Mapping the Electronic Structure and the Reactivity Trends for Stabilized α‐Boryl Carbanions

Abstract

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.

Site‐Selective (Z)‐α‐Borylalkenyl Copper Systems for Nucleophilic Stereodefined Allylic Coupling

“…In order to disclose the different reactivity between α‐borylalkenyl and β‐borylalkenyl copper systems, we analyzed here their electronic structures using the atomic charge at the carbanionic carbon ( qC ) and the C−B bond order ( C − B bo ) as descriptors to evaluate their nucleophilicity. This would place these carbanionic species within a general reactivity map for α‐boryl carbanions that we have established previously [12] . Figure 1 plots the descriptor values for α‐borylalkenyl copper species A and the phenyl‐substituted A ‐ Z and A ‐ E isomers, to be compared with the phenyl‐substituted β‐borylalkenyl copper intermediates B ‐ Z and B ‐ E , using in all cases Cu I ‐PPh 3 as metal fragment.…”

“…This would place these carbanionic species within a general reactivity map for α-boryl carbanions that we have established previously. [12] Figure 1 plots the descriptor values for α-borylalkenyl copper species A and the phenyl-substituted A-Z and A-E isomers, to be compared with the phenylsubstituted β-borylalkenyl copper intermediates B-Z and B-E, using in all cases Cu I -PPh 3 as metal fragment. To have a total picture of the reactivity trends, we have also studied the atomic charge at carbanionic carbon and the CÀ B bond order of α-borylalkyl copper systems C.…”

1,2‐Dialkylation of 1,1‐Arylboryl Alkenes Via Borata‐Alkene Intermediate

“…Under this panoramic overview, we envisioned a polar addition of t BuLi reagent to the terminal carbon of 1‐phenylvinylboronic acid pinacol ester 2 , promoted by the stability of the resulting α‐boryl carbanion 3 , followed by electrophilic trapping with C( sp 3 )X (X=I, Br and Cl), via substitution pathways (Scheme 1D). It has been demonstrated that α‐boryl carbanions show a remarkable stability due to the valence deficiency of the adjacent three coordinate boron center, as illustrated in the borata‐alkene resonance forms (Scheme 1D) [19,20] …”

“…It has been demonstrated that α-boryl carbanions show a remarkable stability due to the valence deficiency of the adjacent three coordinate boron center, as illustrated in the borata-alkene resonance forms (Scheme 1D). [19,20] This "all-alkyl" cross-coupling reaction inverts the trends of t BuLi addition to form alkenylboronates and is capable of generate two new C(sp 3 )À C(sp 3 ) bonds across the alkene, delivering valuable tetrasubstituted carbon centers, in the absence of catalyst, additives or any type of radical initiators. To the best of our knowledge, the tert-butyl motif can form CÀ C(sp 3 ) bond at the terminal position of 1, only as radical tertbutyl generated from t BuI and AIBN/Bu 3 SnH, [21,22] or generated from visible light-activated Ir or Ru catalysts.…”

Alkenylboronic Ester Activation to Nucleophilic Addition and Electrophilic Trapping with Carbonyl Groups

“…We based our new reactivity on the remarkable stability of the -arylboryl carbanion, which is due to the valence deficiency of the adjacent three-coordinate boron center, postulating the corresponding alkylideneborane resonance form (Scheme 1d). 20,21 Next, we considered nucleophilic addition to vinylboranes C, followed by electrophilic trapping with a carbonyl functional group (aldehyde or ketone). Murakami and coworkers were pioneers in studying the allylation reaction of aldehydes with 1-alkenylboronates, which acted as synthetic equivalents of -substituted allylboronates, in the presence of cationic rhodium(I) catalysts.…”