Front Cover: Non-conventional Behavior of a 2,1-Benzazaphosphole: Heterodiene or Hidden Phosphinidene? (Chem. Eur. J. 52/2021)

The titled 2,1-benzazaphosphole ( 1 ) (i. e. ArP, where Ar=2-(DippN=CH)C₆H₄, Dipp=2,6-iPr₂C₆H₃) showed a spectacular reactivity behaving both as a reactive heterodiene in hetero-Diels-Alder (DA) reactions or as a hidden phosphinidene in the coordination toward selected transition metals (TMs). Thus, 1 reacts with electron-deficient alkynes RC≡CR (R=CO₂Me, C₅F₄N) giving 1-phospha-1,4-dihydro-iminonaphthalenes 2 and 3 , that undergo hydrogen migration producing 1-phosphanaphthalenes 4 and 5 . Compound 1 is also able to activate the C=C double bond in selected N-alkyl/aryl-maleimides RN(C(O)CH)₂ (R=Me, tBu, Ph) resulting in the addition products 7–9 with bridged bicyclic 2.2.1] structures. The binding of the maleimides to 1 is semi-reversible upon heating. By contrast, when 1 was treated with selected TM complexes, it serves as a 4e donor bridging two TMs thus producing complexes μ-ArP(AuCl)₂] ( 10 ), (μ-ArP)₄Ag₄]X]₄ (X=BF₄ ( 11 ), OTf ( 12 )) and μ-ArP(Co₂(CO)₆)] ( 13 ). The structure and electron distribution of the starting material 1 as well as of other compounds were also studied from the theoretical point of view.