- Stable Singlet Carbenes as Organic Superbases
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A simple experimental procedure for scaling carbene Br?nsted 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.
- Bertrand, Guy,Grotjahn, Douglas B.,Jazzar, Rodolphe,Junor, Glen P.,Vermersch, Fran?ois,Yazdani, Sima
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p. 27253 - 27257
(2021/11/22)
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- NHC-copper-thiophene-2-carboxylate complex for the hydroboration of terminal alkynes
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An air-stable N-heterocyclic carbene-copper thiophene-2-carboxylate (CuTC) complex has been prepared for the stereoselective hydroboration of terminal alkynes using pinacolborane (HBpin) or 1,8-naphthalenediaminatoborane (HBdan). The newly synthesized complex can be directly activated by hydroboranes without a cocatalyst such as a base, and exhibits high reactivity for the hydroboration of alkynes under mild conditions. A gram-scale hydroboration of terminal phenylacetylene demonstrated the applicability of the copper complex for the preparation of alkenyl boronates.
- Jang, Won Jun,Kang, Byung-Nam,Lee, Ji Hun,Choi, Yoon Mi,Kim, Chong-Hyeak,Yun, Jaesook
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supporting information
p. 5249 - 5252
(2019/06/07)
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- A closer look at the reactivity between N-heterocyclic carbenes and fluoroalkenes
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The fundamental reactivity leading to N-heterocyclic fluoroalkene adducts is explored in detail, featuring a total of 15 N-heterocyclic carbenes (NHCs) with various electronic and steric environments. The activity of these carbenes towards tetrafluoroethylene (TFE), hexafluoropropene (HFP), trifluoroethylene (HTFE) and vinylidene fluoride (VDF) is assessed in THF and toluene. Attempts were made to correlate the observed reactivity with electronic (Tolman Electronic Parameters) and steric (% buried volume) parameters unique to each NHC, but a trend has yet to be fully determined. However, the unique steric constraints of a cyclic (alkyl)(amino)carbene (CAAC) were shown to modify the initial point of nucleophilic attack on HTFE, providing selective transformation to a different adduct than has been observed to date with all reactions involving this fluoroalkene.
- Leclerc, Matthew C.,Da Gama, Jason G.,Gabidullin, Bulat M.,Baker, R. Tom
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- Selective Activation of Fluoroalkenes with N-Heterocyclic Carbenes: Synthesis of N-Heterocyclic Fluoroalkenes and Polyfluoroalkenyl Imidazolium Salts
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Selective reactions between nucleophilic N,N′-diaryl-heterocyclic carbenes (NHCs) and electrophilic fluorinated alkenes afford NHC fluoroalkenes in high yields. These stable compounds undergo efficient and selective fluoride abstraction with Lewis acids to give polyfluoroalkenyl imidazolium salts. These salts react at Cβ with pyrrolidine to give ammonium fluoride-substituted salts, which give rise to conjugated imidazolium-enamine salts through loss of HF. Alternatively, reaction with 4-(dimethylamino)-pyridine provides a Cα-pyridinium-substituted NHC fluoroalkene. These compounds were studied using multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. Insight into their electronic structure and reactivity was gained through the use of DFT calculations. The unusual suspects: N,N′-Diaryl-heterocyclic carbenes react with fluorinated alkenes to afford N-heterocyclic fluoroalkenes in high yields. Efficient and selective fluoride abstraction with Lewis acids affords the first examples of polyfluoralkenyl imidazolium salts. These salts react with pyrrolidine and DMAP through different pathways to provide unique compounds.
- Leclerc, Matthew C.,Gorelsky, Serge I.,Gabidullin, Bulat M.,Korobkov, Ilia,Baker, R. Tom
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supporting information
p. 8063 - 8067
(2016/06/14)
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- Expanded ring diaminocarbene palladium complexes: Synthesis, structure, and Suzuki-Miyaura cross-coupling of heteroaryl chlorides in water
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A series of new 6- and 7-membered N-heterocyclic carbene (NHC) complexes of palladium (NHC)Pd(cinn)Cl (cinn = cinnamyl = 3-phenylallyl) were synthesized and characterized structurally in the solid state. The influence of ring size (5, 6 or 7) and bulkiness of N-aryl substituents (Mes = 2,4,6-trimethylphenyl, or Dipp = 2,6-diisopropylphenyl) in carbenes on palladium catalysed Suzuki-Miyaura cross-coupling was revealed. Due to the unique stereoelectronic properties of expanded ring NHCs, a versatile, highly efficient green protocol of coupling of heteroaromatic chlorides and bromides with boronic acids has been developed. High quantitative yields of biaryls were achieved with water as solvent, under air, using low catalyst and phase transfer agent loadings, and with mild and environmentally benign base NaHCO3. The Royal Society of Chemistry 2013.
- Kolychev, Eugene L.,Asachenko, Andrey F.,Dzhevakov, Pavel B.,Bush, Alexander A.,Shuntikov, Viacheslav V.,Khrustalev, Victor N.,Nechaev, Mikhail S.
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p. 6859 - 6866
(2013/08/25)
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- Chromium-silicon multiple bonds: The chemistry of terminal N-heterocyclic-carbene-stabilized halosilylidyne ligands
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An efficient method for the synthesis of the first N-heterocyclic carbene (NHC)-stabilized halosilylidyne complexes is reported that starts from SiBr 4. In the first step, SiBr4 was treated with one equivalent of the N-heterocyclic c
- Filippou, Alexander C.,Chernov, Oleg,Schnakenburg, Gregor
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p. 13574 - 13583
(2012/01/05)
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- Imidazolylidenes, imidazolinylidenes and imidazolidines
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Starting from glyoxal, 1,3-diarylimidazolinium chlorides 3 were obtained in a three-step sequence via the diimines (1) and ethylene diamine dihydrochlorides (2). Reduction of 1,3-diarylimidazolinium chlorides (3) with lithium alumnium hydride furnished the 1,3- diarylimidazolidines (4) while their deprotonation with potassium hydride in thf gave access to stable carbenes (1,3-diarylimidazolin-2-ylidenes, 5). Similarly substituted imidazol-2-ylidenes are described for comparison.
- Arduengo III, Anthony J.,Krafczyk, Roland,Schmutzler, Reinhard,Craig, Hugh A.,Goerlich, Jens R.,Marshall, William J.,Unverzagt, Markus
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p. 14523 - 14534
(2007/10/03)
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