1. INTRODUCTION The focus of this review is on the reactivity of cationic, tricoordinated boron electrophiles and on recent applications that rely on intermediates having sextet boron and net positive charge. Older reviews have appeared that address early studies involving a variety of boron cations,1 or that emphasize the structural, bonding, and theoretical aspects of di-, tri-, and
Review
2a is the same as or different from the borenium salt 2b. Whether the authors intended to distinguish 2a from representations such as 3a or 3b is also uncertain. On the other hand, a 1947 paper by Wiberg and Hertwig shows a similar reaction (R2BNHR + HCl, eq 2) and provides two
Figure 1. Boron cation nomenclature and structural representations in current use.
and Nöth is a milestone in the field of boron cation chemistry. It systematizes a number of earlier observations and defines the currently accepted nomenclature for boron-containing cations. A more recent (2005) review by Piers et al. updates the structural and bonding aspects in depth,3 discusses reactions of boron cations in the gas phase as well as in solution, and explores several other topics of special interest to the organoelement community. We will take a somewhat different approach to follow developments from the organic chemistry perspective, including selected historical aspects of reactivity, some of which have not appeared in prior reviews. Several topics will necessarily overlap with the excellent overview by Piers et al., but detailed coverage will be limited to the solution chemistry of tricoordinate boron cations, the reactive intermediates known as borenium ions according to Nöth’s classification.2 This terminology depends on the number of ligands at boron and avoids distinctions based on bond order or resonance contributions. Nöth’s treatment also recognizes the iso-electronic relationship between borenium and carbenium ions, both of which feature a formally vacant p-orbital at the central atom (boron and carbon, respectively) and the same overall positive charge. Consistent with the carbenium analogy, borenium ions have long been suspected as intermediates in classical nucleophilic substitution chemistry involving B−X bonds in tetracoordinate boron structures, although we will find that these suspicions were often unfounded. On the other hand, recent studies have revealed fascinating new roles for borenium intermediates, ranging from enantioselective catalysis and memory of chirality applications to hints of C−F activation and C−H insertion chemistry. It would be fair to say that these recent developments were slow to unfold, given the long history of borenium chemistry.
Review /CR
Cationic Tricoordinate Boron Intermediates: Borenium Chemistry from the Organic Perspective
Timothy S. De Vries, Aleksandrs Prokofjevs, and Edwin Vedejs*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
6.2. The Role of Borenium Species in Diels−Alder Catalysis 6.3. Reactivity and Scope of OxazaborolidineDerived Diels−Alder Catalysts 6.4. Miscellaneous Uses of Chiral Borenium Lewis Acid Catalysts 7. Miscellaneous Applications of Borenium Lewis Acids 7.1. B−O Activation 7.2. Lewis Acidity and π-Conjugated Borenium Analogues 7.3. π-Conjugated Borenium Analogues as Selective Anion Sensors 7.4. Tricoordinate Boron in Structures Containing Metal Cations 8. Borenium Reagents for C−B Bond Formation 8.1. Borenium Ion Equivalents 8.2. Do Borenium Ions Participate in Hydroboration Chemistry? 8.3. Borenium Equivalents in Electrophilic Aromatic Borylation 8.3.1. Intramolecular Borylation 8.3.2. Intermolecular Borylation 8.3.3. Recent Developments in Muetterties Borylation (BX3 plus Proton Scavenger) 9. Extension of Borenium Chemistry to Neighboring Fields and Unusual Environments 9.1. N-Heterocyclic Carbenes as Stabilizing Ligands for Borenium Salts 9.2. Miscellaneous Related Topics Involving BH Species 10. Summary Author Information Corresponding Author Notes Biographies Acknowledgments References P R T U U V W X X X Y Z Z AA AD AE AE AF AG AH AH AH AH AH AH
Received: April 20, 2011
© XXXX American Chemical Society
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Chemical Reviews tetracoordinate boron cations (borinium, borenium, and boronium, respectively; Figure 1).2 The latter review by Kölle