This manuscript includes a comprehensive study of the synthesis and spectroscopic features of 2,5-disilyl boroles.
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.
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‐(MeNHC)‐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.
Establishing access to a bulky tetraaryl dilithiobutadiene (Ph*C)4Li2 (Ph*=3,5‐tBu2(C6H3)) 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)4BAr 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)4BPh. 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‐C6H3 (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.
Structurally authenticated free B-alkylb oroles are presented and electronic implications of alkylsubstitution were assessed. Deprotonation of ab oron-bound exocyclic methyl group in aB -methyl borole yields the first 5-boratafulvene anion-an isomer to boratabenzene.Boratafulvene was structurally characterized and its electronic structure probed by DFT calculations.T he pK a value of the exocyclic B À CH 3 in as et of boroles was computationally approximated and confirmed apronounced acidic character caused by the boron atom embedded in an anti-aromatic moiety.The non-aromatic boratafulvene reacts as aC-centered nucleophile with the mild electrophile Me 3 SnCl to give as tannylmethyl borole,r egenerating the anti-aromaticity.A sn ucleophilic synthons for boroles,b oratafulvenes thus open an entirely new avenue for synthetic strategies towardt his highly reactive class of heterocycles.Boratafulvene reacts as amethylene transfer reagent in abora-Wittig-type reaction generating aborole oxide.
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