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Synlett 2010(14): 2087-2092  
DOI: 10.1055/s-0030-1258131
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Mild, Efficient, and Robust Method for Stereocomplementary Iron-Catalyzed Cross-Coupling Using (E)- and (Z)-Enol Tosylates

Hiroshi Nishikado, Hidefumi Nakatsuji, Kanako Ueno, Ryohei Nagase, Yoo Tanabe*
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-1337, Japan
Fax: +81(79)5659077; e-Mail: tanabe@kwansei.ac.jp;
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Publication History

Received 3 May 2010
Publication Date:
09 July 2010 (online)

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Abstract

Iron-catalyzed cross-coupling of Grignard reagents (RMgX) with (E)- and (Z)-enol tosylates proceeded smoothly to give a variety of the corresponding (E)- and (Z)-trisubstituted α,β-unsaturated methyl esters (total 30 examples; 55-98% yield). The simple, mild, stereoretentive method utilized iron(III) chloride (FeCl3), iron(III) acetylacetonate [Fe(acac)3], and iron(III) tris(dibenzylmethane) [Fe(dbm)3]. The (E)- and (Z)-enol tosylates were readily prepared by the reported stereocomplementary tosylation method from methyl β-keto esters or α-formyl esters. Methyl α-formyl esters were obtained via a practical and robust TiCl4-Et3N-mediated α-formylation of methyl esters with methyl formate.

Key words

cross-coupling - iron - stereoselective synthesis - enol ­tosylate - α,β-unsaturated ester

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4

Ref. 2b, p. 1501.

9

In the absence of Fe(III) catalysts, the major side reaction was an addition to the ester moiety and the desired coupling products were not obtained. The basic reactivity order of the Fe(III) catalysts is as follows: Fe(dbm)3 > Fe(acac)3 with NMP > Fe(acac)3 > FeCl3. Due to the accessibility and cheapness, the choice order was FeCl3, Fe(acac)3, and Fe(dbm)3; For (E)-1, (E)-3, and (Z)-3, FeCl3 sufficiently worked well, whereas reactive Fe(dbm)3 was required for (Z)-1.

11

Yields of the traditional basic method range from 0 to 50%. Reexamination of NaH-promoted α-formylation using an aliphatic simple esters, Me(CH2)4CO2Me, in our hands, however, was not reproducible under identical conditions. These strongly basic and heterogeneous conditions might be troublesome.