Tris(2,4,6‐trifluorophenyl)borane: An Efficient Hydroboration Catalyst

Abstract The metal‐free catalyst tris(2,4,6‐trifluorophenyl)borane has demonstrated its extensive applications in the 1,2‐hydroboration of numerous unsaturated reagents, namely alkynes, aldehydes and imines, consisting of a wide array of electron‐withdrawing and donating functionalities. A range of over 50 borylated products are reported, with many reactions proceeding with low catalyst loading under ambient conditions. These pinacol boronate esters, in the case of aldehydes and imines, can be readily hydrolyzed to leave the respective alcohol and amine, whereas alkynyl substrates result in vinyl boranes. This is of great synthetic use to the organic chemist.


General experimental
With the exception of the starting materials, all reactions and manipulations were carried out under an atmosphere of dry, O2-free nitrogen using standard double-manifold techniques with a rotary oil pump.
A nitrogen-filled glove box (MBraun) was used to manipulate solids including the storage of starting materials, room temperature reactions, product recovery and sample preparation for analysis. All starting materials other than those synthesized as outlined below were purchased from commercial sources (Sigma-Aldrich, Alfa Aesar, Acros, Fluorochem, TCI) and were used as received without subsequent purification. All solvents (toluene, CH2Cl2, hexane) were dried by employing a solvent purification system MB SPS-800 and stored under a nitrogen atmosphere. Deuterated solvents were distilled and/or dried over molecular sieves before use. Chemicals were purchased from commercial suppliers and used as received. 1 H, 13 C, 11 B, 19 F and 29 Si NMR spectra were recorded on a Bruker Avance II 400 or Bruker Avance 500 spectrometers. Chemical shifts are expressed as parts per million (ppm, δ) downfield of tetramethylsilane (TMS) and are referenced to CDCl3 (7.26/77.16 ppm) as internal standards. NMR spectra were referenced to BF3·Et2O/CDCl3 ( 11 B), CFCl3 ( 19 F) and Me4Si ( 29 Si). The description of signals include: s = singlet, d = doublet, t = triplet, q = quartet, sep = septet, m = multiplet and br. = broad. All coupling constants are absolute values and are expressed in Hertz (Hz). 13 C NMR was measured as 1 H decoupled. Yields are given as isolated yields. All spectra were analyzed assuming a first order approximation. Mass spectra were measured in house on a Waters LCT Premier/XE or a Waters GCT Premier spectrometer.

General procedure d:
In accordance with the literature known procedure [7] the requisite aldehyde (10 mmol) was dissolved in CH2Cl2 (10 mL) along with 3 Å molecular sieves. To this the necessary amine (10 mmol) was added. The reaction was left at ambient temperature for 2 hours at which point MgSO4 was added with subsequent filtration. Volatiles were removed in vacuo to leave the pure imine in quantitative yields.
General procedure g: As per general procedure e and f, at room temperature.
In accordance with general procedure f using prop-2-yn-
Spectroscopic data agrees with literature values. [
Spectroscopic data agrees with literature values. [
Crystallographic studies were undertaken of a single crystal mounted in paratone and studied on an Agilent SuperNova Dual Atlas three-circle diffractometer using Cu-Kα radiation and a CCD detector. Measurements were carried out at 150(2) K with temperatures maintained using an Oxford cryostream unless otherwise stated. Data were collected and integrated and data corrected for absorption using a numerical absorption correction based on gaussian integration over a multifaceted crystal model within CrysAlisPro. [26] The structures were solved by direct methods and refined against F 2 within SHELXL-2013. [27] A summary of crystallographic data are available as ESI and the structures deposited with the Cambridge Structural Database (CCDC deposition numbers 1556722-1556723). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.