Organic Superbases Research Articles

Novel Organic Superbase Dopants for Ultraefficient N‐Doping of Organic Semiconductors (Adv. Mater. 22/2023)

Organic Semiconductors In article number 2300084, Huan Wei, Zebing Zeng, Yugang Bai, Yuanyuan Hu, and co-workers report the discovery of organic superbases as ultraefficient n-dopants for organic semiconductors. The dopants result in record-high conductivity in typical n-type organic semiconductors, showing doping efficiency three times higher than the benchmark n-dopant N-DMBI. The image illustrates n-doping of P61CBM by the dopants.

Novel Organic Superbase Dopants for Ultraefficient N-Doping of Organic Semiconductors.

Doping is a powerful technique for engineering the electrical properties of organic semiconductors (OSCs), yet efficient n-doping of OSCs remains a central challenge. Herein, the discovery of two organic superbase dopants, namely P2-t-Bu and P4-t-Bu as ultra-efficient n-dopants for OSCs is reported. Typical n-type semiconductors such as N2200 and PC61 BM are shown to experience a significant increase of conductivity upon doping by the two dopants. In particular, the optimized electrical conductivity of P2-t-Bu-doped PC61 BM reaches a record-high value of 2.64Scm-1 . The polaron generation efficiency of P2-t-Bu-doped in PC61 BM is found to be over 35%, which is 2-3 times higher than that of benchmark n-dopant N-DMBI. In addition, a deprotonation-initiated, nucleophilic-attack-based n-doping mechanism is proposed for the organic superbases, which involves the deprotonation of OSC molecules, the nucleophilic attack of the resulting carbanions on the OSC's π-bonds, and the subsequent n-doping through single electron transfer process between the anionized and neutral OSCs. This work highlights organic superbases as promising n-dopants for OSCs and opens up opportunities to explore and develop highly efficient n-dopants.

Compact Rotaxane Superbases.

Challenges for the development of efficacious new superbases include their ease of synthesis, chemical stability, and high basicity, while minimizing nucleophilicity is important for reducing unwanted side reactions. Here, we introduce a new family of organic superbases, compact amine-crown ether rotaxanes, which show desirable characteristics in all these respects. Metal-free active template synthesis provides access to a range of rotaxanes with as little as three atoms between the stoppering groups, locking the location of a small crown ether (21C7 and 24C8 derivatives) over the amine group of the axle. The forced proximity of the interlocked protophilic components results in pKaH+ values as high as 32.2 in acetonitrile, which is up to 13 pKaH+ units greater than the pKaH+ values of the non-interlocked components, and brings the free base rotaxanes into the basicity realm of phosphazene superbases. The rotaxane superbases are generally chemically stable and, in a model reaction for superbases, eliminate HBr from a primary alkyl bromide with complete selectivity for deprotonation over alkylation. Their modest size, ease of synthesis, high basicity, low nucleophilicity, and, in the best cases, rapid substrate deprotonation kinetics and excellent hydrolytic stability make compact amine-crown ether rotaxane superbases intriguing candidates for potential applications in synthesis and supramolecular and materials chemistry.

Organic superbase-mediated synthesis of borohydride ionic liquids as novel composite hypergolic fuels

This study prepared a series of novel hypergolic fluids based on borohydride ionic liquids and organic superbase using an in situ synthetic method. In these hypergolic fluids, ionic liquids in 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) acted as triggers for the self-ignition of DBN and DBU upon contact with high-concentration hydrogen peroxide (H2O2). These hypergolic fluids had high densities (>1.000 ​g ​cm−3), low viscosities (as low as 34.03 ​cP), and acceptable ignition delay times (IDT). The ignition processes of the hypergolic fluids with 90% H2O2 as an oxidizer were first investigated in this study, and they differed from the previously reported ignition phenomena. Different from the case with white fuming acid (WFNA) as an oxidizer, the ignition processes of hypergolic fluids with 90% H2O2 as an oxidizer did not exhibit secondary rebound and splashing and formed a homogeneous mixed layer when the droplets were in contact with 90% H2O2. The different ignition processes significantly influenced the properties of hypergolic fluids. Compared with the hypergolic fluids with WFNA as an oxidizer, those with 90% H2O2 as an oxidizer showed a shorter IDT (IDTmin[90% H2O2]=28.3 ​ms, IDTmin[WFNA]=126 ​ms) and formed stable flames without secondary combustion. These results demonstrate that the in-situ synthesized fuels in this study hold great promise as green fuels in hypergolic propulsion systems.

Superbasicity of imines with bicyclo[5.1.0]octa-1,3,5,7-tetraene scaffold due to electron delocalization in the conjugated acids

Gas phase basicity of organic superbases with bicyclo[5.1.0]octa-1,3,5,7-tetraene (fused cyclopropene and cycloheptatriene) scaffold and an imine group as proton acceptor site was computationally assessed using B3LYP/6-311++G(d,p) method. The basicity of these imines originates from the electron delocalization and consequently delocalization of the positive charge upon protonation leading to stability of the conjugated acid. Also, the effects of different electron donating groups (EDGs) and hydrogen bond formation on the basicity enhancement were studied. The calculated PAs were 1000–1307 kJ mol−1 indicating superbasicity and hyperbasicity of the studied organic bases. These organic bases can deprotonate H2SO4 and FHSO4 in the gas phase and produce stable ion pair clusters.

Ring-Opening Polymerization of Trimethylene Carbonate with Phosphazene Organocatalyst.

Aliphatic polycarbonate (APC) compounds are an important class of biodegradable materials with excellent biocompatibility, good biodegradability, and low toxicity, and the study of these compounds and their modification products aims to obtain biodegradable materials with better performance. In this context, the ring-opening polymerization (ROP) of trimethylene carbonate (TMC) from a low nucleophilic organic superbase of phosphazene (t-BuP4) as a catalyst and benzyl alcohol (BnOH) as an initiator at room temperature was carefully studied to prepare poly(trimethylene carbonate) (PTMC) which is one of the most studied APC. 1H NMR and SEC measurements clearly demonstrate the presence of a benzyloxy group at the α-terminus of the obtained PTMC homopolymers while investigation of the polymerization kinetics confirms the controlled/living nature of t-BuP4-catalyzed ROP of TMC. On the basis of this, the block copolymerization of TMC and δ-valerolactone (VL)/ε-caprolactone (CL) was successfully carried out to give PTMC-b-PCL and PTMC-b-PVL copolymers. Furthermore, PTMC with terminal functionality was also prepared with the organocatalytic ROP of TMC through functional initiators. We believe that the present ROP system is a robust, highly efficient, and practical strategy for producing excellent biocompatible and biodegradable PTMC-based materials.

Polyetheretherketone fiber-supported TBD as an efficient fibrous superbase catalyst for organic conversions in continuous-flow processing

The development of clean catalytic technologies in continuous-flow processing for efficient organic conversions has a positive influence on the fine chemicals and pharmaceutical manufacturing. Herein, the commonly used organic superbase 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was immobilized on the commercially available ultra-high strength textile fiber polyetheretherketone (PEEK) to report a polymer-supported basic catalyst (PEEK-TBD) for three types of organic conversions in cleaner continuous-flow processing. The fiber catalyst was prepared through an improvement strategy and verified as a heterogeneous fibrous superbase catalyst in the microtubule reactor to mediate the Knoevenagel condensation-cyclization between salicylaldehydes and activated nitriles for the synthesis of iminocoumarins in ethanol (10 samples, yields 91–97%) after turning the flow rate, substrate concentration, and catalyst padding quantity, additionally, PEEK-TBD was also effective for the cyanosilylation of carbonyl compounds for the synthesis of cyanogen compounds in acetonitrile, as well as the substitution reaction of aniline and carbonate for the synthesis of N,N’-substituted urea in toluene in continuous-flow processing. Moreover, the resulting fiber samples were characterized in detail by sorts of analytical methods and techniques to further confirm the dependability of this methodology, and the clean and valid gram-scalable procedure of PEEK-TBD in continuous-flow processing has extensive application prospects on the chemical productions.

Utilizing the synergistic effect between imidazole aromaticity and guanidine structure for the computational design of novel uncharged organic superbases

Proton affinities (PA) and gas phase basicities (GB) of novel structure of non-ionic organic superbases on the basis of guanidine embedded imidazole framework, were computed at B3LYP/6-311+G(d,p) level of theory. The used logic for designing these structures is synergistic effect of guanidine-imidazole twin in order to aromatization and subsequent stabilization of imidazole ring upon protonation which is strengthen with push-pull effect of variety electron-donating groups on the five-membered ring. This strategy caused to the design of organobases with considerable basicity in the range of 985.39–1177.68 kJ mol−1. Majority of the designed structures show the PA values more than 1028 kJ mo1−1, as the threshold of superbasicity. The HOMA/HOMED, and NICS(1)zz analyses were used for each structure and its conjugated acid in order to better understand and discuss the basicity values.

Preconcentration of Superbase Ionic Liquid from Aqueous Solution by Membrane Filtration.

Certain organic superbase ionic liquids (ILs) have shown good cellulose dissolution and fiber regeneration performance, allowing us to obtain high-quality textile fibers. However, there is a lack regarding the IL recovery from the spinning bath and its purification, which is essential for the economic viability of the process. Aiming to understand methods to separate ILs from water for reuse/recycle, the use of pressure-driven membrane processes to recycle ionic liquids from aqueous solution was investigated. The recovery of two superbase ILs, 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-enium acetate, [mTBDH][OAc], and 5-methyl-1,5,7-triaza-bicyclo[4.3.0]non-6-enium acetate, [mTBNH][OAc], were studied using different types of membranes (microfiltration, ultrafiltration, nanofiltration, and reverse osmosis, RO). Additionally, pressure, IL concentration, temperature, and multicycle effect were evaluated. Significant retentions (>45%) were obtained for the nanofiltration and RO membranes (NF270-NF and BW30LE-RO). The increase in pressure and temperature resulted in an increase in volumetric flux and a decrease in IL retention. On the other hand, IL concentration decreased the volumetric flow and rejection. For the serial filtration tests, a three-fold ionic liquid concentration was achieved, for a maximum concentration of 14 wt % of the ionic liquid. The membrane filtration methodology proved to be an efficient technique for carrying out the preconcentration of the IL from dilute solutions.

Bicyclic-ring base doping induces n-type conduction in carbon nanotubes with outstanding thermal stability in air

The preparation of air and thermally stable n-type carbon nanotubes is desirable for their further implementation in electronic and energy devices that rely on both p- and n-type material. Here, a series of guanidine and amidine bases with bicyclic-ring structures are used as n-doping reagents. Aided by their rigid alkyl functionality and stable conjugate acid structure, these organic superbases can easily reduce carbon nanotubes. n-Type nanotubes doped with guanidine bases show excellent thermal stability in air, lasting for more than 6 months at 100 °C. As an example of energy device, a thermoelectric p/n junction module is constructed with a power output of ca. 4.7 μW from a temperature difference of 40 °C.

Mechanistic Studies Yield Improved Protocols for Base-Catalyzed Anti-Markovnikov Alcohol Addition Reactions.

The catalytic anti-Markovnikov addition of alcohols to simple alkenes is a longstanding synthetic challenge. We recently disclosed the use of organic superbase catalysis for the nucleophilic addition of alcohols to activated styrene derivatives. This article describes mechanistic studies on this reversible reaction, including thermodynamic and kinetic profiling as well as computational modeling. Our findings show the negative entropy of addition is counterbalanced by an enthalpy that is most favored in nonpolar solvents. However, a large negative alcohol rate order under these conditions indicates excess alcohol sequesters the active alkoxide ion pairs, slowing the reaction rate. These observations led to an unexpected solution to a thermodynamically challenging reaction: use of less alcohol enables faster addition, which in turn allows for lower reaction temperatures to counteract Le Chatelier's principle. Thus, our original method has been improved with new protocols that do not require excess alcohol stoichiometry, enable an expanded alkene substrate scope, and allow for the use of more practical catalyst systems. The generality of this insight for other challenging hydroetherification reactions is also demonstrated through new alkenol cyclization and oxa-Michael addition reactions.

Pretreatment of Corn Stover by Levulinic Acid-Based Protic Ionic Liquids for Enhanced Enzymatic Hydrolysis

Economically competitive and sustainable dissolution pretreatment technology is in high demand due to the remarkable achievement in the pretreatment of lignocellulose using ionic liquids (ILs) for enhanced enzymatic hydrolysis. Herein, levulinic acid (Lev), a well-known bio-based platform chemical derived from carbohydrates, in conjunction with a series of organic superbases, was used as a raw chemical to prepare a new class of protic ILs (PILs). The findings indicated that the obtained PILs exhibited a good solubility of up to 10 wt % toward corn stover-based lignocellulose at 140 °C in 40 mins, based on which a facile and efficient dissolution pretreatment technology was developed for enhanced enzymatic hydrolysis of corn stover. The facile dissolution and regeneration of corn stover resulted in significant changes in the composition and physical–chemical structures. These changes led to significantly enhanced enzymatic hydrolysis behavior of the pretreated sample with total reducing sugar and glucose yields of 0.8 and 0.49 g/g within 48 h under optimal conditions, respectively. The Kamlet–Taft solvent parameters of the PILs were systematically evaluated, and the interaction between the lignocellulose and PILs was studied by 13C NMR, indicating that the satisfactory performance of PILs was benefited from the higher ß parameter value and the stronger interactions, particularly the existence of the extra hydrogen-bond interaction from the potential keto–enol tautomerism of the ketone groups in levulinate anions. The compositional and physicochemical changes in the lignocellulose were systematically evaluated to achieve an in-depth understanding of the dissolution activation mechanism using various characterization techniques. The findings also showed that 44.3% of lignin could be fractionated during the facile dissolution and regeneration process, together with a transformation of the crystalline structure of cellulose I to cellulose II and of the packed morphology to a porous one. The structure of the fractionated lignin was also characterized for a better understanding of the fractionation process. The study demonstrated a new sustainable dissolution pretreatment technology for enhanced enzymatic hydrolysis, which can provide significant insights into the design of new solvents for biomass dissolution and processing.

Organic superbaset-Bu-P4-catalyzed demethylations of methoxyarenes

The organic superbaset-Bu-P4 catalyzes the demethylation reactions of methoxyarenes in the presence of alkanethiol and hexamethyldisilazane.

Stable Singlet Carbenes as Organic Superbases

AbstractA simple experimental procedure for scaling carbene Brønsted basicity is described. The results highlight the strong basicity of pyrazol‐4‐ylidenes, a type of mesoionic carbene, also named cyclic‐bentallenes (CBA). They are more basic (pKaH >42.7 in acetonitrile) than the popular proazaphosphatrane Verkade bases, and even the Schwesinger phosphazene superbase P4(tBu). The basicity of these compounds can readily be tuned, and they are accessible in multigram quantities. These results open new avenues for carbon centered superbases.

Stable Singlet Carbenes as Organic Superbases.

A simple experimental procedure for scaling carbene Brønsted basicity is described. The results highlight the strong basicity of pyrazol-4-ylidenes, a type of mesoionic carbene, also named cyclic-bentallenes (CBA). They are more basic (pKaH >42.7 in acetonitrile) than the popular proazaphosphatrane Verkade bases, and even the Schwesinger phosphazene superbase P4 (t Bu). The basicity of these compounds can readily be tuned, and they are accessible in multigram quantities. These results open new avenues for carbon centered superbases.

Harnessing aromaticity to obtain new powerful organic superbases based on phosphaallene ylide scaffold: A density functional theory study

A density-functional theory study of phosphaallenes basicity in the gas phase has been carried out using the B3LYP/6-311+G(d,p) computational method. The origin of the high basicity is identified as the aromatization of the unsaturated ring upon protonation of central carbon of phosphaallene ylide framework. Basicities are enhanced by placing electron-donating substituents such as dimethylamino and methoxy group on the ring. These phosphaallene ylide derivatives have proton affinities more than about 90 kJ mol−1, which is greater than the PA of DMAN. Hence, they introduced as candidates for strong neutral organic superbases and hyperbases.

Integrated and Metal Free Synthesis of Dimethyl Carbonate and Glycidol from Glycerol Derived 1,3-Dichloro-2-propanol via CO2 Capture

Dimethyl carbonate (DMC) and glycidol are considered industrially important chemical entities and there is a great benefit if these moieties can be synthesized from biomass-derived feedstocks such as glycerol or its derivatives. In this report, both DMC and glycidol were synthesized in an integrated process from glycerol derived 1,3-dichloro-2-propanol and CO2 through a metal-free reaction approach and at mild reaction conditions. Initially, the chlorinated cyclic carbonate, i.e., 3-chloro-1,2-propylenecarbonate was synthesized using the equivalent interaction of organic superbase 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) and 1,3-dichloro-2-propanol with CO2 at room temperature. Further, DMC and glycidol were synthesized by the base-catalyzed transesterification of 3-chloro-1,2-propylenecarbonate using DBU in methanol. The synthesis of 3-chloro-1,2-propylenecarbonate was performed in different solvents such as dimethyl sulfoxide (DMSO) and 2-methyltetrahydrofuran (2-Me-THF). In this case, 2-Me-THF further facilitated an easy separation of the product where a 97% recovery of the 3-chloro-1,2-propylenecarbonate was obtained compared to 63% with DMSO. The use of DBU as the base in the transformation of 3-chloro-1,2-propylenecarbonate further facilitates the conversion of the 3-chloro-1,2 propandiol that forms in situ during the transesterification process. Hence, in this synthetic approach, DBU not only eased the CO2 capture and served as a base catalyst in the transesterification process, but it also performed as a reservoir for chloride ions, which further facilitates the synthesis of 3-chloro-1,2-propylenecarbonate and glycidol in the overall process. The separation of the reaction components proceeded through the solvent extraction technique where a 93 and 89% recovery of the DMC and glycidol, respectively, were obtained. The DBU superbase was recovered from its chlorinated salt, [DBUH][Cl], via a neutralization technique. The progress of the reactions as well as the purity of the recovered chemical species was confirmed by means of the NMR analysis technique. Hence, a single base, as well as a renewable solvent comprising an integrated process approach was carried out under mild reaction conditions where CO2 sequestration along with industrially important chemicals such as dimethyl carbonate and glycidol were synthesized.

Design of Novel Uncharged Organic Superbases: Merging Basicity and Functionality.

ConspectusOne of the constant challenges of synthetic chemistry is the molecular design and synthesis of nonionic, metal-free superbases as chemically stable neutral organic compounds of moderate molecular weight, intrinsically high thermodynamic basicity, adaptable kinetic basicity, and weak or tunable nucleophilicity at their nitrogen, phosphorus, or carbon basicity centers. Such superbases can catalyze numerous reactions, ranging from C-C bond formation to cycloadditions and polymerization, to name just a few. Additional benefits of organic superbases, as opposed to their inorganic counterparts, are their solubility in organic reaction media, mild reaction conditions, and higher selectivity. Approaching such superbasic compounds remains a continuous challenge. However, recent advances in synthetic methodology and theoretical understanding have resulted in new design principles and synthetic strategies toward superbases. Our computational contributions have demonstrated that the gas-phase basicity region of 350 kcal mol-1 and even beyond is easily reachable by organosuperbases. However, despite record-high basicities, the physical limitations of many of these compounds become quickly evident. The typically large molecular weight of these molecules and their sensitivity to ordinary reaction conditions prevent them from being practical, even though their preparation is often not too difficult. Thus, obviously structural limitations with respect to molecular weight and structural complexity must be imposed on the design of new synthetically useful organic superbases, but strategies for increasing their basicity remain important.The contemporary design of novel organic superbases is illustrated by phosphazenyl phosphanes displaying gas-phase basicities (GB) above 300 kcal mol-1 but having molecular weights well below 1000 g·mol-1. This approach is based on a reconsideration of phosphorus(III) compounds, which goes along with increasing their stability in solution. Another example is the preparation of carbodiphosphoranes incorporating pyrrolidine, tetramethylguanidine, or hexamethylphosphazene as a substituent. With gas-phase proton affinities of up to 300 kcal mol-1, they are among the top nonionic carbon bases on the basicity scale. Remarkably, the high basicity of these compounds is achieved at molecular weights of around 600 g·mol-1. Another approach to achieving high basicity through the cooperative effect of multiple intramolecular hydrogen bonding, which increases the stabilization of conjugate acids, has recently been confirmed.This Account focuses on our efforts to produce superbasic molecules that embody many desirable traits, but other groups' approaches will also be discussed. We reveal the crucial structural features of superbases and place them on known basicity scales. We discuss the emerging potential and current limits of their application and give a general outlook into the future.

Impact of DBU on the synthesis of cellulose-graft-poly(l-lactide) copolymer in CO2 switchable solvent with different grafting strategies

Modification of cellulose homogeneously by graft copolymerization via grafting-from or grafting-to approaches is an attractive and versatile way to impart specific properties onto this promising renewable material. The grafting efficiency is greatly influenced by the solvent and catalyst used during this process. In this study, cellulose-graft-poly (l-lactic acid) (C-g-PLLA) was prepared by ring opening polymerization of l-lactide (LA) (grafting-from) and transesterification of PLLA (grafting-to) with cellulose dissolved in CO2 switchable solvent. The impact of the presence of 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) was then elucidated by comparing the grafting efficiency of the process with or without adding extra amount of this organic superbase. Results showed that without adding extra DBU, the grafting efficiency increased with an increase in concentration of grafting agents (LA or PLLA). However, when extra DBU was added, the grafting efficiency decreased with increased levels of DBU. These results indicate that the DBU protonated by the hydroxyl group from cellulose participated in the dissolution of cellulose after capture of CO2 in the switchable solvent, and played a role in the graft copolymerization to produce C-g-PLLA. On the other hand, the free DBU that was unprotonated displayed an antagonistic effect towards grafting copolymerization. Detailed graft copolymerization pathways and related modes of action were then considered in attempt to provide new insight into the role of CO2 switchable solvent for cellulose modification by ROP or transesterification.

Azacalix[3]triazines: A Substructure of Triazine‐Based Graphitic Carbon Nitride Featuring Anion‐π Interactions

AbstractGraphitic carbon nitride (GCN) has garnered broad research interest due to its unique catalytic properties. However, GCN, prepared by general methods, possesses myriad structural defects and it has been difficult to elucidate their intrinsic physical properties. We report the development of azacalix[3]triazines (AC3Ts), a substructure of triazine‐based GCN (Tz‐GCN). Despite the electron‐deficient natures of triazine, AC3Ts capture protons as organic superbases. We reveal the unique anion‐π interactions of AC3Ts that alters the ionization potentials of AC3Ts. To the best of our knowledge, these features have not yet been recognized for Tz‐GCN. These unveiled features of AC3Ts are expected to expand the usage scope and possibilities for GCNs.