Tetrabutylammonium Acetate: Applications, Synthesis, and Mixture Properties

Tetrabutylammonium acetate (TBAA) is a phase transfer catalyst (PTC). Tetrabutylammonium acetate can be used to catalyze the alkynylation of carbonyl compounds with trimethylsilylacetylenes providing good yields of propargylic alcohols. Along with tetrabutylammonium bromide (TBAB) as a molten reaction medium in the synthesis of 4-aryl-2-quinolones. Tetrabutylammonium acetate is a promoter in the regioselective direct arylation of azoles (1-methylpyrazole, oxazole, and thiazole) with aryl bromides in presence of palladium acetate catalyst. It’s also an ionic liquid to dissolve cellulose in the presence of DMSO co-solvent. Tetrabutylammonium acetate can act as an activator in the synthesis of disubstituted alkynes through copper-free Sonogashira coupling reaction.

Synthesis of tetrabutylammonium acetate

Ionic liquids refer to substances whose melting point is close to or lower than room temperature and are completely composed of anions and cations. They have the characteristics of both ions and liquids. Their high ionic strength can break the interaction of intermolecular and intramolecular hydrogen bonds, thus achieving the effect of conventional solvents. Dissolution of difficult-to-dissolve natural polymers, including cellulose, lignin, cotton fiber, chitin, cotton fiber, keratin, silk protein, etc. Among them, tetrabutylammonium acetate has good solubility and will not corrode equipment, which is the best One of the feasible ionic liquid solvents. Tetrabutylammonium acetate used as a solvent has higher requirements on the purity of cations, and the presence of inorganic cations such as potassium and sodium will hinder the dissolution of natural polymers (Ema.Rsc Advances, 2014,4(5):2523-2525.). Adding chelating agent 18-crown ether-6 in tetrabutylammonium acetate/dimethyl sulfoxide (DMSO) mixed solvent can improve the solubility of natural polymers such as cellulose (Miao.RSC Advances,2014,4(69) :36721-36724.), but the high price of 18-crown ether-6 limits its large-scale industrial use.[1]

A kind of synthetic method of tetrabutylammonium acetate for solvent, take tetrabutylammonium bromide, alkali metal hydroxide and acetic acid as raw materials, the mass molar ratio of the three is 1:0.9～1.1:0.9～1.3, according to the following steps To synthesize:(1) Dissolve tetrabutylammonium bromide in an alcoholic solvent to form a solution with a concentration of 30% to 60%, add a total amount of 50% to 95% of alkali metal hydroxide, and stir for 0.5h- After 2h, filter to remove the precipitated by-product alkali metal bromide; continue to add the remaining 50% to 5% alkali metal hydroxide to the filtrate, continue to react in an ice-water bath for 0.5h to 2h, then filter to remove the precipitated by-product alkali metal bromide Salt, gained filtrate is the alcoholic solution of tetrabutyl ammonium hydroxide;(2) Add acetic acid dropwise to the alcoholic solution of tetrabutylammonium hydroxide to acidity under ice-water bath, distill and remove the alcoholic solvent after stirring for 0.5h-2h to obtain thick tetrabutylammonium acetate;(3) Dissolving the thick tetrabutylammonium acetate in a polar aprotic solvent to form a solution of 30% to 60%, stirred and dissolved in an ice-water bath for 0.5h to 2h, then filtered to remove undissolved inorganic salts, and distilled off the solvent to obtain Finished tetrabutylammonium acetate.

Binary Mixtures of Choline Acetate and Tetrabutylammonium Acetate with Natural Organic Acids

In this framework, with the aim of understanding the mechanism governing the formation of the deep eutectic solvents, we conducted a systematic study of the chemical–physical properties of mixtures composed of choline acetate (ChAc) or tetrabutylammonium acetate (TBAAc) with three different natural organic acids containing different numbers and typologies of acidic functions. It is worth noting that both acetate salts present cations commonly applied in the formulation of DESs. The three chosen natural organic acids, namely, l-ascorbic acid (AA), citric acid (CA), and maleic acid (MA). The MIR spectra of the tetrabutylammonium acetate mixtures (solid at room temperature) and of their components, instead, were measured using an FT-IR Spectrum Two by PerkinElmer, in attenuated total reflection (ATR) mode. All spectra were acquired by means of 100 scans between 400 and 4000 cm–1, with 1 cm–1 resolution. The starting configurations were obtained putting randomly the components in a cubic box using Packmol software. For ChAc and tetrabutylammonium acetate, 1200 ion pairs were put in cubic boxes with an initial side of 80 Å. For 2–1 mixtures, the boxes were composed by 1200 ion pairs (ChAc or TBAAc) and 600 molecules of acid, with 1–1 composition by 1200 ion pairs and 1200 molecules of acid and 1–2 mixture composition by 600 ion pairs and 1200 molecules of acid.[2]

The systems based on tetrabutylammonium acetate are measured by means of ATR-IR. As expected from the different structures, the spectra of TBAAc and ChAc are very different. In ChAc, the presence of a hydroxyl group causes a hydrogen bond between the cation and anion. An important indication of this strong interaction can be found observing the low frequencies of the OH and C═O vibrations, as previously discussed. Notwithstanding the deep differences in the coordination of choline and TBA with the anion, the frequencies of νC═O of the acetate are the same (1580 cm–1) in both ion pairs. From this evidence, we can hypothesize that the interaction strengths in both ion pairs are comparable. These results agree with the DFT calculations simulating one, two, or three ion pairs at B3LYP/6-31G: these models show the interaction between the alkyl chains of the TBA cation and the carboxylic group of acetate. The bands between 2800 and 2900 cm–1in the ATR spectrum of tetrabutylammonium acetate are assigned to the stretching of CH groups (νCH2 and νCH3). The spectra of all mixtures containing TBAAc do not show any clear shift of νCH and νC═O; this can be interpreted as evidence that the coordination of the ion pair remains almost unchanged. Concerning the OH stretching bands, in the tetrabutylammonium acetate -AA 1–2 spectrum, the peaks in the range of 3500–3200 cm–1 are the same of pure AA.

The structures of the mixtures presented in this work were investigated by means of vibrational spectroscopy (MIR): the results suggest that in all cases, both salts act as a hydrogen bond acceptor (HBA), while the acids act as a hydrogen bond donor (HBD). For both salts, ion couples are slightly destabilized by interactions with acids. The vibrational spectra also suggest that the coordination occurs via H-bonds; these H-bonds are donated by acids, as observable by a progressive blue shift of νOH and a red shift of νC═O of acid bands when the concentration of acids increases. This means that the presence of ChAc or tetrabutylammonium acetate mitigates the self-association phenomenon of acid molecules. MD simulations suggest that the most acidic hydrogen is more prone to forming hydrogen bonds with the anion. In all mixtures, the distance between the anion and cation is slightly increased with the addition of acids, and in tetrabutylammonium acetate -based materials, the TBA–TBA distance strongly increases. The occurrence of a prepeak in the structure function of TBAAc systems has been linked to the tendency of alkyl chains to aggregate, forming some structural inhomogeneous regions. The tendency to vitrify ChAc-based systems could be explained with the similar sizes of ions and acids, leading to large disorder; on the contrary, the tetrabutylammonium acetate systems at low acid concentrations tend to crystallize, similar to the pure ionic solid. Only at high acid concentrations does disorder increase, and the tetrabutylammonium acetate mixture does not display crystallization but only glass transitions.

References

[1]ZHEJIANG KENTE CATALYSTS TECH - CN116425636, 2023, A

[2]Di Muzio, Simone et al. “Binary Mixtures of Choline Acetate and Tetrabutylammonium Acetate with Natural Organic Acids by Vibrational Spectroscopy and Molecular Dynamics Simulations.” The journal of physical chemistry. B vol. 128,3 (2024): 857-870.

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