Palladium as catalyst

Dihydrothebainone, CigHagOgN, produced by the hydrogenation of thebaine in acetic acid in presence of palladium as catalyst, has m.p. 138-43°, — 72-5° (Schopf) gives a hydriodide, m.p. 262-3° (Freund) ... 

For the activation of a substrate such as 19a via coordination of the two carbonyl oxygen atoms to the metal, one should expect that a hard Lewis acid would be more suitable, since the carbonyl oxygens are hard Lewis bases. Nevertheless, Fu-rukawa et al. succeeded in applying the relative soft metal palladium as catalyst for the 1,3-dipolar cycloaddition reaction between 1 and 19a (Scheme 6.36) [79, 80]. They applied the dicationic Pd-BINAP 54 as the catalyst, and whereas this type of catalytic reactions is often carried out at rt or at 0°C, the reactions catalyzed by 54 required heating at 40 °C in order to proceed. In most cases mixtures of endo-21 and exo-21 were obtained, however, high enantioselectivity of up to 93% were obtained for reactions of some derivatives of 1. 

The reaction mechanism differs from that of other catalytic hydrogenations that also are carried out in the presence of palladium as catalyst, e.g. that of olefins. Presumably an organopalladium species is formed as an intermediate, which then reacts with the hydrogen ... 

Gas liquid chromatography of similar hydrogenation reaction mixtures on a 6 foot column, packed with 3.1% S.E. 30 supported on Diatoport S, at 150°C., showed that the proportion of the D-gluco to l-ido isomers formed was in the ratio of approximately 70 30 whereas in methanol and using palladium as catalyst the ratio was 96 4. 

A three-component, one-pot reaction of acyl chloride, propargylic alcohol derivatives and Nal using palladium as catalyst provided trisubstituted furans as depicted in the following scheme <06EJOC2991>. 3-Chloro-4-iodofurans can also be produced when IC1 and NaCl are used in the second step. 

The Diels-Alder additions of methyl y-fluorocrotonate to 2 3-dimethylbuta-l 3-diene and cycZopentadiene were effected by heating the reactants in sealed tubes at 110-120° for about 3 hr. The reduction of the unsaturated products, (XVIII) and (XX), was carried out at room temperature with palladium as catalyst. In both cases the theoretical quantity of hydrogen was absorbed, although the hydrogenation of the dimethyl derivative was much slower than that of the ewdomethylene compound. 

Palladium(II) salts apparently oxidize arylamines to arylpalladium salts since alkenes are arylated by reaction with only an aromatic amine and a palladium salt. However, yields are generally low.100 Much better yields are obtained if /-butyl nitrite is added and, of course, this forms the diazonium salt in situ. This not only saves a step but some diazonium salts which are too unstable to be isolated may be used as well. The reactions are carried out in the presence of acetic or chloroacetic acid with 5-10% bis(di-benzylideneacetone)palladium as catalyst (equation 41).101... 

Thus, for our present purposes a similar approach was followed using Suzuki cross-coupling reactions as the key steps in the synthesis of our target compounds. Symmetrically substituted compounds were synthesized in a twofold Suzuki crosscoupling reaction from commercially available p-substituted phenylboronic acids or esters and 4,4 -dibromobiphenyl or 4,4 -biphenyl-bis-boronic acid ester and a p-substituted arylhalide, respectively, using tetrakis (triphenylphosphino) palladium as catalyst together with cesium fluoride as base in dry tetrahydrofurane as shown in Scheme 8.1. The desired products were obtained in respectable yields after heating at reflux for 50 h. 

Scheme 5. Heck reaction using colloidal palladium as catalyst.

Indoles, pyrroles and carbazoles themselves are suitable substrates for palladium-catalyzed amination. An initial study of this reaction using DPPF-ligated palladium as catalyst showed that these reactions occurred readily with electron-poor aryl halides. With unactivated aryl bromides, the reaction with pyrrole or indole resulted in good yield, but reaction times were long and the temperature was 120 °C. Thus, an improved catalyst system was necessary for reactions to occur in a more general fashion and with temperature- or base-sensitive substrates. 

Colloidal nickelous hydroxide is reduced to colloidal nickel by hydrogen in the presence of colloidal palladium as catalyst. The freshly precipitated hydroxide may also be reduced in a similar manner. It is advantageous to add sodium protalbinate to the mixture before reduction in order to increase the stability of the hydrosols produced.4... 

Using palladium as catalyst, the region of minimal bond delocalization is reduced preferentially, whereas platinum tends to catalyze the reduction of terminal rings (equation 40). ... 

When 2-iodoimidazole 381 was treated with 382 bearing both a terminal alkene and a terminal alkyne, in the presence of copper and palladium as catalysts, only the Sonogashira product was obtained in high yield (Scheme 91) <2006T3798>. 

With palladium as catalyst a tetrahydro derivative (106) is obtained,82120 which can be further reduced with a platinum catalyst to the octahydro derivative (107).120... 

The tetramers 18 are stereoisomers of 3,3,6,6,9,9,12,12-octamethylpentacyclo-[9.1.0.0. 0 . 0 °]dodecane, as shown by extensive spectroscopic studies. The product distribution is only slightly dependent on the substituents in the 3-position of the cyclopropene ring. With bis(dibenzylideneacetone)palladium as catalyst, about 6% of the hexamethyl-tron -tris-cr-homobenzene 7 is additionally produced. When R R, a large number of isomeric products is formed and a complete analysis of all the tetrameric products has not yet been possible." ... 

A similar procedure with bis(t/ -allyl)palladium or bis(cycloocta-l,5-diene)palladium as catalyst leads to only slightly different product distributions of the tetramers. The absolute yield of the nonpolymeric products is lower in both cases. ... 

When 3,3-dimethylcyclopropene is exposed to carbon monoxide under pressure in the presence of tetrakis(triphenylphosphane)palladium as catalyst, 3,3,6,6,10,10-hexamethyl-exo,enrfo-tet-racyclo[7.1.0.0 " .0 ]decan-8-one (hexamethyl-tris-cr-homotropone, 10) is formed as the main product (72.2%) along with carbonyl-free di- and oligomerization products. At ambient pressure with an in situ generated palladium(O) catalyst, only a 20% yield of the tetracyclic ketone 10 is obtained. 

Dichlorobis(triphenylphosphane)palladium as catalyst leads to mixtures of the tetracyclic homo-cyclotrimer 11 and the tris-cr-homotropone 12 with both 3,3-dimethyl- and 3,3-diphenyl-cyclopropene in relatively low yields. ... 

Table 1. Hydrogenation of Substituted Cyclopropanes with Palladium as Catalyst...

On the other hand, compounds 86 and 87a can be transformed into 105 and 87b, respectively, without affecting the ring double bonds using palladium as catalyst (80JOC482 87JCS(P1)955). 

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