In the presence of a Pd(0) precatalyst, Pd2(bq)2(nbe)2 or Pd2(dba)3, 1,6-enyne esters were heated in refluxing benzene to give cyclodimers as single regioisomers. On the other hand, the combination of the Pd(0) precatalyst and triphenyl phosphite gave rise to various cycloisomerization products depending on the substitution pattern of the enyne esters. Six-membered ring cycloisomerization products were predominantly obtained from enyne esters bearing methallyl or 2-phenyl-2-propenyl moieties, while other enyne esters afforded normal five-membered ring cycloisomerization products. Intramolecular [2 + 2 + 2] cyclocotrimerization of enediyne esters also proceeded in the presence of the Pd(0) precatalyst and triphenylphosphine to give fused cyclohexadienes.

A novel palladium(IV) complex consisting of a C 2-symmetrical palladaspirocycle framework and an ether ligand was assembled only in a single step from commercially available Pd2(dba)3, tetrachloro-1,2-benzoquinone (o-chloranil), and norbornene. The solid-state structure was confirmed by X-ray analysis of a THF complex. The ligand exchange from diethyl ether to pyridine converted the trigonal-bipyramidal complex to a distorted-octahedral bis(pyridine) complex. Similar palladium(IV) spirocyclic complexes were also formed from the reaction of Pd2(dba)3 with o-chloranil and benzonorbornadiene. The benzonorbornadiene-derived complexes exhibited pronounced stability in both the solid state and solution. The complete decomposition of the mono(pyridine) complex required several hours in C6D6 at 70 °C, and the addition of extra pyridine retarded the decomposition rate. Benzonorbornadiene was obtained as the main organic product, together with two isomeric adducts between o-chloranil and benzonorbornadiene. On the other hand, the decomposition of the pyridine complex took place within 15 min even at −40 to −50 °C, upon exposure to HCl in a CDCl3 solution.

Abstract:The synthesis of azabicycloalkenones bearing a vinylogous amide moiety was achieved by means of the rhodium-catalyzed decarbonylative cycloaddition of cyclobutenediones with a pendant alkene. The starting cyclobutenediones were efficiently prepared from appropriate squaric acid monoesters and N-benzylalkenylamines under microwave heating conditions.

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