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<p>Bridgehead olefins such as anti-Bredt olefins and bicyclo[2.2.0]hex-1(4)-enes possess a highly strained C=C bond, rendering them exceptionally reactive and prone to distinctive chemical behaviour. Almost all reported examples are transient species that have not yet been validated by X-ray crystallography. Formal isoelectronic replacement of carbon with heteroatoms enables the construction of bonding and structural motifs corresponding to their organic counterparts. However, isolated heteroatom-containing analogues of bridgehead olefins have not been structurally validated. Here we report the synthesis of an inorganic bicyclo[2.2.0]hex-1(4)-ene featuring a B<sub>2</sub>N<sub>2</sub>Al<sub>2</sub> framework through a reductive skeletal transformation reaction of diaminodialuminyldiborane. X-ray crystallographic and spectroscopic analyses, supported by quantum chemical calculations, reveal that the nearly planar B<sub>2</sub>N<sub>2</sub>Al<sub>2</sub> skeleton features a highly strained bridgehead B=B bond between tricoordinate and pentacoordinate boron centres. Thermolysis in benzene at 80 °C results in fragmentation accompanied by intramolecular C–H bond activation, whereas sequestration of potassium cations with a cryptand leads to double ring-opening to afford a cumulenyl anion with a B=B=N scaffold.</p><p></p>

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A strained bridgehead unsaturated bond between tricoordinate and pentacoordinate boron

  • 期刊论文

  • 发表日期：

    2026年4月16日

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  • Chenting Yan,
  • Kavita Devi,
  • Rei Kinjo

摘要

Bridgehead olefins such as anti-Bredt olefins and bicyclo[2.2.0]hex-1(4)-enes possess a highly strained C=C bond, rendering them exceptionally reactive and prone to distinctive chemical behaviour. Almost all reported examples are transient species that have not yet been validated by X-ray crystallography. Formal isoelectronic replacement of carbon with heteroatoms enables the construction of bonding and structural motifs corresponding to their organic counterparts. However, isolated heteroatom-containing analogues of bridgehead olefins have not been structurally validated. Here we report the synthesis of an inorganic bicyclo[2.2.0]hex-1(4)-ene featuring a B2N2Al2 framework through a reductive skeletal transformation reaction of diaminodialuminyldiborane. X-ray crystallographic and spectroscopic analyses, supported by quantum chemical calculations, reveal that the nearly planar B2N2Al2 skeleton features a highly strained bridgehead B=B bond between tricoordinate and pentacoordinate boron centres. Thermolysis in benzene at 80 °C results in fragmentation accompanied by intramolecular C–H bond activation, whereas sequestration of potassium cations with a cryptand leads to double ring-opening to afford a cumulenyl anion with a B=B=N scaffold.