Murray et al of AstraZeneca Process R&D in the UK report on their optimization of the Mizoroki-Heck reaction because they feel the world needs an ubiquitous HM protocol which consistently delivers good yields of coupled product. Number of substrates covered: around 30. Number of catalysts covered: 15. Base: methyldicyclohexylamine, solvent: N,N-dimethylacetamide. Main conclusion: of all the catalysts tried only three stand out, bis(tri-ter-butylphospine)palladium on of them.
But what is tetrabutylammonium chloride doing here? The article only mentions in the footnotes that it increases catalyst activity and conversions. For all catalysts tried? and how does it work then? Are all reactions homogeneous?
Equally intriguing is the presence of "homeopathic palladium" as one of the catalysts tried. Palladium acetate is listed twice as catalyst in one of the main tables, one them the homeopathic version. With substrate methyl-4-bromobenzoate against styrene the yield is zero and with the non-homeopathic palladium the yield is 100%. With 4-cyanobromobenze the homeopathic yield is 30% versus 100%. The contrast could not be more stark. The text and supporting info remain silent on the details but the Mighty Google informs us homeopathic palladium may have something to do with successful Pd catalysed reactions at very low loading (review here). Not here.