Palladium‐Catalyzed Oxidative Arylating Carbocyclization of Allenynes

Abstract

Vinylallene oder kreuzkonjugierte Triene werden in der Titelreaktion selektiv gebildet. Zwei mögliche Mechanismen zur Erklärung der unterschiedlichen Produkttypen werden vorgeschlagen. Kontrollexperimente sprechen dafür, dass p‐Benzochinon (BQ) nicht nur als Oxidans wichtig ist, sondern auch als Ligand eine wesentliche Rolle spielt. E=CO2Me.

Control of Selectivity in Palladium‐Catalyzed Oxidative Carbocyclization/Borylation of Allenynes

“…Also, some preliminary results regarding carbocyclization/borylation were obtained with differently substituted 1,5-allenynes ( 1 ; R=H, Me, Ar), which only gave borylated triene products 4 (Scheme 1 b). 13b However, under carbocyclization/arylation conditions alkyl-substituted allenynes afforded two different constitutional isomers (arylated trienes and arylated vinylallenes) in a ratio determined by the substitution on the starting allenyne 13b. The aim of the present study was to develop a carbocyclization/borylation that can be directed towards either a borylated triene or a borylated vinyllallene by control of the reaction conditions (Scheme 1 c).…”

“…Furthermore, by combining the borylation with a C–C bond-forming cyclization, complex molecules suitable for various further functionalizations could be obtained in one step 8–11, 13b,d. Such borylating carbocyclizations have been successfully developed by the group of Cárdenas 9–11.…”

“…Palladium-catalyzed borylating carbocyclizations of allenynes: a) under non-oxidative conditions;11 b) under oxidative conditions;13b c) under selective oxidative conditions. E=CO 2 Me.…”

“…Recently we accomplished the carbocyclization/arylation of allenynes with arylboronic acids 13b. Also, some preliminary results regarding carbocyclization/borylation were obtained with differently substituted 1,5-allenynes ( 1 ; R=H, Me, Ar), which only gave borylated triene products 4 (Scheme 1 b).…”

“…13b The use of a catalytic amount of palladium acetate (2 mol %) and stoichiometric amounts of BQ (1.1 equiv) in THF at 50 °C led to an isomeric mixture of borylated triene 4 a and borylated vinylallene 5 a in 28 % and 14 % yield, respectively (Table 1, entry 1). Analyzing the effect of different solvents showed that a higher selectivity for 4 a was obtained when DCE was used as the solvent (in Table 1, entry 4 vs. entries 1–3).…”