Selective ring expansion and C−H functionalization of azulenes

Abstract

We report a transition metal-catalyzed ring expansion of azulene that can be contrasted with C–H functionalization. This study represents the first example of the successful ring expansion of azulene using Cu(hfacac)2 (hfacac: hexafluoroacetylacetonate) with a diazo reagent. This result is notable for extending the Buchner reaction, previously limited to benzenoid aromatics, to nonbenzenoid compounds. The chemoselectivity of the reaction can be directed towards C–H functionalization by substituting the Cu catalyst with AgOTf. This approach does not require the addition of phosphine, NHC, or related ligands, and prefunctionalization of azulenes is unnecessary. Furthermore, the method exhibits excellent functional group tolerance, allowing for the synthesis of a wide range of 6,7-bicyclic compounds and C–H functionalized azulenes. We also present a theoretical study that explains the experimental observations, explaining why copper afford the ring expansion product while silver forms the C–H alkylation product.

Access to cyclohexadiene and benzofuran derivatives via catalytic arene cyclopropanation of α-cyanodiazocarbonyl compounds

“…5,6 In simple and unconstrained systems, the cyclopropane ring strain (∼28 kcal mol −1 ) associated with the NCD generally drives the equilibrium toward the side of the CHT tautomer, 6 as reflected by the results from many reactions of α-diazoacetates or α-diazoketones with aromatic substrates. 7 However, there are several factors that may alter the position of the equilibrium to instead favour the NCD, which include geometric constraint, 8 decomposition methods, 2 b ,7 a or the electronic nature of the attached substituents 6,9 particularly for those at the C-7 position. 6 The reaction of dicyanodiazomethane in benzene, studied by Ciganek, delivered 7,7-dicyanonorcaradiene as the first reported example of a stable norcaradiene.…”