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 -Et3 N-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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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 .
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11 Yields of the traditional basic method
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using an aliphatic simple esters, Me(CH2 )4 CO2 Me,
in our hands, however, was not reproducible under identical conditions. These
strongly basic and heterogeneous conditions might be troublesome.
