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

Abstract

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-(CF 3 ) 2 C 6 H 3 ] 4 − ) and Cs[CB 11 H 12 ] (CB 11 H 12 − = 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 29 Si 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 29 Si 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.

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

“…In 2013, Hatanaka and co-workers prepared silaoxazolinium ion 405 + , which performed similarly well in challenging Mukaiyama aldol reactions (Scheme 131). 282 However, the catalytic activity of 405 + was strongly dependent on the counteranion and the substituents on the silicon atom. With the weakly coordinating anion [HCB 11 H 11 ] − instead of [BAr F 4 ] − , the aldol reaction between cyclohexanone (183m) and the silyl enol ether 402a to give β-hydroxy ketone 403c was not successful under otherwise identical conditions.…”

“…In the presence of neutral main-group element compounds of groups 15–17, Lewis adduct formation leads to silylated onium ions that are often difficult-to-access using conventional silyl transfer reagents. In this way, numerous silylonium salts of type [R n E–SiR 3 ]­[WCA] have been isolated and structurally characterized, including (per)­silylated pnictonium (E = N, ,,– ,,,,,,,,,,,,,,,,– P, ,,,,,– As, , and Sb), chalconium (E = O, ,,,,,,,,,,,– , S ,,– , Se ,, , and Te ,…”

Nonclassical Applications of closo-Carborane Anions: From Main Group Chemistry and Catalysis to Energy Storage

“…The basic structure and bonding, synthesis, and library of known derivatives containing 3 have been reviewed elsewhere . We define nonclassical applications as uses beyond coordination chemistry of the ionic bonding and σ-type complexation of the anions themselves, − weakly coordinating anions to isolate reactive intermediates, weakly coordinating anions for Lewis acid catalysis, − anions for Li + -initiated olefin polymerization, liquid crystals, and the catalytic functionalization of the cage vertices . Specifically, this review will cover new directions in main group catalysis utilized to achieve some of the most challenging catalytic reactions such as C–F, C–H, and C–C functionalizations that are difficult or impossible to realize with transition metals.…”

Anti-Selective Asymmetric Nitro-Michael Reaction of Furanones: Diastereocontrol by Catalyst

“…The long N(1)–C(2) bond length can be ascribed to the strong electrostatic repulsion between the nitronate moiety and N(1), which bears a considerable negative charge (−0.3868; Mulliken). The formal positive charge on N(1) can be neutralized by electron release from five neighboring hydrogen atoms in the geminal positions relative to N(1) . In contrast, the si -face adduct 8 cannot engage in intramolecular H-bonding between 6′-OH and the nitronate oxygen.…”

“…The formal positive charge on N(1) can be neutralized by electron release from five neighboring hydrogen atoms in the geminal positions relative to N(1). 10 In contrast, the si-face adduct 8 cannot engage in intramolecular H-bonding between 6′-OH and the nitronate oxygen. Consequently, 8 dissociates into the catalyst and the nitroalkene due to the electrostatic repulsion between the negatively charged N(1) and the nitronate moiety.…”