Boroles from alumoles: accessing boroles with alkyl-substituted backbones via transtrielation

The alumole Cp3tAlC4Et4 (Cp3t = 1,2,4-tris(tert-butyl)cyclopentadienyl) is reported to be capable of transferring its butadiene moiety to aryl(dihalo)boranes to generate boroles through aluminum–boron exchange. The products feature a rare alkyl-substituted backbone, which, as shown in other examples, often leads to dimerization due to insufficient steric protection of the antiaromatic borole ring. Sterically crowded aryl groups bound to the boron atom are shown to prevent dimerization, allowing access to the first monomeric derivatives of this type. Results from UV-vis spectroscopy, electrochemistry, and DFT calculations reveal that the alkyl substituents cause remarkable modifications in the optical and electronic properties of the boroles compared to their perarylated counterparts.

Resonances are given as singlet (s), doublet (d), triplet (t), septet (sept) or multiplet (m). Highresolution mass spectrometry (HRMS) data were obtained from a Thermo Scientific Exactive Plus spectrometer. UV-vis spectra were measured on a METTLER TOLEDO UV-vis-Excellence UV5 spectrophotometer at room temperature. Cyclic voltammetry experiments were performed using a Gamry Instruments Reference 600 potentiostat. A standard threeelectrode cell configuration was employed using a platinum disk working electrode, a platinum wire counter electrode, and a silver wire, separated by a Vycor tip, serving as the reference Numbering of the carbon atoms for the assignment of NMR shifts was done in accordance with Figure S1. S3 Figure S1. NMR numbering scheme for the borole compounds.

Synthesis of Me2SnC4Et4
To a solution of C4Et4Li2 (250.0 mg, 1.40 mmol) in benzene (5 mL) was added dropwise a solution of Me2SnCl2 (308.2 mg, 1.40 mmol) in benzene (5 mL), leading to the immediate formation of a colorless solid (LiCl). The reaction mixture was stirred for 30 min at room temperature and then filtered. Evaporation of the solution at a pressure of 1.2 mbar afforded the product as a colorless oil (404.9 mg, 1.29 mmol, 92%). The spectral data match those reported in the literature. 9

NMR spectra of isolated compounds
Note: Additional small signals are observed in the NMR spectra of the boroles, indicating the presence of residual side products that were not completely removed during the workup of the reactions. These signals correspond to either aluminum dibromide or dimethyltin dihalide species. The captions of the NMR spectra also indicate the route (Al or Sn) by which the products were obtained.  Figure S29. UV-vis absorption spectra of 2 (red), 3 (brown), 4 (orange), and 6 (purple) in DCM at 23 °C.

X-ray crystallographic data
The crystal data of 3, 5a and 6a were collected on a RIGAKU XTALAB SYNERGY-R diffractometer with a HPA area detector and multi-layer mirror monochromated CuKa radiation.
The structure was solved using the intrinsic phasing method, 10 refined with the SHELXL program 11 and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically.  Table S2).
These were accomplished by using the gauge-independent atomic orbital (GIAO) 30-32 method.
The corresponding values were obtained by placing dummy atoms in the centers of BC4 rings and at distances of 0.1 Å along the axis perpendicular to the ring centers (see Table S3). For the NICSzz scan, the zz component of the magnetic shielding tensor was used (see Figure S34). 33 Furthermore, the anisotropy of the induced current density (ACID) 34-35 method at the PBE0-D3(BJ)/6-311+G(d,p) level of theory was used to further confirm and support the results (see Figure S35). For this only the relevant π-orbitals were considered.
To analyze the bonding situations in the different systems, the Wiberg bond indices (WBIs) 36 and the Mayer bond order (MBOs) 37 as well as natural bond orbital (NBO) 38 charges were obtained at the PBE0-D3(BJ)/6-311G(d,p) level of theory (see Table S4). For the former two the Multiwfn 3.8 39 tool was used and for the last the NBO 7 38 program package.
The frontier orbitals are plotted in Figure S36 and their energetic level is given at the PBE0-D3(BJ)/6-311+G(d,p)/SMD(benzene) level of theory. For graphic representation GaussView 6.0.16 40 was used.