1.3- Dienes allylpalladium compounds

A functionalized mercury(II) compound like ethyl (acetoxymercurio)acetate (136) allows an easy approach to prostaglandin endoperoxide analogs (equation 52).204 Several organomercury(II) compounds, RHgCl (R = Me, aryl, benzylic), are able to add to 1,3-dienes in the presence of a stoichiometric amount of a palladium(II) salt and affonl ir-allylpalladium compounds of type (137) in variable yields (equation 53).20S A related intramolecular carbomercuration has been reported by Snider.206 It allows a stercospe-cific approach to the chloromercury compound (138 equation 54). Similar palladium-mediated reactions... 

Metallo-ene reactions involving the transfer of palladium and nickel have been described since the early 1960s, mostly in cormection with their crucid role in the Pd- and Ni-catalyzed polymerization of butadiene." Hence, insertions of 1,3-dienes into allylpalladium compounds were extensively studied. Thus preformed allylic complexes (49) underwent the metallo-ene reactions (49) —> (51) at 20 C (20 h) or 35 °C (<5 min) or 70 °C 0 h) the reaction rate depended on the substituents R and as well as on the ligand L and decreased in the order R = Cl > H > Me R = H > Me L = Feacac > acac > Cl. Further diene insertion into the resulting allylpalladium product (51) (polymerization) was generally slower than the initial step (49) —> (51) and again relies on the nature of the ligand L (Scheme 11). ... 

Metallo-ene reactions involving the transfer of palladium and nickel have been described since the early 1960s, mostly in connection with their crucial role in the Pd- and Ni-catalyzed polymerization of butadiene. Hence, insertions of 1,3-dienes into allylpalladium compounds were extensively studied. Thus preformed allylic complexes (49) underwent the metallo-ene reactions (49) (51) at 20 C (20 h)... 

C.i.b. Six-Centered Processes. Allylpalladium compounds, which are readily accessible, for example, by allylic substitution on allyl esters or by carbopalladation of 1,2-dienes or 1,3-dienes with or without subsequent rearrangement (Scheme 7, Eq. 1), may... 

When allylic compounds are exposed to palladium(0) in the absence of any suitable nucleophiles, 1,4-elimination occurs to yield conjugated dienes." Following oxidative addition, hydride elimination from 7C-allylpalladium compound 93 gives conjugated diene 94, often as a mixture of E- and Z-... 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

The 2,3-alkadienyl esters 839 are reactive compounds toward Pd catalysts and form the a-alkylidene-rr-allylpalladium complexes 840, which react further to give two kinds of products, namely the 1,2- and 1,4-diene derivatives 841 and 842, depending on the reactants. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

A nice and convergent approach to both compounds makes use of RCM to form the 5-membered building block 71, which mimics the carbohydrate part of the nucleosides. The necessary diene precursor 69 is readily assembled via Evans aldol chemistry. RCM then affords the ring in almost quantitative yield (69->70), leaving the chiral centers and the free hydroxyl group intact. Removal of the chiral auxiliary by reductive cleavage, attachment of the base by means of jt-allylpalladium chemistry, and a final deprotection step complete these highly efficient syntheses [46]. 

On the other hand, the methoxyester results from MeOH attack on coordinated double bond, followed by methoxycarbonylation (Scheme 11). In both cases, the formation of 7r-allylpalladium complexes directs the regio-chemistry of the process. By optimizing the reaction conditions, it has been possible to obtain the unsaturated diester selectively. The latter compound is particularly important, since it can be easily transformed after hydrolysis and hydrogenation into adipic acid [52-54], Selective alkoxy-alkoxycarbonylation of 1,3-dienes has also been achieved [55]. 

Metal-Halogen Compounds. An unusual example of the addition of a metal halide to a conjugated diene has been reported. The complex formed from palladium chloride and butadiene has been shown to be a dimer of 1-chloromethyl-7r-allylpalladium chloride, (85). Whether this is a true insertion reaction or some type of ionic reaction has not been determined, but its close analogy with the olefin-palladium chloride insertion reaction mentioned above would suggest an insertion mechanism for the diene reaction also. 

The course of the insertion of diphenylketene into ir-allylpalladium intermediates has been found to depend on the nature of the ir-allyl precursor. Allyl acetates give dienes, while allyl carbonates give car-boxymethylated products (equations 84 and 8S).264 An intermediate allyl Pd—OR species (9) is believed to exist in both cases. When R = Ac, decarbonylation is followed by (J-H elimination, whereas if R = Me, alkoxide attacks the acylpalladium intermediate and yields the methoxycarbonyl compound. 

Both stoichiometric and catalytic reactions involving 7r-allylpalladium complexes are known. Reactions involving 7r-allylpalladium complexes become stoichiometric or catalytic depending on the preparative methods of the 7r-allylpalladium complex. Preparation of the 7r-allylpalladium complexes 6 by the oxidative addition of various allylic compounds 5, mainly esters to Pd(0), and their reactions with nucleophiles are catalytic. This is because Pd(0) is regenerated after the reaction with the nucleophile, and the Pd(0) reacts again with allylic compounds to form the complex 6. These catalytic reactions are treated in Section 4.3. However, the preparation of 7r-allyl complexes 6 from alkenes 7 requires Pd(II) salts. Subsequent reaction with nucleophiles generates Pd(0). As a whole, Pd(II) is consumed, and the reaction ends as the stoichiometric process, because in situ reoxidation of Pd(0) to Pd(II) is not attainable in this case. Also, 7i-allylpalladium complex 9 is formed by the reaction of conjugated dienes 8 with Pd(II), and the reaction of 9 with nucleophiles is stoichiometric. 

Thus activation and functionalization of alkenes, enones and conjugated dienes are possible based on the 7i-allylpalladium complex formation from these unsaturated compounds. 

Photolysis of T -allylpalladium complexes such as (34) gives 1,5-dienes or, if oxygen is present, conjugated carbonyl compounds such as CH2=CHCHO. 

This reaction constitutes a special type of process in which a hydrogen and a nucleophile are added across the diene, with formation 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], hydroamination [21, 22], and addition of active methylene compounds [21a, 23, 24]. These reactions are initiated by an oxidative addition of H-Nu to the palladium(O) catalyst, which produces a palladium hydride species 1 in which the nucleophile is coordinated to the metal (Scheme 11.1). 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 JT-allylpalladium intermediate. Now, depending... 

The added acid most likely plays several roles. First, the acid is necessary for the redox transformation of Pd(0)-BQ to Pd(ll) + HQ in the catalytic cycle [65]. Second, the acid will lead to the formation of a cationic 7t-allylpalladium intermediate which will facilitate coordination of BQ. Third, the acid will protonate the oxygen of the coordinated BQ, and in this way the quinone becomes more electron-withdrawing. It was found that the rate of the reaction increased with the amount of acid, and that there was a linear increase in the range of 0-30 mol% of acid however, adding too much acid catalyzed the destruction of BQ. The stereochemistry of the dialkoxylation is consistent with a trans alkoxypalladation [118] of the diene to give 7t-allyl intermediate 91, followed by external trans attack of alcohol to give the cis-dialkoxy compound 92 (Scheme 11.32). 

Carbon-phosphorus bonds are formed by the Pd-catlyzed allylation of various phosphorus compounds (Scheme 13). The reaction of l-acetoxy-2-cyclohexene with LiPPh2 in refluxing THF provides an allylic phosphine in low yield (<15%). The phosphine produced deactivates the catalyst by coordination, which lowers the yield. In contrast, the reaction of LiP(S)Ph2 with l-acetoxy-2-cyclohexene takes place at room temperature to give allylic diphenylphosphine sulfides in 85% yield. ° Pd-catalyzed Michaelis-Arbuzov reaction of cinnamyl acetate with trimethyl phosphite affords a dimethyl allylic phospho-nate. With the reaction conditions being rather severe, this method may not be applicable to an allylic acetate that can produce a conjugated diene via /3-hydride elimination from the intermediate 7r-allylpalladium complex. 

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). 

In 1967 elimination of phenol from allyl phenyl ethers to form 1,3-diene in the presence of a palladium catalyst was reported briefly by Smutny. Later, Tsuji applied the Pd-catalyzed elimination reaction of terminal allylic compounds for the synthesis of terminal 1,3-dienes.Thus, elimination of acetic acid and phenol from allylic acetates and allyl phenyl ethers was carried out by refluxing the allylic compounds in dioxane or toluene in the presence of catalytic amounts of palladium acetate and PPha as a ligand for the palladium catalyst (Table 1). The allylic isomers were converted to the same products. No reaction takes place with allylic methyl ether, an allylic alcohol, or an allylic amine, which cannot easily form 7r-allylpalladium complexes by oxidative addition. 

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