Palladium-catalyzed 1,4-additions conjugated dienes

Pyrone 104 was obtained when diester-substituted vinylidenecyclopropane 105 reacted in the presence of a rheniumfO) catalyst (Scheme 2.68) [120]. Palladium-catalyzed addition of acetic acid to vinylidenecyclopropanes gave conjugated dienes [121]. 

If the side chain with the nucleophile is situated in the 1-position of the conjugated diene, a palladium-catalyzed spirocyclization occurs. In this case stereoselective oxa-spirocyclizations were obtained from the diene alcohols 59 and 60 (equation 23 -25)58. The reaction worked well for the formation of a tetrahydrofuran and tetrahydropyran in the spirocyclization. In the absence of chloride ions 59 gave high yields of the acetoxy oxaspirocyclic compound 61 via a 1,4-anti addition across the diene (equation 23). In the presence of stoichiometric amounts of LiCl a 1,4-syn oxychlorination took place and allylic chloride 62 was obtained (equation 24). Under chloride-free conditions, cyclohep-tadiene alcohol 60 afforded oxaspirocyclic acetate 63 (equation 25). 

Palladium acetate triarylphosphine complexes catalyze the addition of vinylic groups from vinylic halides to olefinic compounds in the presence of amines. Conjugated dienes are major products from 0,/3-unsaturated acids, esters, or nitriles while unactivated olefinic compounds react best in the presence of secondary amines where allylic amines are major products. The reactions are usually regio- and stereospecific. The synthetic utility of the reaction is illustrated with a wide variety of examples. 

Three-component additions, comprising two conjugated diene molecules and a nucleophile, can be catalyzed by palladium salts, such as palladium acetate. The major products are 1-substituted 2,7-octadienes, along with minor amounts of 3-substituted 1,7-octadienes (equation 65). Water, alcohols, phenols, carboxylic acids, and amines are some of the nucleophiles that have been used in this reaction. 

Larock and co-workers [47] have studied the palladium-catalyzed arylation of 1,3-dienes followed by intramolecular attack by an oxygen nucleophile. o-Iodophenols and o-iodobenzyl alcohol were used as substrates. However these reactions, which are annulation reactions, lead to a 1,2-addition to the conjugated dienes and will not be further treated here. Amides, also, were used as nucleophiles in these reactions. 

Palladium(ll)-catalyzed 1,4-additions to conjugated dienes also involve the foimation of a (jr-allyl)palladium intermediate. These, in all known cases, are oxidation reactions. 

Palladium-catalyzed reaction of conjugated dienes in the presence of a halide anion can be controlled to give a l-acyloxy-4-halo-2-alkene selectively under appropriate reaction conditions. The catalyst for this system is a palladium(II) salt, usually Pd(OAc)2 or Li2PdCl4. The reaction may be intermolecular or intramolecular. In most cases, it is stereoselective and results in a 1,4-cis addition to the diene. The products obtained from such reactions are useful synthetic intermediates since they have two allylic leaving groups with a large difference in reactivity (see below, under Synthetic applications ). 

The reaction conditions are similar to those employed in the diacetoxylation reaction, with the difference that the halide concentration (usually CI ) has been increased. Thus, palladium-catalyzed oxidation of 1,3-dienes with / -benzoquinone in the presence of lithium chloride and lithium acetate gives l-acetoxy-4-chloroalk-2-enes [78]. For example cyclohexa-1,3-diene and cyclohepta-1,3-diene afforded the corresponding chloroacetates 58a and 58b in good yield and >98% cis selectivity [Eq.(41)]. Cycloocta-1,3-diene gave a 61% yield of acetoxychlorination product (>98% cis), but in this case a 3 1 mixture of 1,4-and 1,2-addition products was formed. A number of substituted cyclic conjugated dienes work well, and, in all cases tried, the reaction proceeds with >97-98% cis selectively [52,78-81]. 

A palladium-catalyzed 1,4-dialkoxylation of conjugated dienes was obtained when the 1,4-oxidation was performed in an alcohol as the solvent [105]. In this case it is necessary to run the reaction in the presence of a catalytic amount of a strong acid such as methanesulfonic acid or perchloric acid. Cyclic dienes gave a highly stereoselective 1,4-ds addition of the two alkoxy groups [Eq.(48)j. Also, the reaction with acyclic conjugated dienes proceeded in a, 4-syn addition. Thus, ( , )-hexa-2,4-diene gave E)- 2R, 5R ) dimethoxyhex-3-ene. The mechanism is depicted in Scheme 8-27. 

Palladium-catalyzed 1,4-additions to conjugated dienes can be divided into two classes (1) non-oxidation reactions that (2) oxidation reactions. In the former class, a palladium(O) catalyst is employed and the first step in the catalytic cycle is often an activation of one of the reactants by its oxidative addition to Pd(0). In the second class, a palladium(II) complex is the active catalyst which oxidizes the substrate diene with formation of Pd(0). Reoxidation of Pd(0) to Pd(II) by an oxidant regenerates the active catalyst. 

Palladium-catalyzed hydrocyanation of olefins has been reported [31]. However, the corresponding reactions with conjugated dienes have not been reported explicitly. The analogous nickel-catalyzed hydrocyanation of conjugated dienes has been described [32] and is the basis for the commercial adiponitrile process. In this case, it has been shown [33] that the overall addition of HCN to the 1,3-diene occurs with cis stereochemistry consistent with path B in Scheme 8-1. 

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 asymmetric hydrosilylation of a-methylstyrene with methyldi-chlorosilane has been catalyzed by (/ )-benzylmethylphenylphosphine complexes of platinum(II) 302) or nickel(II) 304) to give a 5 or 17.6% excess of one enantiomer in the addition product, 2-phenylpropyl-methyldichlorosilane. The corresponding palladium(II) complexes were, however, only slightly useful for asymmetric synthesis in hydrosilylation of olefins. Nevertheless, palladium(II) complexes of methyldiphenyl-phosphine or epimeric neomethyldiphenylphosphine, where the dissymmetry is remote from the phosphorus, are especially useful for the induction of asymmetry in the hydrosilylation of styrene and some cyclic conjugated dienes 199). A similar procedure has been used for... 

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