Electronically Driven Regioselective Iridium‐Catalyzed C−H Borylation of Donor‐π‐Acceptor Chromophores Containing Triarylboron Acceptors

Abstract We observed a surprisingly high electronically driven regioselectivity for the iridium‐catalyzed C−H borylation of donor‐π‐acceptor (d‐π‐A) systems with diphenylamino (1) or carbazolyl (2) moieties as the donor, bis(2,6‐bis(trifluoromethyl)phenyl)boryl (B(FXyl)2) as the acceptor, and 1,4‐phenylene as the π‐bridge. Under our conditions, borylation was observed only at the sterically least encumbered para‐positions of the acceptor group. As boronate esters are versatile building blocks for organic synthesis (C−C coupling, functional group transformations) the C−H borylation represents a simple potential method for post‐functionalization by which electronic or other properties of d‐π‐A systems can be fine‐tuned for specific applications. The photophysical and electrochemical properties of the borylated (1‐(Bpin)2) and unborylated (1) diphenylamino‐substituted d‐π‐A systems were investigated. Interestingly, the borylated derivative exhibits coordination of THF to the boronate ester moieties, influencing the photophysical properties and exemplifying the non‐innocence of boronate esters.


General experimental details
Unless otherwise noted, the following conditions apply.
HRMS were recorded using a Thermo Scientific Exactive Plus Orbitrap MS system with an Atmospheric Sample Analysis Probe (ASAP).
Single-crystal X-ray diffraction: Crystals suitable for single-crystal X-ray diffraction were selected, coated in perfluoropolyether oil, and mounted on MiTeGen sample holders. Diffraction data were collected on Bruker X8 Apex II 4-circle diffractometers with CCD area detectors using Mo-Kα radiation monochromated by graphite (1) or multi-layer focusing mirrors (1-(BPin)2). Diffraction data of 2-(Bpin)2 were collected on a RIGAKU OXFORD DIFFRACTION XtaLAB Synergy diffractometer with a semiconductor HPA-detector (HyPix-6000) and multi-layer mirror monochromated Cu-Kα radiation. The crystals were cooled using an open flow N2 Oxford Cryostream or a Bruker Kryoflex II low-temperature device. Data were collected at 100 K. The images were processed and corrected for Lorentzpolarization effects and absorption as implemented in the Bruker or Rigaku OD (CrysAlis Pro ) software packages. The structures were solved using the intrinsic phasing method (SHELXT) [5] and Fourier expansion technique. All non-hydrogen atoms were refined in anisotropic approximation, with hydrogen atoms 'riding' in idealized positions, by full-matrix least squares against F 2 of all data, using SHELXL [6] software and the SHELXLE graphical user interface. [7] The single crystal of 2-(Bpin)2 was a very thin long needle with long reflections spanning several frames. This led to partly too high reflection intensities from data reduction, which is a result of overlap with other reflections' tails. Though residual values are high, the structure solution can be considered as a proof of the conformation of the molecule. Diamond [8] software was used for graphical representation. Other structural information was extracted using OLEX2 [9] software. Crystal data and experimental details are listed in Table  S1; full structural information has been deposited with the Cambridge Crystallographic Data Centre: CCDC-1999432 (1), 1999433 (1-(Bpin)2), and 2003436 (2-(Bpin)2).
Photophysical measurements: All measurements were performed in standard quartz cuvettes (1 cm x 1 cm cross-section). UV-visible absorption spectra were recorded using an Agilent 8453 diode array UV-visible spectrophotometer.
Emission spectra were recorded using an Edinburgh Instruments FLSP920 spectrometer equipped with a double monochromator for both excitation and emission, operating in rightangle geometry mode, and all spectra were fully corrected for the spectral response of the instrument. All solutions used for photophysical measurements had a concentration lower than 1 × 10 -5 M to minimize inner filter effects during fluorescence measurements.
Fluorescence quantum yields were measured using a calibrated integrating sphere (inner diameter: 150 mm) from Edinburgh Instruments combined with the FLSP920 spectrometer described above. For solution-state measurements, the longest-wavelength absorption maximum of the compound in the respective solvent was chosen as the excitation wavelength.
Fluorescence lifetimes were recorded using the time-correlated single-photon counting (TCSPC) method using the same FLSP920 spectrometer described above. Solutions were excited with a picosecond pulsed diode laser at an emission maximum of 376.6 nm. The full widths at half maximum (FWHM) of the laser pulses were ca. 50-100 ps, while the instrument response function (IRF) had a FWHM of ca. 1.0 ns, measured from the scatter of a Ludox solution at the excitation wavelength. Decays were recorded to at least 10000 counts in the peak channel with a record length of at least 1000 channels. The band pass of the monochromator was adjusted to give a signal count rate of <10 kHz. Iterative reconvolution of the IRF with one decay function and non-linear least-squares analysis were used to analyze the data. The quality of the fit was judged by the calculated value of the reduced χ 2 and visual inspection of the weighted residuals.
Electrochemical measurements: All cyclic voltammetry experiments were conducted in an argon-filled glovebox using a Gamry Instruments Reference 600 potentiostat. A standard three-electrode cell configuration was employed using a platinum disk working electrode, a platinum wire counter electrode, and a silver wire reference electrode separated by a Vycor frit, serving as the reference electrode. The redox potentials are referenced to the ferrocene/ferrocenium ([Fc/Fc + ]) redox couple by using decamethylferrocene ([Cp*2Fe]; E1/2 = -0.532 V in CH2Cl2) as an internal standard. Tetra-n-butylammonium hexafluorophosphate ([nBu4N][PF6]) was employed as the supporting electrolyte. Compensation for resistive losses (iR drop) was employed for all measurements Theoretical Studies: All calculations (DFT and TD-DFT) were carried out with the Gaussian 09 (9.E.01) [10] program package and were performed on a parallel cluster system. GaussView (6.0.16), Avogadro [11] and multiwfn [12] were used to visualize the results, to measure calculated structural parameters, and to plot orbital surfaces (isovalue: ± 0.030 [e a0 -3 ] 1/2 ). The ground-state geometries were optimized using the B3LYP functional [13] in combination with the 6-31+g(d) basis set. [14,15] The D3 dispersion correction of Grimme and coworkers was used. [16] The ultrafine integration grid and symmetry constraints were used for all molecules. Frequency calculations were performed on the optimized structure to confirm that it was a local minimum showing no negative (imaginary) frequencies. Based on the optimized structure, the lowest-energy vertical transitions (gas-phase and solvent correction using the polarizable continuum model) were calculated (singlets, 25 states) by TD-DFT, using the Coulomb attenuated functional CAM-B3LYP. [17] The CAM-B3LYP has been shown to describe ICT systems more accurately than B3LYP. [18]