Methyltrioxorhenium-catalysed oxidation of secondary amines to nitrones in ionic liquids

Nitrones serve as starting materials for the synthesis of many heterocycles. Oxidation of secondary amines using hydrogen peroxide and the catalytic amount of methyltrioxorhenium in ionic liquids is a useful method for the preparation of nitrones. Ultrasonic irradiation and ionic liquids have a positive influence on the reaction. The nitrones required were isolated in good yields. Corresponding hydroxylamines, which can be easily oxidised to nitrones, often accompanied the main products. Methyltrioxorhenium in ionic liquids was re-used in several reaction cycles without any deteriorating effect on the course of the reaction.     

Selenium dioxide catalyzed oxidation of secondary amines with hydrogen peroxide. Simple synthesis of nitrones from secondary amines

Oxidation of secondary amines with hydrogen peroxide in the presence of selenium dioxide catalyst at room temperature gives nitrones, which are versatile synthetic intermediates, highly efficiently.     

Non-nanogold catalyzed aerobic oxidation of secondary amines to imines

Bulk gold powder (approximately 10(3) nm particle size) is a highly active catalyst for the oxidative dehydrogenation of secondary amines to imines under the mild conditions of 1 atm O2 and 60-100 degrees C.     

Chemoselective organocatalytic aerobic oxidation of primary amines to secondary imines

Biomimetic aerobic oxidation of primary benzylic amines has been achieved by using a quinone catalyst. Excellent selectivity is observed for primary, unbranched benzylic amines relative to secondary/tertiary amines, branched benzylic amines, and aliphatic amines. The exquisite selectivity for benzylic amines enables oxidative self-sorting within dynamic mixtures of amines and imines to afford high yields of cross-coupled imine products.     

Perfluoro alkyl 1,3-diketonates of cyclic and acyclic secondary amines

Cyclic and acyclic secondary amines were quaternized with perfluoroalkyl 1,3-diketones to form secondary ammonium perfluoroalkyl 1,3-diketonates in good yields. Saturated cyclic secondary amines, including morpholine, N-acetylmorpholine, 3-methyl-N-acetylpiperidine, 2,2,6,6-tetramethyl-4-aminopiperidine, 4,4-dimethylimidazoline, 1,3-bispiperidine propane, and acyclic secondary amines, di(n-propyl) amine, di(isopropyl) amine, di(n-butyl) amine, and di(isobutyl) amine were reacted with various fluorine-containing 1,3-diketones (1-5) to yield the corresponding 1,3-diketonates (6-29). The compounds were characterized by ¹H, ¹⁹F, and ¹³C NMR, electrospray MS, and elemental analyses. Melting points and T_g values were obtained by DSC. Thermal decomposition data (5% weight loss temperature) were recorded by TGA. X-ray single-crystal structures show that 9 and 26 crystallize in monoclinic space group P2(1)/c.     

The oxidation of secondary amines by alkanesulfonic peracids

Alkanesulfonic peracids are efficient oxidation agents for the conversion of secondary amines to the corresponding nitroxyl radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1162–1165, May, 1990.     

One-pot formation of aza-enolates from secondary amines and condensation to esters and alkyl bromides

Starting from commercial secondary amines, a one-pot procedure allows a direct access to enaminones through a one-pot deprotonation/oxidation/in situ re-deprotonation/acylation sequence without intermediate isolation of the intermediate Schiff base or its corresponding enamine tautomer. An alternative one-pot sequence involving a similar oxidation step followed by one or two alkylation steps yields, after acidic work-up, functionalized ketones directly from the parent amine. This process has been applied to an expeditious synthesis of the male aggregation pheromone of the cereal leaf beetle Oulema melanopus.     

Molybdenum oxide/bipyridine hybrid material {[MoO3(bipy)][MoO3(H2O)]}n as catalyst for the oxidation of secondary amines to nitrones

The inorganic-organic hybrid material {[MoO₃(bipy)][MoO₃(H₂O)]}_n (bipy = 2,2′-bipyridine) can be used as a water-tolerant catalyst for the oxidation of secondary amines under mild conditions using either urea hydrogen peroxide (UHP) or tert-butylhydroperoxide (TBHP) as the oxidant. Under optimized reaction conditions (2 mol % catalyst, 3-4 equiv TBHP, CH₂Cl₂ as the solvent, 40 °C), the corresponding nitrones were obtained with different efficiency depending on the nature of the cyclic or acyclic amine used.     

Selective oxidation of sulfides to sulfoxides using a silica immobilised vanadyl alkyl phosphonate catalyst

A range of sulfides can be selectively oxidised to the corresponding sulfoxides in good yields using catalytic quantities of immobilised vanadyl, cobalt or nickel alkyl phosphonates and the reoxidant sodium bromate or vanadyl alkyl phosphonate with tert-butyl hydroperoxide or aqueous hydrogen peroxide as oxidants.     

Moving protons with pendant amines: proton mobility in a nickel catalyst for oxidation of hydrogen

Proton transport is ubiquitous in chemical and biological processes, including the reduction of dioxygen to water, the reduction of CO(2) to formate, and the production/oxidation of hydrogen. In this work we describe intramolecular proton transfer between Ni and positioned pendant amines for the hydrogen oxidation electrocatalyst [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+) (P(Cy)(2)N(Bn)(2) = 1,5-dibenzyl-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane). Rate constants are determined by variable-temperature one-dimensional NMR techniques and two-dimensional EXSY experiments. Computational studies provide insight into the details of the proton movement and energetics of these complexes. Intramolecular proton exchange processes are observed for two of the three experimentally observable isomers of the doubly protonated Ni(0) complex, [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+), which have N-H bonds but no Ni-H bonds. For these two isomers, with pendant amines positioned endo to the Ni, the rate constants for proton exchange range from 10(4) to 10(5) s(-1) at 25 °C, depending on isomer and solvent. No exchange is observed for protons on pendant amines positioned exo to the Ni. Analysis of the exchange as a function of temperature provides a barrier for proton exchange of ΔG(?) = 11-12 kcal/mol for both isomers, with little dependence on solvent. Density functional theory calculations and molecular dynamics simulations support the experimental observations, suggesting metal-mediated intramolecular proton transfers between nitrogen atoms, with chair-to-boat isomerizations as the rate-limiting steps. Because of the fast rate of proton movement, this catalyst may be considered a metal center surrounded by a cloud of exchanging protons. The high intramolecular proton mobility provides information directly pertinent to the ability of pendant amines to accelerate proton transfers during catalysis of hydrogen oxidation. These results may also have broader implications for proton movement in homogeneous catalysts and enzymes in general, with specific implications for the proton channel in the Ni-Fe hydrogenase enzyme.     

Greener fluorous chemistry: Convenient preparation of new types of ‘CF3-rich’ secondary alkyl mesylates and their use for the synthesis of azides, amines, imidazoles and imidazolium salts

2,2,2-Trifluoroethanol, 1,1,1,3,3,3-hexafluoro-2-propanol, and nonafluoro-tert-butyl alcohol were used as precursors for the preparation of the appropriate bis(polyfluoroalkoxymethyl)carbinols [(R_FHOCH₂)₂CHOH, 1a-c, R_FH = (a) CF₃CH₂, (b) (CF₃)₂CH, and (c) (CF₃)₃C] and the corresponding mesylates [(R_FHOCH₂)₂CHOSO₂CH₃, 2a-c]. This novel design paradigm is introduced to eliminate the persistence and bioaccumulation problems of fluorous chemistry, which are associated with the use of longer linear perfluoroalkyl groups (e.g. R_fn ≥ n-C₈F₁₇, n-C₇F₁₅). Secondary mesylates 2a,b and the primary tosylate [(CF₃)₃COCH₂CH₂OTs, 2d] displayed acceptable reactivity towards azide and imidazole nucleophiles to allow the syntheses of novel fluorous azides, which on hydrogenolysis with H₂/Pd-C offered fluorous amines [(R_FHOCH₂)₂CHNH₂, 8a,b], and 1-(polyfluoroalkyl)imidazoles (5a,b,d), respectively, while 2c showed no reactivity due to steric hindrance. The reaction of 8a,b with formaline, glyoxal and hydrochloric acid gave symmetrical 1,3-dialkylated imidazolium chlorides (9a,b), while 5a,b,d were effectively alkylated using n-C₈F₁₇(CH₂)₃I, methyl iodide, 2-bromoethanol, and 2d to yield the corresponding 1,3-dialkylimidazolium iodides, bromides, and tosylates (7aa-ec). Some physical properties of new compounds including mp, bp and solubility patterns were also analyzed; and the fluorophilicity values of 1a-c, and 2a-c were experimentally determined by GC and/or ¹⁹F NMR spectroscopy.     

Cycloruthenated primary and secondary amines as efficient catalyst precursors for asymmetric transfer hydrogenation

[reaction: see text] Ruthenacycles obtained by cyclometalation of enantiopure aromatic primary or secondary amines with [(eta6-benzene)RuCl2]2 or with [(eta6-p-cymene)RuCl2]2 are efficient catalysts for asymmetric transfer hydrogenation (TOF up to 190 h(-1) at room temperature). Enantioselectivities in the transfer hydrogenation of acetophenone ranged from 38% to 89%. It is possible to prepare the catalysts in situ, which allows the use of high throughput experimentation.     

Regiospecific oxidation by DDQ of unhindered alkyl groups in sterically hindered aromatic amines

DDQ oxidises an unhindered activated alkyl group to a carbonyl or a hydroxymethyl group depending on the nature of the substitution, in a sterically hindered aromatic amine.