A Boratafulvene.

Structurally authenticated free B-alkyl boroles are presented and electronic implications of alkyl substitution were assessed. Deprotonation of a boron-bound exocyclic methyl group in a B-methyl borole yields the first 5-boratafulvene anion-an isomer to boratabenzene. Boratafulvene was structurally characterized and its electronic structure probed by DFT calculations. The pKa value of the exocyclic B-CH3 in a set of boroles was computationally approximated and confirmed a pronounced acidic character caused by the boron atom embedded in an anti-aromatic moiety. The non-aromatic boratafulvene reacts as a C-centered nucleophile with the mild electrophile Me3 SnCl to give a stannylmethyl borole, regenerating the anti-aromaticity. As nucleophilic synthons for boroles, boratafulvenes thus open an entirely new avenue for synthetic strategies toward this highly reactive class of heterocycles. Boratafulvene reacts as a methylene transfer reagent in a bora-Wittig-type reaction generating a borole oxide.

A Neutral Silicon(II) Half-Sandwich Compound.

Metathesis reaction of a dilithio borole dianion, a cyclic π-ligand isoelectronic to ubiquitous cyclopentadienyls, with two equivalents of "silicocenium" cation [Cp*Si]+ as a source of low-valent Si(II), cleanly gives a borole half-sandwich π-complex of Si(II) and silicocene. The resulting half-sandwich complex is a neutral isoelectronic analogue to the iconic silicocenium cation and features the rare structural motif of an apical silicon(II) atom with an energetically high lying lone pair of electrons that is shown to be accessible for coordination chemistry toward tungsten carbonyl. Protonation at the Si(II) atom with [H(OEt2)2][Al{OC(CF3)3}4] induces formal oxidation, and the compound rearranges to incorporate the Si atom into the carbocyclic base to give an unprecedented cationic 5-sila-6-borabicyclo[2.1.1]hex-2-ene. This rearrangement is accompanied by drastic changes in the 11B and 29Si NMR chemical shifts.

A Cationic NHC-Supported Borole.

This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3 )3 ]4 } from the NHC-chloroborole adduct yields the first stable NHC-supported 1-(Me NHC)-2,5-(SiMe3 )2 -3,4-(Ph*)2 -borole cation. Electronically, it features both a five-membered cyclic conjugated 4 π-electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass-spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1-mesityl borole reveals strong Lewis acidity, reduced HOMO-LUMO gaps, and increased anti-aromatic character.

Electronic Push-Pull Modulation by Peripheral Substituents in Pentaaryl Boroles.

Establishing access to a bulky tetraaryl dilithiobutadiene (Ph*C)4 Li2 (Ph*=3,5-tBu2 (C6 H3 )) allowed for the synthesis of five-membered heterocycles with incorporated main-group elements. Along with an amino borole, a set of substituted pentaaryl boroles (Ph*C)4 BAr has been synthesized. The examination of their absorption spectra and computational studies by means of DFT granted insight into the influence of peripheral substituents on the electronic features of the parent pentaphenyl borole (PhC)4 BPh. Introduction of the more electron-rich Ph* residue at the carbon atoms increases the HOMO energy, redshifting the visible π/π*-absorption bands compared with the parent pentaphenyl borole. The influence on the frontier orbitals of three different boron-bound aryls with electronically modulating substituents in the remote 3,5-positions Ar=3,5-R2 -C6 H3 (R=Me, H, CF3 ) was studied. The substituents were found to increase (+I effect, Me) or decrease (-I effect, CF3 ) the LUMO energy, thus directly affecting the visible absorption spectra. This represents the first study on HOMO-LUMO-gap adjustments by a combined push-pull approach of a substituted pentaphenylborole.

A donor-supported silavinylidene and silylium ylides: boroles as a flexible platform for versatile Si(ii) chemistry.

Electron-deficient, anti-aromatic 2,5-disilyl boroles are shown to be a flexibly adaptive molecular platform with regards to SiMe3 mobility in their reaction with the nucleophilic donor-stabilised precursor dichloro silylene SiCl2(IDipp). Depending on the substitution pattern, selective formation of two fundamentally different products of rivalling formation pathways is achieved. Formal addition of the dichlorosilylene gives the 5,5-dichloro-5-sila-6-borabicyclo[2.1.1]hex-2-ene derivatives. Under kinetically controlled conditions, SiCl2(IDipp) induces 1,3-trimethylsilyl migration and adds exocyclically to the generated carbene fragment giving an NHC-supported silylium ylide. In some cases interconversion between these compound classes was triggered by temperature or NHC-addition. Reduction of silaborabicyclo[2.1.1]hex-2-ene derivatives under forcing conditions gave clean access to recently described nido-type cluster Si(ii) half-sandwich complexes of boroles. Reduction of a NHC-supported silylium ylide gave an unprecedented NHC-supported silavinylidene which rearranges to the nido-type cluster at elevated temperatures.

Borole-based half-sandwich complexes of germanium and tin.

The synthesis and initial observations regarding the reactivity of borole-based half-sandwich complexes with apical divalent group 14 elements germanium and tin are described. The 2,5-disilylborole substitution pattern allows their access via salt metathesis of the respective borole dianions.

2,5-Bis-trimethylsilyl substituted boroles.

This manuscript includes a comprehensive study of the synthesis and spectroscopic features of 2,5-disilyl boroles. Reacting boron trichloride BCl3 with 2,3-Ph*2-1,4-(SiMe3)2-1,4-dilithiobuta-1,3-diene (Ph* = 3,5-t-Bu2(C6H3)) allowed reliable access to 1-Chloro-2,5-(SiMe3)2-3,4-(Ph*)2-borole in good yields (60%). Unlike 2,3,4,5-tetraphenyl haloboroles, this 2,5-bis-trimethylsilyl substituted chloroborole is thermally stable in solution to up to 130 °C. Metathesis reactions of the chloroborole with metal aryls or of the dilithiobutadiene with arylboron dihalides grant access to 1-Ar-2,5-(SiMe3)2-3,4-(Ph*)2 boroles (Ar = Ph, Mes, Ph*, C6F5). Unlike the generally intensely blue-green 2,3,4,5-tetraaryl boroles, brightly orange/red 2,5-bis-trimethylsilyl substituted boroles reveal blue-shifted π/π*-transitions due to a lack of π-system interaction between borole and 2,5-bound aryls. Light is shed on the synthetic peculiarities for the synthesis of 2,5-disilyl-boroles. While direct treatment of the respective 1,1-dimethyl-stannole with ArBCl2via otherwise well-established B/Sn exchange reactions fails, the selectivity of reactions of 2,3-Ph*2-1,4-(SiMe3)2-1,4-dilithiobuta-1,3-diene with ArBCl2 is solvent dependent and leads to rearranged 3-borolenes in hydrocarbons. Gutmann-Beckett analysis reveal reduced Lewis-acidity of disilylboroles compared to pentaphenyl borole.