Directing the regioselectivity of rhodium(I) catalysed cyclisation of 2-alkynyl benzoic acids

Rhodium(I) dicarbonyl complexes 1–4 containing chelating N-donor ligands bis(pyrazolyl)methane (bpm), bis(imidazolyl)methane (bim), tris(pyrazolyl)toluidine (tpt) or tris(imidazolyl)methanol (tim) were investigated as catalysts for the hydroalkoxylation of alkynyl benzoic acids (5a–g). The regioselectivity of the reaction was shown to be highly dependent on the nature of the terminal alkyne substituent (R) of the alkynol substrate. It was also determined that the presence of a third uncoordinated N-donor group in complexes 3 and 4 suppressed the catalytic efficiency of these complexes, and that the selectivity of the reaction for forming either endocyclic (6) or exocyclic (7) hydroalkoxylation products was influenced by the pendant hydroxyl group present in complex 4. We used 13C NMR spectroscopy to quantify the polarity of the alkynyl benzoic acid CC bond and our efforts to correlate this measure of bond polarity to the observed regioselectivity of the reaction are discussed.

The hydroalkoxylation of 2-alkynyl benzoic acids catalysed by Rh(I) complexes containing N-donor ligands affords the endo- and/or exo-cyclic product depending on the terminal alkyne substituent (R), moderation of reaction selectivity is achieved by the inclusion of pendant functional groups on the ligand backbone and via manipulation of the CC bond polarity.Figure optionsDownload as PowerPoint slide