Deoxygenation of tertiary amine N-oxides under metal free condition using phenylboronic acid

A simple and efficient method for the deoxygenation of amine N-oxides to corresponding amines is reported using the green and economical reagent phenylboronic acid. Deoxygenation of N,N-dialkylaniline N-oxides, trialkylamine N-oxides and pyridine N-oxides were achieved in good to excellent yields. The reduction susceptible functional groups such as ketone, amide, ester and nitro groups are well tolerated with phenylboronic acid during the deoxygenation process even at high temperature. In addition, an indirect method for identification and quantification of tert-amine N-oxide is demonstrated using UV–Vis spectrometry which may be useful for drug metabolism studies.     

Synthesis of isatin-conjugated 3H-indole-N-oxides and their serendipitous conversion to spiroindolenines

Isatin-conjugated 3H-indole-N-oxides were synthesized from isatin-derived propargylic alcohols and nitrosobenzenes in moderate yields. Isatin-conjugated 3H-indole-N-oxides were converted to novel spiroindolenines under PPh₃-mediated deoxygenation reaction condition, serendipitously.     

A general method for the deoxygenation of aromatic N-oxides using RuCl3·xH2O

An efficient, simple and selective method for the deoxygenation of aromatic N-oxides, such as N-arylnitrones, azoxybenzenes, N-heteroarene N-oxides using ruthenium(III) chloride to afford deoxygenated products in excellent yields, is described.     

Metal-Free Deoxygenation of Amine N-Oxides: Synthetic and Mechanistic Studies

We report herein an unprecedented combination of light and P(III)/P(V) redox cycling for the efficient deoxygenation of aromatic amine N-oxides. Moreover, we discovered that a large variety of aliphatic amine N-oxides can easily be deoxygenated by using only phenylsilane. These practically simple approaches proceed well under metal-free conditions, tolerate many functionalities and are highly chemoselective. Combined experimental and computational studies enabled a deep understanding of factors controlling the reactivity of both aromatic and aliphatic amine N-oxides.     

A novel method for the reduction of sulfoxides and pyridine N-oxides with the system silane/MoO2Cl2

A novel method for the reduction of sulfoxides and pyridine N-oxides using a silane and a catalytic amount of MoO₂Cl₂ in excellent yields and with a wide functional group tolerance is reported. A green protocol for this reaction was developed in water with the air-stable catalytic system PMHS/MoO₂Cl₂(H₂O)₂.     

Photocatalytic deoxygenation of N–O bonds with rhenium complexes: from the reduction of nitrous oxide to pyridine N-oxides

The accumulation of nitrogen oxides in the environment calls for new pathways to interconvert the various oxidation states of nitrogen, and especially their reduction. However, the large spectrum of reduction potentials covered by nitrogen oxides makes it difficult to find general systems capable of efficiently reducing various N-oxides. Here, photocatalysis unlocks high energy species able both to circumvent the inherent low reactivity of the greenhouse gas and oxidant N₂O (E⁰(N₂O/N₂) = +1.77 V vs. SHE), and to reduce pyridine N-oxides (E_1/2(pyridine N-oxide/pyridine) = −1.04 V vs. SHE). The rhenium complex [Re(4,4′-tBu-bpy)(CO)₃Cl] proved to be efficient in performing both reactions under ambient conditions, enabling the deoxygenation of N₂O as well as synthetically relevant and functionalized pyridine N-oxides.

A rhenium-based photocatalyst enables the deoxygenation of several compounds containing N–O bonds, such as N₂O and pyridine N-oxides.     

Sulfonium ylides derived from 2-hydroxy-benzoquinones: crystal and molecular structure and their one-step conversion into Mannich bases by amine N-oxides

Reaction of 2-hydroxy-para-benzoquinones with DMSO/Ac₂O produced dimethylsulfonium ylides, of which crystal structures as well as solid and liquid state NMR spectra were recorded. The ylides react with tertiary methylamine N-oxides in a one-pot, multi-step process to 3-methylamino-substituted benzoquinones. The mechanism starts with a deoxygenative deprotonation of the amine N-oxides, followed by a formal electrophilic displacement of DMSO by the resulting carbonium-iminium ion.     

Access to lincomycin N-oxide isomers controlled by reaction conditions

Oxidation of lincomycin with H₂O₂ in alkaline media leads to N-oxides, besides the conversion of thiomethyl group into sulfoxides and sulfones. NH₄OH favors formation of the S-isomer; both R- and S-isomers of the N-oxide are formed in the presence of NaOH. Addition of acetonitrile markedly accelerates the reaction.     

Mild and Efficient Deoxygenation of Amine-N-oxides with BiCl3/Indium System

The BiCl₃/indium system was found to be a new reagent for deoxygenation of various amine-N-oxides to the corresponding amines in good to excellent yields under mild conditions.     

Cu(I)-mediated deoxygenation of N-oxides to amines

A mild and highly efficient deoxygenation of variety of N-oxides using an inexpensive CuX, or a CuX-Zn or CuX-Al couple is described.     

‘Reductive ozonolysis’ via a new fragmentation of carbonyl oxides

This account describes the development of methodologies for ‘reductive’ ozonolysis, the direct ozonolytic conversion of alkenes into carbonyl groups without the intermediacy of 1,2,4-trioxolanes (ozonides). Ozonolysis of alkenes in the presence of DMSO produces a mixture of aldehyde and ozonide. The combination of DMSO and Et₃N results in improved yields of carbonyls but still leaves unacceptable levels of residual ozonides; similar results are obtained using secondary or tertiary amines in the absence of DMSO. The influence of amines is believed to result from conversion to the corresponding N-oxides; ozonolysis in the presence of amine N-oxides efficiently suppresses ozonide formation, generating high yields of aldehydes. The reactions with amine oxides are hypothesized to involve an unprecedented trapping of carbonyl oxides to generate a zwitterionic adduct, which fragments to produce the desired carbonyl group, an amine, and ¹O₂.     

KMnO4-mediated direct C2-selective C−H arylation of quinoline N-oxides with aromatic hydrazines

An efficient protocol for the synthesis of 2-arylquinoline N-oxides has been developed via KMnO₄-mediated cross-coupling reaction of quinoline N-oxides with aromatic hydrazines in moderated to good yields. The reactions proceeded efficiently over a broad range of substrates with excellent regioselectivity and functional group tolerance.     

A chemoselective deoxygenation of N-oxides by sodium borohydride-Raney nickel in water

A simple and convenient protocol for deoxygenation of aliphatic and aromatic N-oxides to the corresponding amines in good to excellent yield using sodium borohydride-Raney nickel in water is reported. Other functional moieties such as alkenes, halides, ethers, and amides are unaffected under the present reaction condition.     

Probing the metal complexing ability of bis-quinolizidine N-oxides: Synthesis, spectroscopy and crystal structures

New ZnX₂ (X = Cl, Br) complexes with sparteine N1-oxide, sparteine epi-N16-oxide, lupanine (2-oxosparteine) N-oxide and α-isosparteine N-oxide were obtained and characterized by spectroscopic and crystallographic methods. Complexation with N1-oxides involves inversion of the configuration at the N16 atom of sparteine, converting its C ring from a boat into a chair form, whereas the structure of sparteine epi-N16-oxide, typified by the trans boat/chair C/D quinolizidine moiety, remains unchanged upon complexation. Coordination of zinc salts in the above compounds is realized solely through the oxygen atom of the N-O group and is accompanied by protonation of the “not-N-oxide” nitrogen atom. At variance lupanine N-oxide forms bis-monodentate complexes with ZnX₂ utilizing both the N-oxide and the lactam carbonyl oxygen atoms to bridge the neighboring ZnX₂ units, while reaction of α-isosparteine N-oxide with ZnX₂ leads to formation of complex salts of the general formula [H(−)α-Sp(N-oxide)][(ZnX₃)(H₂O)].     

Biocatalytic and chemical routes to all the stereoisomers of methionine and ethionine sulfoxides

Biotransformations of the N-phthaloyl derivatives of d- and l-methionine and of d- and l-ethionine by Beauveria bassiana ATCC 7159 or Beauveria caledonica ATCC 64970 produce the corresponding (S_S) sulfoxides in good yield and diastereomeric excess. Pure (S_SS_C) diastereomers can be obtained from l-series substrates by crystallisation of the biotransformation extract, and the corresponding (S_SR_C) products obtained from d-series substrates by chromatography of the biotransformation extract. Hydrogen peroxide-catalysed oxidation of the N-phthaloyl derivatives of d- and l-methionine and of d- and l-ethionine gives diastereomeric mixtures from which the (S_SS_C) and (R_SR_C) diastereomers can be obtained by crystallisation, and the (S_SR_C) and (R_SS_C) diastereomers obtained by chromatography. N-Cbz- and N-t-Boc methionines are also converted to sulfoxides with predominant (S_S) configuration by both B. bassiana and B. caledonica, but the isolated yields and d.e. of products were generally lower than those obtained from the N-phthaloyl substrates.Removal of the N-phthaloyl group from diastereomerically pure methionine and ethionine sulfoxides gave the corresponding amino acid sulfoxides in high yield; removal of N-Cbz and N-t-Boc groups from protected methionine sulfoxides was also achieved without loss of configuration at sulfur.     

Comparative chromatographic adsorbabilities of some azines,thioles and thienopyridines,their heteroatom oxides and other derivatives on silica gel

Thin layer chromatographic R_f data on silica gel are reported for 64 compounds containing either a thiole sulfur atom, one or more azine nitrogen atoms, or both. Thiole adsorbates are compared with 28 aromatic hydrocarbons and non-heterocyclic sulfur compounds. In general the orders of R_f values found are thioles > sulfides > sulfones > sulfoxides > azine N-oxides and azines > azine-N-oxides. Results are interpreted in terms of hydrogen bonding from the silica gel to the adsorbate. The different orders of adsorbability found amongst four series of condensed thiophenes and their arene analogs for chromatography on silica gel and on alumina are rationalized in terms of planarity and acid-base interactions. Correlations of the tlc data with separations on columns are presented.     

Palladium-catalyzed C–H alkenylation of quinoxaline N-oxide enabled by a mono-N-protected amino acid

The efficient alkenylation of quinoxaline N-oxide was achieved via Pd-catalyzed C–H activation, using the assistance of a mono-N-protected amino acid. Further deoxygenation of the 2-styrylquinoxaline-N-oxides yielded the corresponding styrylquinoxaline derivatives.     

Mild and Efficient Deoxygenation of Amine-N-oxides with Titanium Tetrachloride-Indium System

Abstract

TiCl₄/In system was found to be a new reagent for deoxygenation of various amine-N-oxides to the corresponding amines in good to excellent yields under mild conditions.     

Sulfoxide-controlled SN2′ displacements between cuprates and vinyl and alkynyl epoxy sulfoxides

The S_N2′ displacement of readily available vinyl epoxy sulfoxides with organocopper reagents takes place in good yields with high anti selectivity and a good degree of E/Z stereocontrol to produce enantiopure α-hydroxy vinyl sulfoxides. A second allylic displacement on the related mesyloxy vinyl sulfoxides allows for the asymmetric construction of two adjacent chiral centers. In addition, cuprate mediated S_N2′ addition to alkynyl epoxy sulfoxides affords α-hydroxy allenyl sulfoxides in good yields.     

The deoxygenation of sulfoxide mediated by the Ph3P/Lewis acid combination and the application to the kinetic resolution of racemic phosphines using optically active sulfoxide

It was found that the combination of Ph₃P/TiCl₄ was an effective promoter for the deoxygenation of sulfoxides and gave the corresponding sulfides in good yield (up to 97%) under mild conditions. This method was applied to the reaction between racemic phosphines and (R)-methyl p-tolyl sulfoxide, and it was found that the kinetic resolution was achieved in moderate selectivities.