Regioselective Couplings of Dibromopyrrole Esters

Abstract

The regioselectivity of the Suzuki couplings of several 4,5- and 3,4-dibromopyrrole-2-carboxylate esters has been studied. In general, regioselectivity can be achieved for initial coupling at the more electron-deficient site (C5 and C3, respectively). At the same time, conversions are often modest (40-60%) and attempts to force the reactions to higher conversions often lead to competitive dicoupling.

Key words

Suzuki cross-coupling - regioselectivity - electronic effects - pyrroles - palladium

References

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1

Current address: Department of Chemistry, Middle Tennessee State University, Murfreesboro, TN 37132, USA.

7

The one remaining isomeric dibromopyrrole ester (3,5-dibromo) has also been prepared. The regioselectivity of the Suzuki couplings of this substrate exhibit an unusual solvent dependency that remains under investigation.

12

Bases tried include the following: Na₂CO₃, K₂CO₃, Cs₂CO₃, K₃PO₄, and Ba(OH)₂. Solvents tried include the following: DMF, EtOH-toluene, acetone, and H₂O.