Preparation Structure, and Role of Tetrakis(methoxycarbonyl)palladiacyclopentadiene Cyclic Diolefin Complexes in Selective Palladium-Catalyzed Cyclocotrimerization of Acetylenes with Olefins

Tetrakis(methoxycarbonyl)palladiacyclopentadiene cyclic diolefin complexes. Pd[C₄(COOCH₃)₄L. for L = 1,5- cyclooctadiene and norbornadiene have been prepared. The latter complex has been characterized crystallographically. The material crystallizes with two molecules in space group C₁¹-P1 in a cell of dimensions a = 10.560 (2), b = 10.583 (2), c = 9.710(2) Å, α= 99.33 (1), β = 113.24(1), and γ = 85.53 (2)°. The structure has been refined by standard methods to a conventional R index of 0.027, based on 3646 reflections above background. There is a nearly symmetrical coordination of the diolefin to the palladium atom of the metallocycle. The olefinic C=C bond lengths of 1.352 (4) Å in the norbornadiene fragment together with spectroscopic results suggest a net transfer of electrons from the electron-rich norbornadiene to the metallocycle which has electron-withdrawing substituents. The fact that the diolefin bonds more strongly to the metallocycle than does an activated acetylene is brought to bear on mechanisms of cyclotrimerization of acetylenes and cyclocotrimerization of two acetylene and one olefin molecule. The parent oligomeric Pd[C₄(COOCH₃)₄ complex is an efficient catalyst for the cyclocotrimerization of two dimethylacetylenedicarboxylate molecules with norbornadiene to give 1,2,3,4-tetrakis(methoxycarbonyl)benzene. Use of norbornene in place of norbornadiene leads to the palladium-catalyzed stereoselective production of a cyclohexa-1,3-diene derivative in 94% yield.