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.

In control: A highly selective carbocyclization/borylation of allenynes with bis(pinacolato)diboron (B2pin2) under palladium catalysis and with p‐benzoquinone (BQ) as the oxidant was developed. The use of either LiOAc⋅2 H2O with 1,2‐dichloroethane (DCE) as the solvent or BF3⋅Et2O together with THF is crucial for the selective formation of borylated trienes and vinylallenes, respectively.

An enantioselective oxidative carbocyclization–borylation of enallenes that is catalyzed by palladium(II) and a Brønsted acid was developed. Biphenol-type chiral phosphoric acids were superior co-catalysts for inducing the enantioselective cyclization. A number of chiral borylated carbocycles were synthesized in high enantiomeric excess.

Vinylallenes or cross-conjugated trienes are obtained selectively in the title reaction. Two possible mechanisms are suggested to rationalize the formation of the different types of products. Control experiments indicate that p-benzoquinone (BQ) plays an important role as a ligand in addition to its role as an oxidant. E=CO(2)Me.

Pd-catalyzed C-C bond-forming reactions under oxidative conditions constitute a class of important and widely used synthetic protocols. This Article describes a mechanistic investigation of the arylating carbocyclization of allenynes using boronic acids and focuses on the correlation between reaction conditions and product selectivity. Isotope effects confirm that either allenic or propargylic C-H activation occurs directly after substrate binding. With an excess of HO, a triene product is selectively formed via allenic C-H activation. The latter C-H activation was found to be turnover-limiting and the reaction zeroth order in reactants as well as the oxidant. A dominant feature is continuous catalyst activation, which was shown to occur even in the absence of substrate. Smaller amounts of HO lead to mixtures of triene and vinylallene products, where the latter is formed via propargylic C-H activation. The formation of triene occurs only in the presence of ArB(OH). Vinylallene, on the other hand, was shown to be formed by consumption of (ArBO) as a first-order reactant. Conditions with sub-stoichiometric BF·OEt gave selectively the vinylallene product, and the reaction is first order in PhB(OH). Both C-H activation and transmetalation influence the reaction rate. However, with electron-deficient ArB(OH), C-H activation is turnover-limiting. It was difficult to establish the order of transmetalation vs C-H activation with certainty, but the results suggest that BF·OEt promotes an early transmetalation. The catalytically active species were found to be dependent on the reaction conditions, and HO is a crucial parameter in the control of selectivity.