Conjugated diene complexes nucleophilic additions

Several types of Pd-catalyzed or -promoted reactions of conjugated dienes via TT-allylpalladium complexes are known. The Pd(II)-promoted oxidative difunctionalization reactions of conjugated dienes with various nucleophiles is treated in Chapter 3, Section 4, and Pd(0)-catalyzed addition reactions of conjugated dienes to aryl and alkenyl halides in this chapter. Section 1.1.1. Other Pd(0)-catalyzed reactions of conjugated dienes are treated in this section. 

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. 

Because of the stability of iron tricarbonyl diene complexes, conjugated dienals are protected from polymerization when complexed, while other reactions can be carried out at the aldehyde functionaUty. A number of synthetically attractive nucleophilic transformations of the aldehyde can be performed on these complexes. These include, aldol reactions, Michael additions, reactions with organozinc, -silicon, -boron, and -tin... 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

Intramolecular reactions of allylic acetates with conjugated dienes catalyzed by Pd(0) lead to a 1,4-addition of a carbon and an oxygen nucleophile to the diene. The reaction, which is formally an isomerization, involves tw different yr-allyl complexes (Scheme 8-4) [44]. Reaction of 22 in the presence of the Pd(0) catalyst Pd2(dba)3-CHCl3 (dba = dibenzyl-ideneacetone) and LiOAc/HOAc in acetonitrile at reflux produces the cyclized isomer 25 in 62% yield. The double bond was exclusively of E stereochemistry, while the ring stereochemistry was a mixture of cis and tram isomers. Oxidative addition of the Pd(0) to the allylic acetate gives the intermediate jr-allyl complex 23. Subsequent insertion of a diene double bond into the allyl-palladium bond produces another jr-allyl intermediate (24), which is subsequendy attacked by acetate to give the product 25. 

Palladium(II)-catalyzed 1,4-additions to conjugated dienes, in which at least one alkoxide function is added, require the presence of an alcohol function. In all cases known so far, this involves an alkoxypalladation of tlie conjugated diene to give an intermediate 4-alkoxy-l,2,3-(jr-allyl)palladium complex. Subsequent nucleophilic attack on the jr-allyl... 

The diene-Br2 complex is again in equilibrium with the reagents, and nucleophilic attack at carbon can be carried out either by the bromide of the ammonium bromide ion pair, formed at the moment of the electrophilic attack, or by the less nucleophilic pyridine added in excess in the reaction medium. It is noteworthy that this mechanism is characterized by a rate- and product-limiting nucleophilic step which should be quite insensitive to steric hindrance around the double bond. In agreement with a weak influence of the steric effects, pyridinium perbromide reacts in chloroform and tetrahydrofuran with substituted conjugated and non-conjugated dienes to give selectively (>95%) bromine addition to the more alkylated double bond (equation 44). 

Conjugated dienes coordinated to palladium(Il) undergo nucleophilic addition of oxygen and nitrogen nucleophiles from the opposite side of the palladium center to give Ti-allylpalladium complexes. Subsequent reaction of the 7C-allyl complexes with another molecule of nucleophile results in the 1.4-addition of two kinds of nucleophiles to dienes. The direction of the second nucleophilic attack is dependent on the nature of the nucleophiles as well as the reaction conditions. Representative examples... 

Reaction of conjugated dienes with aryl and alkenyl halides can be explained by the following mechanism. Insertion of a conjugated 1,3-diene into an aryl or alkenylpalladium bond gives the 7r-allylpalladium complex 330 as an intermediate, which reacts further in two ways. As expected, nucleophiles such as carbon nucleophiles, amines, and alcohols attack the 7r-allylpalladium intermediate to form the 1,4-addition product 331 and 1,2-addition product 332. In the absence of the nucleophiles, y5-H elimination occurs to afford the substituted 1,3-diene 333. In some cases, the substituted 1,3-diene 333 reacts again with aryl halide to form the it -allylpalladium 334. Subsequent elimination affords the 1,4-diarylated 1,3-diene 335. 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

Conjugated dienes coordinated to a transition metal can readily be transformed into a rr-allylmetal complex by functionalization at the 4-position.f This makes dienes useful substrates for catalytic transformations since the 7r-allyl complex formed can undergo further reaction. t A number of Pd-catalyzed reactions of conjugated dienes are known that proceed via rr-allylpalladium intermediates and lead to useful 1,4- or 1,2-funtional-ization of the diene. There are two types of reactions of this kind (i) Pd(0)-catalyzed reactions that involve initial oxidative addition and (ii) Pd(II)-catalyzed reactions that involve electrophilic activation of the diene by the metal followed by nucleophilic attack. This section deals with C— N and C—O bond formation via these two types of reactions. This topic has previously been reviewed in connection with Pd-catalyzed additions to conjugated dienes. [5] [n] -jijg present review will focus mainly on the work published since 1997 but will also briefly discuss previous work. 

Non-conjugated dienes ate the source of many palladium enyl complexes. Nucleophilic attack at one of the double bonds of the coordinated diene has been used most, and many examples can be found in COMC (1982) and COMC (1995). The external attack of the nucleophile leads to an overall /rtf r-addition of Pd and the incoming species to the double bond. Oxidation followed by nucleophilic attack on COD affords <7,7 -enyl derivatives from Pd(l,5-COD)(benzoquinone). In the presence of acid, benzoquinone oxidizes Pd(0) to a Pd(n) diene complex with the concomitant formation of hydroquinone. If a suitable nucleophile is present, such as OAc when acetic acid is used, attack on the diene takes place, and a palladium complex is formed (Scheme 80). A weaker nucleophile like CH3S03 is not capable of adding to the double bond, and the reaction stops at the COD Pd(n) complex. The behavior of other quinones and acids was also studied. ... 

Inter- and intramolecular additions of alkenes and dienes to propargylic alcohols catalyzed by thiolate-bridged diruthenium complexes have been described. The processes, a kind of allenylidene-ene reaction, generate 1,5-enynes and dienynes by reaction of propargylic alcohols with 2-arylpropenes [196] and 1,3-conjugated dienes [197], respectively. The intramolecular version of this reac-ti(Mi has been developed to give diastereo- [196, 198] or enantioselective syn-substituted chromanes (Scheme 58) [199]. Recently, the results of DPT calculations indicated that nucleophilic attack of the olefinic Jt-electrons on a carbocationic... 

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