LiAl(OC(Ph)(CF₃)₂)₄:  A Hydrocarbon-Soluble Catalyst for Carbon−Carbon Bond-Forming Reactions

Abstract

The compound LiAl(OC(Ph) (CF 3 ) 2 ) 4 (1), prepared from LiAlH 4 and HOC(Ph)(CF 3 ) 2 , is an active Lewis acid catalyst in toluene solution for the 1,4conjugate addition of silyl ketene acetals to R,β-unsaturated carbonyl compounds and for the direct substitution of allylic acetates by silyl ketene acetals. The structure of 1 contains a rare trigonal prismatic coordination sphere around the Li + cation, with two Li-O(C, Al) bonds and four Li-F(C) bonds.One of the most important uses of new weakly coordinating anions (WCAs) is to enhance the catalytic activity of metal cations. 2 Two examples that have received considerable attention recently are metallocene-catalyzed olefin polymerization 3 and lithiumcatalyzed Diels-Alder reactions and 1,4-conjugate addition reactions. 4 The best Li + -based catalysts, including LiClO 4 , 4a,b,d LiN(O 2 SCF 3 ) 2 , 4e,g and LiCo(C 2 B 9 H 11 ) 2 , 4c exhibit higher activity as the coordinating ability of the solvent decreases from, for example, diethyl ether to 1,2dichloroethane (DCE). Due to restrictions or hazards associated with DCE and other weakly coordinating halogenated solvents, 5 hydrocarbon-soluble and hydrocarbon-active catalysts would be desirable for large-scale industrial use. However, few lithium salts of WCAs are soluble in hydrocarbon solvents, and none that we know of is catalytically active.We now report that the Li + salt of a new tetrakis-(polyfluoroalkoxy)aluminate shows great promise as a hydrocarbon-active catalyst for 1,4-conjugate addition reactions and the direct substitution of allylic acetates by silyl ketene acetals. The new hydrocarbon-soluble compound LiAl(OC(Ph)(CF 3 ) 2 ) 4 , 1, was prepared by adding 4 equiv of HOC(Ph)(CF 3 ) 2 to LiAlH 4 in toluene solution: 6 The hygroscopic, volatile, white solid, which was purified by vacuum sublimation at 138 °C, is extremely soluble in hydrocarbon solvents including hexane and toluene. Diffraction-quality crystals were grown by cooling a hexane solution of 1. 7 The solid-state structure, shown in Figure 1, consists of discrete LiAl(OC-(Ph)(CF 3 ) 2 ) 4 molecules. The Al atom is tetrahedrally coordinated by four 1,1,1,3,3,3-hexafluoro-2-phenyl-2propoxide ligands. The O1-Al-O2 angle of 91.8(1)°is smaller than the five other O-Al-O angles, which range from 105.4(1) to 116.7(1)°. The Al(OC(Ph)(CF 3 ) 2 ) 4anion is a hexadentate chelating ligand for the Li + cation. The O1-Al-O2 unit forms a four-membered chelate ring with Li + , with Li-O(Al,C) bond distances of 1.966(8) and 1.978(8) Å. Four C-F bonds from four different CF 3 groups form five-membered O-Li-F-C-C chelate rings, with Li-F(C) bond distances of 1.984(9), 2.082(9), 2.098(11), and 2.354 (10) Å. The overall LiO 2 F 4 coordination sphere can best be described

Relative Lewis Basicities of Six Al(ORF)4− Superweak Anions and the Structures of LiAl{OCH(CF3)2}4 and [1-Et-3-Me-1,3-C3H3N2][Li{Al{OCH(CF3)2}4}2]

“…[12] The lithium salt of the Al{OCPh(CF 3 ) 2 } 4 À ion is an active Lewis acid catalyst in toluene for the 1,4-conjugate addition of silyl ketene acetals to a,b-unsaturated carbonyls and for the direct substitution of allylic acetates by silyl ketene acetals. [10] In addition, a solution of LiAl{OC(CF 3 ) 3 } 4 in 1,2-dimethoxyethane (DME; 0.01m) has a direct current (dc) conductivity that is nearly 100 times higher than a solution of LiCF 3 SO 3 in DME (0.01m). [13] We are investigating the relative Lewis basicities of Al(OR F ) 4 À superweak ions and the relative Lewis acidities of LiAl(OR F ) 4 compounds.…”

Homoleptische Silber(I)‐Acetylen‐Komplexe

“…[5,7] Daher konnten WCA-Salze solch ungewöhnlicher Gasphasenkomplexe wie [Au(Xe) 4 ] 2+ , [8,9] oder [M(CO) n ] + (n = 1-4, M = Cu, [10] Ag [11][12][13] ) hergestellt werden. In dieser Weise ermöglichen auch [Al(OR F ) 4 ] À -WCAs (OR F = Fluoralkoxid) [14][15][16][17] die Stabilisierung und vollständige Charakterisierung von Salzen einiger Gasphasenkationen, die zuvor nur durch MS-Experimente bekannt waren. [7,[18][19][20][21] [22] unbekannten Stammverbindungen aller homolep-…”

A Lipophilic, Fluorine‐Free, Thermostable, Inexpensive, S4‐Symmetric, Highly Soluble, Weakly Coordinating, Protolabile Aluminate

“…We attribute the unprecedented solubility to the aliphatic and therefore truly lipophilic molecular surface of the altebate anion. [7] The hydrogen atoms at the top layer of the molecular surface are positively polarized (Figure 2, top). In contrast to more fluorophilic WCAs such as the "Teflon ball" of the Krossing group (Figure 2, bottom), [8] the altebate can be considered as an anionic "wax ball".…”