Synthesis, Stability, and Crystal Structure of an Azulenium Cation Containing an Adamantyl Group

Abstract

AbstractStarting from the trimethylsilyl enol ether of 1‐acetyl‐1,3,5‐cycloheptatriene, the title 1H‐azulenium cation was synthesized by a five‐step sequence that involved a Noyori–Mukaiyama aldol reaction, a Nazarov cyclization, a Shapiro reaction, and a hydride abstraction. The Nazarov reaction of the aldol‐type adduct resulted in the formation of an unusual double‐bond position isomer, which has never been obtained in similar reactions forming tetrahydroazulenones. The pKR+ value of the title cation was found to be 9.8, which is less than that expected by inductive stabilization from the number of carbons at the 1‐position. The X‐ray crystal structure of the title cation (ClO4– salt) reveals CH–O interactions and deformation of the azulenyl ion part.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)