Carbonylation and other metal-catalysed C−C coupling reactions used in industry

Abstract

The formation of new carbon-carbon bonds is probably the most fundamental reaction in organic chemistry. Many of the most selective and energy efficient reactions today involve the use of metals and their complexes as promoters. Key routes to C−C bond formation involve carbonylation (using CO) and coupling reactions (of olefins, dienes, and arenes) following an activation step. The largest carbonylation reactions in volume terms are the hydrogenation of CO, to make long chain hydrocarbons for diesel fuel (over supported Fe, Co, Ru or Rh metal catalysts; the Fischer-Tropsch reaction) or, over a Cu−ZnO catalyst, to make methanol. Many of the important carbonylations are homogeneously catalysed, using soluble complexes of the late transition metals, such as Co, Rh, Ir, or Pd. The carbonylation of methanol to acetic acid, MeOH+CO→MeCOOH was originally developed using a Co/I− catalyst; improvements in conditions (better energy use and selectivity), led by mechanistic considerations resulted in, first the Rh/I− (Monsanto) and more recently, the Ir/I− (BP, Cativa) processes. The hydroformylation of olefins,e.g., MeCH=CH2+H2+CO→MeCH2CH2CHO+MeCH(CHO)CH3 was also originally developed using a soluble cobalt catalyst, but again the use of rhodium catalysts (bearing phosphine ligands) has largely superseded the older processes. Even newer technologies, for example involvingsupported homogeneous catalysts, now promise still cleaner and more selective processes. Direct coupling reactions include Friedel-Crafts type processes, for example, C6H6+CH2=CHR→C6H5−C2H4R Zeolites are now the strong acid catalysts employed heterogeneously rather than the previously used AlCl3 or HF; the new catalysts are much more environmentally friendly («greener») as they do not involve halides which cause corrosion and undesirable side reactions. Valuable highly selective reactions include the Cr-catalysed oligomerization of ethylene to 1-hexene or 1-octene, and of butadiene to cyclo-octadiene or cyclo-dodecatriene, over Ni catalysts. Palladium catalysed coupling and carbonylation reactions have made key steps much more environmentally acceptable in the syntheses of the important anti-inflammatory pharmaceuticalsibuprofen andnaproxen.