A mild and robust heterogeneous palladium‐catalyzed CO bond cleavage of 2‐aryloxy‐1‐arylethanols using formic acid as reducing agent in air was developed. The cleaved products were isolated in 92–98 % yield; and by slightly varying the reaction conditions, a ketone, an alcohol, or an alkane can be generated in near‐quantitative yield. This reaction is applicable to cleaving the β‐O‐4′‐ether bond found in lignin polymers of different origin. The reaction was performed on a lignin polymer model to generate either the monomeric aryl ketone or alkane in a quantitative yield. Moderate depolymerization was achieved with native lignin at similar reaction conditions. Mechanistic studies under kinetic control indicate that an initial palladium‐catalyzed dehydrogenation of the alcohol is followed by insertion of palladium to an enol equivalent. A palladium–formato complex reductively cleaves the palladium–enolate complex to generate the ketone.
A palladium-catalyzed transfer hydrogenolysis of primary, secondary, and tertiary benzylic alcohols by formic acid has been developed and studied. The product hydrocarbons were obtained in excellent yields from both secondary and tertiary benzylic alcohols and in good yields for primary benzylic alcohols. The rate of disappearance of 1-phenylethanol (1) follows zero-order dependence in 1 and first-order dependence in formic acid and palladium. Catalytic amounts of base inhibit a competing disproportionation reaction of alcohol to alkane and ketone, and an optimum was obtained when 5 equiv of base to palladium was used. Deuterium kinetic isotope studies for the transfer hydrogenolysis reveal individual isotope effects for the hydridic position (k CHOH /k CDOH = 2.26 ± 0.24) and the protic position (k CHOH /k CHOD = 0.62 ± 0.06) of the formic acid. Simultaneous deuteration in both positions of formic acid gave a combined isotope effect of (k CHOH /k CDOD = 1.41 ± 0.11). We propose a mechanism involving the following steps: a competitive inhibition of the open palladium site by adsorption of the formate anion to generate formato-palladium species, followed by a reversible protonation and a rate-limiting hydride transfer to obtain the active palladium with chemisorbed hydrogen that performs the hydrogenolysis of the alcohol in a fast reaction step.
Precipitated kraft lignin from black liquor was converted into green diesel in three steps. A mild Ni‐catalyzed transfer hydrogenation/hydrogenolysis using 2‐propanol generated a lignin residue in which the ethers, carbonyls, and olefins were reduced. An organocatalyzed esterification of the lignin residue with an in situ prepared tall oil fatty acid anhydride gave an esterified lignin residue that was soluble in light gas oil. The esterified lignin residue was coprocessed with light gas oil in a continous hydrotreater to produce a green diesel. This approach will enable the development of new techniques to process commercial lignin in existing oil refinery infrastructures to standardized transportation fuels in the future.
Palladium-catalyzed allylic substitution of non-derivatized enantioenriched allylic alcohols with a variety of uncharged N-, S-, C- and O-centered nucleophiles using a bidentate BiPhePhos ligand is described. A remarkable effect of the counter ion (X) of the XPd[κ -BiPhePhos][η -C H ] was observed. When ClPd[κ -BiPhePhos][η -C H ] (complex I) was used as catalyst, non-reproducible results were obtained. Study of the complex by X-ray crystallography, P NMR spectroscopy, and ESI-MS showed that a decomposition occurred where one of the phosphite ligands was oxidized to the corresponding phosphate, generating ClPd[κ -BiPhePhosphite-phosphate][η -C H ] species (complex II). When the chloride was exchanged to the weaker coordinating OTf counter ion the more stable Pd[κ -BiPhePhos][η -C H ] +[OTf] (complex III) was formed. Complex III performed better and gave higher enantiospecificities in the substitution reactions. Complex III was evaluated in Tsuji-Trost reactions of stereogenic non-derivatized allylic alcohols. The desired products were obtained in good to excellent yields (71-98 %) and enantiospecificities (73-99 %) for both inter- and intramolecular substitution reactions with only water generated as a by-product. The methodology was applied to key steps in total synthesis of (S)-cuspareine and (+)-lentiginosine. A reaction mechanism involving a palladium hydride as a key intermediate in the activation of the hydroxyl group is proposed in the overall transformation.
The first naturally occurring atropisomeric flavans, myristinins B (2), C (2a), E (4), and F (4a), together with their corresponding trans-isomers, myristinins A (1) and D (3), were isolated from the CH(2)Cl(2) extract of Myristica cinnamomea fruits. Compounds 1, the mixture of 2 and 2a, and the mixture of 4 and 4a, exhibited antifungal activity against Candida albicans with IC(50) values ranging from 5.9 to 8.8 microg/mL, and they selectively inhibited the enzyme cyclooxygenase-2 (COX-2).
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