Direct ester condensation catalyzed by bulky diarylammonium pentafluorobenzenesulfonates

Abstract

A protocol for ester condensation between equimolar amounts of carboxylic acids and alcohols catalyzed by bulky diarylammonium pentafluorobenzenesulfonate is described. We also present procedures for the synthesis of N-(2,6-diisopropylphenyl)-Nmesitylammonium pentafluorobenzenesulfonate. The present ester condensation proceeds well under mild conditions even without the removal of generated water. The synthesis of N-(2,6-diisopropylphenyl)-N-mesitylammonium pentafluorobenzenesulfonate will take B5 days. The ester condensation reactions will take B6 h to 3 days. INTRODUCTIONThe ester condensation reaction is among the most fundamental organic transformations, and more environmentally benign alternative synthetic approaches to the ones currently used are in strong demand by the chemical industry 1 . Conventionally, the ester condensation reaction of carboxylic acids with alcohols is catalyzed by Brønsted acids such as HCl, H 2 SO 4 , p-toluenesulfonic acid and so on for acid-resistance substrates. For acid-sensitive substrates, weak Brønsted acids such as pyridinium p-toluenesulfonate should be used. However, these have lower catalytic activities and the reactants that can be used are rather limited. With regard to green chemistry, in particular with respect to atom economy and E-factor, several catalytic methods for the ester condensation reaction between equimolar amounts of carboxylic acids and alcohols have been developed [2][3][4][5][6] . Conventionally, in fact, esterifications are conducted with an excess of carboxylic acids or alcohols against its reaction counterpart in the presence of an acid catalyst, or with a stoichiometric dehydrating reagent or activated carboxylic acid derivative in the presence of a stoichiometric amount of base. The use of excess amounts of substrates is a wasteful practice in itself. Furthermore, the use of stoichiometric dehydrating reagents or activated carboxylic acid derivatives leads to the formation of significant amounts of undesired by-products. Purifying the crude products from (excess) substrates or from reaction by-products is a rather demanding task-also in financial termsrequiring additional apparati and additional amounts of materials, energy (e.g., for azeotropic reflux) and time. It is therefore evident why the direct catalytic condensation between equimolar amounts of carboxylic acids and alcohols that does not require the presence of dehydrating agents is, at least in principle, such an attractive synthetic goal. Among these 'green' catalytic condensations, metalfree organocatalytic methods are particularly desirable, especially for industrial processes. [OTf] À is the salt of a superacid (trifluoromethansulfonic acid (TfOH)) and a weak base (Ph 2 NH), it is a strong Brønsted acid, and as such is difficult to use in the reaction of sterically demanding and acid-sensitive alcohols.In the course of our continuing study on environmentally benign dehydration catalysts, we have developed N,N-dimesitylammo-

Developments of biobased plasticizers for compostable polymers in the green packaging applications: A review

“…Most of these methods achieved high yield of TEC, although suffered from catalyst deactivation 47 . On the other hand, ATBC is mainly produced from TBC via two methods: direct esterification in the presence of acetic acid 54 and acetylation in the presence acetic anhydride 55 . Currently, different trademarks are already available in the global market, for example, Citroflex A‐2 and Citroflex A‐4, both widely used in different applications including food contact applications 56 .…”

Direct Dehydrative Glycosylation Catalyzed by Diphenylammonium Triflate

“…Based on the success of DPAT, a series of analogous ammonium salts was similarly prepared and screened for dehydrative glycosylation. In general, catalysts prepared from either dialkylamines (entries 6-7) or less acidic Brønsted acids, such as methanesulfonic acid (pKa (H 2 O) = -1.9, entry 8) [22], benzenesulfonic acid (pKa (H 2 O) = -2.8, entry 9) [17][18][19][20][21], or p-toluenesulfonic acid (pKa (H 2 O) = -2.1, entry 10) [23] were inactive. Only catalysts generated from Brønsted acids having acidity similar to that of triflic acid (pKa (H 2 O) = -14.7), such as perchloric acid (pKa (H 2 O) = -15.2) [24], gave comparable yields (entry 11).…”

“…The shift from metal to metal-free catalysis is the current trend for greener and more sustainable chemistry. Arylammonium triflates and bulky diarylammonium arenesulfonates were introduced by Tanabe [15,16] and Ishihara [17][18][19][20][21], respectively, to be effective catalysts for direct dehydrative esterification between carboxylic acids and alcohols in almost equimolar amounts. The local hydrophobic environment provided by the aryl substituents around the reaction center appears to enable condensation to proceed without the need to remove the water produced.…”

The carbon material functionalized with NH2+ and SO3H groups catalyzed esterification with high activity and selectivity

“…Subsequently, Ishihara's group reported that bulky diarylammonium pentafluorobenzenesulfonates ([Ar 2 NH 2 ] + [O 3 SC 6 F 5 ] − ) were more efficient than [Ph 2 NH 2 ] + [OTf ] − in catalytic activity and selectivity. 3,[5][6][7] They assumed that it is the dimeric cyclic ion pairs composed of two diarylammonium cations ([Ar 2 NH 2 ] + ) and two pentafluorobenzenesulfonate anions ([O 3 SC 6 F 5 ] − ) which could form a "hydrophobic wall" to promote the esterification reactions. Also their steric hindrance could effectively suppress the dehydrative elimination of secondary alcohols to produce alkenes (Scheme 1).…”

“…Also their steric hindrance could effectively suppress the dehydrative elimination of secondary alcohols to produce alkenes (Scheme 1). 6 In order to reuse these catalysts, several groups had tried to immobilize these types of ammonium salts on the polymer, but complex synthetic process and instability of the polymer at high temperatures limited their application. [7][8][9] In 2005, Hara's group found a new carbon-based solid sulfonic acid, which was prepared by incompletely carbonizing glucose at high temperatures (over 300 °C) to form a carbonbased material first, and then sulfonating it with a large amount of concentrated sulfuric acid.…”