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

Abstract

This communication presents the facile and cost-effective synthesis as well as properties of salts of a lipophilic aluminate anion. Its unprecedented solubility in pentane makes possible tetrabutylammonium salt extraction with pentane. Nevertheless, its symmetry leads to high melting points of its thermostable salts. Several single crystals for X-ray diffrac- IntroductionThe term "lipophilic anion" appears to be a paradox. The electric charge separation of salts in cations and anions is best stabilized by a polar, hydrophilic environment. However, lipophilic anions are characterized by a low negative charge, large molecular dimensions, and a lipophilic molecular surface. Related weakly coordinating anions (WCA) usually comprise fluorine substituents, [1] resulting in more fluorophilic behavior. The most important structural features of WCAs are, on the one hand, multiple C-F fragments in order to electronically conceal and, on the other hand, bulky substituents to sterically conceal nucleophilic sites within the anion. In contrast, the main principles of designing lipophilic anions are the minimization of electrostatic contact with the positive counterion and maximization of van der Waals interactions with nonpolar solvents. The permethylated carboranes of the Michl group can be considered as lipophilic anions.[2] Our strategy for a lipophilic aluminate anion avoids expensive and persistent C-F bonds and uses large and inexpensive tert-butylated 2,2Ј-biphenols for an extended hydrocarbon molecular surface. Results and Discussion3,3Ј,5,5Ј-Tetra-tert-butyl-2,2Ј-biphenol can be synthesized under ambient conditions by a modified literature protocol comprising oxidative C-C coupling of commercially available 2,4-di-tert-butylphenol with MnO 2 and recrystallization from acetic acid (Scheme 1). [3] [a]

Chiral selection in the formation of borates from racemic binaphthols and related diols

“…However, even when this effect is removed, the homochiral species is still preferred suggesting that, to a certain extent, energetic penalties associated with the homochiral diastereomer appear to be overcome by solvation, conformational changes and/or crystal packing effects. The recent report by Straub et al, 10 of a heterochiral aluminate suggests that a heterochiral borate may be accessible with sufficient steric bulk at the 3,3 0 -position. Fig.…”

“…This demonstrates nicely that heterochiral frameworks can indeed be obtained with very bulky 3,3 0 substituents; the relative stability of the diastereomeric anions studied was also discussed. 10 …”

Synthesis, structure, and unprecedented solubility of lipophilic borate salts

“…We have recently reported aluminate salts comprising two sterically demanding tert-butylated 2,2 0 -biphenolate ligands ( Fig. 1 down right) [10]. Their low reactivity against electrophiles such as tritylium cations originates in the steric shielding of the aluminate center.…”

Lipophilic Anions

“…A suitable test for the lipophilicity of a salt is, however, its solubility in hydrocarbons [1,2], ultimately in pentane. The latter is presumably the solvent that resembles most closely gas-phase conditions at room temperature.The syntheses and properties of the salts of two highly lipophilic anions are summarized, in this case they are bortebate and altebate [3,4]. The names of these anions are derived from the paraphrases borate ester with eight tert-butyl substituents, and aluminate with eight tert-butyl substituents.…”

“…The syntheses and properties of the salts of two highly lipophilic anions are summarized, in this case they are bortebate and altebate [3,4]. The names of these anions are derived from the paraphrases borate ester with eight tert-butyl substituents, and aluminate with eight tert-butyl substituents.…”