Palladium hydroxyl group effect

Hydrogenation of 19-hydroxy-3a- and 3j5-substituted-A -steroids over platinum or rhodium yields increased amounts of 5j9-products as compared to the corresponding 19-desoxy series (hydroxyl group effect). In contrast, the A -19-carboxaldehyde (27) gives only the 5a-product when hydrogenated over either palladium or platinum. ... 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

A synthesis of (-)-4a,5-dihydrostreptazolin was accomplished in a similar manner only a reductive palladium-catalyzed cyclization was utilized.104 The hydride source was polymethylhydroxysilane (PMHS), and the unprotected hydroxyl group of 174 (Equation (89)) had an accelerating effect, since the reaction was completed in minutes instead of hours, as in the example above. 

Among the unimolecular cyclizations, the synthesis yielding a single product with excellent yields was the cyclization of allyloxy alcohols brought about by a ruthenium complex. The other unimolecular processes exhibited very low yields and/or mixture of products. The most popular synthesis of 1,3-dioxocins involved the cyclization of 1,5-alcohols with the insertion of a carbon unit. Such acetalization of both acyclic dialcohols or hydroxyl groups bound to rings is particularly efficient and the cyclization of methane-diphenols or dihydroxybenzophenones with dihalomethanes was of wide applications. Palladium-promoted cyclization of chloromercurio compounds showed to be certainly less effective even if it presented some cases in reasonable yields. The sole example synthesis of 1,3-dioxocins by transformation of another ring has no preparative interest. 

Reaction of hydroxylic compounds with butadiene has been studied with palladium compounds. A common feature is the dimerization of butadiene with incorporation of functional groups from alcohols (270). l-Methoxyoctadiene-2,7 was obtained from butadiene and methanol in the presence of Pd(maleic anhydride)(PPh3)2 (273). Complex Pt(PPh3)4 has also been used, although platinum compounds were less effective. Octadienyl esters were obtained from butadiene and acetic acid in the presence of Pd(acac)2 and either PPh3 or P(OPh)3 (294). Palladium complexes were effective for the synthesis of jS,y-unsaturated esters from butadiene, methanol, and CO. The favored mechanism involved addition of a hydropalladium complex to butadiene to give an allylpalladium intermediate (46). 

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