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Synthesis 2006(7): 1127-1132  
DOI: 10.1055/s-2006-926378
PAPER
© Georg Thieme Verlag Stuttgart · New York

Erbium(III) Triflate: A Valuable Catalyst for the Synthesis of Aldimines, Ketimines, and Enaminones

Renato Dalpozzo*a, Antonio De Ninoa, Monica Nardia, Beatrice Russob, Antonio Procopiob
a Dipartimento di Chimica, Univesità della Calabria, Ponte Bucci, cubo 12C, 87030 Arcavacata di Rende (Cs), Italy
Fax: +39(0984)493077; e-Mail: dalpozzo@unical.it;
b Dipartimento di Scienze Farmaco-Biologiche, Università della Magna Graecia, Complesso Ninì Barbieri, 88021 Roccelletta di Borgia (Cz), Italy
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Publication History

Received 17 October 2005
Publication Date:
08 March 2006 (online)

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Abstract

Aldimines, ketimines, and enaminones can be obtained under erbium(III) triflate catalysis. The reaction mechanism is that typical of imine synthesis. The role of the catalyst is demonstrated for the synthesis of aromatic imines. In contrast to CeCl3/NaI addition to unsaturated aldehydes, which results in Michael addition, no Michael adduct was observed under erbium(III) triflate catalysis.

Key words

imines - enaminones - erbium triflate - Lewis acids - catalysis

10

Some imines were unstable and a one-pot reduction step was necessary for their identification. This was realized by addition of a THF solution of LiBH4 to the mixture, after monitoring the disappearance of the reactants by GC-MS analysis.

13

A solution of Er(OTf)3 in water is only weakly acidic (pH 5.9) and the aqueous layers from the work-up were even less acidic (pH 6.7); such values are not compatible with triflic acid being the active catalyst.

14

LD50 of erbium salts is 4417 mg/Kg against 4000 mg/Kg for NaCl.

15

It is likely that the catalyst can be recovered in higher yields when the reaction is scaled up; we recovered the catalyst in ÷90% yield.