Cyclohexadiene complexes with palladium

Cycloheptene complexes with gold, 12 348 Cyclohexadiene complexes with cobalt, 12 286 with group VIB metals, 12 225-227 with group VIIB metals, 12 240, 241 with iron, 12 264 with palladium, 12 313 with ruthenium, 12 278 with silver, 12 340... 

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson28 obtained the 1,3-diacetate 25 from oxidation of 1,4-cyclohexadiene by ben-zoquinone in acetic acid with palladium acetate as the catalyst (Scheme 3). Presumably the reaction proceeds via acetoxypalladation-isomerization to give a rr-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers29. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of 1,4-cyclohexadiene to the ( r-allyl)palladium complex 26 by treatment... 

Arylsulfinate anions have been employed as terminators of cascade carbopalladations. Starting from an iodoaryl-substituted 1,4-cyclohexadiene, treatment with a palladium catalyst led to an intermediate rr-allyl complex, which was trapped with sodium benzene-sulflnate to yield a tricyclic allylsulfone (Scheme 31). 

The paUadium-catalyzed reactions occurred with the simple combination of [Pd(allyl) Cl]j and added phosphine, or with Pd(PPh3) and an acid co-catalyst. The scope of these reactions encompassed the additions of arylamines. As shown in Equation 16.81, reaction of various arylamines with cyclohexadiene occurred with high enantioselectivity using Trost s ligand. This ligand is discussed in more detail in Chapter 20. These reactions, and nickel-catalyzed reactions, occur by nucleophilic attack of amines on ir-allyl intermediates generated by protonation of diene complexes or insertion of dienes into palladium hydrides. 

A powerful piece of methodology has been developed involving nucleophilic attack on both an r and an T]p-complex in an adaptation of the Wacker reaction. Treatment of cyclohexadiene 9.268 with palladium acetate in acetic acid gives an -complex 9.281 (Scheme 9.76). If the reaction is done in the presence of sodium acetate, this salt will act as a nucleophile to give an t -complex 9.283 via an initial t -complex 9.282. Reductive elimination to form a bond between a ring carbon and the acetate ligand from palladium then gives the diacetate product 9.280. As the first acetate attacks trans to Pd, and the second acetate comes from Pd, the product is the trans isomer. The palladium is now in its zero oxidation state, but inclusion of benzoquinone reoxidizes it to palladium (II) and makes the entire process catalytic. There is an occasional... 

The rational synthesis of anionic, neutral, and cationic dinuclear palladium complexes containing bridging conjugated dienes is shown in Scheme 32 for butadiene, and consists of the formal redox condensation of Pd(0) and Pd(ii) complexes in the presence of the conjugated diene to give Pd(l) dimers. In addition, some complexes with isoprene and with 1,3-cyclohexadiene were made. The isoprene complexes consisted of the expected pair of regioisomers in the case of the less symmetric neutral molecules. The crystal structures of one neutral complex... 

Other examples that involve intermediate allyl cations are illustrated in Scheme 1.4. The cationic palladium(II) complex [Pd(dppp)(PhCN)2](BF4)2 coordinates the carbonyl oxygen of benzaldehyde and the activated carbonyl carbon attacks the isoprene, forming the allyl cation 10 which then cyclizes to give the 4-methyl-6-phenyl-5,6-dihydro-2H-pyran [22]. 2-Oxopropyl acrylate 11, in the presence of trimethylsilyltrifluoromethane sulfonate (TMSOTf) and methoxytrimethylsilane (MeOSMT), generates the cation 11a which is an efficient dienophile that reacts easily with the cyclohexadiene to give the Diels-Alder adduct in good yield [23]. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

Application of various chiral palladium complexes to the stereoselective hydrosilylation of dienes results in the formation of 3-silylcvcloalkenes irrespective of the isomer of the diene used (in the case of cyclohexadienes). When complexes containing menthyldiphenylphosphine (MDPP) and neomenthyldiphenylphosphine (NMDPP) are used, the chiral products with an excess of the (S)-enantiomer are always formed1011. 

Fischer and Werner (218) have reported the preparation of the Pd(0)-olefin complex (cyclopentadiene)( 1,3-cyclohexadiene)palladium-(0). Initially they prepared dimeric (l,3-cyclohexadiene)dichloropal-ladium(II) by reaction of the diene with Pd(CO)Cl2, which was, in turn, reacted with cyclopentadienylpotassium. The red, air-stable, monomeric complex was diamagnetic, soluble in most organic solvents, and had a dipole moment of 2.11 Z) in agreement with the tetrahedral structure (204). 

In case (3) it cannot be excluded that the intermediate olefin complex would be a chelate with both olefmic bonds of the 1,4-cyclohexadiene coordinated to the palladium, and the oxypalladation product is in fact expected to be trans orientated as rotation of the double bond is blocked as with analogous complexes of cyclic diolefins such as 1,5-cyclooctadiene, dicyclopentadiene, norbomadiene, etc. From these, stable fran -oxymetallation products can be obtained [39 1] others are listed in [12, Table IV]. 

Olefin Activation. Mechanistic studies on the 1,4-oxidation of 1,3-dienes led to the discovery of a new palladium catalyst [Pd(DA)2], which was readily prepared from the reaction of Pd2(dba)3 and the Diels-Alder adduct derived from 1,3-cyclohexadiene and/>-benzoquinone. With p-benzoquinone as the stoichiometric oxidant, this palladium complex proves more reactive and selective than the Pd(II) carboxylate, typically used in... 

More recently, the hydroamination of butadiene, isoprene, and cyclohexadiene to form 1 1 adducts with high selectivity catalyzed by nickel and palladium complexes has... 

In 1983, Dieck and co-workers studied the reactions between aryl and vinyl halides with 1,3-dienes and amines. The catalytic formation of n-allylic palladium complexes via addition reactions to 1,3-dienes was involved, then the complexes formed reacted with amines and gave the final products. In order to explore the potential value of this methodology in organic synthesis, they tested 2-iodoaniline as a substrate as well. % using isoprene and 1,3-cyclohexadiene as the coupling partner, the desired cyclized produets were formed in good yields (Scheme 2.80). 

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