unit was found to be orthogonal to each other.

Non-covalent composites of antiaromatic isophlorin-fullerene.

Cocrystallates of fullerene C60 and antiaromatic planar tetraoxaisophlorins have been characterized by single crystal X-ray diffraction analysis. The ring-juncture bonds of C60 are found to be at a very close distance to the plane of the antiaromatic isophlorins. NMR measurements and MALDI-TOF mass spectrometry show that this interaction can persist in both solution and gaseous states, which can be attributed to van der Waals dispersion forces.

Metal assisted cyclodimerization of doubly N-confused dipyrrins into planar aza-heptalenes.

Metal salts cyclodimerize doubly N-confused dipyrrin into a nornorrole type macrocycle, with a bipyrrolic unit at its center. It also represents an unusual aza-heptalene structure with fused bicyclic seven membered rings. The fused rings can have one or two C-N bonds between the dipyrrin units in the cyclodimer. The (1)H NMR spectrum of these molecules displays aromatic character, rather than antiaromatic behaviour expected of nornorrole, and planar conformation in the solid state.