Unsymmetrische, cyclische Diborene und thermische Umlagerung zu einem Borylborylen

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry

A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B2 Br4 (SMe2 )2 . Theoretical studies revealed that this diborene has a considerably smaller HOMO-LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy3 )] afforded two diborene-AuI π complexes, while reaction with DurBH2 , P4 and a terminal acetylene led to the cleavage of B-H, P-P, and C-C π bonds, respectively. Thermal rearrangement of the diborene gave an electron-rich cyclic alkylideneborane, which readily coordinated to AgI via its B=C double bond.

Metal-Free Phosphorus-Directed Borylation of C(sp2 )-H Bonds

Spectacular progress has recently been achieved in transition metal-catalyzed C-H borylation of phosphines as well as directed electrophilic C-H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine-boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂ P→BBr₂ ⁺ species.

Probing the Boundaries between Lewis-Basic and Redox Behavior of a Parent Borylene

The parent borylene (CAAC)(Me3 P)BH, 1 (CAAC=cyclic alkyl(amino)carbene), acts both as a Lewis base and one-electron reducing agent towards group 13 trichlorides (ECl3 , E=B, Al, Ga, In), yielding the adducts 1-ECl3 and increasing proportions of the radical cation [1]•+ for the heavier group 13 analogues. With boron trihalides (BX3 , X=F, Cl, Br, I) 1 undergoes sequential adduct formation and halide abstraction reactions to yield borylboronium cations and shows an increasing tendency towards redox processes for the heavier halides. Calculations confirm that 1 acts as a strong Lewis base towards EX3 and show a marked increase in the B-E bond dissociation energies down both group 13 and the halide group.

Unsymmetrical sp2 -sp3 Disilenes

Disilenes with differently coordinated silicon atoms are not known. Here, we have shown the high yield synthesis of a range of disilenes (2-4 and 6) upon reaction of a hypersilyl silylene PhC(NtBu)₂ SiSi(SiMe₃ )₃ (1) with aliphatic chlorophosphines. The most striking characteristic of these disilenes is the presence of two differently coordinated Si atoms (one is three-coordinated, the other four-coordinated). The analogous reaction with Ph₂ PCl did not afford the desired disilene, but, surprisingly, led to the first tetraphosphinosilane (8). DFT calculations were performed to understand the bonding in disilenes and differences in reactivity of the complexes.

Bond-Strengthening Backdonation in Aminoborylene-Stabilized Aminoborylenes: At the Intersection of Borylenes and Diborenes

Singly NHC-coordinated (aminoboryl)aminoborenium salts react with Na₂ [Fe(CO)₄ ] to yield stable coordination complexes of aminoborylene-stabilized aminoborylenes, which exhibit exceptional σ-donor properties. Upon photolytic CO extrusion from the metal center, the diboron ligand adopts a novel η³ -BBN coordination mode, where bond-strengthening backdonation from the metal center into the vacant B-B π-orbital is observed. This bonding situation can be alternatively described as a Fe-diaminodiborene complex. In a related reduction of CAAC-stabilized (aminoboryl)aminoborenium with KC₈ , the reduced species can be captured with nucleophiles to form three-coordinate (diaminoboryl)borylenes, where both amino groups have migrated to the distal boron atom. Collectively, these reactions illustrate the isomeric flexibility imparted by amino groups on this reduced diboron system, thus opening multiple avenues of novel reactivity.

Reactivity of a Tetra(o-tolyl)diborane(4) Dianion as a Diarylboryl Anion Equivalent

Lithium and magnesium salts of tetra(o-tolyl)diborane(4) dianion, having B=B double bond character, were synthesized. It was clarified that the lithium salt of the dianion has a high-lying HOMO and a narrow HOMO-LUMO gap, which were perturbed by dissociation of Li⁺ cation, as judged by UV/Vis spectroscopy and DFT calculations. The lithium salt of the dianion reacted as two equivalents of a diarylboryl anion with CH₂ Cl₂ or S₈ to give boryl-substituted products.

Phosphine-Stabilized Diiododiborenes: Isolable Diborenes with Six Labile Bonds

The lability of B=B, B-P, and B-halide bonds is combined in the syntheses of the first diiododiborenes. In a series of reactivity tests, these diiododiborenes undergo cleavage of all six of their central bonds in different ways, leading to products of B=B hydrogenation and dihalogenation as well as halide exchange.

Chelated Diborenes and Their Inverse-Electron-Demand Diels-Alder Reactions with Dienes

A doubly base-stabilised diborane based on a benzylphosphine linker was prepared by a salt elimination reaction between 2-LiC₆ H₄ CH₂ PCy₂ ⋅Et₂ O and B₂ Br₄ . This compound was reduced with KC₈ to its corresponding diborene, with the benzylphosphine forming a five-membered chelate. The diborene reacts with butadiene, 2-trimethylsiloxy-1,3-butadiene, and isoprene to form 4,5-diboracyclohexenes, which interconvert between their 1,1- (geminal) and 1,2- (vicinal) chelated isomers. The 1,1-chelated diborene undergoes a halide-catalysed isomerisation into its thermodynamically favoured 1,2-isomer, which undergoes Diels-Alder reactions more slowly than the kinetic product.

Coordination of Asymmetric Diborenes towards Cationic Coinage Metals (Au, Ag, Cu)

Synthesis of a series of cationic coinage metal (Au: 2-4, Ag: 5, and Cu: 6) complexes of asymmetric diborenes (1 a, 1 b) is reported. X-ray diffraction analysis reveals that the metal center interacts unsymmetrically with the B₂ moiety and the terminal boron atom exhibits pronounced pyramidal geometry. A computational study shows that in complexes 2-6, the bonding between the B₂ units and the metal center is dominated by electrostatic interactions concomitant with non-negligible covalent contributions.