The di-π-methane rearrangement is a well-known photochemical reaction in organic chemistry. It can, for example, convert  barrelene (C₈H₈) to its isomer semibullvalene. This reaction is generally possible for compounds consisting of two vinyl moieties connected to an sp³-hybridized carbon atom. Many examples have been reported. Variations with heteroatoms are also known, such as the oxa-di-π-methane and the aza-di-π-methane rearrangement.

Christian Müller, Freie Universität Berlin, Germany, and colleagues have recently discovered that 1-phosphabarrelenes, in which the central CH-group of the barrelene is replaced by an isolobal, trivalent phosphorus atom, can undergo a photochemical rearrangement to 5-phosphasemibullvalenes. This observation expanded the set of established di-π-methane rearrangements to unsaturated organophosphorus compounds.

As a next step in the chemistry of these phosphabarrelenes, the team has demonstrated that the photochemical di-π-methane rearrangement of a new pyridyl-functionalized and bis-CF₃-substituted 1-phosphabarrelene (pictured left) occurs selectively in the coordination sphere of a metal center (L_nM = RH(CO)Cl or W(CO)₄). This led to the first transition-metal complexes containing chelating and chiral 5-phosphasemibullvalene derivatives (pictured right). Such coordination compounds could play an interesting role in transition-metal-mediated transformations and homogeneous catalysis.

• Pyridyl‐Functionalized 1‐Phosphabarrelene: Synthesis, Coordination Chemistry and Photochemical di‐π‐Methane Rearrangement,
  Marlene Bruce, Martin Papke, Andreas W. Ehlers, Manuela Weber, Dieter Lentz, Nicolas Mézailles, J. Chris Slootweg, Christian Müller,
  Chem. Eur. J.2019.
  https://doi.org/10.1002/chem.201903344