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RESEARCH ARTICLE (Open Access)

Why does the synthesis of N-phenylbenzamide from benzenesulfinate and phenylisocyanate via the palladium-mediated Extrusion–Insertion pathway not work? A mechanistic exploration

Yang Yang A , Allan J. Canty B and Richard A. J. O’Hair https://orcid.org/0000-0002-8044-0502 A *
+ Author Affiliations
- Author Affiliations

A School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Vic. 3010, Australia.

B School of Natural Sciences - Chemistry, University of Tasmania, Private Bag 75, Hobart, Tas. 7001, Australia.

* Correspondence to: rohair@unimelb.edu.au

Handling Editor: Curt Wentrup

Australian Journal of Chemistry 76(1) 49-57 https://doi.org/10.1071/CH22209
Submitted: 27 September 2022  Accepted: 3 November 2022   Published: 16 December 2022

© 2022 The Author(s) (or their employer(s)). Published by CSIRO Publishing.

Abstract

The gas-phase extrusion–insertion (ExIn) reactions of the palladium complexes [(phen)nPd(O2SC6H5)]+ (phen = 1,10-phenanthroline, n = 1 or 2), were investigated in the gas phase by multistage mass spectrometry (MSn) experiments consisting of electrospray ionisation and a linear ion trap combined with density functional theory (DFT) calculations. Desulfination of palladium sulfinate cations under collision-induced dissociation (CID) generates the organopalladium intermediates [(phen)nPd(C6H5)]+. Of these two organometallic cations, only [(phen)Pd(C6H5)]+ reacts with phenyl isocyanate via insertion to yield [(phen)Pd(NPhC(O)C6H5)]+. The formation of a coordinated amidate anion is supported by DFT calculations. In exploring this reactivity in the solution phase, we found that heating a mixture of benzenesulfinic acid, phenylisocyanate and palladium trifluoroacetate under a range of different conditions (ligand free versus with ligand, different solvents, addition of acid or base) failed to lead to the formation N-phenyl-benzamide in all cases. Instead, biphenyl was formed and could be isolated in a yield of 46%. DFT calculations using a solvent continuum reveal that the barrier associated with the insertion reaction lies above the competing sequential reactions of desulfination of a second phenyl sulfinate followed by reductive elimination of biphenyl.

Keywords: biaryl coupling, desulfination, DFT calculations, extrusion, insertion, mass spectrometry, palladium mediated reactions, reaction mechanisms.


References

[1]  O Baudoin, New Approaches for Decarboxylative Biaryl Coupling. Angew Chem Int Ed 2007, 46, 1373.
         | New Approaches for Decarboxylative Biaryl Coupling.Crossref | GoogleScholarGoogle Scholar |

[2]  LJ Gooßen, K Gooßen, N Rodríguez, M Blanchot, C Linder, B Zimmermann, New catalytic transformations of carboxylic acids. Pure Appl Chem 2008, 80, 1725.
         | New catalytic transformations of carboxylic acids.Crossref | GoogleScholarGoogle Scholar |

[3]  LJ Gooßen, N Rodríguez, K Gooßen, Carboxylic acids as substrates in homogeneous catalysis. Angew Chem Int Ed 2008, 47, 3100.
         | Carboxylic acids as substrates in homogeneous catalysis.Crossref | GoogleScholarGoogle Scholar |

[4]  LJ Goossen, F Collet, K Goossen, Decarboxylative Coupling Reactions. Isr J Chem 2010, 50, 617.
         | Decarboxylative Coupling Reactions.Crossref | GoogleScholarGoogle Scholar |

[5]  JD Weaver, A Recio, AJ Grenning, JA Tunge, Transition Metal-Catalyzed Decarboxylative Allylation and Benzylation Reactions. Chem Rev 2011, 111, 1846.
         | Transition Metal-Catalyzed Decarboxylative Allylation and Benzylation Reactions.Crossref | GoogleScholarGoogle Scholar |

[6]  N Rodríguez, LJ Goossen, Decarboxylative coupling reactions: a modern strategy for C-C-bond formation. Chem Soc Rev 2011, 40, 5030.
         | Decarboxylative coupling reactions: a modern strategy for C-C-bond formation.Crossref | GoogleScholarGoogle Scholar |

[7]  J Cornella, I Larrosa, Decarboxylative Carbon-Carbon Bond-Forming Transformations of (Hetero)aromatic Carboxylic Acids. Synthesis 2012, 44, 653.
         | Decarboxylative Carbon-Carbon Bond-Forming Transformations of (Hetero)aromatic Carboxylic Acids.Crossref | GoogleScholarGoogle Scholar |

[8]  K Park, S Lee, Transition metal-catalyzed decarboxylative coupling reactions of alkynyl carboxylic acids. RSC Adv 2013, 3, 14165.
         | Transition metal-catalyzed decarboxylative coupling reactions of alkynyl carboxylic acids.Crossref | GoogleScholarGoogle Scholar |

[9]  XT Yin, WJ Li, BL Zhao, K Cheng, Research Progress on Silver-Catalyzed Decarboxylative Coupling Reaction. Chinese J Org Chem 2018, 38, 2879.
         | Research Progress on Silver-Catalyzed Decarboxylative Coupling Reaction.Crossref | GoogleScholarGoogle Scholar |

[10]  J Aziz, S Messaoudi, M Alami, A Hamze, Sulfinate derivatives: dual and versatile partners in organic synthesis. Org Biomol Chem 2014, 12, 9743.
         | Sulfinate derivatives: dual and versatile partners in organic synthesis.Crossref | GoogleScholarGoogle Scholar |

[11]  K Yuan, J-F Soulé, H Doucet, Functionalization of C–H Bonds via Metal-Catalyzed Desulfitative Coupling: An Alternative Tool for Access to Aryl- or Alkyl-Substituted (Hetero)arenes. ACS Catal 2015, 5, 978.
         | Functionalization of C–H Bonds via Metal-Catalyzed Desulfitative Coupling: An Alternative Tool for Access to Aryl- or Alkyl-Substituted (Hetero)arenes.Crossref | GoogleScholarGoogle Scholar |

[12]  DH Ortgies, A Hassanpour, F Chen, S Woo, P Forgione, Desulfination as an Emerging Strategy in Palladium-Catalyzed C–C Coupling Reactions. Eur J Org Chem 2016, 2016, 408.
         | Desulfination as an Emerging Strategy in Palladium-Catalyzed C–C Coupling Reactions.Crossref | GoogleScholarGoogle Scholar |

[13]  S Sun, J-T Yu, Y Jiang, J Cheng, Copper(I)-Catalyzed Desulfinative Carboxylation of Sodium Sulfinates using Carbon Dioxide. Adv Synth Catal 2015, 357, 2022.
         | Copper(I)-Catalyzed Desulfinative Carboxylation of Sodium Sulfinates using Carbon Dioxide.Crossref | GoogleScholarGoogle Scholar |

[14]  RAJ O’Hair, Dimethylargenate is a stable species in the gas phase. Chem Commun 2002, 38, 20.
         | Dimethylargenate is a stable species in the gas phase.Crossref | GoogleScholarGoogle Scholar |

[15]  PF James, RAJ O’Hair, Dimethyl cuprate undergoes C–C bond coupling with methyliodide in the gas phase but dimethyl argenate does not. Org Lett 2004, 6, 2761.
         | Dimethyl cuprate undergoes C–C bond coupling with methyliodide in the gas phase but dimethyl argenate does not.Crossref | GoogleScholarGoogle Scholar |

[16]  N Rijs, GN Khairallah, T Waters, RAJ O’Hair, Gas-phase synthesis of the homo and hetero organocuprate anions [MeCuMe]−, [EtCuEt]−, and [MeCuR]−. J Am Chem Soc 2008, 130, 1069.
         | Gas-phase synthesis of the homo and hetero organocuprate anions [MeCuMe], [EtCuEt], and [MeCuR].Crossref | GoogleScholarGoogle Scholar |

[17]  NJ Rijs, RAJ O’Hair, Gas-Phase Synthesis of Organoargenate Anions and Comparisons with Their Organocuprate Analogues. Organometallics 2009, 28, 2684.
         | Gas-Phase Synthesis of Organoargenate Anions and Comparisons with Their Organocuprate Analogues.Crossref | GoogleScholarGoogle Scholar |

[18]  NJ Rijs, GB Sanvido, GN Khairallah, RAJ O’Hair, Gas phase synthesis and reactivity of dimethylaurate. Dalton Trans 2010, 39, 8655.
         | Gas phase synthesis and reactivity of dimethylaurate.Crossref | GoogleScholarGoogle Scholar |

[19]  K Vikse, GN Khairallah, JS McIndoe, RAJ O’Hair, Fixed-charge phosphine ligands to explore gas-phase coinage metal-mediated decarboxylation reactions. Dalton Trans 2013, 42, 6440.
         | Fixed-charge phosphine ligands to explore gas-phase coinage metal-mediated decarboxylation reactions.Crossref | GoogleScholarGoogle Scholar |

[20]  MJ Woolley, GN Khairallah, G da Silva, PS Donnelly, BF Yates, RAJ O’Hair, Role of the Metal, Ligand, and Alkyl/Aryl Group in the Hydrolysis Reactions of Group 10 Organometallic Cations [(L)M(R)]+. Organometallics 2013, 32, 6931.
         | Role of the Metal, Ligand, and Alkyl/Aryl Group in the Hydrolysis Reactions of Group 10 Organometallic Cations [(L)M(R)]+.Crossref | GoogleScholarGoogle Scholar |

[21]  M Woolley, A Ariafard, GN Khairallah, KH Kwan, PS Donnelly, JM White, AJ Canty, BF Yates, RAJ O’Hair, Decarboxylative-Coupling of Allyl Acetate Catalyzed by Group 10 Organometallics, [(phen)M(CH3)]+. J Org Chem 2014, 79, 12056.
         | Decarboxylative-Coupling of Allyl Acetate Catalyzed by Group 10 Organometallics, [(phen)M(CH3)]+.Crossref | GoogleScholarGoogle Scholar |

[22]  RAJ O’Hair, NJ Rijs, Gas Phase Studies of the Pesci Decarboxylation Reaction: Synthesis, Structure, and Unimolecular and Bimolecular Reactivity of Organometallic Ions. Acc Chem Res 2015, 48, 329.
         | Gas Phase Studies of the Pesci Decarboxylation Reaction: Synthesis, Structure, and Unimolecular and Bimolecular Reactivity of Organometallic Ions.Crossref | GoogleScholarGoogle Scholar |

[23]  LO Sraj, GN Khairallah, G da Silva, RAJ O’Hair, Who Wins: Pesci, Peters, or Deacon? Intrinsic Reactivity Orders for Organocuprate Formation via Ligand Decomposition. Organometallics 2012, 31, 1801.
         | Who Wins: Pesci, Peters, or Deacon? Intrinsic Reactivity Orders for Organocuprate Formation via Ligand Decomposition.Crossref | GoogleScholarGoogle Scholar |

[24]  Z Wang, Y Yang, PS Donnelly, AJ Canty, RAJ O’Hair, Desulfination versus decarboxylation as a means of generating three- and five-coordinate organopalladium complexes [(phen)nPd(C6H5)]+ (n = 1 and 2) to study their fundamental bimolecular reactivity. J Organomet Chem 2019, 882, 42.
         | Desulfination versus decarboxylation as a means of generating three- and five-coordinate organopalladium complexes [(phen)nPd(C6H5)]+ (n = 1 and 2) to study their fundamental bimolecular reactivity.Crossref | GoogleScholarGoogle Scholar |

[25]  A Noor, JW Li, GN Khairallah, Z Li, H Ghari, AJ Canty, A Ariafard, PS Donnelly, RAJ O’Hair, A one-pot route to thioamides discovered by gas-phase studies: palladium-mediated CO2 extrusion followed by insertion of isothiocyanates. Chem Commun 2017, 53, 3854.
         | A one-pot route to thioamides discovered by gas-phase studies: palladium-mediated CO2 extrusion followed by insertion of isothiocyanates.Crossref | GoogleScholarGoogle Scholar |

[26]  Y Yang, A Noor, AJ Canty, A Ariafard, PS Donnelly, RAJ O’Hair, Synthesis of Amidines by Palladium-Mediated CO2 Extrusion Followed by Insertion of Carbodiimides: Translating Mechanistic Studies to Develop a One-Pot Method. Organometallics 2019, 38, 424.
         | Synthesis of Amidines by Palladium-Mediated CO2 Extrusion Followed by Insertion of Carbodiimides: Translating Mechanistic Studies to Develop a One-Pot Method.Crossref | GoogleScholarGoogle Scholar |

[27]  Y Yang, AJ Canty, AI McKay, PS Donnelly, RAJ O’Hair, Palladium-Mediated CO2 Extrusion Followed by Insertion of Isocyanates for the Synthesis of Benzamides: Translating Fundamental Mechanistic Studies To Develop a Catalytic Protocol. Organometallics 2020, 39, 453.
         | Palladium-Mediated CO2 Extrusion Followed by Insertion of Isocyanates for the Synthesis of Benzamides: Translating Fundamental Mechanistic Studies To Develop a Catalytic Protocol.Crossref | GoogleScholarGoogle Scholar |

[28]  Y Yang, B Spyrou, JM White, AJ Canty, PS Donnelly, RAJ O’Hair, Palladium-mediated CO2 Extrusion Followed by Insertion of Allenes: Translating Mechanistic Studies to Develop a One-Pot Method for the Synthesis of Alkenes. Organometallics 2022, 41, 1595.
         | Palladium-mediated CO2 Extrusion Followed by Insertion of Allenes: Translating Mechanistic Studies to Develop a One-Pot Method for the Synthesis of Alkenes.Crossref | GoogleScholarGoogle Scholar |

[29]  Y Yang, B Spyrou, PS Donnelly, AJ Canty, RAJ O’Hair, The role of silver carbonate as a catalyst in the synthesis of N-phenylbenzamide from benzoic acid and phenyl isocyanate: A mechanistic exploration. Aust J Chem 2022, 75, 495.
         | The role of silver carbonate as a catalyst in the synthesis of N-phenylbenzamide from benzoic acid and phenyl isocyanate: A mechanistic exploration.Crossref | GoogleScholarGoogle Scholar |

[30]  B Skillinghaug, C Sköld, J Rydfjord, F Svensson, M Behrends, J Sävmarker, PJR Sjöberg, M Larhed, Palladium(II)-Catalyzed Desulfitative Synthesis of Aryl Ketones from Sodium Arylsulfinates and Nitriles: Scope, Limitations, and Mechanistic Studies. J Org Chem 2014, 79, 12018.
         | Palladium(II)-Catalyzed Desulfitative Synthesis of Aryl Ketones from Sodium Arylsulfinates and Nitriles: Scope, Limitations, and Mechanistic Studies.Crossref | GoogleScholarGoogle Scholar |

[31]  M Behrends, J Sävmarker, PJR Sjöberg, M Larhed, Microwave-Assisted Palladium(II)-Catalyzed Synthesis of Aryl Ketones from Aryl Sulfinates and Direct ESI-MS Studies Thereof. ACS Catal 2011, 1, 1455.
         | Microwave-Assisted Palladium(II)-Catalyzed Synthesis of Aryl Ketones from Aryl Sulfinates and Direct ESI-MS Studies Thereof.Crossref | GoogleScholarGoogle Scholar |

[32]  K Garves, Coupling, Carbonylation, and Vinylation Reactions of Aromatic Sulfinic Acids via Organopalladium Intermediates. J Org Chem 1970, 35, 3273.
         | Coupling, Carbonylation, and Vinylation Reactions of Aromatic Sulfinic Acids via Organopalladium Intermediates.Crossref | GoogleScholarGoogle Scholar |

[33]  DH Ortgies, F Chen, P Forgione, Palladium and TEMPO as Co-Catalysts in a Desulfinative Homocoupling Reaction. Eur J Org Chem 2014, 2014, 3917.
         | Palladium and TEMPO as Co-Catalysts in a Desulfinative Homocoupling Reaction.Crossref | GoogleScholarGoogle Scholar |

[34]  B Rao, W Zhang, L Hu, M Luo, Catalytic desulfitative homocoupling of sodium arylsulfinates in water using PdCl2 as the recyclable catalyst and O2 as the terminal oxidant. Green Chem 2012, 14, 3436.
         | Catalytic desulfitative homocoupling of sodium arylsulfinates in water using PdCl2 as the recyclable catalyst and O2 as the terminal oxidant.Crossref | GoogleScholarGoogle Scholar |

[35]  A de Gombert, AI McKay, CJ Davis, KM Wheelhouse, MC Willis, Mechanistic Studies of the Palladium-Catalyzed Desulfinative Cross-Coupling of Aryl Bromides and (Hetero)Aryl Sulfinate Salts. J Am Chem Soc 2020, 142, 3564.
         | Mechanistic Studies of the Palladium-Catalyzed Desulfinative Cross-Coupling of Aryl Bromides and (Hetero)Aryl Sulfinate Salts.Crossref | GoogleScholarGoogle Scholar |

[36]  WA Donald, CJ McKenzie, RAJ O’Hair, C–H Bond Activation of Methanol and Ethanol by a High-Spin FeIVO Biomimetic Complex. Angew Chem Int Ed 2011, 50, 8379.
         | C–H Bond Activation of Methanol and Ethanol by a High-Spin FeIVO Biomimetic Complex.Crossref | GoogleScholarGoogle Scholar |

[37]  AKY Lam, C Li, G Khairallah, BB Kirk, SJ Blanksby, AJ Trevitt, U Wille, RAJ O’Hair, G da Silva, Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation. Phys Chem Chem Phys 2012, 14, 2417.
         | Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation.Crossref | GoogleScholarGoogle Scholar |

[38]  Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Petersson GA, Nakatsuji H, Li X, Caricato M, Marenich AV, Bloino J, Janesko BG, Gomperts R, Mennucci B, Hratchian HP, Ortiz JV, Izmaylov AF, Sonnenberg JL, Williams; Ding F, Lipparini F, Egidi F, Goings J, Peng B, Petrone A, Henderson T, Ranasinghe D, Zakrzewski VG, Gao J, Rega N, Zheng G, Liang W, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Throssell K, Montgomery Jr JA, Peralta JE, Ogliaro F, Bearpark MJ, Heyd JJ, Brothers EN, Kudin KN, Staroverov VN, Keith TA, Kobayashi R, Normand J, Raghavachari K, Rendell AP, Burant JC, Iyengar SS, Tomasi J, Cossi M, Millam JM, Klene M, Adamo C, Cammi R, Ochterski JW, Martin RL, Morokuma K, Farkas O, Foresman JB, Fox DJ. Gaussian 16 Rev. C.01. Wallingford, CT; 2016.

[39]  Y Zhao, DG Truhlar, The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor Chem Acc 2008, 120, 215.
         | The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals.Crossref | GoogleScholarGoogle Scholar |

[40]  M Dolg, U Wedig, H Stoll, H Preuss, Energy-Adjusted Ab initio Pseudopotentials for the first Row Transition Elements. J Chem Phys 1987, 86, 866.
         | Energy-Adjusted Ab initio Pseudopotentials for the first Row Transition Elements.Crossref | GoogleScholarGoogle Scholar |

[41]  D Andrae, U Häußermann, M Dolg, H Stoll, H Preuß, Energy-Adjusted Ab initio Pseudopotentials for the second and third row transition elements. Theor Chim Acta 1990, 77, 123.
         | Energy-Adjusted Ab initio Pseudopotentials for the second and third row transition elements.Crossref | GoogleScholarGoogle Scholar |

[42]  PC Harihara, JA Pople, The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theor Chim Acta 1973, 28, 213.
         | The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies.Crossref | GoogleScholarGoogle Scholar |

[43]  AW Ehlers, M Böhme, S Dapprich, A Gobbi, A Höllwarth, V Jonas, KF Köhler, R Stegmann, A Veldkamp, G Frenking, A set of f-polarization functions for pseudo-potential basis sets of the transition metals Sc, Cu, Y, Ag and La, Au. Chem Phys Lett 1993, 208, 111.
         | A set of f-polarization functions for pseudo-potential basis sets of the transition metals Sc, Cu, Y, Ag and La, Au.Crossref | GoogleScholarGoogle Scholar |

[44]  A Höllwarth, M Böhme, S Dapprich, AW Ehlers, A Gobbi, V Jonas, KF Köhler, R Stegmann, A Veldkamp, G Frenking, A set of d-polarization functions for pseudo-potential basis sets of the main group elements AlBi and f-type polarization functions for Zn, Cd, Hg. Chem Phys Lett 1993, 208, 237.
         | A set of d-polarization functions for pseudo-potential basis sets of the main group elements AlBi and f-type polarization functions for Zn, Cd, Hg.Crossref | GoogleScholarGoogle Scholar |

[45]  V Barone, M Cossi, Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model. J Phys Chem A 1998, 102, 1995.
         | Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model.Crossref | GoogleScholarGoogle Scholar |

[46]  K Fukui, Formulation of the Reaction Coordinate. J Phys Chem 1970, 74, 4161.
         | Formulation of the Reaction Coordinate.Crossref | GoogleScholarGoogle Scholar |

[47]  K Fukui, The Path of Chemical Reactions — the Irc Approach. Acc Chem Res 1981, 14, 363.
         | The Path of Chemical Reactions — the Irc Approach.Crossref | GoogleScholarGoogle Scholar |

[48]  AD Becke, Density-Functional Exchange-Energy Approximation with Correct Asymptotic-Behavior. Phys Rev A 1988, 38, 3098.
         | Density-Functional Exchange-Energy Approximation with Correct Asymptotic-Behavior.Crossref | GoogleScholarGoogle Scholar |

[49]  CT Lee, WT Yang, RG Parr, Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron-Density. Phys Rev B 1988, 37, 785.
         | Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron-Density.Crossref | GoogleScholarGoogle Scholar |

[50]  AD Becke, Density-Functional Thermochemistry. III. The Role of Exact Exchange. J Chem Phys 1993, 98, 5648.
         | Density-Functional Thermochemistry. III. The Role of Exact Exchange.Crossref | GoogleScholarGoogle Scholar |

[51]  PJ Stephens, FJ Devlin, CF Chabalowski, MJ Frisch, Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J Phys Chem 1994, 98, 11623.
         | Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields.Crossref | GoogleScholarGoogle Scholar |

[52]  F Weigend, F Furche, R Ahlrichs, Gaussian basis sets of quadruple zeta valence quality for atoms H–Kr. J Chem Phys 2003, 119, 12753.
         | Gaussian basis sets of quadruple zeta valence quality for atoms H–Kr.Crossref | GoogleScholarGoogle Scholar |

[53]  Y Okuno, Theoretical Investigation of the Mechanism of the Baeyer-Villiger Reaction in Nonpolar Solvents. Chem Eur J 1997, 3, 212.
         | Theoretical Investigation of the Mechanism of the Baeyer-Villiger Reaction in Nonpolar Solvents.Crossref | GoogleScholarGoogle Scholar |

[54]  JA Keith, EA Carter, Quantum Chemical Benchmarking, Validation, and Prediction of Acidity Constants for Substituted Pyridinium Ions and Pyridinyl Radicals. J Chem Theory Comput 2012, 8, 3187.
         | Quantum Chemical Benchmarking, Validation, and Prediction of Acidity Constants for Substituted Pyridinium Ions and Pyridinyl Radicals.Crossref | GoogleScholarGoogle Scholar |