Piano-Stool Rhodium Enalcarbenoids: Application to Catalyst-Controlled Metal-Templated Annulations of Diazoenals and 1,3-Dicarbonyls

An electrophilic piano-stool rhodium­(III)-enalcarbenoid resulted from the reaction of diazoenal with the cationic Cp*RhIII in the presence of a 1,3-diketone. The synthetic utility of these transient carbenoids has been demonstrated in the metal-templated [3 + 2] annulation of diazoenals and 1,3-dicarbonyls, thus leading to the enal-functionalized tetrasubstituted furans. The significance of the piano-stool enalcarbenoids has been further exemplified by the mechanistically distinct, complementary Lewis acid templated [2 + 3] annulation of diazoenals and 1,3-dicarbonyls, resulting in the trisubstituted furanyl-enones and acrylates. Mechanistic investigations revealed that these annulations proceed through catalyst-dependent chemoselective activation of diazoenal by the in situ formed metal diketonates. These methodologies gave access to core structures of indeno­[1,2-b]­furans, tetracyclic OLED, and a pan-AKT inhibitor.