page_title: Tellurophenes
section_title: Frustrated Lewis Pair chemistry
hierarchical_section_title: Tellurophenes / Reactivity / Frustrated Lewis Pair chemistry
caption_attribution_description: English: Synthesis of tellurium-boron heterocycles
context_page_description: Tellurophenes are the tellurium analogue of thiophenes and selenophenes.
context_section_description: In 2015, Stephan et al. reported a vinyl telluroether with a pendant borane which acted as an intramolecular frustrated Lewis pair (FLP). This was achieved by reacting tellurium acetylide with B(C₆F₅)₃ in pentane at room temperature, affording bright orange crystals in 94% yield. It was found through ¹¹B NMR that the product had a four-coordinate boron, which indicated a weak Te-B interaction due to the broad signal. Reacting this compound with phenylacetylene at room temperature resulted in a cis-1,2-addition across the alkyne bond, generating a zwitterionic, six-membered Te-B heterocycle, as observed using X-ray diffraction spectroscopy. It was found that the coordination geometries around tellurium and boron were pseudo-trigonal pyramidal, and pseudo-tetrahedral, respectively. The C-C bond length was observed to be 1.326(4) Å, much longer than a C-C triple bond and closer to a C-C double bond, indicating that the compound had activated phenylacetylene through FLP chemistry. However, unlike other reported FLP compounds, it was unable to activate H₂ or bind CO₂, which was attributed to the fact that telluroethers are poor nucleophiles. Although the telluroether did not undergo oxidation by halogens to produce the corresponding Te(IV) dihalide compounds, it was found to react with iodine to afford a five-membered Te-B-I heterocycle.
  Later, Stephan et al. reported the synthesis of various Te-B heterocycles through reaction of 1-bora-4-tellurocyclohexa-2,5-diene and two equivalents of a terminal alkyne upon heating, with loss of a diarylalkyne. X-ray crystal studies revealed that the C-C bond distances in the heterocycle were close to that of C-C double bonds, indicating delocalization within the molecule. The reaction was proceeded with high regioselectivity, with the terminal CH carbon of the alkyne attaching to the boron. Consistent with this observation, it was proposed that the Te and B act as a FLP, undergoing a [4+2] cyclcoaddition to the alkyne such that the boron adds to the CH carbon, due to the steric bulk at the boron center. In 2018, this FLP chemistry was developed further through the synthesis of 4H-1,4-telluraborine, which was found to be a useful hydroboration reagent for alkenes, ketones, and aldehydes.
caption: Synthesis of tellurium-boron heterocycles[32]
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