Halide scavengers absence

However, clear evidence of the IL effect was found for the regioselective arylation of olefins. It is generally accepted that the Heck reaction may proceed via two pathways, a neutral pathway leading to the preferential formation of linear olefins and an ionic counterpart more likely to give rise to branched olefins. Thus, Pd(OAc)2 and l,3-bis(diphenylphosphino)propane (DPPP) immobilized in [G4GiIm]BF4 promote the exclusive a-arylation of several classes of electron-rich olefins with a wide range of aryl iodides and bromides in the absence of halide scavengers (Scheme 3). ... 

In more recent studies by Xiao and coworkers [40m,n], Mizoroki-Heck reactions catalysed by Pd(OAc)2 associated with dppp and performed from the eleclron-rich alkene ( -butylvinyl ether) and aryl halides (without any halide scavenger, i.e. under the conditions of the textbook neutral mechanism of Scheme 1.27 proposed by Cabri and Candiani [Ig]) give a mixture of branched and linear products in DMF, whereas the branched product is exclusively produced in ionic liquids (in the absence of halide scavengers) in a faster reaction. Whatever the medium, the cationic complex ArPd5(dppp)+ is always the sole reactive complex with electron-rich alkene (Scheme 1.33) [53]. Consequently, the regioselectivity should not vary with the experimental conditions. 

Hydrocarbon Radicals. Secondary reactions of radicals can complicate the interpretation of electron scavenging experiments. Thus, methyl radicals produced from the methyl halides will, in the absence of a radical scavenger, react with the solute to give methane. 

In 2010, the groups of Charette and Lei independently reported an iron/diamine-catalyzed reaction of aryl iodides or bromides with a solvent amount of arene at 80-90°C (Eqs. 24 and 25) [60, 61]. A mixture of ortho-, meta-, and para-isomers was obtained when substituted arenes were used as the substrate, the ort/jo-isomer being the major product. The Charette group reported a KIE value of 1.04, while Lei group measured a KIE of 1.7. Based also on reaction inhibition by a radical scavenger, Charette suggested that radical processes are involved. Recent studies have revealed that cross-coupling of an aryl halide with an arene can proceed in the absence of a transition metal catalyst ([62] and references therein). 

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