PALLADIUM-CATALYZED COCYCLIZATION OF ALLE NES WITH ELECTRON-POOR ALKYNES - SOME STUDIES ON COUPLING OF ALLENE DERIVATIVES WITH METHYL ACETYLENEDECARBOXYLATE TO PALLADIUM AND PLATINUM

A diazadiene-stabilized palladacyclopentadiene acts as a catalyst prec ursor in the 2:2-cocyclisation of allene with acetylenedicarboxylic ac id esters or 3-hexyne-2,5-dione to form 2,3,6,7-tetrasubstituted 1,4,5 ,8-tetrahydronaphthalenes at 60-degrees-C in high yields. These produc ts can be oxidized quantitatively with 2,3-dichloro-5,6-dicyano-p-benz oquinone (DDQ) to yield the 2,3,6,7-tetrasubstituted naphthalenes. Nap hthalene-2,3,6,7-tetracarboxylic acid derivatives are interesting star ting compounds for the preparation of thermally stable polyimides but difficult to obtain by conventional organic synthesis. Clasic organic multi-step procedures give yields only between 16% and 40% whereas our catalytic approach enables the formation of the tetramethyl ester in 82% yield following a simple two-step procedure. Allenic ethers even e nable a one-pot synthesis (with lower yields) via catalyst-induced eli mination of alcohol from an initially formed bis(alkoxy)-tetrahydronap hthalene. 4-Methylidene-palladacyclopent-2-enes can be trapped in stoi chiometric reactions with allene derivatives (but not allene itself) a nd dimethyl acetylenedicarboxylate. Coupling of two allene moieties on platinum affords bis(methylidene)-metallacyclopentanes that might ser ve as model compounds for active palladium intermediates of the cataly tic reaction. An unexpected open chain compound is obtained by inserti on of phenoxyallene into the platinacyclopentadiene (RN=CH-CH=NR)PtCE= CE-CE=CE (E = COOCH3), elimination and aromatization of the correspond ing six-membered ring product.