J. Braz. Chem. Soc., Vol. 21, No. 2, 371-374, 2010.
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The Use of Anhydrous CeCl3 as a Recyclable and Selective Catalyst for the
Acetalization of Aldehydes and Ketones
Claudio C. Silveira,*,a Samuel R. Mendes,a Francieli I. Ziembowicz,a
Eder J. Lenardãob and Gelson Perinb
aDepartamento de Química, Universidade Federal de Santa Maria,
CP 5001, 97105-970 Santa Maria-RS, Brazil
bInstituto de Química e Geociências, LASOL, Universidade Federal de Pelotas,
CP 354, 96010-900 Pelotas-RS, Brazil
Um método eficiente, limpo, quimiosseletivo e sem a utilização de solventes para a síntese
de dimetil acetais de aldeídos e cetonas foi desenvolvido usando ortoformiato de trimetila e
CeCl3 anidro como catalisador. O método é geral e, sob condições reacionais brandas, fornece
compostos carbonílicos protegidos em bons rendimentos, incluindo aril e alquil cetonas e aldeídos.
O catalisador pode ser reutilizado diretamente, três vezes, sem perda significativa da atividade.
An efficient, clean, chemoselective and solvent-free method for the synthesis of ketone and
aldehyde dimethyl acetals was developed using trimethyl orthoformate and commercially available
anhydrous CeCl3 as a recyclable catalyst. The method is general and affords the protected carbonyl
compounds in good yields and under mild conditions, including aryl and alkyl ketones and activated
aldehydes. The catalyst could be utilised directly for 3 cycles, without significant loss of activity.
Keywords: cerium(III) chloride, acetals, trimethyl orthoformate, aldehydes, ketones
Introduction
have some drawbacks, such as the use of toxic, strong and
corrosive acids, non-recyclable catalysts, volatile organic
solvents (VOCs) and, in some cases, non-commercially
available catalysts. A greener, atom efficient approach to
acetals is the use of trialkyl orthoesters instead of alcohols,
because they provide both alkoxy groups while scavenging
the water produced in the reaction.3,4,9,10,13,14
In view of our interest in the development of new,
cleaner methods for classical reactions22 and new
applications of cerium(III) in organic synthesis,23 we
decide to study the reaction of carbonyl compounds 1
with trimethyl orthoformate to obtain dimethyl acetals 2
(Tables 1 and 2).24
Acetalization of aldehydes and ketones is the most
frequently used strategy for protection of carbonyl
groups against nucleophilic reagents.1 Besides being very
useful derivatives in total synthesis, acetals can be easily
transformed into several utile functional groups.2 Because
of the versatility and usefulness of acetals in organic
synthesis, several methods for the protection3-20 and the
selective, mild deprotection21 of aldehydes and ketones
have been described. The classical procedure for protection
involves the Lewis3-15 or protic acid-catalyzed16-20 reaction
of a carbonyl compound with a large excess of an alcohol
in the presence of a water scavenger.Among the employed
Lewis acids are metal triflates,3-8 lanthanoid chlorides,9,10
Results and Discussion
SnCl4,11 FeCl3,12 LiBF4,13 ZnCl2 and pyridinium ions15
14
while HCl,16 p-TsOH,17 HBF4,18 2,6-pyridinedicarboxylic
acid19 and silica-supported HClO420 have been used as protic
acids. Despite the advocated advantages of several recently
described methods for the acetalization reaction, they
Initially, we chose benzaldehyde 1a (2.0 mmol) and
trimethyl orthoformate (2.2 mmol) as standard starting
materials to establish the best conditions for the reaction
under solvent-free media. We examined the temperature,
25
amount of anhydrous CeCl3 and the reaction time
(Table 1).
*e-mail: silveira@quimica.ufsm.br