is inevitable in Li-CO 2 batteries, [11,12] Li-CO 2 /O 2 batteries, [8,13] and even Li-air batteries. [14] Exception is that the discharge products could be different without the generation of Li 2 CO 3 under specific conditions including protected anodes and effective electrolytes. [15,16] Li 2 CO 3 decomposes to CO 2 when the potential is higher than 3.8 V versus Li/Li + . Notably, O 2 evolution is not detected, as is expected according to the decomposition reaction 2Li 2 CO 3 → 4Li + + 4 e -+ 2CO 2 + O 2 . Instead, superoxide radicals or "nascent oxygen" form during the self-decomposition of Li 2 CO 3 . [12,17] More accurate verification was performed through chemical probes, which qualitatively detected the existence of singlet oxygen ( 1 O 2 ). [18] Parasitic reactions of electrolytes and catalyst degradation were then induced by Li 2 CO 3 oxidation. [12,18] Therefore, efficient air cathodes are expected to change this situation.The introduction of metal nanoparticles could limit side reactions and promote the interaction between Li 2 CO 3 and C. [11,[19][20][21] Beyond this, metal-organic frameworks or surface modified carbon materials also brought unexpected electrochemical performance. [22,23] Therefore, catalysts play important roles in Li-CO 2 batteries. [24][25][26][27][28][29] As mentioned above, self-decomposition of Li 2 CO 3 induced a series of parasitic reactions, and further influenced the stability of catalysts during the operation of Li-CO 2 batteries. Only by clarifying the changes of catalysts in this process can we design more stable Li-CO 2 batteries. In previous reports, mono metal catalysts (Ru, Cu, Au, and Ni) exhibited outstanding activity toward Li-CO 2 batteries and revealed some changes in electrochemical processes. [19][20][21] Nevertheless, there exist shortcomings with mono metal catalysts from materials preparation to electrochemical processes. First, for example, the preparation for monodispersed Ru nanoparticles was often achieved under mild experimental conditions without the generation of stable crystal surfaces, further affecting catalytic activity in Li-CO 2 batteries. [21,30] Second, the incompatibility between the discharge products and mono metal nanomaterials might lead to severe agglomeration and dropping during long cycles. [20,31] In this work, we designed a composite of ruthenium-copper nanoparticles highly co-dispersed on graphene (Ru-Cu-G), and this composite cathode endows Li-CO 2 batteries with low overpotential and excellent cyclability through their synergistic Li-CO 2 batteries are attractive electrical energy storage devices; however, they still suffer from unsatisfactory electrochemical performance, and the kinetics of CO 2 reduction and evolution reactions must be improved significantly. Herein, a composite of ruthenium-copper nanoparticles highly co-dispersed on graphene (Ru-Cu-G) as efficient air cathodes for Li-CO 2 batteries is designed. The Li-CO 2 batteries with Ru-Cu-G cathodes exhibit ultra-low overpotential and can be operated for 100 cycles with ...
We propose a principle for H-bonding activation in acylation of hydroxyl groups, where the acylation is activated by the formation of hydrogen bonds between hydroxyl groups and anions. With the guidance of this principle, we demonstrate a method for the selective acylation of carbohydrates. By this method, diols and polyols are regioselectively acetylated in high yields under mild conditions using catalytic amounts of acetate. In comparison to other methods involving reagents such as organotin, organoboron, organosilicon, organobase, and metal salts, this method is more environmentally friendly, convenient, and efficient and is also associated with higher regioselectivity. We have performed a thorough quantum chemical study to decipher the mechanism, which suggests that acetate first forms a dual H-bond complex with a diol, which enables subsequent monoacylation by acetic anhydride under mild conditions. The regioselectivity appears to originate from the inherent structure of the diols and polyols and their specific interactions with the coordinating acetate catalyst.
An efficient one‐pot method for the selective benzylation of diols and polyols using 0.1 equiv. of organotin reagents and tetrabutylammonium bromide as catalyst has been developed. The diols and polyols containing a cis‐vicinal diol were regioselectively benzylated in 70–94% isolated yields. A catalytic reaction mechanism was also proposed.magnified image
H-bonding activation in the regioselective acetylation of vicinal and 1,3-diols is presented. Herein, the acetylation of the hydroxyl group with acetic anhydride can be activated by the formation of H-bonds between the hydroxyl group and anions. The reaction exhibits high regioselectivity when a catalytic amount of tetrabutylammonium acetate is employed. Mechanistic studies indicated that acetate anion forms dual H-bonding complexes with the diol, which facilitates the subsequent regioselective monoacetylation.
COMMUNICATION This journal isWe demonstrated that using NaOH and NaOMe in methanol for deacylation are identical, indicating that Zemplén condition has been misleading us for almost 90 years. The traditional base-catalyzed mechanism cannot be used to explain our results. We proposed that H-bond complexes play key roles in the base-catalyzed process, explaining why deacylation in methanol can be catalyzed by hydroxide.Acyl groups are widely used as protecting groups in organic synthesis strategies, especially in carbohydrate chemistry. [1] With the addition of acylation reagents under mild conditions, the hydroxyl groups of substrates readily form esters as intermediate products, and the esters are easily removed under Zemplén conditions when required. 1-2 For almost 90 years, it has been believed that the regular hydrolysis of an ester using a base such as potassium hydroxide (KOH) or sodium hydroxide (NaOH) requires a stoichiometric amount of base for each ester functional group, and generates a large amount of potassium or sodium acetate, whereas Zemplén deacylation uses only a catalytic amount of sodium methoxide (NaOMe). 2 Today, Zemplén deacylation, performed with a catalytic amount of sodium methoxide in methanol, is a standard tool in laboratories and industry settings. 1 Its inherent disadvantage is the retention in solution of sodium ions. As the sodium ions can be removed using H + -exchanged resin, industrial deacylation techniques normally involve ion-exchanged columns. The H + -exchanged resin can be regenerated with acid after ion exchange. The ion exchange procedure would be omitted from deacylation in laboratories, and especially in industry settings, supposed that the reaction were catalyzed by methoxyl anion resin. At first appearance, it seems impossible due to the difficulty of acquiring methoxyl anion resin. In 1981, Goodman's group found that hydroxyl anion exchanged resin could be used for the catalytic deacylation of sugars in methanol. 3 They believed that the deacylation be catalyzed by methoxyl anion and the methoxyl anion be generated by a series of ion exchanges with the hydroxyl anion at the resin surface. Despite its reported efficiency, 4 this method has unfortunately not been extensively adopted, likely because of the unconvincing explanation.In this study, we proved that Zemplén condition had been misleading us for almost 90 years. We found that deacylation using a hydroxyl anion base in methanol does not in fact require a stoichiometric amount of base for each ester functional group. The results of our experiments indicate that the use of NaOH in methanol is identical to that of NaOMe in methanol for deacylation. The development of Zemplén condition was based on the tranditional base-catalyzed mechanim which cannot be used to explain our results (Figure 1a). Therefore, deacylation can be performed with a catalytic amount of hydroxyl anion resin and the resin can be repeatedly reused in the same process after simply filtered. These results cannot be explained by traditional base-...
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