Green carbon-carbon bond-forming reactions by heterogeneous metal catalysts

The organic reactions mediated by solid surface come closest to an ideal simplified version of chemical processes. This article describes development of heterogeneous metal catalysts aiming at 'green' carbon-carbon bond-forming reactions, based on unique properties of inorganic crystals, i.e. hydroxyapatite, montmorillonite, and hydrotalcite. A coordinatively-unsaturated Ru species generated on the hydroxyapatite surface efficiently catalyzes Diels-Alder reaction as well as aldol reaction of nitriles with carbonyl compounds in water solvent as a Lewis acid. A robust Pd2+ species on a Ca-deficient hydroxyapatite is capable of promoting Heck and Suzuki coupling reactions. A Sc(H2O)(6) species within the expansible interlayer space of montmorillonite shows an outstanding catalysis for Michael reaction of 1, 3-dicarbonyls even under solventless conditions. Allylation of carbonyl compounds with allyltrimethylsilane proceeds smoothly using Mn+-montmorillonite catalysts to afford homoallylic alcohols. Self-organized hydrotalcite provides a unique base catalyst in aldol reaction of aldehydes in water. The above functionalized and recyclable solid catalysts offer 'green' protocols of carbon-carbon bond-forming reactions to replace conventional methods using stoichiometric reagents and homogeneous metal complex catalysts.