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DOI: 10.1055/s-0030-1258131
Mild, Efficient, and Robust Method for Stereocomplementary Iron-Catalyzed Cross-Coupling Using (E)- and (Z)-Enol Tosylates
Publication History
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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References and Notes
Ref. 2b, p. 1501.
9In 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.
11Yields 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.