Noncatalytic [2+2] Cycloaddition Providing Donor‐Acceptor Type 2‐Azetines Featuring the 2‐Aminophenyl Groups

In contrast to the previously reported TiCl 4 ‐catalyzed [2+2] synthesis of 2‐azetines using internal alkynes and ethyl 3,3,3‐trifluoro‐2‐((methylsulfonyl)imino)propanoate [CF 3 C(═NMs)CO 2 Et], electron‐abundant terminal alkynes do not necessarily require a catalyst for the [2+2] cycloaddition process. In this study, we demonstrate that the 2‐azetines, which were produced by using 2‐ethynyl‐ N , N ‐dialkylanilines (ArC≡CH, Ar = 2‐R 2 NC 6 H 4 , R = alkyl), showed blue fluorescence (Φ = 4∼9%). The structural elucidation, using X‐ray crystallographic analysis and density functional theory (DFT) calculations, of the fluorescent 2‐azetines involved the pyramidalized flanking amino groups, as steric congestion prevented conventional π‐conjugation between the amino and phenylene units. It is noteworthy that the C═C unit in the 2‐azetine cycle plays a crucial role in the fluorescent character, and saturation of the double bond afforded the nonfluorescent azetidine derivative. Thus, the combination of the electron‐donating pyramidal amino group and the electron‐accepting π‐molecular skeleton, including the appropriate bridging aryl spacer, is a promising molecular design for developing unique photo‐functional N‐heterocyclic materials. In addition, to investigate the o ‐aminophenyl effect on the 2‐azetine framework, we employed the azole‐substituted phenylacetylenes for the [2+2] cycloaddition. 2‐( N ‐Pyrrolyl) and 2‐( N ‐indolyl)phenylacetylenes facilitated the [2+2] cycloaddition process predominantly and afforded the corresponding 2‐azetines exclusively, whereas the [2+2] synthesis reported so far included the byproducts.