Lewis Acid/Base Interaction Matrix Database

π-LUMO Lewis Acids

π-LUMO Lewis Acids

If an extended electron poor π-system species reacts via a single atomic centre, for example when a benzyl cation is reduced to toluene, the species is better considered behaving as an ambidentate, π-stabilised Lobe-LUMO Lewis acid.

However, when when the species reacts via its extended π-system directly, for example during Diels-Alder cycloaddition or when forming a π-organometallic, the the species should be considered as a π-HOMO Lewis base species. Thus, there is an overlap between π-stabilised Lobe-LUMO and π-LUMO classification.

Search for π-LUMO Lewis acid species in The Chemical Thesaurus

FMO Topology:

Delocalised cationic hydrocarbon π-systems and those neutral but electron deficient π-functions which participate in concerted multicentre reactions.

Hückel MO theory gives rise to whole families of π-structure: polyene ribbons, aromatics, etc. Each system is FMO unique. π-Species must be considered at the Hückel level, as well as by VB resonance techniques.

Indeed, quantum mechanics is all about patterns. A particularly striking manifestation is seen with the polyene system of: 1, 2, 3, 4, 5, 6... conjugated p-orbital systems and how they give rise to the cabenium ion, allyl cation & pentatrieneyl canion and alkene, 1,3-diene & 1,3,5-triene π-LUMO Lewis acids:

Charge: Positive or delta+ electron poor π-systems.
HSAB: Intrinsically soft.
Chemistry:

Species behave as π-LUMO Lewis acid species when they undergo FMO controlled multicentre interactions. These most obviously manifest themselves in three situations:

  • Stabilisation of the π-system: certain patterns/structures are associated with stability such as 4n+2 π-electrons in a cyclic array, the allyl cation, etc.

  • Diels-Alder cycloaddition and other pericyclic interactions, Type 20 Lewis acid/base complexation.

  • Having non-nucleophilic complex anion Lewis base anions to stabilise the π-LUMO Lewis acid specie.
Congeneric Series:

Few congeneric series, but the chloronitrobenzene series can be considered congeneric with respect to the nucleophilic displacement of Cl by a nucleophile:

pi-LUMO Lewis acid (generic)
Acrylonitrile

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Alkene LUMO (generic)

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Allyl cation

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Allyl cation (generic)

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Arenium ion (generic)

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Aronium ion (generic)

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Azoniaanthracene ion

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Azulene

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Benzaldehyde

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Benzaldehyde radical cation

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Benzene diazonium ion

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Benzonium ion

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Benzyl cation

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Benzyl cation (generic)

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1,3-Butadiene, s-cis

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1,3-Butadiene, s-trans

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1,3-Cyclohexadiene

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((1-Cyclohexenyl)methylidene)propanedinitrile

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Cyclopentadiene

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alpha-Cyclopropyl cation

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Diazonium ion (generic)

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trans 1,2-Dichloroalkene (generic)

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Dimethylethylcarbenium ion

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Diphenyl carbenium ion

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Enone (generic)

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Furan

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6 Heterocycle (generic)

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Heterocycle, polycyclic (generic)

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1,3,5-Hexatriene

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N5 Cation

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1,3,5,7-Octatetrene

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Pentadienyl cation

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Phenyl cation

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Picric acid

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Polyene (generic)

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Propargylic cation

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Propargylic cation (generic)

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Pyrrole

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Pyrylium ion

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Quinolizinium ion

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7,7,8,8,- Tetracyano-p-quinodimethane

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Tetracyanoethylene

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Thiophene

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Thiopyrylium ion

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Triethyloxonium tetrafluoroborate

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1,3,5-Trinitrobenzene

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Triphenylmethyl carbenium ion

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Tropylium ion

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alpha, beta-Unsaturated EWG function (generic)

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Vinyl chloride (generic)

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Poster
Nucleophiles & Bases

© Mark R. Leach 1999-


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