Palladium catalyzed hydrogenation

Thus, in contrast to benzothiepins, dibenzo compounds can be synthesized by direct acid-catalyzed elimination of water from hydroxy derivatives, or of amines from amino derivatives, at elevated temperatures due to their thermal stability. As in the case of benzothiepins, dibenzo derivatives can also be prepared by base-catalyzed elimination from the corresponding halo derivatives however, the yields are somewhat lower compared to the acid-catalyzed reactions. As a special case, an aziridine derivative was deaminated by palladium-catalyzed hydrogenation to afford the corresponding dibenzothiepin.69... 

Catalysis and Synthesis in the Laboratory. Research on the practical applications of catalysis was not matched in the laboratory. We began a study of metal and non-metal catalyzed reactions early in our sonochemistry program. Our first project was to develop a convenient method of hydrogenating a wide range of olefins. We chose formic acid as our hydrogen source and found it to be effective. For example, with continuous irradiation, palladium catalyzed hydrogenations of olefins are complete in one hour(44). 

An unexpected dimerization product was obtained in 30% yield during the palladium-catalyzed hydrogenation of 2-(2-pyridylmethylene)-3(2H)-benzo[b]furan-3-one. A couple of similar types of substrates were investigated, indicating the scope of the substrates was limited... 

It was concluded that the high selectivity observed in the hydrogenation experiments using 26 b is explained by the relatively strong coordination of the alkyne to the palladium center, which only allows for the presence of small amounts of alkene complexes. Only the latter are responsible for the observed minor amounts of ( )-alkene, which was shown to be a secondary reaction product formed by a subsequent palladium-catalyzed, hydrogen-assisted isomerization reaction. Since no n-octane was detected in the reaction mixture, only a tiny... 

In this chapter, we will focus on the rhodium-catalyzed hydrogenation of functionalized ketones and the development of chiral phosphorous ligands for this process. Although there are other chiral phosphorous ligands which are effective for ruthenium-, iridium-, platinum-, titanium-, zirconium-, and palladium-catalyzed hydrogenation, they will not be discussed here. For details of these chemistries, the reader should refer to other chapters of this book. 

The structure, and the trans-relative stereochemistry, of dihydroarcyriarubin B (352) was confirmed by comparison of the product obtained from arcyriarubin B (350) by palladium-catalyzed hydrogen transfer from cyclohexene in boiling xylene. Under these conditions, only the thermodynamically more stable trans-diastereomer was formed. Based on these data, and the spectroscopic comparison with the hydrogenation product of arcyriarubin B (350), the structure 352 was assigned to dihydroarcyriarubin B (252) (Scheme 2.90). 

This conversion of readily available 2-furyl alcohols into unsaturated pyranosuloses proved a very effective route to racemic monosaccharides, through stepwise, selective functionalization of the enone grouping in 325. The shortest synthesis of a natural compound by following this scheme involves palladium-catalyzed hydrogenation of the aldosulose (325, R = Me) obtained from l-(2-furyl)ethanol, resulting209 in cinerulose A, the sugar component of the antibiotic cine-rubin. 

Basically, the synthesized target molecules were achieved in amounts of 200-500 mg with a standard on purity of >95%. The purities of the compounds were routinely checked by analytical HPLC. The core template 1 was prepared by palladium-catalyzed hydrogenation of commercially available glucal 10 followed by deacetylation, according to the literature, which afforded derivative 11 [27] (Scheme 3.2.3). Free hydroxy compound 11 was selectively silylated with two equivalents of TBDMSC1 in the presence of imidazole 12, followed by benzylation and subsequent cleavage of the TBDMS groups 1. 

The synthesis of linker-head intermediate 16, illustrated in Scheme 3.2.5, began with Boc-tranexamic acid 15 [29] which, on amide coupling with Cbz-piperazine and EDC, again followed by palladium-catalyzed hydrogenation of the Cbz group as in the latter case, furnished derivative 16. 

To ensure complete oxidation of aldehydes to carboxylic acids, Aulin-Erdtman and Tomita (1963) treated the ozonation products of lignin model compounds with hydrogen peroxide. Soteland (1971) applied hydrogen peroxide to a groundwood ozonation product and found that it degraded the components to unidentified volatile substances. To simplify the product mixture, reduction is sometimes used to stop further oxidation. Tanahashi et al. (1975) applied palladium-catalyzed hydrogenation, while Kondo et al. (1987) added dimethyl sulfide. 

Substituted-2-phenylbenzo[. ]furans were able to undergo a palladium-catalyzed hydrogenation reaction to give their corresponding 2,3-dihydrobenzo[ ]furans, but the yields are quite low (Scheme 51) <2002T4261>. 

On the basis of the solid-state conformations of acryloyl6 and 2-butenoyl7 sultams, the major product is assumed to arise from attack of 0s04 on the side opposite to the pseudoaxial sulfonylic oxygen 6. The same model is applicable to other nonchelation-controlled (V-enoylsul-tam transformations such as nitrile oxide cycloadditions (see Section D. 1.6.1.2.) and palladium-catalyzed hydrogenations (see Section D.2.5.1.)8. 

The Ox group can be removed by reduction, either by palladium-catalyzed hydrogenation (quantitative) or by use of sodium in liquid ammonia (75-85% yield). The Ox group can also be cleaved by oxidation with excess m-chloroperbenzoic acid followed by hydrolytic workup (70%). Simple Ox dipeptides have been prepared using l-ethyl-3-(3 -dimcthylaminopropyl)carbodiimide hydrochloride (I, 371). The free dipcpiidcs... 

The palladium-catalyzed hydrogenations of 1,2-cyclononadiene and 1,2-cyclodecadiene form substantial amounts of trans- as well as the ct s-cycloalkene. - The only unhindered approaches to either double bond of these dienes lead to a cis isomer. Because the trans is the less stable geometrical isomer in rings with fewer than 11 carbon atoms, its formation implies that an intermediate is produced which is capable of yielding both cis- and trans-cycloalkenes at comparable rates. Moore proposed that the intermediate has the syn-anti- n- y structure (43 equation 35). 

Grigg and co-workers [203] have very recently described an ingenious cascade process. Tliis is initiated by the addition of Bu3SnH to alkyne cyclization and anion capture follow and afford polycyclic products (Scheme 4-52). A palladium-catalyzed hydrogenation of the double bond in ajS-unsaturated A -acyloxy azolidinones was described by Wu et al. [204] the diastereoselectivity was low. 

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