posted on 2014-12-19, 00:00authored byMia N. Burhardt, Andreas Ahlburg, Troels Skrydstrup
A catalytic protocol for synthesis of thioesters from aryl, vinyl,
and benzyl bromides as well as benzyl chlorides was developed using
only stoichiometric amounts of carbon monoxide, produced from a solid
CO precursor inside a two-chamber system. As a catalytic system, the
combination of bis(benzonitrile) palladium(II) chloride and Xantphos
furnished the highest yields of the desired compounds, along with
the weak base, NaOAc, in anisole at 120 °C. The choice of catalytic
system as well as solvent turned out to be important in order to ensure
a high chemoselectivity in the reaction. Both electron-rich and electron-deficient
aryl bromides worked well in this reaction. Addition of 1 equiv of
sodium iodide to the reaction improved the chemoselectivity with the
electron-deficient aryl bromides. The thiol scope included both aryl
and alkyl thiols, including 2-mercaptobenzophenones, whereby a thiocarbonylation
followed by a subsequent McMurry coupling yielded differently substituted
benzothiophenes. It was demonstrated that the methodology could be
applied for 13C introduction into the thiophene ring.