Cationic Tricoordinate Boron Intermediates: Borenium Chemistry from the Organic Perspective

Abstract

CONTENTS 1. Introduction 4246 2. History of Borenium Ions: Often Considered, Seldom Confirmed 4247 2.1. Suspected Intermediates in B−N Protonation 4247 2.2. Hypothetical Isomers of Cl 3 B•NHMe 2 4247 2.3. The First Observable Borenium Salt 4247 2.4. Nucleophilic Substitution of X 3 B•NR 3 and Py•BF 2 X 4248 2.5. Aminoborenium Ions, Stabilization by n-Delocalization 4249 3. Recent Developments in the Generation of Observable Borenium Intermediates 4249 3.1. Electrophilic Activation by Protonation or by Lewis Acid Catalysis 4249 3.2. Halide Abstraction by a Halophile 4250 3.3. The Borinium−Borenium Interface: Gas-Phase Oxyborenium Ions 4251 3.4. Borenium Salt Generation by Hydride Abstraction 4251 3 . 4 . 1 . H y d r i d e A b s t r a c t i o n b y T r i s -(pentafluorophenyl)borane 4251 3.4.2. Hydride Abstracton by Trityl Salts 4252 3.4.3. Hydride-Bridged Borenium Salt Dimers 4252 3.5. Borenium Ion Generation by Nucleophilic Addition−Heterolysis 4253 4. Borenium Lewis Acidity vs Structure 4254 4.1. Experimental Evaluation of Lewis Acidity 4254 4.2. Computational Evaluation of Borenium Lewis Acidity 4255 5. Borenium Ions and Stereogenic Boron 4257 5.1. Racemization by Heterolysis 4257 5.2. Asymmetric Memory at Stereogenic Boron 4257 5.3. Chiral Salicylaldimine Boronate Complexes 4258 6. Enantioselective Catalysis and Chiral Borenium Ions 4259 6.1. The Dual Function of Oxazaborolidines in the CBS Reduction 4260 6.2. The Role of Borenium Species in Diels−Alder Catalysis 4261 6.3. Reactivity and Scope of Oxazaborolidine-Derived Diels−Alder Catalysts 4263 6.4. Miscellaneous Uses of Chiral Borenium Lewis Acid Catalysts 4265 7. Miscellaneous Applications of Borenium Lewis Acids 4266 7.1. B−O Activation 4266 7.2. Lewis Acidity and π-Conjugated Borenium Analogues 4267 7.3. π-Conjugated Borenium Analogues as Selective Anion Sensors 4268 7.4. Tricoordinate Boron in Structures Containing Metal Cations 4269 8. Borenium Reagents for C−B Bond Formation 4269 8.1. Borenium Ion Equivalents 4269 8.2. Do Borenium Ions Participate in Hydroboration Chemistry? 4270 8.3. Borenium Equivalents in Electrophilic Aromatic Borylation 4271 8.3.1. Intramolecular Borylation 4271 8.3.2. Intermolecular Borylation 4272 8.3.3. Recent Developments in Muetterties Borylation (BX3 plus Proton Scavenger) 4275 9. Extension of Borenium Chemistry to Neighboring Fields and Unusual Environments 4276 9.1. N-Heterocyclic Carbenes as Stabilizing Ligands for Borenium Salts 4276 9.2. Miscellaneous Related Topics Involving BH Species 4277 10. Summary 4278 Author Information 4279 Corresponding Author 4279 Notes 4279 Biographies 4279 Acknowledgments 4279 References 4279

Frustrated Lewis Pair Chemistry: Development and Perspectives

“…One strategy to replace electrophilic boranes is to exploit a lesser studied class of boron-based Lewis acids, the borenium cations. [190] To this end, the NHC adduct of 9-BBN, (I 4 ] (Scheme 132). This ability to activate H 2 led to the application of the borenium cation as a catalyst for the hydrogenation of imines and enamines.…”

Lewis Superacidic Ionic Liquids with Tricoordinate Borenium Cations

“…The most important routes to generate borenium cations are hydride or halide abstraction from tetracoordinate boron adducts, [BX3L], using an excess of abstracting agent (Figure 1). [4,5,7,10] Many borenium complexes have been reported as 'oily materials'. [4] Despite the increasing number of known crystal structures, [5,8,9] they are generally difficult to crystallize, and are typically studied in solution, using 11 B NMR spectroscopy.…”

“…In AN scale, hexane has AN = 0, SbCl5 in 1,2-dichloroethane has AN = 100, and acids with AN > 100 are considered Lewis superacids. The method is well established in Lewis acidity studies of both borenium cations [7,22] and of ionic liquids. [18,20] Unambiguous results could only be obtained for systems with N-donors, because the 31 P NMR signals from tepo (used in 1-3% quantity) were obscured by signals from the P888 ligand.…”

Formation of C(sp2)Boronate Esters by Borylative Cyclization of Alkynes Using BCl3

“…[2] Whilst this reaction has developed into atruly powerful transformation, the discovery of new routes,p articularly transition-metal-free methods,t o efficiently generate C(sp 2 ) À boronate esters,w hich are challenging to access by iridium catalysis,r emains desirable. Recent advances in transition-metal-free borylation include benzannulations, [3] radical mediated borylation, [4] electrophilic borylation, [5] and carbanion-mediated borylation. [6] One underexplored approach to metal-free C(sp 2 )ÀB bond formation proceeding from simple hydrocarbon precursors is the borylative cyclization of alkynes,w herein ab oron electrophile activates an alkyne for intramolecular electrophilic cyclization with as econd p-system.…”