Rhodium(III)-Catalyzed C–H/N–H Alkyne Annulation of Nonsymmetric 2-Aryl (Benz)imidazole Derivatives: Photophysical and Mechanistic Insights

Abstract

Rhodium(III) catalysis enabled C−H/N−H alkyne annulation of nonsymmetric imidazole derivatives. This study encompasses the synthesis of imidazoles from a naturally occurring quinoidal compound and their use for the preparation of rigid πextended imidazole derivatives with outstanding fluorescence. Our study also brings to light the photophysical aspects and the mechanism of the reaction studied via computational calculations. This method provided an efficient and versatile tool for the synthesis of fluorescent compounds with a wide range of chemical and biological applications.

Ruthenium(II)‐Catalyzed C−H/N−H Alkyne Annulation of Nonsymmetric Imidazoles: Mechanistic Insights by Computation and Photophysical Properties

“…The time‐resolved measurements of the excited states of the solutions and films, presented next, can provide us, at least in a qualitative way, a better understanding of the conformational states of the studied systems and their possible aggregation character (J‐type or H‐type, vide infra ) [36] . The red‐shift of the ABS band for the 3 d’ solution and film is still larger than the one for the 3 d solution (see the arrows in Figure 5 (II)), indicating a larger aggregation and, consequently, a higher delocalization or larger averaged conjugation length for this system; there is an inversion of their emission efficiencies, the corresponding Φ value for the 3 d solution is smaller than the one for the 3 d’ solution, contrary of was observed for the other pairs of isomer solutions [21] …”

“…In our previous work, [21] we demonstrated the formation of the desired product 3 a by rhodium‐based catalysis. Compound 3 a was obtained in 59 % yield and 3 a′ in 19 % yield.…”

“…The formation of G from A is exergonic by −14.9 kcal mol −1 , and its demetalation leads to the desired product 3 a . The preference for the formation of 3 a in comparison to its isomer 3 a’ is related to a larger steric hindrance at the aliphatic site of 1 a , which is known to increase the energy of intermediates and transition states associated with this reaction profile [21] …”

“…In our previous work, we demonstrated that the products resulting from the annulation reaction herein described display outstanding fluorescence. Experimental investigations by means of UV−vis absorption (ABS), time‐resolved and steady‐state fluorescence (FL) measurements were performed in a series of solutions and dropcast films at room temperature [21] …”

“…Examples of new methods for the synthesis of nonsymmetric imidazoles are rare in the literature [20] . Based on this observation, in a recent study described by us, we demonstrated the synthesis of nonsymmetric imidazoles via C−H/N−H alkyne annulation reactions using the precatalyst [RhCp*Cl 2 ] 2 , a high‐cost reaction mediator (Scheme 1B) [21] . In connection with this study, we are concerned about the demand for C−H bond activation reactions mediated by cheaper and affordable metal‐based catalysts [22] .…”