Allyl Palladium Species from Allylic Compounds

7r-Allyl Palladium Species from Allylic Compounds 

An interesting one-pot sequential three-component reaction involving the nude- 

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Since 7r-allylpaIIadium complexes are formed by oxidative addition of allylic compounds to zerovalent palladium species, and the eliminated HPdX from tt-allylpalladium complexes readily decomposes to regenerate a Pd(0) species with liberation of HX, the elimination processes to 1,3-dienes is catalyzed by palladium complexes. It is considered that the elimination step from HPdX to Pd(0) and HX is reversible therefore, normally the elimination is carried out in the presence of suitable base (B) to capture HX. The catalytic elimination of HX from allylic compounds for the synthesis of 1,3-dienes under mild conditions provides a useful method (Scheme 2). 

Another easily available palladium compound is PdCl2 however, it has low or no activity. The chloride ion in the coordination sphere of palladium seems to inhibit the coordination of two moles of butadiene to form the bis-77-allylic complex. However, PdCl2 can be used in the presence of an excess of bases, such as KOH, NaOH, sodium phenoxide, sodium acetate, potassium acetate, sodium methoxide, or tertiary amines. These bases deprive the chloride ion from the coordination sphere of palladium to form the active species. Thus, very stable and easily prepared... 

It is reasonable to assume that the identical complex will be generated whether it be done stoichiometrically from an alkene, to give a chloride or carboxylate dimer followed by the addition of 2 equiv. of a phosphine per Pd, or by the addition of an allyl-X compound to give a phosphine-Pd0 complex. This assumption is supported by the fact that complexes generated in either manner have been found to exhibit identical reaction profiles.380 Furthermore, for the vast majority of allylpalladium reactions studied, it is most likely that the reactive species is a cationic bisphosphine-palladium complex (13).13 Calculations... 

This reaction constitutes a special type of process in which a hydrogen atom and a nucleophile are added across the diene with fonnation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20] amination [21,22], and addition of active methylene compounds [21 a,23,24], These reactions are initiated by an oxidative addition of H-Nu to the palladium(0) catalyst, which produces a palladium hydride species 1 where the nucleophile is coordinated to the metal (Scheme 8-1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a (jr-allyl)palladium intermediate. Now depending on the nature of the nucleophile (Nu) the attack on the jr-allyl complex may occur either by external trans-aVtBck (path A) or via a cw-migration from palladium to carbon (path B). 

Oxidative addition [1, 38] of 1-alkenyl, i-alkynyl, allyl, benzyl, and aiyl halides to a palladium(O) complex affords a stable rra .s-<7-palladium(II) complex (11). The reaction proceeds with complete retention of configuration for alkenyl halides and with inversion for allylic and benzylic halides. Alkyl halides having /3-hydrogens are rarely useful because the oxidative addition step is very slow and may compete with /3-hydride elimination from the a-organopalladium(II) species. However, it has been recently shown that iodoalkanes undergo the cross-coupling reaction with organoboron compounds (Section 2.4.5). 

Palladium(0)-catalyzed intramolecular reductive coupling of the diacetates 1 and 3 with hexa-methyldistannane leads to tricyclic compounds 2 and 4, respectively, in a completely regioselec-tive manner2. Intermediate formation of a monoallyltin species followed by attack of this allyl tin moiety on the rc-allyl complex generated from the remaining allyl acetate is a likely route to the products. Since a substrate similar to 3 has been shown to isomcrizc to a nonstereogenic terminal diacetate under the conditions listed, the stereochemical results obtained for both 1 and 3 reflect the simple diastereoselectivity of these cyclizations. 

The second type of organopalladium intermediates are 7r-allyl complexes. These complexes can be obtained from Pd(II) salts and allylic acetates and other compounds with potential leaving groups in an allylic position." The same type of TT-allyl complexes can be prepared from alkenes by reaction with PdCl2 or Pd(02CCF3)2. The reaction is formulated as an electrophilic attack on the tt electrons followed by loss of a proton. The proton loss probably proceeds via an unstable species in which the hydrogen is bound to palladium." ... 

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