The cationic ruthenium−hydride complex [(C 6 H 6 )(PCy 3 )(CO)RuH] + BF 4 − catalyzes selective etherification of two different alcohols to form unsymmetrically substituted ethers. The catalytic method exhibits a broad substrate scope while tolerating a range of heteroatom functional groups in forming unsymmetrical ethers, and it is successfully used to directly synthesize a number of highly functionalized chiral nonracemic ethers.
A cationic ruthenium hydride complex, [(C6H6)(PCy3)(CO)RuH] + BF4- (1), with a phenol ligand was found to exhibit high catalytic activity for the hydrogenolysis of carbonyl compounds to yield the corresponding aliphatic products. The catalytic method showed exceptionally high chemoselectivity toward the carbonyl reduction over alkene hydrogenation. Kinetic and spectroscopic studies revealed a strong electronic influence of the phenol ligand on the catalyst activity. The Hammett plot of the hydrogenolysis of 4-methoxyacetophenone displayed two opposite linear slopes for the catalytic system 1/p-X-C6H4OH (ρ = −3.3 for X = OMe, t-Bu, Et, and Me; ρ = +1.5 for X = F, Cl, and CF3). A normal deuterium isotope effect was observed for the hydrogenolysis reaction catalyzed by 1/p-X-C6H4OH with an electron-releasing group (kH/kD = 1.7-2.5; X = OMe, Et), whereas an inverse isotope effect was measured for 1/p-X-C6H4OH with an electron-withdrawing group (kH/kD = 0.6-0.7; X = Cl, CF3). The empirical rate law was determined from the hydrogenolysis of 4-methoxyacetophenone: rate = kobsd [Ru][ketone] [H2] −1 for the reaction catalyzed by 1/p-OMe-C6H4OH, and rate = kobsd [Ru][ketone] [H2] 0 for the reaction catalyzed by 1/p-CF3-C6H4OH. Catalytically relevant dinuclear ruthenium hydride and hydroxo complexes were synthesized, and their structures were established by X-ray crystallography. Two distinct mechanistic pathways are presented for the hydrogenolysis reaction on the basis of these kinetic and spectroscopic data.
A well-defined cationic Ru-H complex catalyzes reductive etherification of aldehydes and ketones with alcohols. The catalytic method employs environmentally benign water as the solvent and cheaply available molecular hydrogen as the reducing agent to afford unsymmetrical ethers in a highly chemoselective manner.Etherification of oxygenated organic compounds is an ubiquitous organic transformation in both industrial and fine chemical syntheses. 1 Strong Brønsted acid and heterogeneous acid catalysts are commonly employed for industrial-scale etherification of alcohols, 2 while the Williamson ether synthesis has long been used for laboratory-scale synthesis of unsymmetrically substituted ethers. 3 Seminal catalytic C-O bond formation methods such as Ullmann-and Mitsunobu-type coupling reactions have been extensively utilized for the synthesis of aryl-substituted ethers. 4 More recently, a number of highly effective catalytic methods for unsymmetrical ethers have been developed from use of hydroalkoxylation of alkenes 5 and oxidative C-H alkoxylation of arenes. 6 The reductive etherification of carbonyl compounds has also been shown to be a synthetically powerful etherification method, but this method requires a stoichiometric amount of silane as the reducing agent. 7 Despite such remarkable progress, these catalytic etherification methods pose major synthetic and environmental problems in that they employ reactive reagents such as inorganic acids and organic alkoxide substrates, which result in the formation of copious amounts of wasteful byproducts. From the viewpoint of achieving green and sustainable catalysis, the development of an efficient and broadly applicable catalytic etherification process that NOT THE PUBLISHED VERSION; this is the author's final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.Organic Letters, Vol 17, No. 7 (2015): pg. 1778-1781. DOI. This article is © American Chemical Society and permission has been granted for this version to appear in e-Publications@Marquette. American Chemical Society does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from American Chemical Society. 3does not form any wasteful byproducts remains a high priority goal, particularly for the synthesis of unsymmetrically substituted ethers. 8 We recently discovered that a well-defined cationic ruthenium hydride complex [(C6H6)(PCy3)(CO)RuH] + BF4 -(1) is a highly selective catalyst precursor for the etherification of two different alcohols to form unsymmetrically substituted ethers. 9 While this etherification provides unsymmetrical ethers without forming any wasteful byproducts, it was not effective for the coupling between electronically similar or sterically demanding aliphatic alcohols, as it gave a mixture of symmetrical and unsymmetrical ethers. In an effort to extend the scope of the etherification reaction, we explored the analogous reductiv...
It cuts two ways: The cationic [Ru-H] complex catalyzes selective coupling of α- and β-amino acids with ketones to form α-alkylated ketone products. The reaction involves CC and CN bond cleavage which result in regio- and stereoselective alkylation using amino acids. A broad substrate scope and high functional-group tolerance is demonstrated.
Biosynthesis using plant extract is known as one of the potential techniques to synthesize different zinc oxide nanoparticles (ZnO-NPs) in different size ranges. ZnO-NPs were synthesized using Plumeria leaf extract with laboratory chemical reagent Zn(CH3COO)2 and followed by the micro-encapsulation of biosynthesized ZnO-NPs using chitosan and cellulose with TEOF as a cross-linker employing freeze gelation method. Both neat and encapsulated ZnO-NPs have been characterized by FT-IR, UV spectroscopy, XRD, and SEM techniques. The UV-spectroscopic analysis confirmed the characteristic band of ZnO-NPs at 356.0 nm, and FIIR showed the peaks at 544 cm−1 and 545 cm−1 corresponding to the Zn–O bond. Powder XRD pattern showed the wurtzite structure of ZnO and gave the calculated average crystallite size as of 27.23 nm. In the case of encapsulated ZnO-NPs, the UV–visible spectrum showed two strong absorption peaks at 232.5 nm, 242.5 nm, and a weak peak at 357 nm. A broad peak at 3333 cm−1 in FT-IR spectra is either due to N–H stretching in the amide group of chitosan or hydroxyl group in encapsulated ZnO-NPs. It was observed that chitosan loaded ZnO-NPs had higher entrapment efficiency (81.98%) at 15 mL of plant extract. The kinetic profile in the release of ZnO particles out from encapsulated ZnO-NPs was observed to follow four kinetic paths in 120 min at pH 1.2. The particle release followed the zero-order kinetic in the first 50 min and then followed by Hixson–Crowell kinetic in the next 50 min with two different rate constants, 2.6 × 10−3 min−1 and 13 × 10−3 min−1, before it backs to the zero-order kinetics. This study shows that ZnO nanoparticles can easily be biosynthesized and encapsulated for use in the pharmaceutical industry.
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