Abstractgem‐Diborylalkanes have emerged as efficient reagents for synthesizing organoboron compounds through selective C−C bond‐forming reactions. Activation of the 1,1‐diborylalkanes generates carbanions with enhanced stability that are able to react with a series of electrophiles, carbonyl compounds, imines and epoxides to promote formation of a new C−C bond. These new sets of reactions have become general for a wide range of substrates and they can be understood by alternative mechanisms that justify the potential use of these reagents. The formation of C–C(B) bonds can be achieved with chemo‐, diastereo‐ and enantioselectivity, because the nucleophilc α‐boryl or α‐diboryl carbanions attack in a stereoselective manner, by means of the catalyst involved. The synthesis of gem‐diborylalkanes has also been promoted by innovative methods and facilitates access to multiborylated reagents with different substituents and properties.magnified image
α‐Monoboryl anions show remarkable stability due to the valence deficiency of the adjacent three‐coordinate boron center and can be expressed as its resonance form, the borata‐alkene systems [R2B=CH2]−. The diversity of synthetic approaches as well as the properties of the C=B bond are disclosed in this review, dealing with both electronic and structural properties of the boryl moieties involved. Full characterization in solid state by X‐ray diffraction demonstrated the short distances between B and C, as a consequence of the boron ylide character in the boron‐stabilized carbanions. This review includes a collection of C−B length distances as well as 11B NMR data that can be useful for diagnostic evidence of the partial boron‐carbon double‐bond character. Natural bond orbital (NBO) analysis on DFT computed structures also supports and justifies the borata‐alkene character. The reactivity of the C=B bond acting as nucleophilic synthon is unveiled through extensive electrophilic trapping examples. The homologation protocols with diborylmethane, via single carbon chain extension, involves a facile introduction of the C(sp3)−B bonds, which can be subsequently transformed into functionalized target products, containing C−O, C−N or C−C bonds.
The chemistry of stabilized α‐boryl carbanions shows remarkable diversity, and can enable many different synthetic routes towards efficient C−C bond formation. The electron‐deficient, trivalent boron center stabilizes the carbanion facilitating its generation and tuning its reactivity. Here, the electronic structure and the reactivity trends of a large dataset of α‐boryl carbanions are described. DFT‐derived parameters were used to capture their electronic and steric properties, computational reactivity towards model substrates, and crystallographic analysis within the Cambridge Structural Dataset. This study maps the reactivity space by systematically varying the nature of the boryl moiety, the substituents of the carbanionic center, the number of α‐boryl motifs, and the metal counterion. In general, the free carbanionic intermediates are described as borata‐alkene species with C−B π interactions polarized towards the carbon. Furthermore, it was possible to classify the α‐boryl alkylidene metal precursors into three classes directly related to their reactivity: 1) nucleophilic borata‐alkene salts with alkali and alkaline earth metals, 2) nucleophilic η2‐(C−B) borata‐alkene complexes with early transition metals, Cu and Ag, and 3) α‐boryl alkyl complexes with late transition metals. This trend map aids selection of the appropriate reactive synthon depending on the reactivity sought.
Exchange of boryl moieties is possible between alkenylboranes and diboron reagents as a stereospecific cross-metathesis with formation of mixed diboron reagents. DFT calculations propose a mechanism with both symmetric and non-symmetric diboron reagents.
1,1‐Diborylalkenes can be transformed into (Z)‐skipped dienes through CuI‐phosphine catalyzed allylic coupling reactions. The energetically preferred formation of (Z)‐α‐borylalkenyl copper (I) species and the subsequent nucleophilic attack, explains the stereoselective nucleophilic substitution with allyl bromides. The eventual treatment of (Z)‐skipped dienes with NaOtBu promotes cyclization/aromatization patterns via enyne intermediates.
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