Synthesis of Silaoxazolinium Salts Bearing Weakly Coordinating Anions: Structures and Catalytic Activities in the Aldol Reaction

The synthesis and structures of silaoxazolinium salts 2 and their application to the catalytic Mukaiyama aldol reaction are described. The reaction of (N-amidomethyl)­dimethylchlorosilane (1a) or (N-amidomethyl)­bis­(trimethylsilyl)­chlorosilane (1b) with metal salts of weakly coordinating anions such as Na­[TFPB] (TFPB = B­[3,5-(CF₃)₂C₆H₃]₄^–) and Cs­[CB₁₁H₁₂] (CB₁₁H₁₂^– = carba-closo-dodecaborate) gave the corresponding five-membered-ring silaoxazolinium salts 2 in high yields (93–97%). The structures of a series of silaoxazolinium salts 2 were determined by X-ray crystal analysis as well as ²⁹Si NMR spectra. It was proved that the silicon atoms of silaoxazolinium salts 2a,b are nearly completely free from the coordination of anions, and the geometries of the silicon centers are distorted tetrahedra. The ²⁹Si chemical shifts of salts 2 appeared in the range +31 to +49 ppm, revealing the appreciable silylium cation character of the silicon. The silaoxazolinium salt of TFPB anion 2a exhibited effective catalytic activity for the Mukaiyama aldol reaction of low reactive unactivated ketones such as cyclohexanone and acetophenone, giving the corresponding aldol products in high yields. The catalytic activity is highly dependent on the nature of the counteranions and the substituents on the silicon. DFT calculations of silaoxazolinium cation have disclosed that positive charges of silaoxazolinium cations are mainly located at the silicon atoms, while the nitrogen atoms and oxygen atoms are negatively charged.