N-Heterocyclic carbenes

Editor: Prof. Steven P. Nolan
King Saud University

N-heterocyclic carbenes now hold a preferred ligand status in organic and organometallic chemistry. Their role in catalysis continues to grow. The use of these ligands now extends to numerous fields spanning from fine chemicals to polymer synthesis. Main group chemistry has also benefited from these ligands as stabilizing entities. The action of state-of-the-art catalysts are nowadays better understood through detailed mechanistic work, which permits the design of ever-better performing catalytic systems. This Thematic Series highlights the diversity of fields that are affected and enhanced by N-heterocyclic carbenes.

N-Heterocyclic carbenes

Steven P. Nolan

Editorial
Published 07 Dec 2015

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Beilstein J. Org. Chem. 2015, 11, 2474–2475, doi:10.3762/bjoc.11.268

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Synthesis and structures of ruthenium–NHC complexes and their catalysis in hydrogen transfer reaction

Chao Chen,
Chunxin Lu,
Qing Zheng,
Shengliang Ni,
Min Zhang and
Wanzhi Chen

Full Research Paper
Published 30 Sep 2015

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1. Graphical Abstract
2. Scheme 1: Synthesis of complexes 1 and 2.
  
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3. Figure 1: Structural view of 1 showing 30% thermal ellipsoids. All hydrogen atoms and PF₆⁻ were omitted for c...
  
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4. Figure 2: Structural view of 2 showing 30% thermal ellipsoids. All hydrogen atoms and PF₆⁻ were omitted for c...
  
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5. Scheme 2: Synthesis of 3.
  
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6. Figure 3: Structural view of 3 showing 50% thermal ellipsoids. All hydrogen atoms and PF₆⁻ were omitted for c...
  
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7. Scheme 3: Synthesis of complexes 4 and 5.
  
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8. Figure 4: Structural view of 4 showing 30% thermal ellipsoids. All hydrogen atoms and PF₆⁻ were omitted for c...
  
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9. Figure 5: Structural view of 5 showing 30% thermal ellipsoids. All hydrogen atoms and PF₆⁻ were omitted for c...
  
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Beilstein J. Org. Chem. 2015, 11, 1786–1795, doi:10.3762/bjoc.11.194

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Influence of bulky yet flexible N-heterocyclic carbene ligands in gold catalysis

Alba Collado,
Scott R. Patrick,
Danila Gasperini,
Sebastien Meiries and
Steven P. Nolan

Full Research Paper
Published 02 Oct 2015

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1. Graphical Abstract
2. Figure 1: ‘ITent’ family of ligands, including IPr. First row: percentage buried volume (% V_bur) calculated i...
  
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3. Scheme 1: Synthesis of gold complexes bearing the ITent ligands.
  
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4. Scheme 2: Silver-free synthesis of [Au(ITent)(NTf₂)] complexes.
  
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Beilstein J. Org. Chem. 2015, 11, 1809–1814, doi:10.3762/bjoc.11.196

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Half-sandwich nickel(II) complexes bearing 1,3-di(cycloalkyl)imidazol-2-ylidene ligands

Johnathon Yau,
Kaarel E. Hunt,
Laura McDougall,
Alan R. Kennedy and
David J. Nelson

Full Research Paper
Published 12 Nov 2015

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1. Graphical Abstract
2. Scheme 1: Synthesis of [Ni(η¹-Cp)(η⁵-Cp)(IMes)] (1) and [NiCl(Cp)(IMes)] (2).
  
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3. Scheme 2: Synthesis of [NiCl(Cp)(ICy)] using conventional heating.
  
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4. Scheme 3: Synthesis of (a) [NiCl(Cp)(ICy)] (3) and (b) [NiCl(Cp)(IDD)] (4) with microwave heating.
  
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5. Figure 1: Molecular structures of complexes [NiCl(Cp)(ICy)] (3) (left) and [NiCl(Cp)(IDD)] (4) (right) as det...
  
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6. Scheme 4: Model Suzuki–Miyaura reaction for the evaluation of the new complexes as cross-coupling pre-catalys...
  
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Beilstein J. Org. Chem. 2015, 11, 2171–2178, doi:10.3762/bjoc.11.235

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Evidencing an inner-sphere mechanism for NHC-Au(I)-catalyzed carbene-transfer reactions from ethyl diazoacetate

Manuel R. Fructos,
Juan Urbano,
M. Mar Díaz-Requejo and
Pedro J. Pérez

Full Research Paper
Published 20 Nov 2015

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1. Graphical Abstract
2. Scheme 1: The Au(I)-catalyzed skeletal rearrangement of the [2 + 2] cycloaddition of 1,6-enynes that involves...
  
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3. Scheme 2: The catalytic activity of IPrAuCl + NaBAr^F₄ in the carbene-transfer reaction to styrene or methanol....
  
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4. Scheme 3: The gold-promoted decarbenation reaction described by Echavarren and co-workers.
  
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5. Scheme 4: (a) General representation of the metal-catalyzed carbene-transfer reaction (olefin cyclopropanatio...
  
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6. Figure 1: Plot of evolved nitrogen with time for the reactions of EDA with styrene or methanol.
  
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7. Figure 2: Top: Plots of evolved nitrogen with time for the reactions of EDA with styrene (left) or methanol (...
  
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8. Scheme 5: The outer- and inner-sphere routes for this transformation.
  
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9. Figure 3: The experimental device for the measurement of N₂ evolution.
  
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Beilstein J. Org. Chem. 2015, 11, 2254–2260, doi:10.3762/bjoc.11.245

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Convenient preparation of high molecular weight poly(dimethylsiloxane) using thermally latent NHC-catalysis: a structure-activity correlation

Stefan Naumann,
Johannes Klein,
Dongren Wang and
Michael R. Buchmeiser

Full Research Paper
Published 20 Nov 2015

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1. Graphical Abstract
2. Scheme 1: NHC-carboxylates part of this study (top) and polymerization scheme with initial thermal decarboxyl...
  
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3. Figure 1: Comparison of conversion over time for D₄ polymerization (80 °C, bulk) using 5_Me-Me-CO₂. Note that ...
  
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4. Scheme 2: Discussed mechanisms proposed to operate in NHC-mediated polymerization of D₄ in presence/absence o...
  
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5. Figure 2: Thermal activation of a 5_Me-Me-CO₂/BnOH/D₄ (1:5:500) composition after a latency period of 72 h.
  
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Beilstein J. Org. Chem. 2015, 11, 2261–2266, doi:10.3762/bjoc.11.246

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Efficient synthetic protocols for the preparation of common N-heterocyclic carbene precursors

Morgan Hans,
Jan Lorkowski,
Albert Demonceau and
Lionel Delaude

Full Research Paper
Published 25 Nov 2015

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1. Graphical Abstract
2. Scheme 1: Various synthetic paths leading to the formation of NHCs.
  
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3. Scheme 2: Retrosynthetic path for the preparation of symmetrical imidazolium and imidazolinium salts from sim...
  
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4. Figure 1: Structures of the imidazolium and imidazolinium salts discussed in this study and their acronyms.
  
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5. Scheme 3: Synthesis of 1,3-dicyclohexylimidazolium tetrafluoroborate (ICy·HBF₄).
  
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6. Scheme 4: Synthesis of 1,3-dibenzylimidazolium tetrafluoroborate (IBn·HBF₄).
  
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7. Scheme 5: Synthesis of 1,3-dimesitylimidazolium salts (IMes·HCl and IMes·HBF₄).
  
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8. Scheme 6: Synthesis of 1,3-dimesitylimidazolinium chloride (SIMes·HCl).
  
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9. Scheme 7: Synthesis of 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IDip·HCl).
  
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10. Scheme 8: Synthesis of 1,3-bis(2,6-diisopropylphenyl)imidazolinium chloride (SIDip·HCl).
  
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11. Scheme 9: Synthesis of 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazolium chloride (IDip*·HCl).
  
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Beilstein J. Org. Chem. 2015, 11, 2318–2325, doi:10.3762/bjoc.11.252

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Comparison of the catalytic activity for the Suzuki–Miyaura reaction of (η⁵-Cp)Pd(IPr)Cl with (η³-cinnamyl)Pd(IPr)(Cl) and (η³-1-t-Bu-indenyl)Pd(IPr)(Cl)

Patrick R. Melvin,
Nilay Hazari,
Hannah M. C. Lant,
Ian L. Peczak and
Hemali P. Shah

Full Research Paper
Published 08 Dec 2015

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1. Graphical Abstract
2. Figure 1: General depictions of allyl and related precatalysts that are highly active for the Suzuki–Miyaura ...
  
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3. Scheme 1: Synthesis of Cp.
  
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4. Figure 2: Comparison of catalytic performance of Cin, Cp and ^tBuInd for a series of Suzuki–Miyaura reactions ...
  
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5. Figure 3: Comparison of catalytic performance of Cin, Cp and ^tBuInd for a series of Suzuki–Miyaura reactions ...
  
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6. Scheme 2: Proposed mechanism for the activation of Cp to monoligated Pd(0).
  
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7. Scheme 3: Synthesis of Cp^Dim.
  
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8. Figure 4: ORTEP of Cp^Dim at 30% probability. Hydrogen atoms and isopropyl groups of IPr are omitted for clari...
  
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9. Scheme 4: Observation of Cp^Dim under modified catalytic conditions.
  
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10. Scheme 5: Cp^Dim is not an active precatalyst for a Suzuki–Miyaura reaction at room temperature.
  
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11. Scheme 6: Disproportionation of Cp^Dim with dvds.
  
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12. Scheme 7: a) Crossover experiment between Cp and (μ-allyl)(μ-Cl)Pd₂(IPr)₂. b) Crossover experiment expressed ...
  
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Beilstein J. Org. Chem. 2015, 11, 2476–2486, doi:10.3762/bjoc.11.269

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Rhodium, iridium and nickel complexes with a 1,3,5-triphenylbenzene tris-MIC ligand. Study of the electronic properties and catalytic activities

Carmen Mejuto,
Beatriz Royo,
Gregorio Guisado-Barrios and
Eduardo Peris

Full Research Paper
Published 14 Dec 2015

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1. Graphical Abstract
2. Scheme 1: Schematic representation of ligands A and B.
  
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3. Scheme 2: Synthesis of rhodium(I), iridium(I), and nickel(II) complexes of ligand B.
  
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4. Scheme 3: Tolman electronic parameters (TEP) for A, B and their related monocarbenes.
  
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5. Figure 1: CV plots of complexes 2 (a), and 3 (b). Experiments were carried out using 1 mM solutions of the co...
  
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6. Figure 2: CV plot (a) and relevant DPV section (b) of complex 4. Experiments were carried out using 1 mM solu...
  
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7. Scheme 4: Schematic representation of complex 6.
  
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Beilstein J. Org. Chem. 2015, 11, 2584–2590, doi:10.3762/bjoc.11.278

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Direct estimate of the internal π-donation to the carbene centre within N-heterocyclic carbenes and related molecules

Diego M. Andrada,
Nicole Holzmann,
Thomas Hamadi and
Gernot Frenking

Full Research Paper
Published 24 Dec 2015

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1. Graphical Abstract
2. Scheme 1: Schematic view of the calculated carbenes 1–15.
  
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3. Figure 1: Optimized geometries of carbenes 1–15 at the BP86/def2-TZVPP level of theory. Bond lengths and angl...
  
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4. Figure 2: Frontier orbitals (BP86/def2-TZVPP) and eigenvalues (in eV) of the carbenes 1–15. The isosurfaces w...
  
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5. Scheme 2: Schematic view of the major orbital interactions between a carbon atom in the ³P electronic ground ...
  
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6. Figure 3: Plot of deformation densities ∆ρ of the pairwise orbital interactions between C(³P) and N(Me)HC=CHN...
  
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7. Figure 4: Plot of the ΔE_π values against NBO p_π occupation for the NHC family 1–15.
  
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Beilstein J. Org. Chem. 2015, 11, 2727–2736, doi:10.3762/bjoc.11.294

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New metathesis catalyst bearing chromanyl moieties at the N-heterocyclic carbene ligand

Agnieszka Hryniewicka,
Szymon Suchodolski,
Agnieszka Wojtkielewicz,
Jacek W. Morzycki and
Stanisław Witkowski

Full Research Paper
Published 30 Dec 2015

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1. Graphical Abstract
2. Figure 1: Examples of olefin metathesis ruthenium catalysts.
  
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3. Figure 2: Selected ruthenium metathesis catalyst bearing chromanyl moieties.
  
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4. Scheme 1: Synthesis of the new NHC precursor. Reagents and conditions: a) HNO₃, CH₂Cl₂, 0 °C, 58%; b) HOCH₂SO...
  
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5. Scheme 2: CM with electron-deficient olefin.
  
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6. Scheme 3: Possible products of metathesis reaction between diene and alkene.
  
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7. Figure 3: π-Complex and rutenacyclobutane intermediate with a five-membered ring chelate.
  
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8. Scheme 4: CM reaction of β-carotene and retinyl acetate with ethyl (2E,4Z/E)-3-methylhexa-2,4-dienoate. React...
  
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9. Figure 4: Numbering of carbon atoms in the chromanyl moiety.
  
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Beilstein J. Org. Chem. 2015, 11, 2795–2804, doi:10.3762/bjoc.11.300

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Effective immobilisation of a metathesis catalyst bearing an ammonium-tagged NHC ligand on various solid supports

Krzysztof Skowerski,
Jacek Białecki,
Stefan J. Czarnocki,
Karolina Żukowska and
Karol Grela

Full Research Paper
Published 05 Jan 2016

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1. Graphical Abstract
2. Figure 1: Selected classical and heterogeneous ruthenium complexes.
  
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3. Figure 2: Applications of NHC ammonium-tagged catalysts.
  
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4. Scheme 1: Synthesis of ammonium-tagged complex 8.
  
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5. Scheme 2: Model RCM reaction.
  
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6. Figure 3: Influence of temperature and concentration on RCM of 9. Conditions: 1 mol % of 8-C* (5 wt % on C*),...
  
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7. Figure 4: Presentation of various Ru-based catalysts. From the left: 20 mg of Gre-II powder, 20 mg of 8 as fi...
  
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8. Figure 5: Influence of the support type on the metathesis outcome. Conditions: 1 mol % 8, toluene 80 °C; [9] ...
  
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9. Figure 6: Filtration of the reaction mixture after RCM of 9 catalysed by 1 mol % of 8-powder.
  
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10. Figure 7: Split test during RCM of 9 (1 mol % cat, toluene 80 °C, [9] = 0.2 M). The reaction mixtures were fi...
  
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11. Scheme 3: Model metathesis reactions used in tests.
  
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12. Figure 8: RCM of 9 catalysed by 8 and 8-Fe. Conditions: 1 mol % catalyst, toluene 80 °C, [9] = 0.2 M.
  
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13. Figure 9: Removal of 8-Fe and subsequent recovery of 8. A: stirred reaction mixture containing 8-Fe, B: the s...
  
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14. Scheme 4: Supported catalyst 8 in sequential cross metathesis and reduction.
  
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Beilstein J. Org. Chem. 2016, 12, 5–15, doi:10.3762/bjoc.12.2

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N-Methylphthalimide-substituted benzimidazolium salts and PEPPSI Pd–NHC complexes: synthesis, characterization and catalytic activity in carbon–carbon bond-forming reactions

Senem Akkoç,
Yetkin Gök,
İlhan Özer İlhan and
Veysel Kayser

Full Research Paper
Published 15 Jan 2016

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1. Graphical Abstract
2. Scheme 1: Synthesis of benzimidazolium salts and their PEPPSI Pd–NHC complexes.
  
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3. Figure 1: (A) UV–vis absorbance spectra were taken in DMSO. (B) The second derivative of the compound 5 calcu...
  
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Beilstein J. Org. Chem. 2016, 12, 81–88, doi:10.3762/bjoc.12.9

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Versatile deprotonated NHC: C,N-bridged dinuclear iridium and rhodium complexes

Albert Poater

Full Research Paper
Published 22 Jan 2016

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1. Graphical Abstract
2. Scheme 1: Equilibrium between the monoiridum complex bearing a C-bound anionic imidazolide and its correspond...
  
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3. Scheme 2: Experimental routes to the “equilibrium” between 3_H_-_H and 3_H_-_T.
  
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4. Figure 1: View of the molecular structure of a) 3_H-H and b) 3_H-T. Hydrogen atoms have been omitted for the sa...
  
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5. Figure 2: Steric maps for the NHC ligand of 3_H_-_H, coordinated to iridium by a) carbene or b) nitrogen. The is...
  
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6. Scheme 3: Equilibrium between complexes 3–6, in the presence of CO, PMe₃, and MeI.
  
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7. Figure 3: View of the molecular structure of a) 4_H_-_H and b) 4_H_-_T (main distances in Å).
  
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8. Figure 4: View of the molecular structure of 6. Hydrogen atoms have been omitted for the sake of clarity (mai...
  
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Beilstein J. Org. Chem. 2016, 12, 117–124, doi:10.3762/bjoc.12.13

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Simple activation by acid of latent Ru-NHC-based metathesis initiators bearing 8-quinolinolate co-ligands

Julia Wappel,
Roland C. Fischer,
Luigi Cavallo,
Christian Slugovc and
Albert Poater

Full Research Paper
Published 28 Jan 2016

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1. Graphical Abstract
2. Scheme 1: Synthesis of 1-4; only the isolated and characterized complexes are shown.
  
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3. Figure 1: Solid state structure of complexes 2a and 2b as retrieved from single crystal X-ray diffraction.
  
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4. Figure 2: Time/conversion plot for the polymerization of 5 by preinitiators 1–4 in the presence of HCl ([5]:[...
  
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5. Figure 3: ¹H NMR spectrum in the low-field region of the active species for complexes 4 and M32.
  
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6. Scheme 2: Energetics of 2a and 2b protonation in kcal/mol.
  
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7. Figure 4: Reaction pathway of the transformation of 2b to HovII (energies in kcal/mol; main distances in Å).
  
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8. Figure 5: DTA-TGA measurements for polymerizations of DCPD with catalysts 1b and 2b; Reaction conditions: [ca...
  
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Beilstein J. Org. Chem. 2016, 12, 154–165, doi:10.3762/bjoc.12.17

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Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions

Rajeev S. Menon,
Akkattu T. Biju and
Vijay Nair

Review
Published 09 Mar 2016

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1. Graphical Abstract
2. Scheme 1: Breslow’s proposal on the mechanism of the benzoin condensation.
  
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3. Scheme 2: Imidazolium carbene-catalysed homo-benzoin condensation.
  
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4. Scheme 3: Homo-benzoin condensation in aqueous medium.
  
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5. Scheme 4: Homobenzoin condensation catalysed by bis(benzimidazolium) salt 8.
  
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6. Scheme 5: List of assorted chiral NHC-catalysts used for asymmetric homobenzoin condensation.
  
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7. Scheme 6: A rigid bicyclic triazole precatalyst 15 in an efficient enantioselective benzoin reaction.
  
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8. Scheme 7: Inoue’s report of cross-benzoin reactions.
  
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9. Scheme 8: Cross-benzoin reactions catalysed by thiazolium salt 17.
  
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10. Scheme 9: Catalyst-controlled divergence in cross-benzoin reactions.
  
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11. Scheme 10: Chemoselective cross-benzoin reactions catalysed by a bulky NHC.
  
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12. Scheme 11: Selective intermolecular cross-benzoin condensation reactions of aromatic and aliphatic aldehydes.
  
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13. Scheme 12: Chemoselective cross-benzoin reaction of aliphatic and aromatic aldehydes.
  
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14. Scheme 13: Cross-benzoin reactions of trifluoromethyl ketones developed by Enders.
  
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15. Scheme 14: Cross-benzoin reactions of aldehydes and α-ketoesters.
  
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16. Scheme 15: Enantioselective cross-benzoin reactions of aliphatic aldehydes and α-ketoesters.
  
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17. Scheme 16: Dynamic kinetic resolution of β-halo-α-ketoesters via cross-benzoin reaction.
  
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18. Scheme 17: Enantioselective benzoin reaction of aldehydes and alkynones.
  
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19. Scheme 18: Aza-benzoin reaction of aldehydes and acylimines.
  
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20. Scheme 19: NHC-catalysed diastereoselective synthesis of cis-2-amino 3-hydroxyindanones.
  
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21. Scheme 20: Cross-aza-benzoin reactions of aldehydes with aromatic imines.
  
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22. Scheme 21: Enantioselective cross aza-benzoin reaction of aliphatic aldehydes with N-Boc-imines.
  
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23. Scheme 22: Chemoselective cross aza-benzoin reaction of aldehydes with N-PMP-imino esters.
  
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24. Scheme 23: NHC-catalysed coupling reaction of acylsilanes with imines.
  
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25. Scheme 24: Thiazolium salt-mediated enantioselective cross-aza-benzoin reaction.
  
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26. Scheme 25: Aza-benzoin reaction of enals with activated ketimines.
  
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27. Scheme 26: Isatin derived ketimines as electrophiles in cross aza-benzoin reaction with enals.
  
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28. Scheme 27: Aza-benzoin reaction of aldehydes and phosphinoylimines catalysed by the BAC-carbene.
  
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29. Scheme 28: Nitrosoarenes as the electrophilic component in benzoin-initiated cascade reaction.
  
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30. Scheme 29: One-pot synthesis of hydroxamic esters via aza-benzoin reaction.
  
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31. Scheme 30: Cookson and Lane’s report of intramolecular benzoin condensation.
  
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32. Scheme 31: Intramolecular cross-benzoin condensation between aldehyde and ketone moieties.
  
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33. Scheme 32: Intramolecular crossed aldehyde-ketone benzoin reactions.
  
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34. Scheme 33: Enantioselective intramolecular crossed aldehyde-ketone benzoin reaction.
  
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35. Scheme 34: Chromanone synthesis via enantioselective intramolecular cross-benzoin reaction.
  
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36. Scheme 35: Intramolecular cross-benzoin reaction of chalcones.
  
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37. Scheme 36: Synthesis of bicyclic tertiary alcohols by intramolecular benzoin reaction.
  
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38. Scheme 37: A multicatalytic Michael–benzoin cascade process for cyclopentanone synthesis.
  
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39. Scheme 38: Enamine-NHC dual-catalytic, Michael–benzoin cascade reaction.
  
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40. Scheme 39: Iminium-cross-benzoin cascade reaction of enals and β-oxo sulfones.
  
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41. Scheme 40: Intramolecular benzoin condensation of carbohydrate-derived dialdehydes.
  
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42. Scheme 41: Enantioselective intramolecular benzoin reactions of N-tethered keto-aldehydes.
  
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43. Scheme 42: Asymmetric cross-benzoin reactions promoted by camphor-derived catalysts.
  
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44. Scheme 43: NHC-Brønsted base co-catalysis in a benzoin–Michael–Michael cascade.
  
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45. Scheme 44: Divergent catalytic dimerization of 2-formylcinnamates.
  
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46. Scheme 45: One-pot, multicatalytic asymmetric synthesis of tetrahydrocarbazole derivatives.
  
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47. Scheme 46: NHC-chiral secondary amine co-catalysis for the synthesis of complex spirocyclic scaffolds.
  
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Correction

Graphical Abstract

Beilstein J. Org. Chem. 2016, 12, 444–461, doi:10.3762/bjoc.12.47

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Reactivity studies of pincer bis-protic N-heterocyclic carbene complexes of platinum and palladium under basic conditions

David C. Marelius,
Curtis E. Moore,
Arnold L. Rheingold and
Douglas B. Grotjahn

Full Research Paper
Published 28 Jun 2016

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1. Graphical Abstract
2. Figure 1: Previously reported PNHC complexes of interest for this work, along with the targeted complex 5.
  
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3. Figure 2: Crystal structure of 6-Pd. The atoms of the tert-butyl groups, C–H bonds, and the solvent have been...
  
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4. Scheme 1: Formation of dimers 6-Pd and 6-Pt by addition of NaOt-Bu.
  
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5. Scheme 2: Formation of 7 and 8 by addition of LiN(iPr)₂ (1 or 2 equiv) (R = tert-butyl).
  
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Beilstein J. Org. Chem. 2016, 12, 1334–1339, doi:10.3762/bjoc.12.126

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