Mordecai Rabinovitz

Paratropicity and antiaromaticity: role of the homo-lumo energy gap

Abstract Doubly charged systems derived from fused benzenoid polycycles reveal an unquenched delocalization of 4 n π-electrons and hence are predicted to possess antiaromatic character. The magnitude of the paratropic 1 H NMR chemical shifts, due solely to the paramagnetic secondary field sustained in these species, was found to depend linearly upon the magnitude of LUMO-HOMO energy gaps of the corresponding systems. The existence of such a correlation enables a comprehensive treatment of the various factors which determine the antiaromatic character and the subtle interrelations between those factors. This, in turn, leads to a deeper understanding of antiaromaticity.

HYDROXIDE ION INITIATED REACTIONS UNDER PHASE TRANSFER CATALYSIS CONDITIONS---IV

Abstract The nature of the quat-anion interactions in a PTC/OH − system was probed by examining the reactivity and selectivity of a CA/O-ambident anion towards an alkylating agent in the presence of various quats. It is suggested that the accessibility of the positive N center of the qual for association with the anion is the major factor in determining the behavior of PTC/OH − reactions proceeding through the Makosza mechanism.

Synthetic approaches to aromatic belts: building up strain in macrocyclic polyarenes

This tutorial review discusses synthetic strategies towards aromatic belts, defined here as double-stranded conjugated macrocycles, such as [n]cyclacenes, [n]cyclophenacenes, Schlüter belt, and Vögtle belt. Their appeal stems, firstly, from the unique nature of their conjugation, having p orbitals oriented radially rather than perpendicular to the plane of the macrocycle. Secondly, as aromatic belts are model compounds of carbon nanotubes of different chiralities, a synthetic strategy towards the buildup of structural strain in these compounds could finally open a route towards rational chemical synthesis of carbon nanotubes. The elusiveness of these compounds has stimulated fascinating and ingenious synthetic strategies over the last decades. The various strategies are classified here by their approach to the buildup of structural strain, which is the main obstacle in the preparation of these curved polyarenes.

Bicorannulenyl: Stereochemistry of a C40H18 Biaryl Composed of Two Chiral Bowls

The bicorannulenyl molecule is composed of two chiral bowls tethered by a single bond in a helical fashion. This simple combination of two chiral motifs gives rise to rich dynamic stereochemistry, where 12 conformers interconvert through bowl inversions and central bond rotation, and enantiomerizations occur via multistep processes. Interestingly, 8 out of 10 transition states are chiral, giving rise to mostly chiral enantiomerization pathways, where the molecule changes chirality without passing through an achiral conformation. However, analysis of the stereochemical landscape by DFT calculations and variable temperature NMR spectroscopy reveals that the energetically most favorable enantiomerization pathway passes through one of the two achiral transition states. Single-crystal X-ray diffraction corroborates the DFT results and provides information on packing modes of bicorannulenyl molecules in the solid state that have not been seen previously for other buckybowls.

Dianions and Tetraanions of Bowl-Shaped Fullerene Fragments Dibenzo[a,g]corannulene and Dibenzo[a,g]cyclopenta[kl]corannulene

The fullerene fragment 1 (C28H14) gives a paratropic dianion and a diatropic tetraanion on alkali metal reduction, whereas the C30H14 hydrocarbon 2, which contains just one more ring from the C60 network, gives a diatropic dianion and a paratropic tetraanion.

π‐Conjugated Anions: From Carbon‐Rich Anions to Charged Carbon Allotropes

Stable π-conjugated anions are formed in the reaction of π-conjugated systems with alkali metals. Reactivity, aromatic properties and aggregation of the anions are described. Bowl-shaped and fullerene anions, as well as helicene and planar polycyclic anions, are discussed vis-a-vis their aromaticity and charge delocalization pattern.

Bis-quaternary ammonium salts as phase transfer catalysts

Bis-quaternary ammonium salts are used for the extraction of polyanions and show better extractive abilities than common phase transfer catalysts.

1H NMR patterns of antiaromatic systems: Paratropic displacement dependence upon LUMO-HOMO energy gap

Abstract 1 H NMR chemical shifts of antiaromatic species reveal an enhanced paratropic displacement. The high-field shifts exhibited by doubly charged benzenoid polycycles were shown to be strongly related to the LUMO-HOMO energy gaps in these antiaromatic systems. As the gap decreases, a larger paratropic shift was observed.

Probing the nature of polycyclic conjugated dianions: from carbocyclic to heterocyclic dianions: NMR studies, π-delocalization and electronic structure

NMR studies (proton and carbon) afford an experimental tool for the evaluation of the patterns of charge delocalization and charge distribution in π-conjugated polycyclic dianions. In several cases a partitioning of charge could be observed (as well as a segregation of M.O.s), a situation unprecedented in π-conjugated systems. This charge partitioning is rationalized by the relative paratropicities of the charged and uncharged components

Reduction of 1, 3, 5, 7, 9-penta-t-butyl-corannulene: Isomeric sandwiches with multiple lithium cations inside and out

Abstract Four reduction stages were observed for 1, 3, 5, 7, 9-penta-t-Bu-corannulene(2), two of which exhibited diamagnetic character: a paratropic dianion 2 2− and a diatropic tetraanion 2 4− . In the last stage of reduction three distinct tetraionic species are detected. Two of them are sandwhich-type compounds with eight Li cations each and a tetraionic monomer.