Evidence on palladacycle-retaining pathway for Suzuki coupling. Inapplicability of Hg-drop test for palladacycle catalysed reactions

Investigation of the model atroposelective Suzuki reaction catalysed by chiral CN-palladacycles have shown that two types of catalytic cycle may operate simultaneously, with strong dependence of their contributions upon the palladacycle structure and conditions used. Dominant contribution of palladacycle retaining pathway was provided by the non-metallocene planar chiral iminate CN-palladacycle (Rpl)-4a using as catalyst, in reaction performed in toluene under aerobic conditions with KF as a base, affording (Sa)-2-methoxy-1,1′-binaphthalene with enantioselectivity up to 53% ee. The catalyst was recovered almost quantitatively as an iodide-bridged dimer, whose structure was confirmed by an X-ray diffraction study of its phosphine derivative. It was also shown that the common Hg drop test was unsuitable for mechanistic testing of palladacycle-catalysed reactions because of the transmetallation product formation, whose structure was confirmed by an X-ray diffraction study.