C(2)-symmetric Lewis acid catalysts: The role of imidazole in the stereoselective hydrosilylation of carbonyl compounds

Abstract

<p>The demand for functionalized materials of high optical purity has led to an intense research effort in asymmetric synthesis. Much of the attention has focused on the application of chiral main group and transition metal complexes to promote enantioselective transformations. The successful development of asymmetric catalysts is an iterative, multistep, process involving ligand design, catalyst synthesis, and substrate screening. In order to facilitate this process, new ligands, which are easily prepared and modified, must be advanced. In the realm of tetradentate ligands, the classical example is Jacobsen's salen complexes. The ligands introduced here contain an imidazole moiety, which for years have been utilized by organic chemists as additives when protecting alcohols with silicon reagents. We advance here novel ligands that allow for enantioselective hydrosilylation of ketones. In particular we take advantage of the high reactivity of pentacoordinate silicon species. Initially, we provide evidence for pentacoordination of hydridosilicon compounds by imidazole, histidine, and C2 -symmetric imidazole ligands. We subsequently show that these species can be used for the reduction of carbonyl compounds; in the case of histidine and a C2 -symmetric ligand reduces enantioselectively. The lack of reactivity of chiral C2 symmetric N,N' -bis(N -methyl-2-methylene-imidazole)-1,2-cylcohexanediamine derivatives prompted us to look at the metal binding properties of these ligands, and their subsequent use as enantioselective reduction catalysts. Thus the second phase of this work involved the study of the complexation of these tetradentate ligands with metal species. The copper complex of the achiral N,N' -bis(N -methyl-2-methylene-imidazole)-1,2-ethanediamine and chiral N,N' -bis(N -methyl-2-methylene-imidazole)-1,2-cyclohexyldiamine were prepared and characterized by electrospray mass spectrometry and X-ray crystallography. The titanium adduct of two chiral cyclohexyl diamine ligands was utilized as catalyst for the enantioselective hydrosilylation of ketones. This reaction may or may not involve the intermediacy of pentacoordinate silicon compounds. The rhodium complex of these ligands was then utilized for the hydrostanylation of aldehydes. The third part of this thesis involved preliminary work on developing a catalyst for the polymerization of alkoxysilane. Recent reports of an enzyme responsible for assembling the silicate surfaces of a marine sponge, prompted us to explore the possibility of our ligand system being utilized for the polymerization of alkoxysilanes.</p>