Structural and Electronic Properties of Boranes Containing Boron‐Chalcogen Multiple Bonds and Stabilized by Amido Imidazoline‐2‐imine Ligands

Iminoboranes With Parent B=NH Entity: Imino Group Metathesis, Nucleophilic Reactivity and N-N Coupling

While R2 C=N-R double bonds in organic imines are well established, the related iminoboranes R-B=N-R are either prone to oligomerization or can only be stabilized at sufficient steric congestion. In particular, the examples of unsubstituted parent B=N-H entity are rare. We demonstrate that the amino imidazoline-2-imine ligand system (HAmIm) not only gives rise to the isolation of a parent (AmIm)B=N-H iminoborane, but also to species of type (AmIm)B=N-SiMe3 with concomitant stabilization by lithium bromide. The double bond character in these systems is unambiguously corroborated by DFT calculations. The steric accessibility of the (AmIm)B=NH unit allows facile reactivity including metathesis reactions with C=O and C=S bonds, nucleophilic addition toward organic and organometallic carbonyl compounds, but also oxidative N-N coupling within a dimeric unit. Thus, the chemical behavior of the (AmIm)B=N-H and (AmIm)B=N-SiMe3 is distinctly different from that of organic imines.

Base-Stabilized Neutral Oxoborane and Thioxoborane Supported by a Bis(oxazolinyl)(phenyl)methanide Ligand

Base-stabilized neutral oxoborane and thioxoborane supported by a bis(oxazolinyl)(phenyl)methanide ligand have been synthesized and structurally characterized. While previous synthetic attempts of oxoborane ligated by β-diketiminate (NacNac) did not allow for its isolation in acid-free form, oxoborane supported by a bis(oxazolinyl)(phenyl)methanide ligand is isolable, due to the absence of imine ɑ-protons and steric protection of enamine carbon. Crystallographic analysis revealed the presence of a B-O double bond close to the shortest end of the reported lengths. Its reactivity has also been examined, and it was majorly governed by the nucleophilicity and basicity of the oxygen atom. The chemical inertness and synthetic convenience of the bis(oxazolinyl)(phenyl)methanide scaffold presented in this work suggest its utility as an innocent alternative to the NacNac scaffold.

Electrophilicity of Hoveyda-Grubbs Olefin Metathesis Catalysts as the Driving Force that Controls Initiation Rates

The dissociative mechanism of initiation for a series of Hoveyda-Grubbs type metathesis catalysts modified at the para and meta positions in the isopropoxybenzylidene ligand is investigated by means of DFT calculations. The electron donating/withdrawing capacity of the ligand was screened through the incorporation of various substituents such as halogens, nitro, alkoxides, ketones, esters, amines, and amides. Variations in structural parameters, energy barriers for the Ru-O bond dissociation, and Ru-O bond strength were examined as a function of the Hammett constant. It was found that electronic properties of the catalysts such as chemical potential, hardness, and electrophilicity correlate linearly with the dissociative energy barriers. These findings enable a systematic rationalization and prediction of rate of precatalyst initiation through the calculation of only the HOMO-LUMO gap of catalysts, as the faster the initiation, the more electrophilic the catalyst.

Reductive Al-B σ-Bond Formation in Alumaboranes: Facile Scission of Polar Multiple Bonds

We present facile access to an alumaborane species with electron precise Al-B σ-bond. The reductive rearrangement of 1-(AlI2 ), 8-(BMes2 ) naphthalene (Mes=2,4,6-Me3 C6 H2 ) affords the alumaborane species cyclo-(1,8-C10 H6 )-[1-Al(Mes)(OEt2 )-8-B(Mes)] with a covalent Al-B σ-bond. The Al-B σ-bond performs the reductive scission of multiple bonds: S=C(NiPrCMe)2 affords the naphthalene bridged motif B-S-Al(NHC), NHC=N-heterocyclic carbene, while O=CPh2 is deoxygenated to afford an B-O-Al bridged species with incorporation of the remaining ≡CPh2 fragment into the naphthalene scaffold. The reaction with isonitrile Xyl-N≡C (Xyl=2,6-Me2 C6 H4 ) proceeds via a proposed (amino boryl) carbene species; which adds a second equivalent of isonitrile to ultimately form the Al-N-B bridged species cyclo-(1,8-C10 H6 )-[1-Al(Mes)-N(Xyl)-8-B{C(Mes)=C-N-Xyl}] with complete scission of the C≡N triple bond. The latter reaction is supported with isolated intermediates and by DFT calculations.