Comproportionation reaction

Israeli, A., Patt, M., Oron, M., Samuni, A., Kohen, R., and Goldstein, S. 2005. Kinetics and mechanism of the comproportionation reaction between oxoammonium cation and hydroxy-lamine derived from cyclic nitrones. Free Radical Biology and Medicine 38 317-324. 

It should be recalled that in the case of the EE mechanism, in solution one can have the comproportionation reaction ... 

The next group of [Ru(bpy)2L2] complexes to be considered involves pyridine-based ligands. The structural and spectroscopic properties of cM-[Ru(bpy)2(py)2] have been reported." Hydrolysis of ci5-[Ru(bpy)2(py)Cl] gives cw-[Ru(bpy)2(py)(H20)] with 80% retention of configuration. Retention of configuration also accompanies the oxidation of the latter to cis-[Ru(bpy)2(py)(0)] " ", racemization of which is slow the use of this Ru complex as a chiral oxidant has been examined." The kinetics of the comproportionation reaction between [Ru (bpy)2(py)(0)] " " and [Ru(bpy)2(py)(H20)] + in MeCN have been studied using stopped-fiow methods. The Ru product is unstable, and in MeCN solution undergoes disproportionation... 

The formation of N2 can be described as a comproportionation reaction, with a three-electron interchange between the reactants and further proton and OH- loss. The end-on linear coordination mode, t)1-N2, is well known to be a stable form for N2 bound to transition metal centers (80). 

The catalytic significance of this observation is not known since no deviation from a two-electron Nemst plot is observed with NADH as reductant and no kinetic studies have been done to compare the rate of the NAD -facilitated comproportionation reaction with the rate of catalytic turnover. No comparable studies on the effect of NADP on the oxidation-reduction potential of ferredoxin-NADP reductase have been, to our knowledge, published. Inasmuch as the physiological role for this enzyme is reduction of the pyridine nucleotide rather than its oxidation, the potential of the enzyme should be significantly lower than that of the pyridine nucleotide couple. Indeed, a value of —445 mV has been determined for this flavoenzyme with the driving force for its reduction being due to a decrease of 90 mV in the one-electron potential of the ferredoxin reductant. This increase... 

A general treatment has been published of oxygen atom transfer kinetics for complexes wherein the dimerization/comproportionation reaction is a complicating feature.165 This treatment critically summarizes all pertinent earlier information. The assumption that dimerization is very fast with respect to oxygen atom transfer is born out by the agreement of the observed and calculated kinetic parameters. 

In this section, the electrochemical behavior of an EE mechanism with two reversible electron transfer reactions will be studied. It will also be shown that for this electrode process (given in reaction scheme (3.II)) in both cases, i.e., normal ordering and potential inversion, the disproportionation/comproportionation reaction (3) can take place in the diffusion layer. 

As discussed in Sect. 3.3.4, the incidence of comproportionation reactions on the voltammetric response can be disregarded if diffusion is the only mass transport... 

It is interesting that a Mo2(II,III) intercalate is obtained by either Schemes I or II. Our studies on Mo2(II,II) coordinated by tetraphosphate anion, Mo2(HPC>4)44, show that it is extremely susceptible to oxidation to the mixed-valence Mo2(HP04>43" dimer (36). Indeed, solutions of Mo2(HP04>44 are smoothly converted to Mo2(HP04>43 by the weakly oxidizing M—M complex Mo2(HP04)42 in a simple comproportionation reaction. Presumably, the Mo2(II,II) core is not stable in a phosphate environment, and parallel to the molecular solution chemistry, the mixed-valence Mo2(II,III) bimetallic center is the thermodynamic product. 

Holm and co-workers (51, 52) developed sterically hindered NS 2 - and NS-donor ligands designed to suppress the comproportionation reaction [Eq. 3] that leads to a p-oxo-molybdenum(V) dimer and showed that these systems were capable of catalyzing OAT reactions, including those involving some substrates of the Mo—MPT enzymes. 

The redox characteristics of these azaviolenes and of analogs with polyene linkages between the heterocycles have been determined by Hiinig and co-workers.462,596 The rate constant for the comproportionation reaction yielding 180 (X = CH) was evaluated as (6.5 2.0) x 108 M l sec-1 by Bennion et al.597 using the temperature-jump technique. 

The titanocene(lll) complexes Cp2TrX(PMe3) (X = F, Cl, Br, 1, Me, SMe) are readily prepared by comproportionation reactions of Cp2Ti(PMc3)2 and CP2TLX2. The products are characterized by EPR spectroscopy. ... 

A frequently used indirect method involves cyclizable (cf. (7)) or other mechanistic probes which should provide evidence for free radical intermediates and thus for SET [19,37,59]. However, Newcomb and Curran have pointed out the pitfalls of such an approach especially if iodide precursors are used [17]. The supposedly radical-indicative reaction may come about albeit slower by a different, nonradical mechanism or the radical formation may occur via a secondary process which is not directly related to the first reaction step. A similar side-route can be made responsible for the appearance of stable radical compounds which may arise via a comproportionation reaction between non-reduced starting material and the doubly reduced species which can be formed from a hydro form (the normal product, Eq. (5)) and the usually strongly basic organometallic or hydridic reagents (Eq. (9)) [58]. The ability of strong bases to produce reduced radical species via complicated electron/proton transfer processes has been known for some time in the chemistry of quinones and quaternary salts [60,61]. 

Reactions of basic lithium compounds LiR or LiER with halides [19,37,59], pseudohalides [62] or unsaturated acceptors [63-65] were frequently reported to involve SET steps. The strongly basic character of such lithium compounds may, however, cause the observation of apparent one electron-reduced species such as ketyls or semiquinones via deprotonation of hydrolysis products and ensuing comproportionation reactions (Eq. (9)) [58]. 

Disproportionation reactions of [(> -Cp)2TiCl]2 to (>/ -Cp)2TiCl2 and (r/ -Cp)2Ti(CO>2 have been described . A comproportionation reaction has also been reported ... 

The possible role of oxygen atom transfer in molybdenum enzyme catalysis was recognized in the early 1970s (190-194). In the ensuing years, a wealth of chemistry has established molybdenum as the premier exponent of such reactions (7, 195). Importantly, related dioxo-Mo(VI) and oxo-Mo(IV) complexes are interconverted by oxygen atom transfer reactions (Eq. (13)). These reactions are effected by reductants (X) such as tertiary alkyl and aryl compounds of the group 15 elements (especially phosphines) and oxidants (XO) such as S- and N-oxides. In many cases, however, the Mo(VI) and Mo(IV) compounds participate in a comproportionation reaction yielding dinuclear Mo(V) complexes (Eq. (15)). 

The comproportionation reaction (15) is biologically unimportant and must be prevented in meaningful enzyme models. Accordingly, much effort has been invested in ligand design aimed to sterically inhibit the formation of dinuclear species several successful and significant... 

Imido-thiolato-complexes of vanadium of the type [V(Bu N)(SR)3] have been made by the reaction of [V(Bu N)Cl3] with Li[SR] (R = Bu , SiPhj) 18). Comproportionation reactions of the trithiolate with the trihalide precursor were then used to derive the related complexes... 

Furthermore, a chlorobis(allyl)- and a dichloromono(allyl)neodymium complex could be prepared by the comproportionation reactions formulated in eqs. (3) and (4) [39]. 

Allylstannanes rapidly undergo the Kocheshkov comproportionation reaction with SnCU, even at -50 °C. The initial product is apparently formed with allylic inversion, but further transmetallation may give other allylic isomers.35 The SnCl3 group which is formed enhances the electrophilic power of the allyl group. This has been exploited in aromatic allylation (equation 9-13),36 and transmetallations of this type provide the basis for some, but not all, of the examples of the catalysis of the allylation of carbonyl compounds by allylstannanes in the presence of Lewis acids37,38 (Section 9.1.3.4). 

The whole range of mixed [Mo30 S3 (H20)9]4+ (n = 0-4) aqua ions is also known. They are obtained as by-products from the preparations of [Mo3S4(H20)9]4+ by reduction of [Mo202S2(cys)2]2 or [MoS4]2 as indicated above.102 Comproportionation reactions between... 

Several different methods have been described to prepare doubly alkynylide-bridged dinuclear zirconocene(lll) compounds. The first approach reported was the comproportionation reaction between bis(alkynyl)metallocenes (M = Zr, Hf) and (butadiene)zirconocene, which has been utilized to prepare the examples summarized in Scheme 10.26 27 The solid-state structure of the representative example (MeC 5H 4 )2Zr(f I h ))2 22 was... 

P. M. S. Monk, R. D. Fairweather, M. D. Ingram, and J. A. Duffy, Evidence for the product of the viologen comproportionation reaction being a spin-paired radical cation dimer, J. Chem. Soc.,... 

This reaction can also be performed in liquid sulfur(IV) oxide at -22 "C, to give CO(NCS)j in 75% yield [300], Attempts to prepare CO(NCS)Cl by the comproportionation reaction between phosgene and CO(NCS)j were unsuccessful [1018]. 

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