Robust Zinc Complexes that Contain Pyrrolidine‐Based Ligands as Recyclable Catalysts for the Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides

Abstract

New zinc catalysts bearing ligands that contain a pyrrolidine scaffold were synthesized and tested successfully in the coupling of CO2 with terminal and internal epoxides. These catalysts provide excellent activity and total selectivity to the corresponding cyclic carbonates, even for challenging substrates such as cyclohexene oxide, trans‐1,2‐epoxybutane, and methyl epoxioleate. The robustness of the most active catalytic system was demonstrated by turnover numbers of up to 1840 for propylene oxide. The recycling of this catalytic system was performed up to five times without relevant loss of activity using this substrate.

Zinc–Azatrane Complexes as Efficient Catalysts for the Conversion of Carbon Dioxide into Cyclic Carbonates

“…As clearly observed from Ta ble 3, 2a was applicable to av ariety of terminal and internal epoxides,a ffording exclusivelyt he corresponding cyclic carbonate productsi n good to high yields.M onosubstituted terminal epoxides were readily converted within a3ht imeframe into their related carbonates with yields ranging from 75 to 95 %, whereas al onger reactiont ime (24 h) was necessary to achieves ufficient conversion for more challengingd isubstituted and terminal 1,2epoxy-2-methylpropane (55 %y ield) and for sterically hindered cyclohexeneo xide and cyclopentene oxide (23 and 67 %y ield, respectively), both obtained with full retention of configuration (cis > 99 %). Although the reactionc onditions reported in the literaturea re slightly different from ours to provide meaningful comparison, our binary 2a/nBu 4 NI catalytic system compares favorably with other N 4 -chelated Zn catalysts, [42,43] exhibiting, in most cases,g reater efficiency for the conversion of terminal and even internal epoxides than the Zn-bpb catalyst reported recently by Adolph et al [42] The reuse of metallic catalysts is also an important issue to investigate from economical and practical use viewpoints. The reusability of 2a and 2b was thus examined under the optimized reaction conditions by using 1,2-epoxyhexane as a model substrate over three catalytic cycles.…”

“…Here again, the lower steric hindrance around the Zn catalytic center in 2 a could account for this enhanced reactivity, highlighting the importance of the available space above the Lewis acidic site to synergistically activate the epoxide. Relative to that shown by other N 4 ‐chelated Zn catalysts, Zn–azatrane catalysts 2 a – c showed better performance in the coupling of CO 2 with epoxyhexane with, in the case of best catalyst 2 a , turnover frequencies (TOFs) on average 47 and 6 times higher than those obtained with Zn–bpb [bpb= N , N ‐bis‐(2‐pyridinecarboxamide)‐1,2‐benzene, TOF: 10 h −1 , 80 °C, 20 h, 50 bar, Zn/ n Bu 4 NBr=0.2/0.2] and Zn–pyrrolidine‐based catalysts (TOF: 75 h −1 , 80 °C, 20 h, 35 bar, Zn/ n Bu 4 NI=0.005/0.1), respectively. The activation of CO 2 at room temperature is highly challenging and few catalytic systems are able to achieve the conversion of CO 2 under such mild conditions.…”

“…Monosubstituted terminal epoxides were readily converted within a 3 h timeframe into their related carbonates with yields ranging from 75 to 95 %, whereas a longer reaction time (24 h) was necessary to achieve sufficient conversion for more challenging disubstituted and terminal 1,2‐epoxy‐2‐methylpropane (55 % yield) and for sterically hindered cyclohexene oxide and cyclopentene oxide (23 and 67 % yield, respectively), both obtained with full retention of configuration ( cis >99 %). Although the reaction conditions reported in the literature are slightly different from ours to provide meaningful comparison, our binary 2 a / n Bu 4 NI catalytic system compares favorably with other N 4 ‐chelated Zn catalysts, exhibiting, in most cases, greater efficiency for the conversion of terminal and even internal epoxides than the Zn–bpb catalyst reported recently by Adolph et al …”

Bifunctional Boron Phosphate as an Efficient Catalyst for Epoxide Activation to Synthesize Cyclic Carbonates with CO2

“…In this context, 100 %a tom economical cycloaddition of CO 2 with epoxidesi so ne of the most promising and studied reactions [7][8][9] because the generated five-membered cyclic carbonates can be used as polar solvents, important chemical intermediates, and electrolytes in lithiumb atteries. For the cycloadditiono fC O 2 with epoxides, many homogeneous and heterogeneous catalyst systems have been developed, including alkali metal salts, [10] ionic liquids, [11][12][13] quaternary ammonium and phosphoniums alts, [14][15][16] metal oxides, [17] transition-metalc omplexes, [18][19][20][21][22] functional polymers, [23][24][25][26] and metal-organic frameworks (MOF), [27][28][29][30][31] etc. We have also synthesized Gd-based MOF materials for activating epoxides in the cycloaddition with CO 2 .…”

Solventless Coupling of Epoxides and CO2 in Compressed Medium Catalysed by Fluorinated Metalloporphyrins

“…Particularly, the coupling of carbon dioxide (CO 2 ) with epoxides [1][2][3][4][5] is a topic of great interest due to the economic and environmental benefits arising from the utilisation of renewable sources for the preparation of polycarbonates or cyclic carbonates [6][7][8][9], and the growing concern on the greenhouse effect [10][11][12][13][14][15]. There is substantial literature on catalyst development for CO 2 insertions into epoxides, most of which consist of transition-metal complexes modified with N-donor ligands [16][17][18][19][20][21][22][23][24][25][26][27]. However, the low polarity of CO 2 often generates insolubility problems when transition-metal complexes are employed as catalysts.…”