Nitrone Complexes Research Articles

Au(I)-catalyzed cycloaddition pathways of non-terminal propargyl substrates

Novel chiral menthol-based pyridyl nitrone ligands were synthesized and Au(I) coordination of the ligands gave chiral Au(I)–nitrone complexes. 1H NMR studies of the gold(I) coordination experiments with nitrone ligands afforded a convenient method for monitoring complex formation. The catalytic effect of Au(I)–nitrone complexes, shown to tune catalytic systems to produce uncommon products, was evaluated in [2 + 2 + 2] cyclotrimerization and [2 + 4] cyclodimerization reactions of non-terminal propargyl acetals. Alternative gold(I)-catalyzed [2 + 2], [2 + 4] and [3 + 4] cycloaddition reaction pathways of non-terminal propargyl acetals with imine substrates gave a diverse range of N-heterocyclic products. The present screening study demonstrates the potential and the versatility of non-terminal propargyl acetals in gold(I)-catalyzed cycloaddition reactions.

Modulation of emission and singlet oxygen photosensitisation in live cells utilising bioorthogonal phosphorogenic probes and protein tag technology.

Herein we report a strategy to utilise the bioorthogonal reactivity and phosphorogenic property of iridium(iii) polypyridine nitrone complexes and SNAP-tag protein for the modulation of emission and singlet oxygen (1O2) photosensitisation in live cells.

Studies on gold-nitrone systems.

A series of novel Au(i)-nitrone complexes with specific catalytic properties were prepared. Furthermore, Au(i)- and Au(iii)-oxadiazole complexes were formed by a novel Au-generated nitrile-nitrone [3 + 2] cycloaddition, and the crystal structures of Au(i)-nitrones as well as the Au(i)- and Au(iii)-oxadiazole complexes were studied (X-ray). A useful one-pot Au(iii)-mediated cycloaddition method was developed for the formation of a number of dihydro-1,2,4-oxadiazoles, involving in situ formation of Au(iii)-oxadiazole complexes. The observed Au(i) and Au(iii) dual selective reactivity gives new understanding about the Au(i)- and Au(iii)-nitrone chemistry.

Conferring Phosphorogenic Properties on Iridium(III)-Based Bioorthogonal Probes through Modification with a Nitrone Unit.

The use of bioorthogonal probes that display fluorogenic or phosphorogenic properties is advantageous to the labeling and imaging of biomolecules in live cells and organisms. Herein we present the design of three iridium(III) complexes containing a nitrone moiety as novel phosphorogenic bioorthogonal probes. These probes were non-emissive owing to isomerization of the C=N group but showed significant emission enhancement upon cycloaddition reaction with strained cyclooctynes. Interestingly, the connection of the nitrone ligand to the cationic iridium(III) center led to accelerated reaction kinetics. These nitrone complexes were also identified as phosphorogenic bioorthogonal labels and imaging reagents for cyclooctyne-modified proteins. These findings contribute to the development of phosphorogenic bioorthogonal probes and imaging reagents.

Conferring Phosphorogenic Properties on Iridium(III)‐Based Bioorthogonal Probes through Modification with a Nitrone Unit

AbstractThe use of bioorthogonal probes that display fluorogenic or phosphorogenic properties is advantageous to the labeling and imaging of biomolecules in live cells and organisms. Herein we present the design of three iridium(III) complexes containing a nitrone moiety as novel phosphorogenic bioorthogonal probes. These probes were non‐emissive owing to isomerization of the C=N group but showed significant emission enhancement upon cycloaddition reaction with strained cyclooctynes. Interestingly, the connection of the nitrone ligand to the cationic iridium(III) center led to accelerated reaction kinetics. These nitrone complexes were also identified as phosphorogenic bioorthogonal labels and imaging reagents for cyclooctyne‐modified proteins. These findings contribute to the development of phosphorogenic bioorthogonal probes and imaging reagents.

Enantioselective 1,3-Dipolar Cycloaddition of Nitrones to Methacrolein Catalyzed by (η5-C5Me5)M{(R)-Prophos} Containing Complexes (M = Rh, Ir; (R)-Prophos = 1,2-bis(Diphenylphosphino)propane): On the Origin of the Enantioselectivity

The rhodium and iridium Lewis-acid cations [(eta(5)-C(5)Me(5))M{(R)-Prophos}(H(2)O)](2+) ((R)-Prophos = 1,2-bis(diphenylphosphino)propane) efficiently catalyze the enantioselective 1,3-dipolar cycloaddition of nitrones to methacrolein. Reactions occur with perfect endo selectivity and with enantiomeric excesses up to 96%. Intermediates [(eta(5)-C(5)Me(5))M{(R)-Prophos}(methacrolein)](SbF(6))(2) (M = Rh (3), Ir (4)) have been spectroscopically and crystallographically characterized. The nitrone complexes [(eta(5)-C(5)Me(5))M{(R)-Prophos}(nitrone)](SbF(6))(2) (M = Rh, nitrone = 1-pyrrolidine N-oxide (5), 2,3,4,5,-tetrahydropyridine N-oxide (6), 3,4-dihydroisoquinoline N-oxide (7); M = Ir, nitrone = 1-pyrrolidine N-oxide (8)) have been isolated and characterized including the X-ray crystal structure of compounds 6 and 8. The equilibrium between methacrolein and nitrone complexes is also studied. [Ir]-adduct complexes are detected by (31)P NMR spectroscopy. A catalytic cycle involving [M]-methacrolein, [M]-nitrone, as well as [M]-adduct species is proposed, the first complex being the true catalyst. The absolute configuration of the adduct 4-methyl-2-N,3-diphenyl-isoxazolidine-4-carbaldehyde (9) was determined through its (S)-(-)-alpha-methylbenzylamine derivative diastereomer. Structural parameters strongly suggest that the disposition of the methacrolein in 3 and 4 is fixed by CH/pi attractive interactions between the pro-S phenyl ring of the Ph(2)PCH(CH(3)) moiety of the (R)-Prophos ligand and the CHO aldehyde proton. Proton NMR data indicate that this conformation is maintained in solution. From the structural data and the results of catalysis the origin of the enantioselectivity is discussed.

Inclusion Complexes of PBN-Type Nitrone Spin Traps and Their Superoxide Spin Adducts with Cyclodextrin Derivatives: Parallel Determination of the Association Constants by NMR Titrations and 2D-EPR Simulations

(1)H NMR and electron paramagnetic resonance (EPR) titrations were used to determine the association constants of the complexes of alpha-phenyl-N-tert-butylnitrone (PBN) analogues and their superoxide spin adducts, respectively, with methylated beta-cyclodextrins. A 1:1 stoichiometry for the nitrones with randomly methylated beta-cyclodextrin and 2,6-di-O-methyl-beta-cyclodextrin and 1:1 and 1:2 stoichiometries for the corresponding cyclodextrin-nitroxide complexes were observed. After the superoxide radical spin trapping reaction, EPR titrations afforded the association constants of the corresponding cyclodextrin-nitroxide complexes. Two-dimensional EPR simulations indicated a bimodal inclusion of the nitroxide free radical spin adducts into the cyclodextrins. For all the nitrone-cyclodextrin and nitroxide-cyclodextrin complexes, the association constants were always higher for the nitroxide complexes than for the nitrone complexes. A cooperative system concerning the complexation of the nitroxide spin adduct with a cyclodextrin was evidenced by EPR titrations. The efficiency of the cyclodextrin inclusion technique to trap superoxide and to resist bioreduction by sodium l-ascorbate was also investigated.

First Asymmetric SmI2‐Induced Cross‐Coupling of Cr(CO)3 Aromatic Nitrone Complexes with Carbonyl Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

First asymmetric SmI2-induced cross-coupling of Cr(CO)3 aromatic nitrone complexes with carbonyl compounds.

The SmI(2)-induced cross-coupling of Cr(CO)(3)-complexed nitrones with carbonyl compounds is described. This highly chemo- and diastereoselective reaction affords enantiopure [small beta]-amino alcohol complexes in excellent yields.

Reactions of nitrones with transition metal nitrile complexes: cycloaddition, ligand substitution, or hydrolysis

The reaction of nitrones with transition metal nitrile complexes of the type [MCl2(PhCN)2] and [MCl4(MeCN)2] leads to different products, depending on the metal. Reaction of [PdCl2(PhCN)2] with RCH=N(Me)O (R=Ph, p-C6H4Me) afford Δ4-1,2,4-oxadiazoline complexes ▪ as products of [2+3] cycloaddition of the nitrone across the C≡N bond of the nitrile. The oxophilic transition metal compounds [TiCl4(MeCN)2] and [ZrCl4(MeCN)2] undergo rapid ligand exchange to form nitrone complexes from which the nitrone can be released without decomposition. The closely related compounds [MoCl4(MeCN)2] and [WCl4(MeCN)2] mediate the hydrolysis of nitrones to the corresponding aldehydes.

Cyclic voltammetry of metal complexes of nitrones and nitroxides

Increasing interest in radicals bound to metals led to the present study of electron transfer to and from metal nitrone complexes and metal nitroxides. These are species involved in biological spin trapping and materials science applications, respectively. Cyclic voltammetry was used to study the redox behavior of complexes of the bidentate N- tert-butyl-α-(2-pyridyl)nitrone (2-PyBN) and the monodentate nitrone 2,5,5-trimethyl-1-pyrroline- N-oxide (M 3PO) with metal hexafluoroacetylacetonates M(hfac) 2 in CH 2Cl 2. Bidentate complexes M(2-PyBN)(hfac) 2 with M=Mn II and Co II were easier to oxidize in CH 2Cl 2 (less positive E pa) than the corresponding M(hfac) 2 and harder to reduce (more negative E pc). In MeCN, this behavior was also observed for M=Co II and Ni II which indicate more facile oxidation when M 3PO is present as co-ligand compared with M(hfac) 2 alone. CV's of M(2-PyBN)(hfac) 2 and [M(M 3PO)(hfac) 2] 2 in MeCN gave common reduction peaks due to hfac. Nitroxides (aminoxyls) 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-1-oxyl (CPNO) gave quasi-reversible oxidations to oxoammonium cations, while N- tert-butyl-(2-pyridyl)phenylmethanaminoxyl (2-PyBNO) showed only irreversible oxidations. CV of solutions of CPNO in the presence of M(hfac) 2, where M is Mn II, Co II, or Ni II, indicated formation of metal–nitroxide complexes for Co II and Ni II. For stable metal nitroxides prepared independently, the CV of Mn(TEMPO) 2 (hfac) 2 showed dissociation to free nitroxide and Mn(hfac) 2 in CH 2Cl 2, while M(2-PyBNO)(hfac) 2 gave distinctive CV's for M=Mn II, Co II, and Ni II.

Nitrones are suitable ligands for heme models: X-ray crystal structure of the first metalloporphyrin nitrone complex.

The metalloporphyrin nitrone complexes [M(oep)-(CO)(DMPO)] (M = Ru, Os; oep = octaethylporphyrinato dianion; DMPO = 5,5-dimethyl-1-pyrroline N-oxide) have been prepared: the crystal structure of the Ru complex reveals a sole eta 1-O binding mode of the nitrone ligand to the metal center.

Rhodium nitrone complexes: efficient catalyst for carbonylation of methanol

Newly synthesized rhodium(I) carbonyl complexes of the type [Rh(CO) 2ClL] ( 1), where L=α, N-diphenylnitrone ( a), α-styryl- N-phenylnitrone ( b), N, N′-diphenyldinitrone ( c) and α-(2-furyl)- N-phenylnitrone ( d), act as efficient catalyst precursors (maximum turn over number (TON) 1200–1400 per 90 min) for carbonylation of methanol to produce acetic acid and its ester at temperature 150±10 °C and pressure 20 bar.

Synthesis, structure and hydrolysis of some [( η5-cyclopentadienyl)( η6-arene) iron(II)][PF 6] complexes bearing an imine or a nitrone function in α-position of the arene ligand

Condensation between [( η 5-Cp)( η 6-fluorene)Fe] u+ ] I and various nitrosoarenes RPhNO (R  H, 2-Me, 3-Me, 4-Me, 3-COMe, 4-COMe and 4-NMe 2) were initiated with 5% t-BuOK and led to [( η 5-Cp)( η 6-fluorenone-anil)Fe] + complexes Ia-g (57–88%) as syn-anti mixtures (ca. 30:70 respectively). [( η 5-Cp)( η 6-diphenylmethane)Fe] + II with nitrosobenzene or nitrosotoluenes gave mixtures of [( η 5-Cp)( η 6-benzophenone- N-phenylnitrone)Fe] + IIa-d and [( η 5-Cp)( η 6-benzophenone-anil(Fe] + IIa-d complexes (ca. 80:20) with good yields (80–92%). Nitrone complexes IIa,c,d were isolated as pure Z-isomer by fractional recrystallisation (22–35%). 2D- 1H, 13C NMR correlation and X-ray analysis were used to identify the new compounds. Owing to the high nitrosobenzene oxidative character, condensation with other [( η 5-Cp)( η 6-arene(Fe] + complexes (arene = toluene, ethylbenzene, indane and tetraline) failed. Acidic hydrolysis of these imine and nitrone complexes has been investigated kinetically using polarography. Macroscale hydrolysis yielded [( η 5-Cp)( η 6-fluorenone)Fe] + II (82%) and [( η 5-Cp( η 6-benzophenone)Fe] + III (91%).

Reactions of [Cp(CO)2Fe:CHAr]+ (Ar = p-C6H4OMe) with O:N-Ar'(Ar' = C6H5, p-C6H4NMe2) and PhN:NPh

Organometallic products formed from the reaction of an electrophilic iron carbene complex with nitrosoarenes or azobenzene reflect net insertion of the ArN{double bond}X moiety into the Fe{double bond}CHAr bond. Cp(CO){sub 2}Fe-O-N(Ar{prime}){double bond}CHAr{sup +} and Cp(CO){sub 2}FeN(Ph)-N(Ph){double bond}CHAr{sup +} (Ar = p-C{sub 6}H{sub 4}OMe, Ar{prime} = p-C{sub 6}H{sub 4}NMe{sub 2}) have been isolated and spectroscopically characterized; the crystal structure of Cp(CO){sub 2}Fe-O-N(Ph){double bond}CHAr{sup +} is reported. Exposure of acetone solutions of Cp(CO){sub 2}Fe-O-N(Ar{prime}){double bond}CHAr{sup +} or Cp(CO){sub 2}FeN(Ph)-N(Ph){double bond}char{sup +} to light yields imine products Ar{prime}N{double bond}CHAr or PhN{double bond}CHAr, respectively. There is no evidence to support the formation of the simple stoichiometric iron-containing products of these reactions, the oxo and nitrene complexes Cp(CO){sub 2}Fe{double bond}O{sup +} and Cp(CO){sub 2}Fe{double bond}NPh{sup +}. Hydrolysis of the nitrone complexes Cp(CO){sub 2}Fe-O-N(Ar{prime}){double bond}CHAr{sup +} in aqueous acetone yields aldehyde products Ar{prime}CHO. 30 refs., 1 fig., 4 tabs.

Spin delocalization in nitrones and imines coordinated to metal acethylacetonates

Nitrone bi(2,4-pentanedionato) metal complexes of 1:1 and 2:1 stoichiometry have been isolated and characterized. Spin desity in the nickel complexes is delocalized mainly in the nitrone π orbital. Nmr studies of both imine and nitrone complexes provide evidence for inhibition of spin density distribution due to twisting from colanarity of phenyl group in the π system.