Novel Multidentate N-heterocyclic Carbene-rhodium Complexes As Efficient Catalysts For The Addition Of Arylboronic Acids To Aldehydes

N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, but also to main group elements such as beryllium, sulfur, and iodine. Because of their diversity structure and the excellent stablility, and activated metal complexes of N-heterocyclic cabene, they have been used to catalyze a lot of kinds of organic reactions, especially in C-C, C-H, C-O, and C-N bond formation. Now N-heterocyclic carbenes are not only the supplement of the classical organophosphanes, but also replaced them to catalyze many kinds of organic reactions in part.Diphenylmethyl alcohol is one of important intermediates in organic synthesis. The addition of arylboronic acids to aldehydes is the most efficient method.To our knowledge, hexadentate N-heterocyclic imidazole salts have not been reported. At the same time, multidentate N-heterocyclic carbene metal complex as catalyst for organic reactions is still scarce. Furthermore, hexadentate N-heterocyclic carbene rhodium complexes as catalyst for the addition of arylboronic acids to aldehydes have not been reported. In this paper, we reported an efficient synthetic method for a series of hexadentate N-heterocyclic imidazolium salts. Afterbromomethylation and bromination, 1, 3, 5 - trimethyl benzene was converted to hexabromomethyl benzene which reacted with 1-substituted imidazoles to yield the corresponding hexadentate imidazolium salts. The hexadentate imidazolium salts were also ligated to rhodium species as catalyst for addition of arylboronic acids to aldehydes reaction and gave excellent yields.Five hexadentate N-heterocyclic imidazolium salts, one bidentate N-heterocyclic imidazolium salts and one tridentate N-heterocyclic imidazolium salts were ligated to rhodium species as catalysts for addition of arylboronic acids to aldehydes. Under the optimized condictions: [Rh(COD)Cl]2 (3 mol%), imidazolium salts (1 mol%), KOBu-r (2 equivalents to the aldehyde), the addition of arylboronic acids to aldehydes afforded diarylmethanols in 99% yields.After optimized the reaction condition, we compared the catalytic result by using didentate N-heterocyclic carbene-rhodium complexes, tridentate N-heterocyclic carbine-rhodium complexes and hexadentate carbine-rhodium complexes formed in situ and found that hexadentate NHC-rhodium complexes as catalysts gave the best result. We also found that during the reaction water is not only the solvent but also one of necessary reactant took park in the reaction. In addition, it is found that the property and concentration of the base played an important role on the addition of arylboronic acids to aldehydes.In conclusion, we have successfully prepared a series of new hexadentate N-heterocyclic carbine-rhodium complex catalysts for the addition of arylboronic acids to aldehydes giving excellent yields.