The use of supported acidic ionic liquids in organic synthesis

Abstract

Catalysts obtained by the immobilisation of acidic ionic liquids (ILs) on solid supports offer several advantages compared to the use of catalytically active ILs themselves. Immobilisation may result in an increase in the number of accessible active sites of the catalyst and a reduction of the amount of the IL required. The ionic liquid films on the carrier surfaces provide a homogeneous environment for catalytic reactions but the catalyst appears macroscopically as a dry solid, so it can simply be separated from the reaction mixture. As another advantage, it can easily be applied in a continuous fixed bed reactor. In the present review the main synthetic strategies towards the preparation of supported Lewis acidic and Brønsted acidic ILs are summarised. The most important characterisation methods and structural features of the supported ionic liquids are presented. Their efficiency in catalytic reactions is discussed with special emphasis on their recyclability.

Figure 1

Cations of ionic liquids used during the synthesis of acidic SILPs.

Scheme 1

Supported Lewis acidic IL with a covalently bonded anion.

Scheme 2

Immobilisation of a Lewis acidic IL via forming a covalent bond between the silica support and the cation.

Scheme 3

Preparation of Lewis acidic SILP material by carrying out ion exchange prior to the grafting step.

Scheme 4

Immobilisation of a Lewis acidic IL by a stepwise procedure.

Scheme 5

Synthesis of a Lewis acidic IL with a sol-gel method.

Scheme 6

Conversion of Brønsted acidic [(C₁=C₂)(HSO₃)³C₃im][OTf] or [(C₁=C₂)(HSO₃)⁴C₄im][OTf] to the Lewis acidic sulfonyl chloride derivatives ([(C₁=C₂)(ClO₂S)³C₃im][OTf] or [(C₁=C₂)(ClO₂S)⁴C₄im][OTf]) and immobilisation on mercaptopropylated silica.

Scheme 7

Immobilisation of an imidazolium IL on a Merrifield resin.

Scheme 8

Preparation of a chloroaluminate IL on a poly(1-vinylimidazole) support.

Scheme 9

Polymerisation of zinc tetrahalide ILs.

Scheme 10

Preparation of a SILP with a Brønsted acidic IL grafted on a silica support.

Scheme 11

Synthesis of a silica supported IL with a Brønsted acidic side chain [52].

Scheme 12

SILP material prepared by grafting of a dual acidic IL on a silica support [55].

Scheme 13

Preparation of phosphonium IL functionalised magnetic nanoparticles [59].

Scheme 14

Grafting of a ferrocenyl IL on Merrifield resin [63].

Scheme 15

Procedures for the immobilisation of dual acidic ILs on polymers [64,65].

Scheme 16

Preparation of SILP starting from poly(4-vinylpyridine) [67,68].

Scheme 17

Preparation of a SILP with heteropolyanions by polymerisation.

Figure 2

Supported ILs with both Lewis acidic and Brønsted acidic sites.

Figure 3

Activation of epoxides (A) and aldehydes (B) by supported Lewis acidic ILs.

Figure 4

Activation of aldehydes via proton transfer from a SO₃H-functionalised cation (A) [46], and via coordination of a HSO₄⁻ anion (B) [56] of acidic SILPs.

Scheme 18

Friedel-Crafts alkylation of cumene.

Scheme 19

Formation of nitronium ions from HNO₃ in the presence of supported [(HSO₃)³C₃Him][HSO₄].

Scheme 20

Supported IL-catalysed transesterification of ethylacetoacetate and octanol in an IL solvent.

Scheme 21

Acetal formation of aldehydes and side reaction of p-hydroxybenzaldehyde (4).

Scheme 22

Ring-opening of epoxides with POCl₃.

Scheme 23

Proposed mechanism for the synthesis of cyclic carbonates in the presence of polystyrene-supported [C₁Him][FeCl₄].

Scheme 24

Cross-aldol condensation of acetophenone in the presence of supported [(HSO₃)³C₃pyr][HSO₄].

Scheme 25

Domino Knoevenagel condensation/Michael addition of 4-hydroxycoumarin (7) and benzaldehyde.

Scheme 26

Synthesis of 4,4'-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol)s (9) by a domino Knoevenagel condensation/Michael addition.

Scheme 27

Synthesis of bis(indolyl)methanes (10) with a catalyst supported by a radical chain transfer reaction.

Scheme 28

Synthesis of 2,3-dihydroquinazolin-4(1H)-ones (11) by a cyclocondensation reaction.

Scheme 29

Synthesis of 1-amidoalkyl-2-naphthols (12).

Scheme 30

Synthesis of β-acetamido ketones by a multicomponent reaction.

Scheme 31

One-pot condensation of aromatic aldehydes, primary amines, and ammonium acetate.

Scheme 32

Multicomponent condensation leading to pyrano[3,2-b]pyrrole (17), pyrano[2,3-b]pyrrole (18) and pyrano[3,2-b]indole (19) derivatives.

Scheme 33

Synthesis of pyrano[2,3-b]indole derivatives (20).

Scheme 34

Multicomponent synthesis of acenaphtho[1,2-b]furans (22).

Scheme 35

Synthesis of trisubstituted pyridines (23) in the presence of [((EtO)₃Si)³C₃C₄im][HSO4] grafted on cellulose.

Scheme 36

Condensation of aldehydes, ethyl acetoacetate and ammonium acetate to form 1,4-dihydropyridines (24).

Scheme 37

Biginelli synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones (25).

Scheme 38

Synthesis of (2H)indazolo[2,1-b]phthalazine-triones (26).

Scheme 39

Synthesis of benzoxanthenes (27) by a three component condensation.

Scheme 40

Synthesis of a spirooxindole (31) in the presence of [((EtO)₃Si)³C₃(HSO₃)⁴C₄im][HSO4] supported on magnetic nanoparticles.

Scheme 41

Synthesis of homoallylic alcohols from aromatic aldehydes and allyltrimethylsilane.