Insertions of Allenes into Palladium−Carbon Bonds of Complexes Containing Bidentate Nitrogen Ligands. Structural and Mechanistic Studies

Abstract

The insertion reactions of the allenes propadiene and 1,2-heptadiene in the Pd-C bond of complexes (NkN)Pd(R)X Cl, Br) have been investigated. An X-ray crystal structure determination of (8-PQ)Pd{(1-3-η)-2-methylpropenyl}Cl exhibited the unexpected monodentate coordination of the nitrogen ligand. The monodentate coordination in apolar solvents and bidentate coordination in polar solvents was demonstrated by means of NOE NMR experiments. Kinetic measurements revealed that the reactions are first order in the palladium concentration and occur via an allene concentration independent and dependent pathway. Reactions of complexes containing flexible bidentate nitrogen ligands were retarded by additional free bidentate nitrogen ligand indicating that initial dissociation of a nitrogen donor is an important step in the reaction. We have strong indications that the migration of the R group to the precoordinated allene is the rate-determining step. Instead of masslaw retardation by excess X -(X ) Cl -, Br -), an enhancement of the reaction has been observed in case of the complexes (8-PQ)Pd(Me)Cl, (8-PQ)Pd(Me)Br, and (i-Pr-DAB)Pd(C(O)-Me)Cl. Flexible bidentate nitrogen ligands greatly enhance the reaction, owing to the easy formation of an accessible site on the metal center. The insertion of allenes into the Pd-C bonds of complexes containing rigid bidentate nitrogen ligands probably proceeds via initial allene association followed by either halide or nitrogen dissociation and subsequent migration of the R group to the precoordinated allene.

Insertion Reactions into Palladium–Carbon Bonds of Complexes Containing Terdentate Nitrogen Ligands; Experimental and Ab initio MO Studies

“…provided strong evidence that insertion reactions in neutral complexes containing bidentate nitrogen ligands occur via Results and Discussion intermediates in which these ligands are coordinated in a unidentate fashion. [12] [13] [14] [15] …”

Apparent Allyl Rotation and Pd−N Bond Rupture in Allylpalladium Complexes with N‐Donor Ligands − Evidence of an Associative Mechanism

“…In some examples it has been reported that a ligand of higher basicity makes the apparent rotation more difficult, [21] while increased steric hindrance of the N-donor ligand can facilitate [20,29] or slow down [21] the process. Negative entropies of activation have been found in some cases [29,32] and it has been reported that the process takes place more easily with coordinating anions [19,29,31] or solvents such as DMSO. [20b] It has also been reported that the addition of ligand or water has a negligible effect on the process in a number of examples; [18,21] other examples have been described where the effect of adding ligand or chlorides is accelerative.…”

Isocyanide Insertion into the Palladium−Carbon Bond of Complexes Containing Bidentate Nitrogen Ligands: A Structural and Mechanistic Study

“…The latter possibility might well occur as structural studies on Pd-η 3 -allyl complexes and (2,9-dimethyl-1,10-phenanthroline)Pt(X) 2 L complexes (L ) C 2 H 4 , CO, PPh 3 , ONPh) revealed monodentate coordination of a nitrogen ligand for flexible ligands as well as for rigid ones. [31][32][33] To obtain more insight into the mechanism of CO insertion, we turned our attention to insertion of the isoelectronic isocyanides. Isocyanides might allow the observation of intermediate complexes and variation of electronic and steric factors by variation of the R group on the nitrogen atom, and moreover, it would be of great interest to look at the possibility for the direct synthesis of the polyimine analogue of polyketone.…”