THE PALLADIUM-CATALYZED ADDITION OF VARIOUS VINYL COMPOUNDS TO DIMETHYL ACETYLENEDICARBOXYLATE

The catalyst precursor L2Pd(CE=CE-CE=CE) (E=COOCH3) reacts with simple vinyl compounds and dimethyl acetylenedicarboxylate to give cyclic 1: 2-, 2: 1- and 2: 2-addition products, namely substituted cyclohexadie ne or benzene, cyclopentene and cyclooctadiene derivatives, which can be separated by column chromatography. Palladacyclopent-2-enes, pallad acyclohepta-2,4-dienes and palladacyclohept-4-enes, which cannot be tr apped, are suggested to be intermediates in the reaction. Different ty pes of products are obtained depending on the nature of the vinyl comp ound: while a-methyl styrene gives only a cyclohexadiene species follo wing 1:2-addition, vinyl ethers along with 1:2-addition undergo 2:1- a nd even 2:2-addition, and cyclopentene and 1,5-cyclooctadiene derivati ves are formed. In the course of the reaction with vinyl ethers, alcoh ols are formed, probably in a reductive elimination step from Pd-H- in termediates in which an alkoxy group occupies a beta-position and also in palladium-mediated aromatization of initially formed alkoxy cycloh exadiene and bisalkoxy cyclohexene products. Yields and product ratios can be influenced by changing the fixed ligand in the catalyst comple x.