Cationic tricoordinate boron intermediates: borenium chemistry from the organic perspective.

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 tetracoordinate boron cations (borinium, borenium, and boronium, respectively; Fig. 1).2 The latter review by Kolle and Noth 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 Noth’s classification.2 This terminology depends on the number of ligands at boron and avoids distinctions based on bond order or resonance contributions. Noth’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. Fig. 1 Boron cation nomenclature and structural representations in current use. 2. History of borenium ions: often considered, seldom confirmed 2.1 Suspected intermediates in B-N protonation The first reports to our knowledge where cationic sextet boron was implicated are the two 1933 papers by Wiberg and Schuster that describe the reaction of dichloro(dimethylamino)borane 1 with hydrogen chloride.4 The experiment was said to produce the hydrochloride (chlorhydrat) of 1, drawn using the notation shown as 2a (eq.1). This representation does not specify connectivity, and nothing in the 1933 papers indicates whether 2a is the same or different compared to 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 generalized drawings of a single ionic structure (5a and 5b) that is clearly identified as the unstable intermediate leading to an isolable covalent adduct, drawn by Wiberg and Hertwig in two unambiguous representations 6a and 6b.5 These drawings and appended comments leave no question about the structure of 5 and the greater stability of 6. However, they may have escaped the notice of subsequent authors, some of whom accepted the borenium structure 2b as a more stable alternative compared to 3b and attributed this conjecture to Wiberg.