Oxygen Palladium, isomerism

Some synthetically useful isomerization reactions of alkenes, other than nitrogen- or oxygen-substituted allylic compounds, were reported by the use of a catalytic amount of transition metal complexes. The palladium complex, /ra r-Pd(C6HsCN)2Gl2, effectively catalyzed the stereoselective isomerization of /3,7-unsaturated esters to a,/3-unsaturated esters (Equation (26)). 

The arylalkylamine 188 undergoes palladium(ll)-catalyzed carbonylation with Pd(OAc)2 and Cu(OAc)2 in the presence of carbon monoxide gas containing oxygen to produce equal quantities of the isomeric benzolactams (Equation 124) <2004JA14342>. 

It is now possible to understand the curious phenomenon whereby the reaction of palladium acetate with I in vacuo first rapidly produces a metal precipitate and then slows at about 20% conversion and finally stops with much of the palladium (II) unreacted. These stages in the reaction correspond to oxidation first by Pd3(OAc)6 and then by IVa with ultimate formation of the inert species Va. A complex mixture of hexenyl acetates is formed in the oxidation of which the major constituent l-hexen-2-yl acetate (VI) is 0.68 mole fraction of the whole mixture. Overall the mixture is closely similar to that obtained in the catalytic reactions of 02 described later, suggesting that the same active palladium-containing species is involved. Much of I is isomerized to a 5 1 mixture of trans- and ds-2-hexene (85% at 6 hrs) with only 3% each of the 3-hexene isomers. This aspect of the selectivity problem in which only one shift of the double bond takes place is also reproduced in the catalytic reaction, but oxygen suppresses the rate of isomerization relative to oxidation. 

The reaction can be carried out at least partially catalytically under oxygen pressure (38). In addition to the problem of making the reaction catalytic in palladium, it appears that only certain olefins react well and only activated aromatic hydrocarbons are useful. Even with activated aromatics the pal-ladation is often not very selective and isomeric mixtures result. Thus, this variation is not as generally useful as the organic halide reaction either. 

Fortunately, most of the palladium addition reactions with olefins can be carried out catalytically in the palladium compound so that large amounts of the expensive palladium compounds are not needed. As in the inorganic palladium salt additions, cupric chloride is a useful reoxidant. This, of course, limits the catalytic reaction to cases where olefin isomerization is not a problem. The cupric chloride is reduced to cuprous chloride during the reaction. As in the acetaldehyde synthesis, the reaction may be made catalytic in copper as well as palladium by adding oxygen and, in this case, hydrogen chloride also. 

Isomerization of 1,3-diene epoxides. Noyori etalf have reported three types of isomerization of allylic oxides in the presence of a catalytic amount of this palladium(O) complex. One class, exemplified by the isomerization formulated in equation (I), involves transfer of a hydrogen atom from the Ca-alkyl group and results in a dienol of the type obtained in the ene reaction of singlet oxygen with 1,3-dienes. 

The eosin-sensitized photo-oxygenation of 3jS-hydroxyandrosta-5,7-dien-17-one (181) provides a mixture of 5a,8-epidioxide (182a), 5,8-dien-7-one (182b), and both isomeric allylic alcohols (183a) and (183b). Palladium-catalysed treatment of (182a) gave the triol (184), which was then converted into the triene-dione (185) °. 

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. 

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