Reaction media according to the invention are high-boiling, dipolar, protic solvents such as ethylene glycol or propylene glycol, or else higher diols or polyalcohols, for example glycerol, or else polyether alcohols such as polyethylene glycols, for example PEG600 and PEG1000, and also their etherified analogs, for example triethylene glycol dimethyl ether or poly(ethylene glycol) dimethyl ether,