Ethylene glycol, palladium® bromide

Lipshutz and colleagues presented recently palladium-catalyzed direct coupling reactions of alkyl iodides and vinyl bromides or iodides catalyzed by 1 mol% Pd(amphos)Cl2 in the presence of zinc and TMEDA in a biphasic aqueous/poly-(ethylene glycol tocopheryl sebacate) reaction medium [198], Internal olefins were obtained in 51-95% yield. For aryl-substituted (Aj-vinyl bromides, retention of double bond geometry was observed, while different degrees of isomerization occurred for (Z)-isomers, which may indicate the intervention of a radical addition process in the course of the coupling process. Alkyl-substituted (Z)-vinyl halides were transformed in contrast with retention of alkene geometry. Aryl halides also reacted [199],... 

Although isomerization of alkenes occurs simultaneously with the oxidation, rhodium and ruthenium complexes can also be used instead of palladium for the oxidation of terminal alkene [15]. With these catalysts, symmetrical quaternary ammonium salts such as tetrabutylammonium hydrogensulfate are effective. Interestingly, the rate of palladium-catalyzed oxidation of terminal alkenes can be improved by using poly(ethylene glycol) (PEG) instead of quaternary ammonium salts [16]. Thus, the rates of PEG-400-induced oxidation of 1-decene are up three times faster than those observed with cetyltrimethylammonium bromide under the same conditions. Interestingly, internal alkenes can be efficiently oxidized in this polyethylene glycol/water mixture. 

Another biphasic Heck reaction was described by Beller et al. [25]. The medium consisted of xylene and ethylene glycol. The catalyst was a palladium complex with a carbohydrate-substituted triphenylphosphine (9 and 10). Aryl bromide (15 mmol), styrene (22.5 mmol), and NaOAc (16.5 mmol) were suspended in 10 mL of xylene and 10 mL of ethylene glycol. The catalyst precursor (Pd(OAc)2) and ligand (Pd/ ligand ratio 1 3) were added and the mixture was heated to 130 °C for 20 h. Both ligands A and B showed better results than the TPPTS ligand (cf Section 2.2.3.2) in the case of activated aryl bromides (for instance, p-nitrobromobenzene). However, for deactivated aryl bromides (for instance, 2-bromo-6-methoxynaphthalene) TPPTS proved to generate a more stable and thus more productive catalyst system. 

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