- Br?nsted Basicities and Nucleophilicities of N-Heterocyclic Olefins in Solution: N-Heterocyclic Carbene versus N-Heterocyclic Olefin. Which Is More Basic, and Which Is More Nucleophilic?
-
A Br?nsted basicity scale comprising nine representative N-heterocyclic olefins (NHOs) was established by measuring the equilibrium acidities of their corresponding precursors in DMSO using an ultraviolet-visible spectroscopic method. The basicities (pKaHs) of the investigated NHOs cover a range from 14.7 to 24.1. The basicities of unsaturated NHOs are stronger than those of their N-heterocyclic carbene (NHC) analogues; however, the basicities for the saturated ones are much weaker than those of their NHC analogues, which is largely due to the aromatization effect that intrinsically influences the acidic dissociations of NHC and NHO precursors. The nucleophilicities of four NHOs were measured photometrically by monitoring the kinetics of reactions of these NHOs with common reference electrophiles for quantifying nucleophilic reactivities. In general, the nucleophilicity of the NHOs is much stronger than that of commonly used Lewis bases such as Ph3P or DMAP [4-(dimethylamino)pyridine] but weaker than that of their NHC analogues; however, caution should be taken when generalizing this conclusion to a wide range of electrophiles with distinctively electronic and structural properties.
- Li, Zhen,Ji, Pengju,Cheng, Jin-Pei
-
-
Read Online
- Synthesis of RTH-type zeolites using a diverse library of imidazolium cations
-
RTH-type zeolites are promising catalytic materials for applications that include the important methanol-to-olefins (MTO) and NOX reduction reactions. Here, RTH-type zeolites are prepared using a wide-range of imidazolium-based, cationic organic structure directing agents (OSDAs), that greatly expand the methodologies and compositions that can be used to synthesize these materials. The abilities of the OSDAs to produce RTH-type zeolites agree well with results from molecular modeling studies of predicted stabilization energies of the OSDAs in the RTH framework. The RTH-type zeolites are stable to steaming up to 900 °C and are shown to be active MTO catalysts.
- Schmidt, Joel E.,Deimund, Mark A.,Xie, Dan,Davis, Mark E.
-
-
Read Online
- Anisotropic, Organic Ionic Plastic Crystal Mesophases from Persubstituted Imidazolium Pentacyanocyclopentadienide Salts
-
We describe the synthesis, supramolecular organization, and thermal characteristics of an unprecedented family of symmetric 1,2,3,4,5-pentaalkylimidazolium ([(Cn)5im]+) salts equipped with halide, nitrate, or pentacyanocyclopentadienide ([Cp(CN)5]-) counterions. Salts containing relatively small anions were obtained as low-melting solids, whereas those with [Cp(CN)5]- anions were found to be ionic liquids even below room temperature. A permethylated derivative, [(C1)5im][Cp(CN)5], proved to be exceptional. Upon heating, the salt self-organized into a new type of organic ionic plastic crystal (OIPC) mesophase, which was termed Mhex and whose anisotropic structure featured hexagonally ordered, rotating anionic stacks positioned within a continuum composed of disordered cations. The structure of the mesophase resembles that of classical columnar liquid-crystalline phases, despite the absence of long, flexible chains. In the Mhex phase, the cations surrounding the anionic columns effectively fulfill the role of "softening" structural constituents, much in the same way as flexible chains. The discovery of the novel mesophase, which displays a two-dimensional, and thus intrinsically anisotropic, lattice resulting from the rotation of entire ionic assemblies around a columnar axis, represents a new paradigm in the field of OIPCs. Relatively high ionic conductivities were measured in the Mhex phase, particularly after doping with the corresponding sodium salt, Na[Cp(CN)5], demonstrating the materials' potential for use in electrochemical applications such as sodium-ion batteries.
- Goossens, Karel,Rakers, Lena,Heinrich, Beno?t,Ahumada, Guillermo,Ichikawa, Takahiro,Donnio, Bertrand,Shin, Tae Joo,Bielawski, Christopher W.,Glorius, Frank
-
p. 9593 - 9603
(2019/12/24)
-
- Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High-Molar-Mass Polyethers
-
A dual catalytic setup based on N-heterocyclic olefins (NHOs) and magnesium bis(hexamethyldisilazide) (Mg(HMDS)2) was used to prepare poly(propylene oxide) with a molar mass (Mn) >500 000 g mol?1, in some cases even >106 g mol?1, as determined by GPC/light scattering. This is achieved by combining the rapid polymerization characteristics of a zwitterionic, Lewis pair type mechanism with the efficient epoxide activation by the MgII species. Transfer-to-monomer, traditionally frustrating attempts at synthesizing polyethers with a high degree of polymerization, is practically removed as a limiting factor by this approach. NMR and MALDI-ToF MS experiments reveal key aspects of the proposed mechanism, whereby the polymerization is initiated via nucleophilic attack by the NHO on the activated monomer, generating a zwitterionic species. This strategy can also be extended to other epoxides, including functionalized monomers.
- Walther, Patrick,Krau?, Annabelle,Naumann, Stefan
-
supporting information
p. 10737 - 10741
(2019/07/04)
-
- Controlled preparation of amphiphilic triblock-copolyether in a metal- and solvent-free approach for tailored structure-directing agents
-
Under mild conditions, PPO-PEO-PPO ("reverse Pluronics") and PBO-PEO-PBO copolyether were generated by way of N-heterocyclic olefin-based organocatalysis. Reverse Pluronics with molar masses > 20 000 g mol-1 could be synthesized with excellent control (DM ≤ 1.03) and were converted into (ordered) mesoporous carbons via organic self-assembly to showcase the need for tailor-made copolymer as structure-directing agent.
- Balint, Alexander,Papendick, Marius,Clauss, Manuel,Müller, Carsten,Giesselmann, Frank,Naumann, Stefan
-
supporting information
p. 2220 - 2223
(2018/03/06)
-
- N-Heterocyclic olefins as efficient phase-transfer catalysts for base-promoted alkylation reactions
-
N-Heterocyclic olefins (NHOs) have very recently emerged as efficient promoters for several chemical reactions due to their strong Br?nsted/Lewis basicities. Here we report the novel application of NHOs as efficient phase-transfer organocatalysts for synt
- Blümel, Marcus,Crocker, Reece D.,Harper, Jason B.,Enders, Dieter,Nguyen, Thanh V.
-
p. 7958 - 7961
(2016/07/06)
-
- Development and Applications of Transesterification Reactions Catalyzed by N-Heterocyclic Olefins
-
A novel method to utilize N-heterocyclic olefins (NHOs), the alkylidene derivatives of N-heterocycic carbenes, as organocatalysts to promote transesterification reactions has been developed. Because of their strong Br?nsted/Lewis basicity, NHOs can enhance the nucleophilicity of alcohols for their acylation reactions with carboxylic esters. This transformation can be employed in industrially relevant processes such as the production of biodiesel, the depolymerization of polyethylene terephthalate (PET) from plastic bottles for recycling purposes, and the ring-opening polymerization of cyclic esters to form biodegradable polymers such as polylactide (PLA) and polycaprolactone (PCL).
- Blümel, Marcus,Noy, Janina-Miriam,Enders, Dieter,Stenzel, Martina H.,Nguyen, Thanh V.
-
supporting information
p. 2208 - 2211
(2016/06/01)
-
- N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide)
-
The metal-free polymerization of propylene oxide (PO) using a special class of alkene - N-heterocyclic olefins (NHOs) - as catalysts is described. Manipulation of the chemical structure of the NHO organocatalyst allows for the preparation of the poly(propylene oxide) in high yields with high turnover (TON>2000), which renders this the most active metal-free system for the polymerization of PO reported to date. The resulting polyether displays predictable end groups, molar mass, and a low dispersity (D strok signM1.09). NHOs with an unsaturated backbone are essential for polymerization to occur, while substitution at the exocyclic carbon atom has an impact on the reaction pathway and ensures the suppression of side reactions. One carbon makes a difference: The efficient and controlled formation of poly(propylene oxide) (PPO) at a very low catalyst loading of N-heterocyclic olefins showcases the use of this group of highly polarized alkenes as catalysts for organopolymerization. A strong structure-activity relationship is found, which is fundamentally different from the reactivity of N-heterocyclic carbenes (TON=turnover number, PDI=polydispersity index).
- Naumann, Stefan,Thomas, Anthony W.,Dove, Andrew P.
-
supporting information
p. 9550 - 9554
(2015/08/11)
-
- The influence of hydrogen-bond defects on the properties of ionic liquids
-
Counterintuitive: The preformation of ion pairs can explain the low melting points of imidazolium-based ionic liquids (ILs) and the resulting expanded range of working temperatures. This quasi-ion-pair formation is possible for ILs having cations with only one interaction site leading to local and directional hydrogen bonds with the corresponding anion (see structure; O red, N blue, F green, S yellow). Copyright
- Peppel, Tim,Roth, Christian,Fumino, Koichi,Paschek, Dietmar,Koeckerling, Martin,Ludwig, Ralf
-
supporting information; experimental part
p. 6661 - 6665
(2011/09/13)
-
- Process for the production of carboxylic acid anhydrides
-
The solubility and stability of rhodium catalysts in rhodium-catalysed carbonylation of alkyl esters or alkyl ethers under substantially anhydrous conditions to produce carboxylic acid anhydrides is improved by the use of co-promoters selected from the group: 1,3-dialkyl-4-methylimidazolium iodide; 1,3-dialkyl-4-ethylimidazolium iodide; 1,3-dialkyl-4-n-propylimidazolium iodide; 1,3-dialkyl-4-isopropylimidazolium iodide; 1,3-dialkyl-4-n-butylimidazolium iodide 1,3-dialkyl-4-sec-butylimidazolium iodide 1,3-dialkyl-4-tert-butylimidazolium iodide; 1,3-dialkyl-2,4,5-trimethylimidazolium iodide and mixtures thereof where the alkyl groups are independently C1 to C20 alkyl.
- -
-
-
- Synthesis and Reactions of 1,2,4,5-Tetramethylimidazole; the Crystal Structure of Pentamethylimidazolium Iodide
-
1,2,4,5-Tetramethylimidazole Derivatives, Synthesis, X-Ray1,2,4,5-Tetramethylimidazole (3) is obtained by metalation of 2,4,5-trimethylimidazole (2) with sodium in liquid ammonia and subsequent addition of methyl iodide. 3 reacts with Lewis acids to form the compounds Me4C3N2*BH3 (8) and Me4C3N2*E+ (4, 5, 7, 10; E = Me, H, SiMe3, C(O)Me).The X-ray structure reveals the ionic nature of pentamethylimidazolium iodide (4) in the solid state.
- Kuhn, Norbert,Henkel, Gerald,Kreutzberg, Joerg
-
p. 1706 - 1712
(2007/10/02)
-