Benzoquinone metal complexes

Benzophenone, o-mercapto-metal complexes, 654 Benzoquinone metal complexes spectra, 397... 

Transition metal complexes of o-benzoquinone, o-semiquinone and catecholate ligands. C. G. Pier-pontand R. M. Buchanan, Coord. Chem. Rev., 1981, 38, 45-87 (72). 

Benzoquinone, 3,5-di-t-butyl-cobalt complexes, 2, 398 Benzoquinone dioximates metal complexes... 

In the case of the naphthoquinone methine-type near-IR dye 55, reduction with tin(II) chloride under acidic conditions gives the leuco dye 56, which has weak absorption maxima at 350-359nm in methanol. The leuco dye 56 can be isolated as a stable pale yellow compound. The oxidation behavior of 56 has been studied by adding benzoquinone as oxidant in methanol solution. Compound 56 immediately produced new absorption at 760 nm which is consistent with the absorption maximum of 55 (Scheme 19).30 The absorption spectra of the leuco, quinone, and metal complex forms are summarized in Table 3. 

Another type of dimerization was observed by Japanese authors198. In the presence of Ni°, compounds like bis(l,5-cyclooctadiene) nickel(0), diphenyl and di-n-propyl cyclopropenone, and cyclohepteno cyclopropenone are transformed to tetra-substituted p-benzoquinones (261/262) by formal (2 + 2) or (3 + 3) cycloaddition of two cyclopropenone moieties effected by metal complexing. 

The mechanism of this new reaction is shown in Scheme 14. Coordination of the diene to palladium(II) makes the diene double bond electrophilic enough to be attacked by the allylsilane. The attack by the allylsilane takes place on the face of the diene opposite to that of the palladium (anti). This is the first example of an anti attack by an allylsilane on a 7T-(olefin)metal complex. Benzoquinone (BQ)-induced anti attack by chloride ion produces the product 58. 

Transition metal complexes with o-dioxolene ligands constitute one of the most intriguing classes of complexes as far as their electrochemical behaviour is concerned, in that, as already mentioned in Chapter 5, Section 1, such ligands are able to shuttle through the oxidation states o-benzoquinone)o-benzosemiquinone/catecholate illustrated in Scheme 250 (a process carried out in nature by the dicopper (I I)-based enzyme catechol oxidase through a single two-electron step see Chapter 9, Section 1.2). 

Fullerenes, among which the representative and most abundant is the 4 symmetrical Cgg with 30 double bonds and 60 single bonds, are known to behave as electron-deficient polyenes rather than aromatic compounds [7]. The energy level of the triply degenerate LUMO of Cgg is almost as low as those of p-benzoquinone or tetracyanoethylene. Thus, a wide variety of reactions have been reported for Cgg such as nucleophilic addition, [4-1-2] cycloaddition, 1,3-dipolar addition, radical and carbene additions, metal complexation, and so on [7]. Fullerene Cgg also undergoes supramolecular complexation with various host molecules having electron-donating ability and an adequate cavity size [8]. 

Benzoquinone, 3,5-di-t-butyl-cobalt complexes, 398 Bcnzoselenophene, 2-(2-pyridyl)-metal complexes, 807 Benzothiazole-2-thiol metal complexes, 802 Benzotriazoles metal complexes, 78 P a, 77... 

Due to some special structural and magnetic peculiarities, in particular, free-radical properties, the quinone ligands and their metal complexes are apart from the other kinds of coordination compounds [125a, 138a], as will be shown below. Here we present an overview of the main methods for the synthesis of complexes containing benzoquinone, semiquinone, and catecholate ligands, and peculiarities of the products. 

Transition metal complexes containing catecholate and semiquinone ligands have been reviewed by Pierpont and Buchanan.336 The syntheses of catecholate adducts of platinum group metals by oxidative addition of benzoquinone (BQ) have been reported (reaction 92).337 The coordinated catechol can then be oxidized to the semiquinone with silver ion (reaction 93). In reactions (92) and (93), L M = lr(Cl)(CO)(PPh3)2] and the benzoquinones (BQ) are shown in (146). The oxidative addition of phenanthrenequinone to [Ir(Cl)(CO)(PPh3)2] has also been cited.338... 

Insertion of redox-active functions in /3-diketonato-metal complexes increases their redox activity. Particularly useful has been the introduction of ferrocenyl or tetrathio-fulvenyl substituents other redox active fragments such as benzoquinone diimines or polypyridines have been also exploited. Frequently, the electrochemical investigation has been focussed on such redox-active substituents rather than on the whole metal-diketonato framework. This section will be limited to well known, representative, mononuclear complexes, but we shall omit complexes such as MCp2(acac) (M = Ti, the elec-... 

A reoxidation of the catalytic amounts of hydroquinone (HQ) to benzoquinone (BQ) in Scheme 8-11 by molecular dioxygen was realized by the use of an oxygen-activating macrocyclic metal complex as cocatalyst [53,62-65]. This leads to a mild biomimetic aerobic oxidation which is now based on a triple catalytic system (Scheme 8-12). With this system cyclohexa-1,3-diene is oxidized to frans-l,4-diacetoxycyclohex-2-ene at room temperature in 85-89% (>91% tmns) [62]. With the use of 2-phenylsulfonyl-l,4-benzoquinone as quinone, the trans selectivity of this process was >97% [53]. 

Macrocyclic metal complexes have recently attracted attention as dioxygen activating catalysts in oxidation reactions. A triple catalytic procedure [1,2] involving three redox systems Pd(II)/Pd(0) - benzoquinone/hydroquinone - ML° /ML was developed for the aerobic oxidation reactions. The multistep electron transfer occurs in the following way electron transfer occurs from the substrate to Pd (II), giving Pd (0), followed by another electron transfer from Pd (0) to benzoquinone. The hydroquinone thus formed, transfers electrons to the oxidized form of the metal macrocycle, which is reduced. The latter is reoxidized by electron transfer to molecular oxygen. 

The activities of the zeolite-encapsulated catalysts were compared to the fi ee complexes. In the oxidation of hydroquinone to benzoquinone the amount of Co-salophen/zeolite catalyst just necessary for the same reaction rate as in the case of fi ee metal complex was used. The same amount of catalysts was used for the triple catalytic oxidation reactions, too. 

Transition metal complexes of benzosemiquinonediimine (s-BQDI) are important in terms of redox properties and in relation to biologically relevant catechol and benzoquinone derivatives. 

This triple catalytic system allows the aerobic oxidation via a multistep electron transfer involving three redox systems Pdll)/Pd(0) - benzoquinone/hydroquinone -MLox/ML, where ML is an oxygen activating macrocyclic transition metal complex. 

Benzoic acid, mercapto-lithographic printing plates, 101 Benzonitrile hydrolysis metal catalysis, 449 Benzoquinone dioximates metal complexes non-integral oxidation states, 144 Benzylamine oxidase, 700 Benzyl halides carbonylation... 

C.G. Pieipont and R.M. Buchanan, Transition metal complexes of < -benzoquinone, f -semiquinone, and... 

A variety of transition metal complexes, especially those of cobalt and copper, catalyze the oxidation of phenols. Autoxidations of 2,6-disubstituted phenols in organic solvents produce mainly the 2,6-disubstituted-1,4-benzoquinone and the 3,5,3, 5 -tetrasubstituted-4,4 -diphenoquinone with Co catalysts and certain copper catalysts (eq. Poly(2,6-... 

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