Adsorption at the liquid-liquid interface in the biphasic rhodium catalyzed hydroformylation of olefins promoted by cyclodextrins: a molecular dynamics study

Using molecular dynamics (MD) simulations, we investigate the interfacial distribution of partners involved in the phase transfer rhodium catalyzed hydroformylation of olefins promoted by beta-cyclodextrins (beta-CDs). The beta-CDs, the reactant (alkene), product (aldehyde), several rhodium complexes (the catalyst, its precursor, and its alkene adduct) are simulated at the water-"oil" interface, where oil is represented by chloroform or hexane. It is shown that unsubstituted beta-CD and its 6-methylated and 2,6-dimethylated analogues adsorb at the interface, whereas the liposoluble permethylated CD does not. The precursor of the catalyst RhH(CO)(TPPTS)3]9- (with triphenylphosphine trisulfonated TPPTS3- ligands) sits in water, but the less charged RhH(CO)(TPPTS)2]6- catalyst and the RhH(CO)(TPPTS)2(alkene)]6- reaction intermediate are clearly surface active. The TPPTS3- anions also concentrate at the interface, where they adopt an amphiphilic conformation, forming an electrical double layer with their Na+ counterions. Thus, the most important key partners involved in the hydroformylation reaction concentrate at the interface, thereby facilitating the reaction, a process which may be further facilitated upon complexation by CDs. These results point to the importance of adsorption at the liquid-liquid interface in the two-phase hydroformylation reaction of olefins promoted by beta-CDs and provide microscopic pictures of this peculiar region of the solution.