Effects of Donor Molecules on the Palladium-Catalyzed Cyclocotrimerization of Acetylenes with Olefins. Preparation of Dimeric Tetrakis(methoxycarbonyl)palladiacyclopentadiene(base) Complexes and the Structure with Base=2,6-Lutidine

A tetrakis(methoxycarbonyl)cyclohexadiene annulation of norbornene, cyclopentene, and cyclohexene with two molecules of dimethyl acetylenedicarboxylate (DMAD) is effectively catalyzed by a combination of an oligomeric palladiacyclopentadiene complex, [Pd(C4(COOCH3)4)]n (1), with 1 equiv of triphenylphosphine. The reaction of 1 with L=pyridine, α-picoline, 2,5-lutidine, 2,6-lutidine, or triphenylphosphine led to new dimeric complexes of formula [Pd(C4(COOCH3)4)L]2. In these complexes there is intermolecular coordination through the carbonyl oxygen atom of the α-methoxycarbonyl group as deduced from spectroscopic results and as shown in an X-ray structure determination of the L=2,6-lutidine complex. This complex crystallizes with four dimers in space group C52-P21/c of the monoclinic system in a cell of dimensions [formula omitted] The structure has been refined by full-matrix least-squares methods to values of R and Rw of 0.036 and 0.050, respectively, based on 440 variables and 8287 observations. Acceleration of the rate of cyclocotrimerization of two DMAD molecules with norbornene by the addition of 1 equiv of triphenylphosphine to the catalyst, 1, and the formation of the dimeric complex are explained in terms of a common three-coordinate intermediate, “Pd(C4(COOCH3)4)L”, which is effectively trapped by an entering cycloolefin in the palladium-catalyzed cyclohexadiene annulation.