In this work, we present a powerful synthetic strategy for selectively converting phosphole oxides by reaction with sulfonyl isocyanates into the corresponding sulfonylimino phospholes, which show a decrease or increase in solid-state emission.

The unprecedented phospha‐aza‐Diels–Alder reaction between an activated electron‐poor imine and 2H‐phospholes yields 1‐phospha‐2‐azanorbornenes in a highly chemoselective and moderately diastereoselective reaction. The intermediate 2H‐phospholes, which act as dienes, are formed in situ from the corresponding 1H‐phospholes. Theoretical calculations confirm that the phospha‐aza‐Diels–Alder reaction is of normal electron demand. The reactive P−N bond in 1‐phospha‐2‐azanorbornenes can be cleaved by nucleophiles leading to the formation of 2,3‐dihydrophospholes.

The reduction of the 1-phospha-2-azanorbornene derivate endo-1 with lithium aluminium hydride leads to an unprecedented 1-phosphabicyclo[3.2.1]octa-2,5-diene, while a phospholide anion is formed with lithium. The latter can be protonated resulting in formation of an unusual 2H-phosphole dimer. Furthermore, 3H-phospholes, previously assumed to have no synthetic relevance as intermediates, were proposed to act as dienophile in the dimerisation of 3,4dimethyl-1-phenylphosphole at elevated temperatures based on theoretical calculations.

When fed the right food, the hermit crab depicted in the cover image grows bigger and finally needs a larger shell. This work reports the reactivity of a 1‐phospha‐2‐azanorbornene, the product of a hetero Diels‐Alder reaction between a 2H‐phosphole and an activated imine. Thus, reductive cleavage of the phosphorus−nitrogen bond with LiAlH4 leads to an unprecedented ring expansion with formation of a 1‐phosphabicyclo[3.2.1]octa‐2,5‐diene. The cover image was designed by Dr. Christoph Selg. More information can be found in the Full Paper by E. Hey‐Hawkins et al. (DOI: 10.1002/chem.202100898).

Das erste Beispiel für eine Phospha‐Aza‐Diels‐Alder‐Reaktion zwischen einem elektronenarmen Imin und 2H‐Phospholen liefert 1‐Phospha‐2‐azanorbornene in einer hoch chemoselektiven und mäßig diastereoselektiven Reaktion. Die als Diene fungierenden intermediären 2H‐Phosphole entstehen in situ aus den entsprechenden 1H‐Phospholen. Theoretische Berechnungen bestätigen, dass die Phospha‐Aza‐Diels‐Alder‐Reaktion einen normalen Elektronenbedarf aufweist. Die reaktive P‐N‐Bindung in 1‐Phospha‐2‐azanorbornenen kann durch Nucleophile unter Bildung von 2,3‐Dihydrophospholen gespalten werden.