Organometallic Chemistry of N-heterocyclic Carbenes

HAN VINH HUYNH Department of Chemistry, National University of Singapore, Republic of Singapore

• Foreword ixPreface xList of Abbreviations and Definitions xii1 General Introduction 11.1 Definition of Carbenes 11.2 Historical Overview of Carbenes, N‐Heterocyclic Carbenes, and Their Complexes 31.2.1 The Quest for Free Stable Carbenes 31.2.2 The Quest for Carbene Complexes 11References 152 General Properties of Classical NHCs and Their Complexes 172.1 Stabilization in NHCs (Push‐Pull Effect) 192.2 Backbone Differences and Their Implications 202.3 Dimerization of Carbenes 222.4 Nomenclature of N‐Heterocyclic Carbenes 252.5 Electronic Properties of NHCs and Different Electronic Parameters 292.5.1 Tolman’s Electronic Parameter (TEP) and Related Carbonyl Based Systems 292.5.2 Lever’s Electronic Parameter (LEP) 362.5.3 Huynh’s Electronic Parameter (HEP) 382.6 Steric Properties of NHCs 422.7 Structural Diversity of NHC Ligands and Their Complexes 452.7.1 Donor‐Functionalized NHCs 452.7.2 Multidentate NHCs 462.7.3 Pincer‐Type NHC Ligands 472.7.4 Tripodal and Macrocyclic Ligands 48References 493 Synthetic Aspects 523.1 General Routes to Azolium Salts as NHC Precursors 523.1.1 N‐Alkylation of Neutral Azoles 523.1.2 Multicomponent Condensation Reactions 553.1.3 Cyclization of Diamines 583.1.4 Cyclization of Formamidines 643.2 General Routes to Free NHCs 663.2.1 NHCs via Deprotonation of Azolium Salts 683.2.2 NHCs via Reduction of Thiones 713.2.3 NHCs via α‐Elimination of Small Molecules 733.3 General Synthetic Routes to NHC Complexes 743.3.1 Coordination of Free NHCs 763.3.2 Cleavage of Electron‐Rich Entetramines by Transition Metals 783.3.3 In Situ Deprotonation of Azolium Salts in the Presence of Transition Metals 783.3.4 Carbene Transfer Routes 813.3.5 Oxidative Addition of an Azolium Salt to a Low‐Valent Metal Complex 833.3.6 Metal‐Template Synthesis Using Isocyanide Complexes as Precursors 853.3.7 NHC Complexes by Small Molecule Elimination 893.3.8 NHC Complexes by Protonation/Alkylation of Azolyl Complexes 93References 954 Group 10 Metal(0)‐NHC Complexes 994.1 Nickel(0)‐NHC Complexes 994.1.1 Reactions of Enetetramines and Free NHCs 994.1.2 Reduction of Nickel(II)‐NHC Complexes 1064.2 Palladium(0)‐NHC Complexes 1074.2.1 Reactions of Free NHCs 1074.2.2 Reduction of Palladium(II)‐NHC Complexes 1124.3 Platinum(0)‐NHC Complexes 1154.3.1 Homoleptic Complexes 1154.3.2 Heteroleptic Complexes 117References 1205 Group 10 Metal(II)‐NHC Complexes 1225.1 Nickel(II)‐NHC Complexes 1225.1.1 Cleavage of Enetetramines and the Free Carbene Route 1225.1.2 In Situ Deprotonation of Azolium Salts with Basic Metal Salts 1245.1.3 The Silver‐Carbene Transfer Route 1275.2 Palladium(II)‐NHC Complexes 1295.2.1 Cleavage of Entetramines and the Free Carbene Route 1305.2.2 In Situ Deprotonation of Azolium Salts with External Base 1335.2.3 The “Palladium Acetate” Route 1355.2.4 The Silver‐Carbene Transfer Route 1395.2.5 Isomers of Bis(NHC) Palladium(II) Complexes 1435.3 Platinum(II)‐NHC Complexes 1505.3.1 Cleavage of Entetramines 1515.3.2 Cyclization of Isocyanide Complexes 1525.3.3 The Oxidative Addition Route 1535.3.4 The Free Carbene Route 1575.3.5 In Situ Deprotonation of Azolium Salts with External Base 1595.3.6 In Situ Deprotonation with Basic Platinum Precursors 1625.3.7 Carbene Transfer Reactions 164References 1686 Group 11 Metal‐NHC Complexes 1716.1 Copper(I)‐NHC Complexes 1716.1.1 The Free Carbene Route 1716.1.2 Alkylation of Copper‐Azolate Complexes 1726.1.3 In Situ Deprotonation with External Base 1736.1.4 In Situ Deprotonation with Basic Copper Precursors 1776.1.5 The Silver Carbene Transfer Route 1806.1.6 The Copper Powder Route 1826.2 Silver(I)‐NHC Complexes 1836.2.1 The Free Carbene Route 1836.2.2 In Situ Deprotonation with Basic Silver Precursors 1866.2.3 Silver‐Carbene Transfer Reactions 1926.3 Gold(I)‐NHC Complexes 1946.3.1 Cleavage of Enetetramines 1946.3.2 Protonation/Alkylation of Azolato Complexes 1946.3.3 The Free Carbene Route 1966.3.4 The Silver‐Carbene Transfer Route 2006.3.5 Ligand Redistribution and Autoionization of Gold(I) NHC Complexes 2106.4 Gold(III)‐NHC Complexes 212References 2187 Ruthenium, Rhodium, and Iridium Metal‐NHC Complexes 2207.1 Ruthenium(II)‐NHC Complexes 2207.1.1 Cleavage of Enetetramines and the Free Carbene Route 2207.1.2 In Situ α‐Elimination 2277.1.3 In Situ Deprotonation of Azolium Salts 2287.1.4 The Silver‐Carbene Transfer Route 2307.2 Rhodium(I)‐ and Rhodium(III)‐NHC Complexes 2327.2.1 Cleavage of Enetetramines and the Free Carbene Route 2327.2.2 In Situ Deprotonation of Azolium Salts 2387.2.3 The Silver‐Carbene Transfer Route 2437.3 Iridium(I)‐ and Iridium(III) NHC Complexes 2457.3.1 Cleavage of Enetetramines and the Free Carbene Route 2457.3.2 In Situ α‐Elimination 2507.3.3 In Situ Deprotonation of Azolium Salts 2517.3.4 The Silver‐Carbene Transfer Route 256References 2618 Beyond Classical N‐heterocyclic Carbenes I 2638.1 N,S‐Heterocyclic Carbenes (NSHCs) 2638.2 N,O‐Heterocyclic Carbenes (NOHCs) 2708.3 Expanded Six‐Membered NHCs 2778.4 Expanded Seven‐ and Eight‐Membered NHCs 2828.5 Expanded Diamidocarbenes (DAC) 287References 2919 Beyond Classical N‐heterocyclic Carbenes II 2939.1 Abnormal Imidazolin‐4/5‐ylidenes (aNHC) 2949.2 Mesoionic 1,2,3‐Triazolin‐5‐ylidenes (MIC) 3039.3 Pyrazolin‐3/5‐ylidenes (Pyry) and Indazolin‐3‐ylidenes (Indy) 3109.3.1 Pyrazolin‐3/5‐ylidenes 3109.3.2 Indazolin‐3‐ylidenes 3179.4 Cyclic (Alkyl)(Amino)Carbenes (CAACs) Or Pyrrolidin‐2‐ylidenes 3209.5 Remote N‐Heterocyclic Carbenes (rNHC) 3249.5.1 Pyridin‐3/4‐ylidenes 3249.5.2 Pyrazolin‐4‐ylidenes 326References 328Index 330