Russell P. Hughes

Transition metal promoted reactions of unsaturated hydrocarbons : III. Insertion of 1,2-dienes into allylic palladium bonds

Abstract The insertions of allene, methylallene, 1,1-dimethylallene and 1,3-dimethylallene into the allylic palladium bonds of the complexes (π-All)Pd(X) (I): X = chloride, acetylacetonate, or hexafluoroacetylacetonate, to give new π-allylic products (II) [e.g. 2-C3H4(CR1R2CCR3R4)Pd(X)] has been investigated. Reaction of (π-All) Pd(acac) complexes with liquid allene yields, as a side-product, bis(acetylacetonato)-2,2′-bi-π-allyldipalladium(II). Addition of 1,2-dienes to CDCl3 solutions of complexes (I) promotes exchange on the NMR time scale of the allylic syn- and anti-protons via a σ-allylic intermediate. The rate of syn—anti proton exchange decreases in the order: All = 2-chloroallyl > allyl > 2-methylallyl > 2-tert-butylallyl; 1,2-Diene = 1,1-dimethylallene > 1,3-dimethylallene > methylallene > allene > tetramethylallene ≈ 1,3-di-t-butylallene; X = Hfacac ⪢ Acac > Cl. The rate of formation of the insertion product (II) decreases in the order: All = 2-chloroallyl > allyl > 2-methylallyl ⪢ 2-tert-butylallyl; 1,2-Diene = 1,3-dimethylallene > 1,1-dimethylallene > methylallene > allene ⪢ tetramethylallene ≈ 1,3-di-t-butylallene; X = Hfacac > Acac ≈ Cl. To account for the stereochemical features of this reaction, and the unusual order of coordinative abilities and reactivities of the different 1,2-dienes, a mechanism is proposed in which the 1,2-diene preferentially coordinates to palladium via its less substituted olefinic function to generate a gs-allylic intermediate (σ-All) (1,2- diene)Pd(X). Direct carbon migration of the σ-bonded carbon of the σ-allyl to the central allenic carbon occurs to give the product (II). The relative reactivities of methyl substituted allenes may be rationalized in terms of a small degree of polarization of the π-1,2-dienePd bond in the transition state for the σ-allyl migration.

Transition metal promoted reactions of unsaturated hydrocarbons : II. Insertion of bicyclic olefins into allylic-palladium and -platinum bonds

Abstract Addition of strained olefins, based on norbornene, norbornadiene,benzonorbornadiene or bicyclo [2.2.2] octene skeletons to π-allylic(hexafluoroacetylacetonato) palladium(II) complexes [(π-All)Pd(Hfacac)], gives “enyl” products derived from “insertion” of the olefin into the least substituted terminal allylicpalladium bond. The reaction involves an initial rapid and reversible formation of (gs-allyl)(π-olefin)Pd(Hfacac). The rate-determining step involves migration of a σ-allylic carbon atom from Pd to the coordinated olefin in a concerted cis — exo addition of PdC across the double bond. Remote electronegative substituents on the olefin do not affect the coordinative ability of the olefin towards Pd. They do however inhibit the migration of the σ-allylic ligand to the coordinated olefin. This observation is interpreted in terms of a small degree of polarization of the π-olefin—Pd bond in the transition state for the σ-allyl migration.

Transition metal-promoted reactions of unsaturated hydrocarbons : IV. Reactions of norbornenyl complexes of palladium(II) with group V donor ligands, olefins, 1,3-dienes and 1,2-dienes

Abstract 6-Acetoxynorbornenylpalladium-hexafluoroacetylacetonate and -benzoyl-trifluoroacetonate complexes (I) react with neutral donor ligands containing Group V atoms to yield 5-acetoxynortricyclenylpalladium complexes. Reactions of (I) with mono-olefins or 1,3-dienes produce analogous nortricyclenyl complexes containing a coordinated olefinic ligand, which are in equilibtrium with the starting materials. The role of steric and electronic factors in the olefinic ligand in determining the equilibrium constant in these systems has been evaluated. Reactions of (I) with 1,2-dienes at −10° generate σ-nortricyclenyl (π-1,2-diene) palladium complexes which rapidly rearrange above 0° to yield new 2-(5′-acetoxynortricyclenyl)allyl complexes of palladium(II). A comparison of these reactions to the known interactions of allylic palladium complexes with olefins, 1,3-dienes, and 1,2-dienes is presented.

A 13C and 1H NMR investigation of the bonding in norbornenyl complexes of palladium(II) and platinum(II)

Abstract The modes of bonding in several norbornenyl complexes of palladinm(II) and platinum(II) has been investigated by both 1H and 13C NMR spectroscopy. Analysis of internal 1H1H, 13C1H, and 195Pt13C coupling constants indicate considerable distortion of the norbornenyl ligand in these complexes relative to organic norbornenes. The nature of this distortion is compatible with a π-homoallylic bonding scheme.

A study of trihapto-pentahapto equilibria in heptadienylpalladium(ii) complexes and the possible role of such complexes in metal complex catalysed 1,4-polymerizations of 1,3-butadiene

Summary 1 H and 13 C NMR spectroscopy and molecular weight studies of a series of hepta-2,6-dienylpalladium(II) carboxylate complexes have demonstrated the existence in solution of an equilibrium between trihapto-dimeric complexes with bridging carboxylates and pentahapto -monomeric complexes with terminal carboxylate ligands. At constant temperature, the position of equilibrium is a function of carboxylate ligand, substituents at position 6 on the heptadienyl ligand, and the solvent. The dimer monomer rearrangement is fast on the 1 H NMR time scale. These observations together with previously reported data concerning the reactivity of allyl and carboxylate ligands coordinated to palladium(II) are considered as model systems for the π-allylnickel carboxylate catalysed cis -1,4-polymerization of 1,3-butadiene. A mechanism is proposed that provides a rationalization for the majority of the factors affecting the initial rate of the nickel complex catalysed polymerization of 1,3-butadiene as reported by Dawans.

2,2′-BI-π-allyl complexes of palladium(II)

Abstract The preparation and properties of a series of dinuclear palladium(II) complexes containing a bridging 2,2′-biπ-allyl ligand are described. When the complexes contain bridging carboxylate ligands the bridging 2,2′-biπ-allyl ligand is asymmetrically distorted. The distortion is different when the carboxylate ligands are replaced by μ-1,3-diphenyltriazene.

The dynamic stereochemistry of the allylic ligand in a μ-allylic complex of platinum(II)

1H NMR studies of an equilibrium mixture of 1,1,1,5,5,5-hexafluoropentane-2,4-dionato-π-allylplatinum(II) [IA] and bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)bis(μ-allyl)diplatinum(II) [IB] show that the μ-allylic ligands of (IB) exhibit temperature dependent tautomeric behaviour, giving rise to a site-exchange between protons on these ligands.

Transition-metal complexes of methylenecyclopropanes: ring-opening reactions promoted by palladium(II)

A variety of stable 1,2-η-bonded methylene cyclopropane complexes of RhI, IrI, Pto, and PtII are reported; isomers of Feists acid dimethyl ester react with PdCl2(MeCN)2 to yield 1,3-enyl complexes of palladium-(II); stable alkyl complexes of PdII; containing β-hydrogen atoms are reported, and their stability is discussed.

The role of electronic effects in determining the thermodynamic stabilities of olefin complexes of palladium(II) and platinum(II)

The dominant factor in determining the thermodynamic stability of the metal–olefin bond in p-substituted styrene complexes of palladium(II) has been shown to be the olefin-π to metal-d component of the bond, rather than the metal-d to olefin-π* component.