Bifunctional mesoporous silica catalysts

Similarly, chiral Cp Rh-TsDPEN was incorporated within flower-like mesoporous silica through the assembling of chiral 4-((trimethoxysilyl)ethyl)phenylsul-fonyl-l,2-diphenylethylenediamine (133) and Si(OEt)4 under a cooperative dualtemplate approach followed by complexation with the Rh complex in the presence of surfactant cetyltrimethylammonium bromide [112]. The obtained bifunction-alised sohd catalyst showed excellent catalytic efficiency in the ATH of aromatic ketones in aqueous medium. The residual surfactant within the silicate network acted as a phase transfer catalyst thus enhancing the catalytic activity, while well-... 

Two years later, thiourea 21, functionalized with a cinchona alkaloid, was efficiently used in the synthesis of 3-indolylmethanamines in good yields and selec-tivities (up to 96% ee) [25]. The transformation takes place in the presence of aromatic, aliphatic, and heteroaromatic imine analogs. Chiral phosphoric acids 26a, mf-26d, and ent-26f were also identified as efficient catalysts for the F-C alkylation between indoles and imines [35, 38, 40]. In these cases, in contrast with thiourea 21, the processes were only successful in the presence of aromatic imines. More recently, the fmitful use of a thiourea supported on mesoporous silica (heterogeneous bifunctional catalyst) was reported [61]. The catalyst can be easily separated from the reaction mixture by simple filtration and reused several times without erasing the enantioselectivity. 

SCHEME 8.14 Synthesis of a bifunctional amine/Pd(II)-diamine catalyst supported on mesoporous silica. 

---