Asymmetric allyl-metal bonding in substituted (eta(3)-allyl)palladium complexes: X-ray structures of cis- and trans-4-acetoxy-[eta(3)-(1,2,3)-cyclohexenyl]palladium chloride dimers

Enantiomerically pure cis and trans isomers of 4-acetoxy-[eta(3)-(1,2,3)-cy clohexenyl]palladium chloride dimers (cis-1 and trans-1) were prepared from enantiomerically pure trans-1-acetoxy-4-chloro-2-cyclohexene. X-ray analys es of these complexes show that in the trans complex (trans-1) the six-memb ered ring prefers a chair conformation, whereas in the cis complex (cis-1) the cyclohexenyl ring has a boat conformation. According to the X-ray struc ture of trans-1 the Pd-C3 bond is shorter than the other allylic terminal p alladium-carbon bond (Pd-Cl). On the other hand, in cis-1 the Pd-C3 and Pd- Cl bond lengths are identical within the experimental error. The calculated structures (B3PW91/LANL2DZ + P) of trans-1 and cis-1 also display differen ces in the allylpalladium bonding. The asymmetric allylpalladium bonding in trans-1 is explained on the basis of pi - sigma* electronic interactions b etween the 4-acetoxy substituent and the allyl-metal moiety.