| Abstract
| - Palladium-catalyzed condensation of benzene-1,2-diol with various propargylic carbonates affordedregio- and stereoselectively 2,3-dihydro-2-ylidene-1,4-benzodioxins. The reaction is suggested toproceed by the formation of a (σ-allenyl)palladium complex, followed by the intermolecular attackof the phenoxide ion on this complex to generate a new (σ-allyl)palladium complex in equilibriumwith the corresponding (η3-allyl)palladium complex. Intramolecular attack of the phenoxide ionafforded the corresponding benzodioxan compound. This last attack occurs predominantly at themore electrophilic end of the (η3-allyl)palladium intermediate. The Z- or E-stereochemistry of theproducts was established by 1H NMR and proton NOE measurements and also by X-ray analysison an example. The Z-stereochemistry generally observed is in agreement with the formation ofthis (η3-allyl)palladium intermediate. However, in the case of tertiary propargylic carbonates, theE-stereochemistry generally observed could be explained by an intramolecular attack of thephenoxide ion on the intermediate (σ-allyl)palladium complex, in slow equilibrium with the (η3-allyl)palladium complex.
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