pubs.acs.org/joc
Nucleophilic Iron Catalysis in Transesterifications: Scope and Limitations
Silja Magens and Bernd Plietker*
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Institut fu€r Organische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
Received March 15, 2010
The ester bond is one of the most common structural motifs found in nature. Apart from the
condensation between an acid and an alcohol, transesterifications represent another mechanistic
alternative for the preparation of this compound class. The present paper summarizes our most
recent investigations in this field, using nucleophilic iron complexes as catalysts for transesterifica-
tions under neutral conditions. This new type of metal catalyst complements the existing methodo-
logies, which rely on Lewis acidic metal complexes. Investigations on scope and limitations,
stereochemical course, and chemoselectivities will be presented.
Introduction
seminal contributions from Hieber,3 Roustan,4 and Zhou5
we were able to develop a regioselective allylic substitution
by employing a combination of [Bu4N][Fe(CO)3(NO)]
(TBAFe)6 and triaryl phosphines7 or N-heterocyclic car-
benes8 as monodentate basic ligands. Upon the basis of the
assumption that a nucleophilic attack of the electron-rich
catalyst at a positively charged olefinic carbon atom is
involved in the first step of this transformation we recently
were able to extend the reaction scope by using esters as
reactive substrates in transesterifications (Scheme 1).9
Apart from the importance of transesterifications in organic
chemistry the Fe-catalyzed version of this reaction represents
the first example for the use of an acyl-iron complex in
catalysis.10 A deeper understanding on factors influencing the
reactivity and stability of these complexes might pave the way
toward the development of further catalytic transformations
in which similar complexes are reactive intermediates (e.g.,
carbonylation, cross-coupling, etc.).
Transition metal-catalyzed reactions are among the most
powerful tools in modern organic synthesis. The use of only
catalytic amounts of a metal complex often allows for a
significant reduction of the activation energy barrier, which
in turn allows reactions to be done at lower temperature. The
cost savings by use of catalytic reactions can be increased if
metal complexes based upon readily available, nontoxic
metals are used. With regard to these both economical and
ecological arguments iron catalysis has faced a tremendous
resurrection within the past years.
Most of the catalytic reactions known today employ iron
salts which function as either Lewis acids or alkyl transfer
agents after in situ transformation to tetraalkyl ferrates.1
However, whereas the metal center in this type of ferrate still
possesses the oxidation state of þII or þIII, catalysis in
which the metal itself is reduced to the oxidation state -II is
still somewhat uncommon.2 Our group got involved in the
latter type of catalysis three years ago. Upon the basis of
Hence we initiated an in-depth investigation on the scope
and limitations of this new catalytic transformation with the
(1) (a) Review on Fe-catalysis: Bolm, C.; Legros, J.; Le Paih, J.; Zani, L.
Chem. Rev. 2004, 104, 6217. (b) Review on Fe-catalyzed cross-couplings:
(6) For the preparation of TBAFe see ref 8b.
€
Furstner, A.; Martin, R. Chem. Lett. 2005, 34, 624. (c) Plietker, B.; Iron
(7) (a) Allylic alkylation: Plietker, B. Angew. Chem. 2006, 118, 1497.
Angew. Chem., Int. Ed. 2006, 45,1469. (b) Allylic amination: Plietker, B.
Angew. Chem. 2006, 118, 6200. Angew. Chem., Int. Ed. 2006, 45, 6053;
(c) Allylic sulfonation: Jegelka, M.; Plietker, B. Org. Lett. 2009, 11, 3462.
Catalysis in Organic Synthesis; Wiley-VCH, Weinheim, Germany, 2008.
(2) Plietker, B.; Dieskau, A. Eur. J. Org. Chem. 2009, 775.
(3) (a) Hieber, W.; Kahlen, N. Chem. Ber. 1958, 91, 2223. (b) Hieber, W.;
Kahlen, N. Chem. Ber. 1958, 91, 2234.
€
(8) (a) Plietker, B.; Dieskau, A.; Mows, K.; Jatsch, A. Angew. Chem. 2008,
(4) (a) Roustan, J. L. A.; Houlihan, F. J. Organomet. Chem. 1988, 353,
215. (b) Cygler, M.; Ahmed, F. R.; Forgues, A.; Roustan, J. L. Inorg. Chem.
1983, 22, 1026. (c) Roustan, J. L. A.; Forgues, A. J. Organomet. Chem. 1980,
C13-C16, 184.
(5) (a) Xu, Y.; Zhou, B. J. Org. Chem. 1987, 52, 974. (b) Zhou, B.; Xu, Y.
J. Org. Chem. 1988, 53, 4421.
120, 204. Angew. Chem., Int. Ed. 2008, 47, 198. (b) Holzwarth, M.; Dieskau,
A.; Tabassam, M.; Plietker, B. Angew. Chem. 2009, 121, 7387. Angew.
Chem., Int. Ed. 2009, 48, 7251.
(9) Magens, S.; Ertelt, M.; Jatsch, A.; Plietker, B. Org. Lett. 2008, 10, 53.
(10) Collman, J. P. Acc. Chem. Res. 1975, 342 and references cited
therein.
DOI: 10.1021/jo1004636
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Published on Web 05/12/2010
J. Org. Chem. 2010, 75, 3715–3721 3715
2010 American Chemical Society