While cyclopropane rings have become very common in organic compounds, three-membered rings with heavier main-group elements are often challenging to synthesize. There are examples of aluminirenes, which contain an AlC₂ ring with a double bond, and of phosphirenes, which contain unsaturated PC₂ rings. However, no main-group metallaphosphirenes—which contain both phosphorus and a metal atom—had been synthesized so far.

Douglas W. Stephan and colleagues, University of Toronto, ON, Canada, and colleagues have synthesized phosphaaluminirenes (example pictured). The team reacted the Al(I) complex HC[(CMe)(NDipp)]₂Al (Dipp = 2,6-ⁱPr₂C₆H₃) with either ^tBuC≡P or AdC≡P (Ad = adamantyl) in toluene at room temperature and obtained the desired products in yields of 74 % and 70 %, respectively.

The products are the first examples of both heteroalumirenes and metallaphosphirenes. The compounds were characterized using X-ray diffraction as well as ¹H, ¹³C, and ³¹P NMR spectroscopy, and the bonding situation was investigated using density functional theory (DFT) calculations. According to the researchers, both compounds’ three-membered rings have a moderate three-centered 2π-electron aromaticity. The aluminium atoms in the rings retain Lewis-acidic reactivity.

• Phosphaaluminirenes: Synthons for Main Group Heterocycles,
  Liu Leo Liu, Jiliang Zhou, Levy L. Cao, Douglas W. Stephan,
  J. Am. Chem. Soc. 2019.
  https://doi.org/10.1021/jacs.9b09330