Synthesis of Unsymmetrical Diboranes by Diborane Metathesis

Lithium, Tin(II), and Zinc Amino-Boryloxy Complexes: Synthesis and Characterization

Analogous to the ubiquitous alkoxide ligand, metal boroxide and boryloxy complexes are an underexplored class of hard anionic O- ligand. A new series of amine-stabilized Li, Sn(II), and Zn boryloxy complexes, comprising electron-rich tetrahedral boron centers have been synthesized and characterized. All complexes have been characterized by one-dimensional (1D), two-dimensional (2D), and DOSY NMR, which are consistent with the solid-state structures unambiguously determined via single-crystal X-ray diffraction. Electron-rich μ2- (Sn and Zn) and μ3- (Li) boryloxy binding modes are observed. Compounds 6-9 are the first complexes of this class, with the chelating bis- and tris-phenol ligands providing a scaffold that can be easily functionalized and provides access to the boronic acid pro-ligand, hence allowing facile direct synthesis of the resulting compounds. Computational quantum chemical studies suggest a significant enhancement of the π-donor ability of the amine-stabilized boryloxy ligand because of electron donation from the amine functionality into the p-orbital of the boron atom.

Bonding in Barium Boryloxides, Siloxides, Phenoxides and Silazides: A Comparison with the Lighter Alkaline Earths

Barium complexes ligated by bulky boryloxides [OBR₂ ]^- (where R=CH(SiMe₃ )₂ , 2,4,6-ⁱ Pr₃ -C₆ H₂ or 2,4,6-(CF₃ )₃ -C₆ H₂ ), siloxide [OSi(SiMe₃ )₃ ]^- , and/or phenoxide [O-2,6-Ph₂ -C₆ H₃ ]^- , have been prepared. A diversity of coordination patterns is observed in the solid state for both homoleptic and heteroleptic complexes, with coordination numbers ranging between 2 and 4. The identity of the bridging ligand in heteroleptic dimers [Ba(μ₂ -X₁ )(X₂ )]₂ depends largely on the given pair of ligands X₁ and X₂ . Experimentally, the propensity to fill the bridging position increases according to [OB{CH(SiMe₃ )₂ }₂ )]^- <[N(SiMe₃ )₂ ]^- <[OSi(SiMe₃ )₃ ]^- <[O(2,6-Ph₂ -C₆ H₃ )]^- <[OB(2,4,6-ⁱ Pr₃ -C₆ H₂ )₂ ]^- . This trend is the overall expression of 3 properties: steric constraints, electronic density and σ- and π-donating capability of the negatively charged atom, and ability to generate Ba ⋅ ⋅ ⋅ F, Ba ⋅ ⋅ ⋅ C(π) or Ba ⋅ ⋅ ⋅ H-C secondary interactions. The comparison of the structural motifs in the complexes [Ae{μ₂ -N(SiMe₃ )₂ }(OB{CH(SiMe₃ )₂ }₂ )]₂ (Ae = Mg, Ca, Sr and Ba) suggest that these observations may be extended to all alkaline earths. DFT calculations highlight the largely prevailing ionic character of ligand-Ae bonding in all compounds. The ionic character of the Ae-ligand bond encourages bridging coordination, whereas the number of bridging ligands is controlled by steric factors. DFT computations also indicate that in [Ba(μ₂ -X₁ )(X₂ )]₂ heteroleptic dimers, ligand predilection for bridging vs. terminal positions is dictated by the ability to establish secondary interactions between the metals and the ligands.

Reactivity of a magnesium diboranate with organic nitriles

A series of complexes generated through reactions of the β-diketiminato magnesium diboranate species, [(BDI)Mg{(n-Bu)pinB-Bpin}] (BDI = HC{(Me)CNDipp}2; Dipp = 2,6-di-iso-propylphenyl), and a variety of organic nitriles are reported. Although, in every case, the diboranate anion acts as a surrogate source of the {Bpin} nucleophile, resulting in B-C bond formation at the electrophilic sp-hydridised nitrile carbon, the resultant compounds display a variable propensity to undergo subsequent reaction with additional nitrile equivalents. This behaviour is rationalised to be a consequence of substituent-dependent modulation in the basicity and resultant electrophilicity of magnesium-coordinated nitrile intermediates.

[BO2 ]- as a Synthon for the Generation of Boron-Centered Carbamate and Carboxylate Isosteres

Oxoborane carbamate and carboxylate analogues result from the in situ trapping of [BO₂]⁻ produced by elimination of 2,3‐dimethyl‐2‐butene from a pinacolatoboryl anion.

Desymmetrization of Dicationic Diboranes by Isomerization Catalyzed by a Nucleophile

Cationic monoboranes exhibit a rich chemistry. By constrast, only a few cationic diboranes are known, that all are symmetrically substituted. In this work, the first unsymmetrically substituted dicationic diboranes, featuring sp2 -sp2 -hybridized boron atoms, are reported. The compounds are formed by intramolecular rearrangement from preceding isomeric symmetrically substituted dicationic diboranes, a process that is catalyzed by nucleophiles. From the temperature-dependence of the isomerization rate, activation parameters for this unprecedented rearrangement are derived. The difference in fluoride ion affinity between the two boron atoms and the bonding situation in these unique unsymmetrical dicationic diboranes are evaluated.

Snapshots of magnesium-centred diborane heterolysis by an outer sphere SN2 process

Reactions of a magnesium diboranate as a source of [Bpin]^– anions are initiated by ‘outer sphere’ attack of C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N bonded substrates.

Reactions of the β-diketiminato magnesium diboranate derivative, [(BDI)Mg{pinB(n-Bu)Bpin}] (BDI = HC{(Me)CNDipp}₂; Dipp = 2,6-i-Pr₂C₆H₃), with N,N′-dialkyl and N,N′-diaryl carbodiimides provided the corresponding C-borylated magnesium borylamidinates. This reactivity occurs with the displacement of n-BuBpin and with the apparent addition of a nucleophilic {Bpin} anion to the electrophilic unsaturated carbodiimide carbon centres. In contrast, while analogous reactions of [(BDI)Mg{pinB(n-Bu)Bpin}] with N-alkyl or N-aryl aldimines and ketimines also resulted in facile displacement of n-BuBpin, they provided the organomagnesium products of {Bpin} addition to the imine nitrogen atom rather than the more electrophilic trigonal imine carbon. Computational assessment by density functional theory (DFT) indicated that, although the energetic differences are marginal, the organomagnesium products may be considered as the kinetic outcome of these reactions with respect to the generation of alternative amidomagnesium regioisomers. This latter deduction was borne out by the thermally-induced conversion of two such organomagnesium species to their C-borylated amidomagnesium isomers, both of which occur with negligible entropies of activation indicative of purely intramolecular processes. Detailed analysis by DFT of the reaction of [(BDI)Mg{pinB(n-Bu)Bpin}] with PhN Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHPh indicated that B–N bond formation is initiated by attack of the imine nitrogen at the three-coordinate boron atom of the diboranate anion rather than the more crowded magnesium centre. Consistent with an effectively spontaneous reaction, the resultant cleavage of the B–B bond of the diboranate unit is accomplished via the traversal of two very modest barriers of only 8.3 and 6.7 kcal mol^–1. This analysis was also supportive of a subsequent intramolecular B–N to B–C isomerisation process. Of greater general significance, however, the addition of the {Bpin}^– anion to the reducible aldimine is best rationalised as a consequence of the electrophilic character of this three-coordinate boron centre rather than any intrinsic nucleophilicity associated with the B–B bond of the [pinBB(n-Bu)pin]^– anion.

Diborane heterolysis and P(v) reduction by Ph3PO coordination to magnesium

Inner sphere attack of Ph₃PO provides a terminal magnesium boryl, which is a potent reagent for the deoxygenation of P(v).

Selective Boryl-Anion Migration in a Vinyl sp2 -sp3 Diborane Induced by Soft Borane Lewis Acids

An intramolecular 1,2‐boryl‐anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B₂Pin₂)]⁻ using “soft” BR₃ electrophiles (BR₃=BPh₃ or 9‐aryl‐BBN). The soft character is key to ensure 1,2‐migration proceeds instead of oxygen coordination/B−O activation. The BR₃‐induced 1,2‐boryl‐anion migration represents a triple borylation of a vinyl Grignard reagent using only B₂Pin₂ and BR₃ and forms differentially protected 1,1,2‐triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B₂Pin₂, 1,2‐boryl‐anion migration can be suppressed in favor of intermolecular {BPin}⁻ transfer to BPh₃, thus enabling simple access to unsymmetrical sp²−sp³ diboranes.

Reactivity Enhancement of a Zerovalent Diboron Compound by Desymmetrization

The desymmetrization of the cyclic (alkyl)(amino)carbene-supported diboracumulene B₂(cAAC^Me)₂ (cAAC^Me = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) by monoadduct formation with IMe^Me (1,3-dimethylimidazol-2-ylidene) yields the zerovalent sp-sp² diboron compound B₂(cAAC^Me)₂(IMe^Me), which provides a versatile platform for the synthesis of novel symmetrical and unsymmetrical zerovalent sp²-sp² diboron compounds by adduct formation with IMe^Me and CO, respectively. Furthermore, B₂(cAAC^Me)₂(IMe^Me) displays enhanced reactivity compared to its symmetrical precursor, undergoing spontaneous intramolecular C-H activation and facile twofold hydrogenation, the latter resulting in B-B bond cleavage and the formation of the mixed-base parent borylene (cAAC^Me)(IMe^Me)BH.