Palladium motion in cyclomeric compounds: a theoretical study.

Density functional theory (DFT)/B3LYP calculations have been carried out to study successive intramolecular 1,n palladium shifts (n = 3-5) in palladium complexes of organic cyclomers. Such shifts of the PdBr(phosphine) moiety, which is bound to the cyclomer and which exchanges concomitantly with a hydrogen atom distant by n carbon atoms from palladium, might lead to an endless motion around the cyclomer. The cyclomers that have been analyzed are either the [1,1,1,1]paracyclophane 1, a THF-based 16-crown-4 structure 2, or the THF-ethylene cyclic dimer 3 (THF = tetrahydrofuran). We show that the [1,1,1,1]paracyclophane 1 is not a good candidate for a circular motion of the metallic moiety. This is due to the very high barrier of the 1,2 Pd/H exchange within the phenyl ring. Hence only a pendulum movement of the metallic moiety between two adjacent phenyl rings can easily take place. For the THF-based systems 2 and 3, the processes along the exo face are found to involve high energy barriers. Processes along the endo face are more accessible, especially for 2 where an endless motion made of successive 1,2 and 1,5 shifts is characterized by barriers that are somewhat less than 30 kcal mol(-1).