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    Home > SCI. Adv.: Han Buxing, academician team, Institute of chemistry, Chinese Academy of Sciences, reports on the self esterification and cross esterification of Alcohols Catalyzed by ionic liquids under the conditions of no metal and oxygen

    SCI. Adv.: Han Buxing, academician team, Institute of chemistry, Chinese Academy of Sciences, reports on the self esterification and cross esterification of Alcohols Catalyzed by ionic liquids under the conditions of no metal and oxygen

    • Last Update: 2018-11-14
    • Source: Internet
    • Author: User
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    Esterification is one of the most important reactions in organic synthesis and chemical industry In practice, esters are usually synthesized from alcohols and carboxylic acids or carboxylic acid derivatives (such as acyl chloride or anhydrides) under acidic conditions Although the method has been well developed, there are still some shortcomings, such as the treatment of corrosive acids and / or their derivatives and a large number of by-products Therefore, from a scientific and industrial point of view, it is necessary to develop a simpler, effective and economic esterification method The direct conversion of alcohol to ester can avoid the use of harmful acids and their derivatives, eliminate the production of adverse products (such as aldehydes and carboxylic acids), and improve the reaction efficiency The conversion of alcohol to ester can be realized under the conditions of homogeneous transition metal catalysts such as Ru, PD, Au, IR or toxic oxidants such as iodine and bromide In recent years, the heterogeneous catalyst of cobalt can also be used to oxidize alcohol to ester directly Therefore, it is more attractive and challenging to develop a green, simple, effective and metal free catalytic system with molecular oxygen as oxidant Ionic liquids (ILS) is an environmentally friendly green solvent, which has the characteristics of no vapor pressure, no combustion and easy recovery Among many ILs, imidazole ILs, such as imidazolyl acetate, have many applications in biomass dissolution, chemical catalysis and Co / SO2 absorption Recently, the team of academician Han Buxing from the Institute of chemistry, Chinese Academy of Sciences has developed for the first time the self esterification and cross esterification of benzyl alcohol or aliphatic alcohol with O2 as oxidant, ILs as catalyst and solvent under the condition of no metal The mechanism study shows that the acid proton cation and basic acetate anion of ionic liquid 1-ethyl-3-methylimidazolylacetate ([EMIM] OAC) can form multiple hydrogen bonds with the hydroxyl of alcohol at the same time, thus effectively catalyzing the reaction This is the first case of this type of reaction without metal The research results were published on science advanced (DOI: 10.1126/sciadv.aas9319) Firstly, the author optimized the reaction conditions by using the self esterification of benzyl alcohol as the model reaction (Table 1) Through the screening of ILs, the author found that the basic 1-ethyl-3-methylimidazolylacetate ([EMIM] OAC) has excellent catalytic performance, and the yield of the target product benzyl benzoate is up to 94% In order to study the effect of anions on the reaction, imidazole ILS containing different anions were used, including [EMIM] (TFA), [EMIM] HSO 4, [EMIM] BF 4 and [EMIM] n (CN) 2, but these ILS could not catalyze the reaction The above results show that acetate anion plays a key role in the transformation On the other hand, 1-octyl-3-methylimidazolylacetate [(OMIM) OAC] or [n 4,4,4,4] OAC can not catalyze the reaction, which shows that [EMIM] cation is also very important for the self esterification of benzyl alcohol In addition, nh4ac / DMSO system also showed no catalytic activity These results fully show that [EMIM] OAC, which is composed of [EMIM] cation and acetate anion, is an excellent catalyst for the reaction (source: Science Advanced) then, the author studied the self esterification reactivity of various alcohols (Table 2) 4-methylbenzyl alcohol can be effectively converted into the corresponding self esterified product 4-methylbenzyl benzoate (2b), with a yield of 92% Benzyl alcohol with electron absorbing groups (CL and no 2) and electron donating groups (och 3) can also yield corresponding esters (2c, 2D and 2e) It is worth noting that the oxidative self esterification of benzyl alcohol can be carried out in the gram scale (200 mmol, 21.6 g) Fatty alcohols with different chain lengths can also be effectively converted into corresponding self esterified esters in [EMIM] OAC, including ethanol, propanol, butanol, hexanol and octanol In general, the reactivity of fatty alcohol is lower than that of benzyl alcohol With the increase of the chain length of aliphatic alcohols, the yield of corresponding esters decreases, and a slightly higher reaction temperature is needed In addition, the cross esterification of benzyl group and fatty alcohol (Table 3) was also studied In the presence of excess ethanol, ethyl benzoate (3a) can be obtained from benzyl alcohol with a yield of 94% In addition, benzyl alcohol substituted by methyl, chlorine, nitro and methoxy can also be converted into ethyl benzoate with high yield and selectivity The high selectivity of the reaction is mainly attributed to the higher activity of benzyl alcohol than that of fatty alcohol In addition, the cross esterification between benzyl alcohol and other long-chain fatty alcohols such as n-butanol, n-hexanol and n-octanol can also be carried out smoothly (3f - 3H) When two different benzyl alcohols are used as substrates, due to their similar activity, the resulting product is a mixture of self esterification and cross esterification In addition, the mechanism of oxidative esterification was studied The reaction was not affected by the free radical scavenger tempo or BHT, and the free radical reaction pathway was excluded Combined with the literature reports, the author speculated a reasonable reaction pathway (Fig.1) First, the [EMIM] cation and the acetate anion form hydrogen bond, and the alcohol IL complex a is obtained by activating the hydroxyl of the alcohol substrate Then, O 2 oxidizes a to get water and corresponding aldehyde B Because of the carbene equilibrium in [EMIM] OAC, carbene attacks aldehyde B to form complex C; its Oh can form hydrogen bond with [EMIM] cation and acetate anion to form complex D Finally, D is converted into intermediate e, and the ester product and carbene are released by substitution reaction with alcohol Using 18 o to label benzyl alcohol, the mechanism was further confirmed (source: Science Advanced) conclusion: the team of academician Han Buxing has developed the self esterification and cross esterification of alcohol catalyzed by [EMIM] OAC under aerobic and nonmetal conditions for the first time The synergistic effect of [EMIM] cations and acetate anions plays a key role in initiating and accelerating the reaction This work has opened up a way for the self esterification reaction under the condition of metal free The author predicts that this simple, efficient and metal free reaction route will have great application potential.
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