Mechanosynthesis of Odd‐Numbered Tetraaryl[n]cumulenes

Abstract A mechanochemical synthesis of one‐dimensional carbon allotrope carbyne model compounds, namely tetraaryl[n]cumulenes (n=3, 5) was realized. Central for the mechanosynthesis of the cumulenic carbon nanostructures were the development of a mechanochemical Favorskii alkynylation‐type reaction and the implementation of a solvent‐free, acid‐free reductive elimination with tin(II) chloride by ball milling.


General information
All chemicals are commercial available and they were used as received unless otherwise stated. Calcium carbide was purchased from Alfa Aesar® (technical grade CaC 2 , 74.24 % purity). Thin-layer chromatography (TLC) was performed using TLC plates (silica gel 60 on aluminum or glass with fluorescence indicator F254) from MERCK. Qualitative analysis of the TLC plates was carried out using UV light (λ = 254 nm and λ = 366 nm) and/or by immersion in an aqueous solution of potassium permanganate (KMnO 4 ) and heating of the stained plates with a heat-gun at 300 °C until dryness. Products were purified by column chromatography using silica gel 60 (40−63 µm) from ACROS Organics. Solvents for column chromatography were distilled prior to use. Mechanochemical reactions were carried out in a in a FRITSCH planetary micro mill model "Pulverisette 7 classic line". All NMR spectra were recorded on a VNMRS 400 or on a VNMRS 600 spectrometer. Proton chemical shifts are reported in parts per million on the δ scale and are calibrated using the residual non-deuterated solvent signal as an internal reference. Spectral data is provided as follows: chemical shift in ppm (from downfield to upfield), multiplicity (s = singlet, d = doublet, m = multiplet), integration and coupling constant J. IR-spectra were recorded on a PERKIN ELMER 100 FT/IR spectrometer with an ATR-Unit. Mass spectra were recorded on a FINNIGAN SSQ7000 (EI 70 eV) spectrometer and high-resolution mass spectra on a THERMO FISHER Scientific Orbitrap XL spectrometer. Powder X-Ray Diffraction (PXRD) patterns were collected on a Bruker D2 phaser X-ray diffractometer.

Mechanochemical Favorskii reaction
Inside a glovebox, CaC 2 (70.4 mg, 1.098 mmol) (of technical grade CaC 2 , 74.24 % purity, real amount used 0.815 mmol) and KOH (123.16 mg, 2.19 mmol) were added in a 12 mL ZrO 2 milling jar with 20 ZrO 2 balls of 5 mm. The jar was closed tightly, sealed with electrical tape, taken out of the glovebox, and milled for 30 min at 800 rpm. The jar, still taped, was placed again inside the glovebox, there, it was opened and 1 (0.54 mmol) was added. The jar was closed and as mentioned before, electrical tape was used to seal it. Once the container was outside the glovebox, the mixture was milled for 3 h at 800 rpm. After the milling was stopped, the milling jar was open outside the glovebox and HCl 1M (10 mL) was added. Then, the aqueous phase was extracted with ethyl acetate (3 × 15 mL). The organic fraction was concentrated under vacuum and the product was purified by flash column chromatography on silica gel using a gradient of ethyl acetate in npentane.
Five-fold scale up experiments were carried out in a 45 mL ZrO 2 milling jar with 5 ZrO 2 balls of 10 mm, and 14 ZrO 2 balls of 5 mm, under otherwise identical conditions.

Mechanosynthesis of [3]cumulenes
A mixture of 2 (0.12 mmol, 1.0 equiv) and SnCl 2 ·2H 2 O (31.8 mg, 0.14 mmol, 1.1 equiv), were transferred into a ZrO 2 milling jar (12 mL volume) equipped with 20 ZrO 2 balls of 5 mm and milled at 400 rpm for 60 min. After the milling was stopped, the reaction mixture was dissolved in a minimal amount of dichloromethane and filtered through a pad of Celite® to remove the inorganic residue. Alternatively, extraction using water (20 mL) and ethyl acetate (3 × 15 mL) or purification by flash column chromatography on silica gel using a gradient of ethyl acetate in n-pentane also led to the isolation of cumulenes 4 in similar yields.

Mechanochemical homocoupling of 3a
A mixture of 3a (100 mg, 0.48 mmol, 1.0 equiv), CuCl (47.6 mg, 0.48 mmol, 1.0 equiv) and TMEDA (55.8 mg, 0.48 mmol, 1.0 equiv), were milled in a 12 mL ZrO 2 milling jar with 20 ZrO 2 balls of 5 mm. Once the milling was stopped, the reaction mixture was recovered and extracted with water (20 mL) and ethyl acetate (3 × 15 mL). Then the organic phase was concentrated in vacuum and the product was purified by flash column chromatography on silica gel using a gradient of ethyl acetate in n-pentane.

Mechanosynthesis of [5]cumulene 6a
A mixture of 5a (50 mg, 0.12 mmol, 1.0 equiv) and SnCl 2 ·2H 2 O (32.6 mg, 0.14 mmol, 1.2 equiv), were transferred into a ZrO 2 milling jar (12 mL volume) equipped with 20 ZrO 2 balls of 5 mm and milled at 400 rpm for 60 min. After the milling was stopped, the reaction mixture was dissolved in a minimal amount of dichloromethane and filtered through a pad of Celite® to remove the inorganic residue.     The spectral data for this compound match that reported in the literature. [3]   The spectral data for this compound match that reported in the literature. [4] IR (ATR) ν (cm  The spectral data for this compound match that reported in the literature. [5] IR (ATR) ν (cm          The spectral data for this compound match that reported in the literature. [5] IR (ATR) ν (cm  Chloro-1,1,4,4-tetraphenylbuta-2,3-dien-1-ol (4a  Cl OH The spectral data for this compound match that reported in the literature. [  The spectral data for this compound match that reported in the literature. [ The spectral data for this compound match that reported in the literature. [3] IR (ATR) ν