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Type 20 Lewis Acid/Base Complexation Chemistry

If the π-LUMO Lewis acid and π-HOMO Lewis base species have suitable:

• Shape (geometry)
• Charge
• and Phase matched FMOs

an orbital phase-symmetry controlled multi-centre π/π complexation reaction can take place with various the participating atoms rehybridizing to give a largely σ-bonded product. The transition state is deemed to proceed via a pericyclic intermediate, where "peri-" is a prefix meaning around or surrounding here.

The classic pericyclic interaction is the Diels-Alder cycloaddition between a diene and a dienophile:

The orbital phase symmetry arguments required for cycloaddition to take place are interchangeable. Cycloaddition can either involve the HOMO of the diene + the LUMO of the dieneophile or the LUMO of the diene + the HOMO of the dieneophile:

Pericyclic reactions are concerted: they take place in a single step. As a consequence, concerted processes provide allow for great Stereochemical control, and pericyclic processes are amongst the most useful of all synthetic methodologies available to the synthetic organic chemist.

Pericyclic chemistry is discussed in detail elsewhere in this webbook.

Electron-poor δ⁺ π-systems can interact with electron-rich δ^– π-systems. The initial attraction between is electrostatic (ionic) in nature to form a π/πcharge transfer complex.

FMO Controlled Pericyclic Interactions

Diels-Alder cycloaddition can be considered as multicentre π-LUMO plus π-HOMO Lewis acid/base Complexation chemistry. However, it is sometimes difficult to decide which species is acting as the Lewis acid and which is the Lewis base.

In normal electron demand Diels-Alder cycloaddition chemistry the diene is electron rich, implying Lewis base character, and the alkene, the dieneophile, is electron deficient implying that the species is the Lewis acid:

In reverse electron demand cycloaddition, the diene acts as the electron deficient Lewis acid and the dieneophile is the electron rich Lewis base:

There are four general classes of pericyclic reaction process:

• Cycloaddition
• Electrocyclic Reactions
• Sigmatropic Rearrangements
• Group Transfer Reactions

Pericyclic chemistry is discussed in detail elsewhere in this webbook.

Charge Transfer Complexes

π/π-Interactions can also lead to the formation of a charge-transfer complex, for example between the electron poor 1,3,5-trinitrobenzene and electron rich benzidine.

π/π-Interactions can also lead to the formation or conductive organic metals such as stacked TTF/TCNQ materials: