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Synlett 2003(14): 2101-2116  
DOI: 10.1055/s-2003-42098
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© Georg Thieme Verlag Stuttgart · New York

The CeCl3·nH2O/NaI System in Organic Synthesis: An Efficient Water Tolerant Lewis Acid Promoter

Giuseppe Bartoli*a, Enrico Marcantonib, Letizia Sambria
a Dipartimento di Chimica Organica ‘A. Mangini’, Università di Bologna, v.le Risorgimento 4, 40136 Bologna Italy
Fax: +39(051)2093654; e-Mail: giuseppe.bartoli@unibo.it;
b Dipartimento di Scienze Chimiche, Università di Camerino, v. S. Agostino 1, 62032 Camerino (MC), Italy
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Publikationsverlauf

Received 13 June 2003
Publikationsdatum:
15. Oktober 2003 (online)

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Abstract

The ability of CeCl3·nH2O/NaI system to work as an useful water tolerant promoter and/or reagent in many organic transformations is reviewed. They include Michael additions of 1,3-dicarbonyl compounds, amines and indoles to α,β-unsaturated ketones, and related systems, the cleavage of carbon-oxygen and carbon-silicon bonds, the conversion of alcohols into the corresponding iodides, as well as the stereospecific dehydration of β-hydroxy carbonyl compounds and the hydroxacyclization of unsaturated 3-hydroxy esters. Generally the reactions are carried out at reflux in wet CH3CN, however, in some case, it is possible to work under solvent free conditions and/or in the presence of the catalyst supported on SiO2. In the last case the simple experimental procedures allow the complete recovery of the catalyst and its recycling without loss of activity. Many of these protocols found wide application in organic synthesis and are now methods of common choice, since they are competitive and in some cases superior to the pre-existing procedures. The reasons for this success can be ascribed to the fact that this reagent allows the reaction to be carried out in close to neutral conditions and thus allows the survival of a large variety of functionalities sensitive to acidic hydrolysis. In addition, CeCl3·7H2O and NaI are cheap, non toxic, and stable compounds, and therefore the protocols based on their use represent an environmentally benign alternative to current chemical processes using water intolerant Lewis acids.

  • 1 Introduction

  • 2 Michael Additions

  • 3 Cleavage of Carbon-Oxygen and Silicon-Oxygen Bonds

  • 3.1 General Protocol for the Deprotection of PMB Ethers

  • 3.2 Cleavage of Trityl, Trialkylsilyl, and Allyl Ethers

  • 3.3 Cleavage of tert-Butyl and Prenyl Esters

  • 3.4 Deprotection of Dioxolanes, Oxathiolanes, and Dithiolanes

  • 4 Reactions Involving the Hydroxyl Group

  • 4.1 Conversion of Alcohols into Iodides

  • 4.2 Dehydration of β-Hydroxy Carbonylic Compounds to the
    Corresponding α,β-Unsaturated Derivatives

  • 4.3 Synthesis of E-Alkylidene Cycloalkanones

  • 4.4 Hydroxacyclization of Unsaturated 3-Hydroxy Esters

  • 5 The Origin of the High Lewis Acid Activity of
    CeCl3·nH2O/NaI Combination

  • 6 Conclusions

Key words

cerium(III) chloride - Lewis acids - sodium iodide - Michael additions - cleavage of ethers - hydroxyl group transformation

18

A typical procedure follows. A flask was charge in succession with the 1, 3-dicarbonyl compound (1 equiv), the Michael acceptor (1.1 equiv), CeCl3·7H2O (0.2 equiv) and NaI (0.1 equiv). The mixture was then stirred at room temperature until the 1,3-dicarbonyl compound was consumed. The reaction was treated under stirring with Et2O (20 mL). The catalyst mixture was removed by filtration and reused. The organic layer was concentrated under reduced pressure and the crude product was purified by silica gel chromatography.

23

Typical experimental procedure. Silica gel (30-60 µm 0.63 g) was added to a mixture of CeCl3·7H2O (13.6 mmol) and NaI (11.6 mmol) in CH3CN (50 mL), and the mixture was stirred overnight at room temperature. The CH3CN was removed by rotary evaporation and to the resulting reagent was added dibenzylamine (10 mmol) and methyl vinyl ketone (12.6 mmol). The mixture was stirred at 35 °C. After completion of the reaction and addition of Et2O the supported catalyst was recovered by filtration and reused. The organic layer was washed with 10% aqueous citric acid with aqueous saturated NaHCO3 solution dried over anhydrous Na2SO4 and the solvent evaporated under reduced pressure. The crude was purified by flash chromatography on SiO2 to give the corresponding β-amino derivative.

76

Bartoli, G.; Bosco, M.; Giuliani, A.; Marcantoni, E.; Palmieri, A.; Petrini, M.; Sambri, L. Organomet. submitted for publication.