Two-Fold Oxidative Coupling of Furan with Indole Provides Modular Access to a New Class of Tetra-(Hetero)Arylated Furans with Up to Four Different Substituents

Highly arylated propeller-shaped heteroarenes constitute an intriguing class of molecular scaffolds for material science applications. Among these, tetraarylated furans demonstrate differentiated properties as compared to other similar heterocyclic cores. The synthetic complexity to access tetraarylated furans increases significantly with increasing number of different peripheral aryl groups. There are only a very limited number of methodologies available to access furans with four different (hetero)aryl substituents. Notably, none of these involve direct oxidative coupling on the furan core as the method of choice. Herein, we report the first methodology based on a sequential two-fold oxidative C-C coupling of furans with indoles to access bis(indolyl)furans (BIFs) - a new class of 'extremely congested' tetra-(hetero)arylated furans with up to four different substituents. The reaction is mediated by inexpensive, earth-abundant FeCl3⋅6H2O and displays high efficiency, wide substrate scope, modularity and aqueous compatibility. Moreover, we also present the first validation of the distinct aggregation-caused quenching (ACQ) property of the tetraarylated furans beyond only phenyls as peripheral groups and disclose new mechanistic underpinnings for the same.

A Versatile Route to Acyl (MIDA)Boronates

A modular approach to highly functional acyl (MIDA)boronates is described. It involves the generation of the hitherto unknown radical derived from acetyl (MIDA)boronate and its capture by various alkenes, including electronically unbiased, unactivated alkenes. In contrast to the anion of acetyl (MIDA)boronate, which has not so far been employed in synthesis, the corresponding radical is well behaved and readily produced from the novel α-xanthyl acetyl (MIDA)boronate. This shelf-stable, easily prepared solid is a convenient acyl (MIDA)boronate transfer agent that provides a direct entry to numerous otherwise inaccessible structures, including latent 1,4-dicarbonyl derivatives that can be transformed into B(MIDA) substituted pyrroles and furans. A competition experiment indicated the acyl (MIDA)boronate substituted radical to be more stable than the all-carbon acetonyl radical but somewhat less reactive in additions to alkenes.

Palladium-Catalyzed Cyclizative Borylation of Allenyl Ketones through Carbene Boryl Migratory Insertion: Access to Densely Substituted Furyl Boronates

Herein the palladium-catalyzed cyclizative borylation of allenyl ketones with diboron compounds is reported which involves the carbene boryl migratory insertion as the key step. This reaction features mild conditions, good functional group tolerance and broad substrate scope. Thus, it represents an efficient methodology for the assembly of diverse tri-substituted furyl boronates. In addition, a series of transformations of the resultant multi-substituted furyl boronates were conducted to provide various densely substituted furan derivatives in good yields, further illustrating the potential synthetic utility of this methodology.