3 research outputs found
Importance of Nonclassical σ‑Hole Interactions for the Reactivity of λ<sup>3</sup>‑Iodane Complexes
Key for the observed
reactivity of λ<sup>3</sup>-iodanes,
powerful reagents for the selective transfer of functional groups
to nucleophiles, are the properties of the 3-center-4-electron bond
involving the iodine atom and the two linearly arranged ligands. This
bond is also involved in the formation of the initial complex between
the λ<sup>3</sup>-iodane and a nucleophile, which can be a solvent
molecule or a reactant. The bonding in such complexes can be described
by means of σ-hole interactions. In halogen compounds, σ-hole
interaction was identified as a force in crystal packing or in the
formation of supramolecular chains. More recently, σ-hole interactions
were also shown to affect the reactivity of the iodine-based hypervalent
reagents. Relative to their monovalent counterparts, where the σ-hole
is located on the extension of the sigma-bond, in the hypervalent
species our DFT calculations reveal the formation of a nonclassical
σ-hole region with one or even two maxima. This observation
is also made in fully relativistic calculations. The SAPT analysis
shows that the σ-hole bond between the λ<sup>3</sup>-iodane
and the nucleophile is not necessarily of purely electrostatic nature
but may also contain a significant covalent component. This covalent
component may facilitate chemical transformation of the compound by
means of reductive elimination or other mechanisms and is therefore
an indicator for its reactivity. Here, we also show that the shape,
location, and strength of the σ-holes can be tuned by the choice
of ligands and measures such as Brønsted activation of the iodane
reagent. At the limit, the tuning transforms the nonclassical σ-hole
regions into coordination sites, which allows us to control how a
nucleophile will bind and react with the iodane
Breaking Down the Reactivity of λ<sup>3</sup>‑Iodanes: The Impact of Structure and Bonding on Competing Reaction Mechanisms
The functionalization of arenes via
diaryliodonium salts has gained
considerable attention in synthesis, as these compounds react under
mild conditions. Mechanistic studies have shown that the formation
of corresponding λ<sup>3</sup>-iodane intermediates takes a
key role, as they determine the course and selectivity of the reaction.
Bridged diaryliodonium salts, featuring a heterocyclic moiety involving
the iodine atom, were shown to exhibit a distinctly different reactivity,
leading to different products. These products are not just the result
of reductive elimination reactions but may also arise via radical
mechanisms. Our quantum chemical calculations reveal that the λ<sup>3</sup>-iodane intermediate is also the “gateway” for
reactions that are observed only for strained bridged systems. At
the same time, we find a remarkable affinity of the hypervalent region
to planarity for all reaction mechanisms. This also explains the correlation
between the size of the bridge connecting the aryl groups and the
reaction products observed. Furthermore, the energetics of these competing
reactions are examined by analysis of the mechanisms. Finally, using
model compounds, some of the basic features governing the reactivity
of λ<sup>3</sup>-iodanes are discussed
Donor- and/or Acceptor-Substituted Expanded Radialenes: Theory, Synthesis, and Properties
The synthesis of donor- (D) and/or
acceptor (A)-expanded [4]radialenes
has been developed on the basis of readily available dibromoolefin
(<b>7</b>), tetraethynylethene (<b>10</b> and <b>20</b>), and vinyl triflate (<b>12</b>) building blocks. The successful
formation of D/A radialenes relies especially on (1) effective use
of a series alkynyl protecting groups, (2) Sonogashira cross-coupling
reactions, and (3) the development of ring closing reactions to form
the desired macrocyclic products. The expanded [4]radialene products
have been investigated by spectroscopic (UV–vis absorption
and emission) and quantum chemical computational methods (density
functional theory and time dependent DFT). The combined use of theory
and experiment provides a basis to evaluate the extent of D/A interactions
via the cross-conjugated radialene framework as well as an interpretation
of the origin of D/A interactions at an orbital level