Synthesis of 2,3,6,7-tetrabromoanthracene

The first synthesis of 2,3,6,7-tetrabromoanthracene is presented, starting from benzene in a straightforward four step synthesis.


Introduction
Anthracene and its derivatives are long known polycyclic aromatic compounds showing a high potential for use in materials science (e.g. fluorescence probing, photochromic systems, electroluminescence) and several reviews have been published so far [1][2][3]. Anthracenes may be readily functionalized in positions 9 and 10 due to their exceptional reactivity. The outer rings, however, can not be functionalized easily. There are some anthracene compounds available with one or two substituents at the 1-or 2-positions (such as 1-or 2-aminoanthracene), but to make the 2-, 3-, 6-and 7-position chemically available for further reactions, great effort is necessary. The only exception is maybe 2,3,6,7-tetramethylanthracene, which was first reported in 1931 by Morgan and Coulson [4].

Results and Discussion
In Scheme 1, the reaction pathway is shown, starting from benzene, which is iodinated four times to give 1 in 71% yield by the method of Mattern [16]. The next step is a fourfold  Sonogashira-Hagihara coupling reaction with trimethylsilylacetylene. We slightly modified the procedure reported by Vollhardt et al. [17] and used tetrakis(triphenylphosphine)palladium(0) as catalyst, obtaining 2 in 92% yield. In the following step, the trimethylsilyl groups were substituted by bromine atoms, analogous to the procedure reported by Bowles and Anthony [6]. The reaction yielded nearly 100% of 3 as a slightly yellow solid after flash chromatography with dichloromethane on silica gel. Although we succeeded in characterizing 3 by 1 H NMR, the neat product is not stable. In fact, dry 3 is explosive and it should be used directly for the final step of the synthesis. In a first attempt, 1,2-dichlorobenzene was used as high boiling solvent and γ-terpinene as hydrogen donor leading to 2,3,6,7-tetrabromoanthracene (4), which was isolated from possible byproducts by column chromatography and recrystallized from ethanol. However, even after this work-up, the solvent was still present according to NMR analysis. In a second attempt, the reaction took place in benzene at 180-200 °C in a steel bomb, using 1,4-cyclohexadiene as hydrogen donor. In this case, the structure and the purity of 4 were proven by NMR and high resolution mass spectrometry.
The UV/VIS-spectra of 2,3,6,7-tetrabromoanthracene and anthracene in cyclohexane are shown in Figure 1. At first glance, both spectra resemble each other; however, all absorption bands are bathochromically shifted (~25 nm difference) and the typical anthracene absorption is broadened for 4 due to the heavy atom effects of the bromo substituents. Both spectra are cut off at 220 nm. Below this wavelength, the absorbance for both substances decreases slightly until the solvent (cyclohexane) starts to absorb.
In conclusion, we developed a straightforward four step synthesis of 2,3,6,7-tetrabromoanthracene, starting from benzene by using a double Bergman cyclization as key step.

Experimental
The NMR spectra were recorded on a 500 MHz NMR spectrometer (Bruker, DRX 500). GC/MS spectra were measured on the Shimadzu GC17A/GCMS-QP5050 mass spectrometer equipped with a standard EI source. High resolution EI mass spectra were recorded using an Autospec X magnetic sector mass spectrometer with EBE geometry (Vacuum Generators, Manchester, UK) equipped with a standard EI source. Samples were introduced by push rod aluminium crucibles. Ions were accelerated by 8 kV in EI mode. Benzene was dried over molecular sieve 4 Å for at least 24 h prior to use. All other solvents and chemicals were used without further purification. Melting points were measured on the B-540 (Büchi) and were not corrected.