Lewis Acid/Base Interaction Matrix Database

π-System Lewis Bases

Electron Rich π-Systems

If an extended electron rich π-system species reacts via a single atomic centre, for example when an allyl anion is protonated to give propene, the species is better considered behaving as an ambidentate, π-stabilised Lobe-HOMO Lewis base.

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

Search for π-HOMO Lewis base species in The Chemical Thesaurus

FMO Topology:

π-HOMO Lewis bases have their electrons in their highest occupied molecular orbital, or HOMO, delocalised over two or more p-orbitals.

Species require modelling by Hückel-FMO techniques as well as by VB-resonance structure methods. Hückel MO modelling gives rise to whole families of π-structure: polyene ribbons, aromatics, etc.:

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 carbanion, allyl anion & pentatrieneyl anion and alkene, 1,3-diene & 1,3,5-triene π-HOMO Lewis bases:

Charge: Negative, delta or electron rich π-systems.
HSAB:

Soft when the entire π-system is acting as the Lewis base, but harder when a single atomic centre is involved and the species is behaving as a Lobe-HOMO Lewis base.

The allyl anion can behave as (or be considered as) 2 π-electrons delocalised over a 3p orbital function, or as a stabilised 2p orbital carbanion. In this latter case it is better to consider the allyl anion to be behaving as a (harder) Lobe-HOMO Lewis base.

Chemistry:

Species behave as π-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 anion, etc.

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

  • Formation of π-organometallic complexes, Type 24 Lewis acid/base complexation.
Congeneric Series:

pi-HOMO Lewis base (generic)
Acetaldehyde enolate

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Acetylene

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Acetylene (generic)

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Acetylene, terminal hydrogen (generic)

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Acrylate ion

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Alkene HOMO (generic)

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Alkene, 1,1-disubstituted (generic)

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cis Alkene (generic)

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Alkene, monosubstituted (generic)

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Alkene, tetrasubstituted (generic)

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trans Alkene (generic)

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Alkene, trisubstituted (generic)

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1° Alkyl benzene (generic)

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Alkyl nitrile alpha-carbanion (generic)

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Allene (generic)

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Allene

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Allyl anion

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Allyl anion (generic)

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1° Allylic alcohol (generic)

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2° Allylic alcohol (generic)

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1° Amide, anion (generic)

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Aniline

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Anisole

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Anthracene

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Arsole

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Azide ion

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Azulene

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2,3-Benzanthracene

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Benzene

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Benzene, 1,2-dialkyl (generic)

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Benzene, substituted (generic)

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Benzidine

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Benzoate ion

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Benzofuran

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Benzoic acid

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Benzophenone radical anion

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Benzo[a]anthracene

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Benzo[a]pyrene

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Benzyl anion

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Benzyl anion (generic)

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Benzyne

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Biphenyl

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Butadiene, 1,3-methoxy

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Carbide ion

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Carbon monoxide

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Carboxylate ion (generic)

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Chrysene

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Coumarin

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Cumulene (generic)

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Cyanate ion

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Cyanide ion

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Cycloheptatriene

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Cycloheptatriene anion

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Cycloheptatriene anion (generic)

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1,3-Cyclohexadiene

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Cyclohexenyl anion

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Cyclooctatetrene dianion

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Cyclooctatetrene dianion (generic)

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Cyclopentadiene

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Cyclopentadienyl anion

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Cyclopentadienyl anion (generic)

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Cyclopropene

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Cyclopropenyl anion

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Cyclopropyl anion

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Dibenz[a,h]anthracene

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trans 1,2-Dichloroalkene (generic)

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Diels-Alder adduct (generic)

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beta-Diketone anion (generic)

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N,N-Dimethyl aniline

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Diphenyl amide anion

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Diphenyl ether

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Diphenyl phosphine anion

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Diphenylamine

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Diphenylmethyl anion

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Enamine anion (generic)

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Enolate anion (generic)

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1,12-Ethanoperylene-13,14-dioic anhydride

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Ethylene

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Fluorene anion

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Formate ion

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Furan

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5 Heterocycle (generic)

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6 Heterocycle (generic)

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Heterocycle, polycyclic (generic)

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Hydrazone (generic)

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Imidazole

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Imidazole anion

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Imido anion (generic)

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Imine, dialkyl (generic)

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Imine, N-alkyl alkyl (generic)

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Imine, trialkyl (generic)

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Indole

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Isocyanide (generic)

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Isoquinoline

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Isothiazole

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Ketone, alkyl phenyl (generic)

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Lithium carbide

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N-Methyl aniline

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Naphthalene

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Naphthalene radical anion

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Oxazole

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Pentacene

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Pentadienyl anion

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Pentalene

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Pentalene dianion

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Perylene

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Phenanthrene

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Phenol

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Phenol (generic)

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Phenoxide ion

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Phenoxide ion (generic)

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Phenyl amide anion

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Phenyl anion

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Phenyl anion (generic)

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Phenyl ether (generic)

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Phenylacetylide anion

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9-Phenylfluorene anion

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Phenylphosphine

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Phenylselenide anion

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Phosphole

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Picrate ion

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Polycyclic aromatic (generic)

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Polyene (generic)

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Prop-2-en-1-imine

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Propene

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Purine

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Pyrazine

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Pyrazole

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Pyrene

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Pyridazine

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Pyridine

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Pyridine (generic)

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Pyrimidine

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Pyrrole

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Quinoline

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Selinole

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cis Stilbene

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trans Stilbene

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Styrene

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alpha-Terpene

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Tetrathiafulvalene

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Thiazole

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Thiocyanate ion

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Thiophene

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Thiophenoxide ion

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1,3,5-Triazine

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Triphenylmethyl anion

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Triphenylphosphine

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Vinyl chloride (generic)

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Vinylnaphthalene

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Poster
Nucleophiles & Bases

© Mark R. Leach 1999-


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