Olefin complexes oxidative arylation

It has been found in the meantime that reaction (1) is generalizable (752), and that oxidative additions of this type occur for such widely differing substrates H2Y as ethylene, benzene 130), cyclic olefins, alkyl and aryl phosphines, aniline 337, 406), and H2S 130), ail of which give the same product structure with a triply-bridging Y ligand. The stability of these third-row transition metal clusters has stiU prevented catalytic reactions of these species, but it is likely that similar ones are involved in olefin and acetylene reactions catalyzed by other metal complexes. 

The palladium-catalyzed reaction of olefins with aryl or vinyl halides or pseudohalides in the presence of base (the Heck reaction) follows a different course from the other crosscoupling reactions after the oxidative addition step. As shown in Scheme 19.5, the olefin coordinates to the palladium after oxidative addition of the aryl or vinyl halide. Tliis coordination of olefin may occur by associative displacement of a monodentate ligand from the palladium, or it may occur by replacement of halide by the olefin to generate a cationic olefin complex. In some cases, these reactions are conducted with aryl or vinyl triflates. In this case, the olefin readily displaces the triflate to generate a cationic palladium-olefin... 

Various palladium species catalyze the reaction, and a common procedure is to start with Pd(OAc>2, PPhj, NaOAc and sometimes NEtj. The reaction generally is done at elevated temperatures (100-150 C) in a polar solvent, such as dimethylacetamide or dimethyformamide. The reaction usually proceeds with formation of at least some Pd metal. The standard reaction sequence involves first reduction of Pd(II), possibly by phosphine or amine, to a ligated Pd(0) species, often represented as PdLj. The latter is proposed to undergo oxidative addition with the aryl halide, and the resulting Pd"(Ar)(X)(L)2 species loses an L and coordinates the olefin. Then, the aryl group inserts into the Pd—olefrn bond. This is followed by p-hydride elimination, possibly assisted by base, liberation of the product and re-coordination of L to regenerate the catalyst. Further details can be found in recent reviews by Crisp, Beletskaya and Cheprakov and Amatore and Jutland. It should be noted that the electrochemical studies of the latter workers indicate that the Pd(0) is present as an anionic complex, such as Pd(L)2(OAc)", and that the oxidative addition gives a 5-coordinate Pd(II) species, such as Pd(Ar)(X)(L)2(OAc)". 

The halogen compound reacts with the Pd(0) species by oxidative addition to form a PdGI) complex. After insertion of the olefin in the aryl-Pd-bond the target molecule is formed by p-H-elimination. At this stage palladium is still in the two valent state. Separation of HBr in presence of the base leads back to the Pd(0) complex. The application of the Pd chemistry in polymerization reactions requires Aat the p-H-elimination can be neglected. 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

Recently, Larock and coworkers used a domino Heck/Suzuki process for the synthesis of a multitude of tamoxifen analogues [48] (Scheme 6/1.20). In their approach, these authors used a three-component coupling reaction of readily available aryl iodides, internal alkynes and aryl boronic acids to give the expected tetrasubsti-tuted olefins in good yields. As an example, treatment of a mixture of phenyliodide, the alkyne 6/1-78 and phenylboronic acid with catalytic amounts of PdCl2(PhCN)2 gave 6/1-79 in 90% yield. In this process, substituted aryl iodides and heteroaromatic boronic acids may also be employed. It can be assumed that, after Pd°-cata-lyzed oxidative addition of the aryl iodide, a ds-carbopalladation of the internal alkyne takes place to form a vinylic palladium intermediate. This then reacts with the ate complex of the aryl boronic acid in a transmetalation, followed by a reductive elimination. 

Organometallic reagents and catalysts continue to be of considerable importance, as illustrated in several procedures CAR-BENE GENERATION BY a-ELIMINATION WITH LITHIUM 2,2,6,6-TETRAMETHYLPIPERIDIDE l-ETHOXY-2-p-TOL-YLCYCLOPROPANE CATALYTIC OSMIUM TETROXIDE OXIDATION OF OLEFINS PREPARATION OF cis-1,2-CYCLOHEXANEDIOL COPPER CATALYZED ARYLA-TION OF /3-DICARBONYL COMPOUNDS 2-(l-ACETYL-2-OXOPROPYL)BENZOIC ACID and PHOSPHINE-NICKEL COMPLEX CATALYZED CROSS-COUPLING OF GRIG-NARD REAGENTS WITH ARYL AND ALKENYL HALIDES 1,2-DIBUTYLBENZENE. 

The [Con(bipy)2 ]2+ species has also been reported to activate hydrogen peroxide and ter -butyl hydroperoxide for the selective ketonization of methylenic carbons, the oxidation of alcohols and aldehydes, and the dioxygenation of aryl olefins and acetylenes (36). Later reports (37), however, while confirming that the cobalt complexes did indeed cata-... 

Palladium (II)-Nucleophile Addition across Olefins. Adding palladium complexes to olefins, either in the presence of an external nucleophile or a ligand which is attached to palladium, produces a palladium-carbon sigma-bonded complex which is not usually isolated in the case of monoolefins. Instead it decomposes and in doing so oxidizes the olefin to an organic carbonyl compound or a vinyl compound, exchanges a substituent group on the olefin, isomerizes the double bond, arylates (alkylates) the olefin, or carboxylates the olefin (2, 3). 

Scheme 2 shows the mechanism generally accepted for the catalytic arylation of olefins with aryl iodides in the presence of a tertiary phosphine-coordinated palladium catalyst and a base (4). Oxidative addition of aryl iodide (Arl) to a Pd(0) species (A), which is most commonly generated from palladium diacetate and a tertiary phosphine ligand, forms an arylpalladium iodide complex (B). Coordination of olefin on B followed by insertion of the coordinated olefin into the Pd-Ar bond forms a a-alkylpalladium species (C), which undergoes p-hydrogen elimination reaction to give the arylation... 

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