Nucleophilic attack on cyclooctadiene complexes of palladium: Stereochemistry of the resulting cyclooctenylpalladium derivatives

Reaction of [PdClMe(cod)] with MeO₂CC=CCO₂Me occurs in two steps, namely, insertion of the alkyne into the Pd-Me bond followed by migratory insertion of one of the double bonds of the coordinated cyclooctadiene into the Pd-alkenyl bond thus formed. The resulting chloride-bridged, cyclooctenyl complex was converted to [Pd(acac){η¹,η²-C₈H ₁₂C(CO₂Me)=CMe(CO₂Me)}], 2a, which was characterized crystallographically. The molecular structure revealed that each insertion took place in a cis manner. Reactions of [PdCl₂(cod)] with a range of nucleophiles were performed, which gave analogous chloride-bridged, cyclooctenylpalladium dimers, 1, which could be converted to their acetylacetonate derivatives, 2. The solid-state structure of the methoxy complex [Pd(acac)(η¹,η₂-C₈H₁₂OMe)], 2b, obtained by reaction of [PdCl₂(cod)] with NaOMe followed by Ag(acac), was determined. This showed that exo attack of the OMe group occurred. In contrast, the structure of the phenyl derivative [Pd(acac)(η¹,η²-C₈H₁₂Ph)], 2e, produced by treatment of [PdCl₂(cod)] with Ph₄Sn followed by Ag(acac), indicated that the phenyl group added in an endo fashion, presumably via a phenyl-palladium intermediate of the type [PdClPh(cod)]. With the larger mesityl group, the complex [PdCl(C₆H₂Me₃)(cod)] could be isolated. Each of the complexes was fully characterized by ¹H and ¹³C{¹H} NMR spectroscopy, and the appearance of the CHPd signal was used to distinguish between the exo and endo isomers. On the basis of the crystallographic and NMR data, it was deduced that the organometallic reagents ArMgX (Ar = Ph, 2-MeC₆H₄, 2,4,6-Me₃C₆H₂), Ph₄Sn, and (4-MeC₆H₄)₂Hg generate the endo products by initial attack at the metal. In contrast, reactions with NaOR (R = Me, Et), NaCH(CO₂Me)₂, AgOAc, and Et₂NH proceed by direct attack of the nucleophile at the coordinated double bond to give the exo products.