Pyridine N-oxide complexes

It effectively hydroxylates hydrocarbons but is free of the explosion risk of the analogous pyridine N-oxide complex. 

Another rather extensive series of similar data, obtained using CS2 solutions and nujol mulls, has been published by Shindo (Fig. 4). His series include considerable data for jS-substituted compounds, for which the question of a choice of substituent constants does not arise. His data also show considerable scatter but seem to suggest strongly that <7+-values are indicated for + M substituents and normal <7-values for —M substituents. The conclusion is confirmed by the short series of similar data reported by Costa and Blasina and by Shupack and Orchin. The data of the latter authors for the NO frequencies in mws-ethylene pyridine N-oxide dichloroplatinum(II) complexes are also moderately well correlated with <7+-values. 

Kobayashi and co-workers reported similar enantioselectivity switch in the bi-nol-yterrbium(III) triflate complex-catalyzed cycloaddition reactions [69] between N-benzylidenebenzylamine N-oxide and 3-crotonoyl-2-oxazolidinone [70]. The reaction in the presence of MS 4 A showed an exclusively high enantioselectivity of 96% ee, while that in the absence of MS 4 A (-50% ee) or in the presence of pyridine N-oxide (-83% ee) had the opposite enantioselectivity (Scheme 7.24). This chirality switch happens generally for the combination of a wide variety of nitrones and dipolarophiles. 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

Copper(II) complexes have been prepared with the 2-acetylpyridine N-oxide 3-azabicyclo[3.2.2.]nonylthiosemicarbazone, 25, and bonding occurs via the pyridine N-oxide oxygen, azomethine nitrogen and thiol sulfur [128]. Based on electronic and ESR spectra, bonding to copper(II) of uninegative, tridentate 25-H is considerably weaker than the related 2-acetylpyridine thiosemicarbazone, 4-H. The other copper(II) complexes reported to date have been prepared... 

The encapsulated complex with bulky alkyl groups was more active than the complex without alkyl groups. The catalytic activity increases on the addition of axial ligands as pyridine N-oxide, and the highest enantiomeric excess, 88%, was also achieved in the presence of the pyridine N-oxide. 

Nickel(II) complexes of a variety of bidentate sulfoxide ligands have been reported (326,378,413) and [NiL3][C104]2 species reported where L is the unusual bidentate ligand 2-(ethysulfinyl)pyridine-iV-oxide. Bidentate 0,0-coordination via sulfoxide and pyridine-N-oxide donors is assigned from infrared data (63). 

Copper(ii) complexes of 8-amino-7-hydroxy-4-methylcoumarin, 1,10-phenanthroline-2-carboxamide, 2-pyridone, 2,3-di-(2-pyridine N-oxide)-quinoxaline," pyridine carboxylates, l-(2 -pyridyl)-2-azonaphthol, and l-(2 -benzothiazolyl)-2-azonaphthol have also been reported. 

Complexes A and B can be simply prepared by passing HI into a solution of TPO in acetic acid at room temperature. Crystals of complex A form immediately and can be isolated by filtration. The suggested composition of complex A follows from NMR and elemental analysis. On standing of the filtrate (in air) further crystals of composition B are formed. Similar complexes were isolated with pyridine N-oxide. 

Extensive studies on diastereoselectivity in the reactions of 1,3-dipoles such as nitrile oxides and nitrones have been carried out over the last 10 years. In contrast, very little work was done on the reactions of nitrile imines with chiral alkenes until the end of the 1990s and very few enantiomerically pure nitrile imines were generated. The greatest degree of selectivity so far has been achieved in cycloadditions to the Fischer chromium carbene complexes (201) to give, initially, the pyrazohne complexes 202 and 203 (111,112). These products proved to be rather unstable and were oxidized in situ with pyridine N-oxide to give predominantly the (4R,5S) product 204 in moderate yield (35-73%). 

Olefin exchange has been used for the preparation of certain plati-num(II) complexes (67). Thus, trimethylvinylsilane replaces ethylene in Zeise s salt to give the compound K[Cl3Pt(H2C=CHSiMe3)]. The SiMe3 group in this complex is readily cleaved by water (wet acetone) with elimination of hexamethyldisiloxane and regeneration of Zeise s salt, but resists attack by pyridine N-oxide, which instead replaces a chlorine atom (67). 

When the donor atom is a part of the aromatic system, one would expect more obvious differences in reactivity. At present relatively little comparative information is available on such heterocyclic systems. Only on pyridine and its derivatives are there any reasonably extensive data. For pyridine a wide variety of coordination processes are available and pyridine-N-oxide as well as metallic complexes and complexes with nonmetallic Lewis acids must be considered. For comparative purposes the great reluctance with which pyridine undergoes electrophilic... 

The preferred position for electrophilic substitution in the pyridine ring is the 3 position. Because of the sluggishness of the reactions of pyridine, these are often carried out at elevated temperatures, where a free radical mechanism may be operative. If these reactions are eliminated from consideration, substitution at the 3 position is found to be general for electrophilic reactions of coordinated pyridine, except for the nitration of pyridine-N-oxide (30, 51). The mercuration of pyridine with mercuric acetate proceeds via the coordination complex and gives the anticipated product with substitution in the 3 position (72). The bromina-tion of pyridine-N-oxide in fuming sulfuric acid goes via a complex with sulfur trioxide and gives 3-bromopyridine-N-oxide as the chief product (80). In this case the coordination presumably deactivates the pyridine nucleus in the 2 and... 

Salts of [ZnLe]2+ (L = pyridine N-oxide) have recently been shown to undergo facile solid state reactions with alkali metal halides, an observation to be taken into account when recording the IR spectra of these and related compounds.706 A crystal structure of the complex [ZnL6] [C104]2 (L = 4-methylpyridine A-oxide) has been reported.707 The metal is in a near-octahedral 06 environment, with an average Zn—O distance of 2.114 A. 

PhlO, cat chiral Mn salen complex, pyridine N-oxide (enantioselective)... 

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