An
efficient gold-catalyzed cyclization of 2-alkynylaldehyde cyclic acetals
has been developed for the synthesis of indenone derivatives. A wide
variety of functionalized indenone derivatives can be obtained in
good-to-excellent yields. HMBC and NOESY NMR analyses and mechanistic
elucidation experiments revealed that the cyclization occurs via a
1,5-H shift. The cyclic acetal group promoted the 1,5-H shift by activating
the benzylic C–H bond and preventing the migration of the alkoxy
group by tethering both alkoxy groups.
The platinum on carbon (Pt/C)-catalyzed deuteration of acrylic and methacrylic acid derivatives in deuterium oxide (D 2 O) efficiently proceeded to give the corresponding acrylic acid-d 3 and methacrylic acid-d 5 derivatives. The olefinic functionality, as well as the methyl group on the unsaturated functionality of the substrate, were satisfactorily deuterated via the hydrogen (H)deuterium (D) exchange reaction. The obtained deuterated compounds are useful building blocks and efficiently converted to the corresponding desired products including a polymer without the degradation of the original deuterium contents.
Deuterium-labeled compounds are widely utilized in various scientific fields. We summarize the recent advances in the direct deuteration of sugar, saturated fatty acid, and arene derivatives using heterogeneous platinum group metal on carbon catalysts by our research group. Hydrogen gas is a key catalyst-activator to facilitate the present H-D exchange reactions. In this review, the direct activation method of catalysts using in situ-generated hydrogen based on the dehydrogenation of alcohols is introduced. The obtained multiple deuterium-labeled products, including bioactive compounds, are expected to contribute to the development of many scientific investigations.
The polyethyleneimine-modified
polymers, polystyrene–divinylbenzene-based
(TAs) and polymethacrylate-based polymers (TAm), were used as palladium
scavengers to eliminate residual palladium species after palladium
on carbon-catalyzed Sonogashira-type coupling reaction. Since both
TAs and TAm indicated relatively favorable elimination abilities toward
residual palladium species in the reaction mixture, the affinities
of TAs and TAm for palladium species were used as supports for palladium
catalysts. The TAm-supported palladium catalyst (Pd/TAm) indicated
better catalyst properties for the chemoselective hydrogenation compared
to those of the corresponding TAs-supported palladium catalyst (Pd/TAs).
Aromatic benzyl ethers; aromatic and aliphatic
N
-Cbzs;
and aromatic carbonyl groups were smoothly hydrogenated in the presence
of 1–5 mol % of Pd/TAm in MeOH or 2-PrOH. In contrast, the
hydrogenation of aromatic ketones was selectively suppressed in morpholine
which act as appropriate catalyst poison and solvent. Furthermore,
Pd/TAm-catalyzed chemoselective hydrogenation was applicable to continuous-flow
reaction.
We have established am ild and direct platinum on carbon (Pt/C)-catalyzed multi-deuterium labeling of various saturated fatty acids including bioactive compounds with high deuterium efficiencies in am ixeds olvent of isopropyla lcohol and deuterium oxide (i-PrOH)/D 2 Ou nder neutral conditions at 120 8 8Cw ithout the external addition of deuterium or hydrogengas.
The catalyst activities of various heterogeneous palladium catalysts supported by anatase-, rutile- and brookite-type titanium oxide for ligand-free Suzuki–Miyaura cross-coupling reactions of aryl chlorides were evaluated. Palladium acetate [Pd(OAc)2], supported on anatase-type titanium oxide (TiO2) via acetonitrile solution impregnation process without reduction [Pd/TiO2 (anatase-type)], demonstrated the highest catalyst activity in comparison to those of other titanium oxide (rutile- or brookite-type) supported Pd(OAc)2 without reduction and reduced Pd/TiO2 (anatase-type) [Pd(red)/TiO2 (anatase-type)]. Various aryl chloride and bromide derivatives were smoothly coupled with arylboronic acids including heteroarylboronic acids in the presence of 5–10 mol% Pd/TiO2 (anatase-type) without the addition of any ligands. Although the fresh Pd/TiO2 (anatase-type) catalyst was surprisingly comprised of ca. 1:2 mixture of palladium(II) and palladium(0) species according to X-ray photoelectron spectroscopy (XPS), in spite of no reduction process, significant further increment of palladium(0) species was observed during the Suzuki–Miyaura coupling reaction, and Pd/TiO2 (anatase-type) was converted into a catalyst, which contained palladium(0) species as the main component [ca. 1:5 mixture of palladium(II) and palladium(0) species]. Therefore, the reduction via the electron donation process to the palladium(II) species may have occurred during the reaction on anatase-type titanium oxide.
Flow reaction methods have been developed to selectively synthesize tertiary, secondary, and primary amines depending on heterogeneous platinum-group metal species under catalytic hydrogenation conditions using nitriles as starting materials. A 10 % Pd/C-packed catalyst cartridge affords symmetrically substituted tertiary amines in good to excellent yields. A 10 % Rh/C-packed catalyst cartridge enables the divergent synthesis of secondary and primary amines, with either cyclohexane or acetic acid as a solvent, respectively. Reaction parameters, such as the metal catalyst, solvent, and reaction temperature, and continuous-flow conditions, such as flow direction and second support of the catalyst in a catalyst cartridge, are quite important for controlling the reaction between the hydrogenation of nitriles and nucleophilic attack of in situ-generated amines to imine intermediates. A wide variety of aliphatic and aromatic nitriles could be highly selectively transformed into the corresponding tertiary, secondary, and primary amines by simply changing the metal species of the catalyst or flow parameters. Furthermore, the selective continuous-flow methodologies are applied over at least 72 h to afford three different types of amines in 80-99 % yield without decrease in catalytic activities.
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