Insight into the Interactions between Azole-Anion-Based Ionic Liquids and Propargylic Alcohol: Influence on the Carboxylative Cyclization of Propargylic Alcohol with Carbon Dioxide

Abstract

To reveal the interactions between azole-anionbased ionic liquids (AILs) and propargyl alcohol (PA) and their effect on the carboxylative cyclization reaction of PA with CO 2 , three AILs, namely, [P 66614 ][Im], [P 66614 ][4-MeIm], and [P 66614 ][4-BrIm], were synthesized and characterized. The interactions between the AILs with 2-methyl-3-butyn-2-ol (MBY) were investigated through IR and 1 H NMR spectra and DFT calculations. The results indicate that azole-based anions can form H-bonds with the hydroxyl (O−H) and the alkynyl hydrogen atoms (CC−H) of MBY simultaneously, with the former much stronger than the latter, which is supported by the interaction energy and the distance of the H-bond from DFT calculations. Interestingly, the strength of the former always increases with the mole fraction of AIL (x AIL ), while that of the latter first decreases and then increases, reaching a minimum at x AIL ≈ 0.4. Furthermore, the order of the strength of the two H-bonds is as follows:which is in good agreement with their basicity. According to the interactions between the AILs with MBY and combined with the reaction mechanism of MBY with CO 2 promoted by AILs, it is evident that the increase in the basicity of the AILs enhances the H-bond between the AILs with O−H, thereby enhancing the activation of MBY and the subsequent CO 2 conversion. However, this will also increase the interaction between AILs and CC−H, resulting in the deprotonation of CC−H and hindering the activation of MBY. Therefore, choosing a suitable basic AIL and controlling its ratio are the keys to the reaction of PA and CO 2 .

Understanding of the interactions between azole-anion-based ionic liquids and 2-methyl-3-butyn-2-ol from the experimental perspective: the cage effect

“…Combined with the results of this study and our previous report, 10 it can be reasonably suggested that both anions and cations of AILs play an important role in activating MBY during the AIL-promoted carboxylative cyclization of MBY with CO 2 .…”

“…[53][54][55] Therefore, d of the alkyl groups of [P 66614 ] + of AILs + MBY mixtures as a function of x AIL are investigated, which have been reported in our previous work, but not focused on and discussed. 10 The results show that d of the terminal CH 3 and the adjacent CH 2 are almost unchanged with the increasing of x AIL (the variation range is within 0.005 ppm, which is close to the measurement accuracy 56 ), indicating that their electron density remains unchanged, which is attributed to these groups being far away from the positive and negative charge centers, and there are no obvious group crossover and charge overlap between them as x AIL increases. As illustrated in Fig.…”

“…NMR measurements are described in our previous work, and the 1 H NMR spectra of the binary mixtures are listed. 10 The sizes of particles of the [P 66614 ][Im] + MBY mixture were measured using dynamic light scattering (Nano ZS90, Malvern Instruments, Ltd, UK) at a constant temperature of T = 298.15 K. Every sample was equilibrated for 10 s before data were collected.…”

“…Meanwhile, 1 H NMR spectroscopy of AILs + MBY mixtures show that when x AIL is less than 0.4, the H-bonds between AILs and O-H of MBY is dominant in the system, and its strength increases with the increase of x AIL , thereby enhancing the activation of MBY and the subsequent CO 2 conversion. 10 Moreover, when x AIL is greater than 0.4, the H-bonds between AILs and CRC-H of MBY in the system will gradually increase and cannot be ignored, resulting in the deprotonation of CRC-H and hindering the activation of MBY. 10 Consequently, the present work proposes that the optimal molar fraction of AILs in the AILspromoted carboxylative cyclization reaction of MBY with CO 2 is about 0.3.…”

“…[7][8][9] The reaction mechanism through DFT calculations and NMR spectroscopy demonstrated that azole-based anions of AILs could not only activate CO 2 , but also activate PA by forming H-bonds with the hydroxyl group (O-H) of PA. [6][7][8] Recently, we have used IR and 1 H NMR spectroscopies and DFT calculations to study the interactions between AILs and MBY and their effects on the reaction of MBY with CO 2 , and surprisingly found that azolebased anions can also form a H-bond with the alkynyl hydrogen atom (CRC-H) of MBY in addition to forming a H-bond with the O-H of MBY. 10 Moreover, as the basicity of AILs strengthened or the molar ratio of AIL/MBY increased, especially when the mole fraction of AIL (x AIL ) was greater than 0.4, the effects of H-bonds between azole-based anions and CRC-H gradually appeared, leading to the deprotonation of CRC-H and hindering the activation of MBY, 10 indicating that the interactions between AILs and MBY play an important role in the carboxylative cyclization of MBY with CO 2 .…”