Alkyl substituted 4-N-oxazadisilinane cations: A new family of Si protic ionic liquids and its application on esterification reactions
Introduction
Ionic liquids (ILs) have attracted much interest during the last two decades. Due to a wide range of anion-cation combinations, the diversity of physicochemical properties and their large variety of applications have made this field revitalized in both academia and industry [1], [2]. The quaternization of a nitrogen or phosphorus atom to form the cation is the initial step in the synthesis of ILs. A few popular cations have been utilized to prepare ILs such as imidazolium [3], pyridinium, ammonium, and phosphonium [4], [5]. A wide range of anions have also been employed [6], offering a huge number of conceivable combinations and distinctive physicochemical properties.
The ILs are classified by their melting points in two categories. One is the room temperature ionic liquids (RTILs) which are defined as ionic salts with melting points below 100 °C [4], [7]. Those salts with melting points above 100 °C can be considered as high-temperature ionic liquids (HTILs) [8], [9], [10]. Some HTILs have been reported with cations such as tetra-alkyl ammonium and phosphonium [11]. HTILs have some favorable properties compared to those of RTILs as they are not as corrosive as RTILs. Moreover, they are solid at room temperature to be easily handled for product separation in organic synthetic applications [9].
The examples of ionic liquid containing silyl or siloxane framework (SiILs) were first reported by Shirota and co-workers [12], [13], [14]. Several groups have also prepared and characterized some new types of SiILs [15], [16], [17], [18]. These SiIL typically have the silyl or siloxane group as a side chain attached to the cation along with various anions. Most SiILs salts have low melting points, and a significant drop in their viscosity and high conductivity were observed.
Oxazadisilinane ring is a six-membered cyclic compound containing a Si-O-Si linkage and a tertiary amine group. For this six-membered heterocyclic oxazadisilinane rings, only a very few examples studies have been presented [19]. The quaternization of the N atom allows the synthesis of a new class of ionic liquids. Herein, six new compounds of 4-N-Alkyl-2,2,6,6-tetramethyl- [1], [4], [2], [6] oxazadisilinane compound have been synthesized. The corresponding ionic liquids were prepared by treating the oxazadisilinane compound with various protic acid and well characterized by NMR, elemental analysis, TGA and HRMS. The Si-PILs were examined for esterification reactions under thermal conditions and microwave irritation.
Section snippets
Synthesis of 4-N-R-tetramethyl-oxazadisilinane compounds
A general method was used to synthesize all cyclic oxazadisilinane; was ring closing of the chloride siloxane chain. An excess amount of amine was mixed with 1,3-bis-chloromethyl-1,1,3,3-tetramethyl-disiloxane and heated at 100 °C overnight under a solvent-free neat condition (Scheme 1). Further distillation was done to purify the products in good yields (Table 1), followed by NMR, HRMS, and elemental analysis characterizations (see ESI for details).
Synthesis of Siloxane Protic Ionic Liquids (Si-PILs)
Si-PILs were prepared by using the equimolar
Conclusion
In summary, 6 4-N-R-2,2,6,6-tetramethyl- [1], [4], [2], [6] oxazadisilinanes have been synthesized successfully by 1,3-bis-chloromethyl-1,1,3,3-tetramethyl-disiloxane as a commercially available chemicals with different amines. Using a neat and simple method, these compounds can be isolated. 21 new Si-PILs were prepared from the corresponding oxazadisilinane compounds with four acids (HCl, MsOH, H2SO4, and HBF4). The new family of Si-PILs was well characterized by using NMR, mass, melting
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We acknowledge the generous financial support from King Abdullah University of Science and Technology (KAUST).
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M.A and P.C contributed equally to this work.