- 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?
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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
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p. 2974 - 2985
(2021/02/06)
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- N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide)
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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.
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supporting information
p. 9550 - 9554
(2015/08/11)
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- Push-pull alkenes by reacting N,N′-dimethyl cyclic ketene N,N′-acetals with isocyanates: synthesis, structures, and reactivities
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N,N′-Dimethyl cyclic ketene N,N′-acetals react with two or three equivalents of isocyanates to generate tetrasubstituted push-pull alkene derivatives in one-pot sequential reactions. X-ray crystallography showed significant elongations and out of plane distorsions of the polarized carbon-carbon double bonds.
- Ye, Guozhong,Henry, William P.,Chen, Chunlong,Zhou, Aihua,Pittman Jr., Charles U.
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supporting information; experimental part
p. 2135 - 2139
(2009/07/26)
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- 2-Alkylidenimidazolidine - Synthesis, Basicity, 1H- and 13C-NMR Spectra
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4,5-Dihydroimidazolium salts 2, 3, and 4 were synthesized by carefully directed alkylation of 4,5-dihydroimidazoles 7 and 8. 2, 3, and 4 were deprotonated by sodium hydride to yield 2-alkylideneimidazolidines 1. 1H and 13C-NMR spectra allow to assign the
- Gruseck, Ursula,Heuschmann, Manfred
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p. 2053 - 2064
(2007/10/02)
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