Dihydrogen activation at non-metallic centers

ChemInformVolume 18, Issue 11 Reviews ChemInform Abstract: Role of Bimetallic Catalysts in Catalytic Hydrogenation and Hydrogenolysis L. GUCZI, L. GUCZI Inst. Isot., Hung. Acad. Sci., H-1525 Budapest, Hung.Search for more papers by this authorZ. SCHAY, Z. SCHAY Inst. Isot., Hung. Acad. Sci., H-1525 Budapest, Hung.Search for more papers by this author L. GUCZI, L. GUCZI Inst. Isot., Hung. Acad. Sci., H-1525 Budapest, Hung.Search for more papers by this authorZ. SCHAY, Z. SCHAY Inst. Isot., Hung. Acad. Sci., H-1525 Budapest, Hung.Search for more papers by this author First published: March 17, 1987 https://doi.org/10.1002/chin.198711365Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume18, Issue11March 17, 1987 RelatedInformation

The enantioselective reductive amination of ketones with Hantzsch ester has been achieved through Bronsted acid catalysis. A novel triphenylsilyl substituted BINOL-derived phosphoric acid catalyst has been developed for this transformation, imparting high levels of selectivity when used with methyl ketones and aromatic amines. A stereochemical model for the observed selectivity based on torsional effects has been developed through molecular modeling and is further supported by a single crystal x-ray structure of an imine-catalyst complex. Mechanistic studies have revealed the importance of catalyst buffering and drying agent on reaction efficiency while a Hammett analysis of acetophenone derivatives offers insight into the key factors involved in the enantiodetermining step. Kinetic studies have shown that imine reduction is rate-determining and follows Michaelis-Menten kinetics. Determination of the Eyring parameters for the imine reduction has also been accomplished and suggests that the phosphoric acid catalyst behaves in a bifunctional manner by activating both the imine electrophile and the Hantzsch ester nucleophile. The intermolecular addition of vinyl, aromatic, and heteroaromatic potassium trifluoroborate salts to non-activating imines and enamines can also be accomplished through Bronsted acid activation. This analog of the Petasis reaction shows a wide substrate scope and is amenable to use with a variety of carbamate protected nitrogen electrophiles in the first example of metal-free 1,2-additions of trifluoroborate nucleophiles. The mechanistic underpinnings of benzyl trifluoroborate addition has also been explored and, in contrast to what is seen with π-nucleophilic species, appears to proceed through an intramolecular alkyl-transfer mechanism.

Eight heterocycles have been prepared in a one‐pot reaction manner based on the Hantzsch dihydropyridine synthesis. The synthesis afforded seven dihydropyridines (DHP) and one unexpected 1,3‐oxazin‐6‐one. Their structures were confirmed based on NMR spectroscopy and mass spectrometry. The obtained products have been evaluated for their cytotoxicity against eight cancer cell lines and one normal cell line. Two halogenated DHPs (7 and 8) displayed cytotoxicity toward all the nine tested cancer cell lines with IC50 values from 4.10 to 58.90 μm, while others showed selective activities. DHPs (7 and 8) bearing a Me group at C(2) and C(6) as well as a halogenated substituent at C(4′) were more antiproliferative than the others.

Use of solvents and environmental friendly materials for applications in Green Chemistry

Four metal complexes of bis(salicyaldimine) dianion, MII(saltBuenpy) (MII = ZnII, NiII, CoII, PdII), have been synthesized and characterized. While Ni(saltBuenpy) exhibit a simple, well know structure in the solid-state, Zn(saltBuenpy) shows an interesting open vast structure of tetrameric assembly [Zn(saltBuenpy)]4 in the solid-state. Photoluminescence studies of Zn(saltBuenpy) exhibit an aggregation-caused quenching (ACQ) effect in DMSO solution. Complex Zn(saltBuenpy) displayed sensing properties in detection of organic nitro compounds (nitrobenzene, NB; 1,4-dinitrobenzene, DNB; 4-nitrotoluene, NT; 2,4-dinitrotoluene, DNT; nitromethane, NM; 2,3-dimethyl-2,3-dinitrobutane, DMDNB) and amines (aniline; m-phenylenediamine; 1-butanamine; 4-aminopyridine; 3-(aminomethyl)pyridine; 4-(aminomethyl)pyridine) through fluorescence quenching, except in the case of 4-(aminomethyl)pyridine that causes fluorescence enhancement. In addition, Zn(saltBuenpy) can also be used to detect Ag+ salts of different anions, as a result of noticeable fluorescene quenching, with exception of Ag(OAc). Detailed studies confirm that anions have interference upon detection of fluorescence change in intensity.

C2-symmetric chiral copper(II)-bisoxazolines function as alcohol dehydrogenase mimics and catalyze highly enantioselective transfer hydrogenations of alpha-ketoesters with Hantzsch esters as a synthetic NADH analogue to give alpha-hydroxy esters in excellent enantioselectivities. [reaction: see text].

A new type of metal-free catalytic hydrogenation has been developed, which allows for the reduction of various nitroarenes to the corresponding anilines with excellent yields and chemoselectivities. The hydrogenation takes place under very mild conditions using dihydrogen in combination with fullerene catalysts.

G. H. Elgemeie and N. H. Metwally, J. Chem. Res. (S), 1999, 208 DOI: 10.1039/A803696I

Effect of anions on catalytic and electrocatalytic hydrogenations and on the electrocatalytic oxidation and evolution of hydrogen on platinum

Recipes for the preparation of some compounded soup powders from tomatoes, potatoes, carrots, green peas, dried peas, chicken and mutton using components like corn starch, skim milk powder, hydrogenated groundnut oil, common salt, mixed spices (black pepper, caraway and ginger) and monosodium glutamate have been developed. Pretreatment and drying conditions for the preparation of pre-cooked dry bases from tomatoes, potatoes, carrots, green peas, dried peas, chicken and mutton have been determined. Except for tomatoes where vacuum drier is necessary, other bases can be easily prepared in a simple cross flow hot air cabinet drier and used subsequently in the preparation of soup powders.

Synthesis of Pyridines by Electrochemical Methods

Cooperation of soft and hard bridging ligands (μ-S and μ-O, respectively) is crucial to the mechanism of heterolytic dihydrogen activation by germanium–ruthenium complex 1 to give syn and anti isomers of the corresponding hydroxogermanium–hydridoruthenium complex 2 (see scheme; dep=2,6-diethylphenyl). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z704899_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

This thesis describes the synthesis of novel neomycin B-tacn (1,4,7-triazacyclonone) and -cyclen (1,4,7,10-tetraazacyclododecane) conjugates, the assessment of the ability of these derivatives and their Cu(II)/ Zn(II) complexes to bind and/or cleave the bacterial ribosomal A-site and the HIV-1 transactivation response (HIV-1 TAR) element, and the evaluation of their antibiotic activities against various Gram-positive and Gram-negative bacterial strains. Six novel neomycin B-tacn/cyclen conjugates, varying in the length of the alkyl spacer between the macrocyclic unit and neomycin B, were prepared by multi-steps syntheses. Tacn and cyclen were initially protected with Boc following literature procedures, leaving one secondary amine free to undergo further reaction. Alkyl esters of varying chain lengths were then introduced onto the free secondary amine, and were subsequently hydrolysed under basic conditions to their corresponding carboxylic acids. In addition, neomycin B was modified at the 5″ position by first converting the primary hydroxyl group into an azide, followed by Pd/C mediated hydrogenation to an amine group. The modified neomycin B bearing the primary amine functionality at the 5″ position was subsequently coupled to the macrocycles bearing the carboxylic group using a standard peptide coupling procedure. Global trifluoroacetic acid (TFA)-mediated Boc-deprotection yielded the final products as their TFA salts, which were purified by high performance liquid chromatography (HPLC) and characterised using nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy and mass spectrometry techniques. Neomycin B-macrocyclic conjugates and their Zn(II) and Cu(II) complexes were tested for their ability to bind to the bacterial ribosomal A-site and the HIV-1 TAR ribonucleic acid (RNA). The conjugates showed subtle enhancement in their binding ability to the A-site over the parent neomycin B. The Zn(II)-cyclen complexes further enhanced the A-site binding, whereas the Cu(II)-tacn complexes were less effective than neomycin B. From the pH dependence studies, it was found that the binding affinity of the conjugates was greater at lower pH. This was attributed to an increase in the charge of conjugate relative to neomycin B, which arises from the partial protonation of the amine groups on the macrocyclic ring. The neomycin B-macrocyclic conjugates and their metal complexes displayed greater binding affinity for the HIV-1 TAR RNA compared to the A-site. This may be due to the fact that the A-site is highly discriminatory of the structure and conformation of the aminoglycoside ligand, while TAR is an inherently less selective RNA target than the A-site and can readily adjust its structure to interact with different aminoglycosides. Both the macrocycles and their metal complexes appear to enhance the binding of neomycin B to TAR RNA, with the Zn(II) neomycin B-cyclen complexes exhibiting the highest affinity. This may be the result of the strong interactions between the Zn(II) ions and the uracil bases within the RNA. This explanation is supported by the fact the Zn(II) neomycin B-cyclen complexes were found to bind more strongly to TARU3 than TARC3, which have three uracil or three cytosine bases in the bulge binding site, respectively. TAR cleavage studies showed that all the neomycin B-macrocyclic conjugates and their metal complexes induce site-specific hydrolysis in the same manner as neomycin B. However, the addition of metal ions did not enhance the hydrolysis of these conjugates. The greatest extent of cleavage under physiological conditions was observed for the neomycin B-cyclen conjugate 38, followed by Zn(II)-38, however, their rate constants for TAR cleavage (2.6±0.9 × 10-⁶ s-¹ and 2.5±0.6 × 10-⁶ s-¹, respectively) were found to be similar to that observed for neomycin B (2.3±1.2 × 10-⁶ s-¹). The preliminary assessments of the antibacterial activities showed the conjugates 37, 39, and 40 to be slightly more effective than neomycin B in inhibiting the growth of the resistant strain of the Gram-negative, P. aeruginosa (ATCC27853). However, their antibiotic activities against the other tested bacterial strains were lower than that of neomycin B.

A variety of Lewis acid-base pairs consisting of tris(pentafluorophenyl)borane, B(C(6)F(5))(3), in combination with sterically demanding five- and six-membered N-heterocyclic carbenes (NHCs) of the imidazolin-2-ylidene, imidazolidin-2-ylidene, and tetrahydropyrimidin-2-ylidene types were investigated with respect to their potential to act as frustrated Lewis pairs (FLP) by reaction with dihydrogen (H(2)) and tetrahydrofuran (THF). A sufficient degree of "frustration" was usually established by introduction of a 1,3-di-tert-butyl or 1,3-diadamantyl carbene substitution pattern, which allows an unquenched acid-base reactivity and thus leads to heterolytic dihydrogen activation and ring-opening of THF. In contrast, 1,3-bis(2,6-diisopropylphenyl)-substituted carbenes showed ambiguous behavior, and the corresponding five-membered imidazolin-2-ylidene formed a stable carbene-B(C(6)F(5))(3) adduct, whereas fast C-F activation and formation of a zwitterionic pyrimidinium-fluoroborate was observed for the six-membered tetrahydropyrimidin-2-ylidene. A stable adduct was also isolated for the combination of the acyclic carbene bis(diisopropylamino)methylene with B(C(6)F(5))(3), and consequently no reactivity toward H(2) and THF was observed. To rationalize the reactivity of the carbene-borane Lewis pairs, the thermodynamics of adduct formation with B(C(6)F(5))(3) were calculated for 10 different carbenes; the stability (or instability) of these adducts can be used as a good measure of the degree of "frustration".

We describe the synthesis and structures of o-C6F4(TeMes)2 (1) and o-C6F4(TeArF)2 (2, ArF = 3,5-(CF3)2C6H3)), two new bifunctional tellurides featuring an electron-withdrawing backbone. While 2 resisted methylation, 1 reacted with Me3O·BF4 in CH2Cl2 to afford o-C6F4(TeMes)(TeMeMes) ([3]+), a mixed-valent telluride/telluronium cation isolated as a tetrafluoroborate salt. Although attempts to methylate the second telluride have been unsuccessful, [3]+ readily catalyzes the hydrogenation of 2-phenyl-quinoline with Hantzsch ester. Comparison with simple telluronium cations including [ArF2TeMe]+ and [MesArFTeMe]+ confirms that the catalytic activity of these compounds originates from the presence of a tetravalent, cationic tellurium center.