Disproportionation reaction catalysis

Most functional groups will tolerate Pd-catalyzed reactions between a halothiophene and hexaalkylditin for the preparation of stannylthiophenes [71, 72]. In practice, however, this method often suffers from consumption of large quantities of hexaalkylditin because of its disproportionation reaction. When bromothiophene 84 was refluxed with hexamethylditin under the agency of Pd(Pli3P)4 catalysis, stannane 85 was obtained in good yield [73]. 

An important use of dinitrosyl complexes of molybdenum has been as catalyst precursors in the alkene disproportionation reaction. Thus [MoCl2(NO)2L2] (L = PPh3 or py) together with AlCl2Et forms an active homogeneous catalyst for alkene disproportionation. The catalysis appears to require loss of NO from the metal and the field has been reviewed.19,31... 

The electrogenerated chemiluminescence (eel) is therefore specific of the triplet-triplet annihilation for A-type compounds. In such processes, the triplet formation, on which the eel phenomenon is based is in competition with the expected reactions already established for redox catalysis, namely homogeneous electron exchange reactions (SET and then disproportionation reactions) ... 

Studies on the catalytic alcohol oxidation process involved investigation of the above systems under an atmosphere of air or O2 The initial rate of production of the ketone product is governed by the rate of formation of the bis(alkoxy) species, while subsequently the catalysis rate is likely determined by regeneration of the dioxo species via disproportionation of the Ru =0 species (see Sections 2 and 3.3.) however, this will depend on the concentration of H2O which increases as the catalysis proceeds, and indeed water has been considered to accelerate the disproportionation reaction by increasing the rate of dioxo formation (eq. 35). Of note, the Ru-... 

Recently, Chen et al. reported a novel hydrogenation of quinoxalines 357 via the convergent disproportionation reaction of dihydroquinoxalines 359 under a ruthe-nium/Br0nsted acid relay catalysis system (Scheme 2.95) [130a]. This transformation was initiated by the reduction of quinoxalines to dihydroquinoxalines 359 under the catalysis of [Rul/i-cymene) ] 360 the active intermediate then underwent phosphoric acid-catalyzed self-transfer hydrogenation to afford primary starting material 357... 

SCHEME 2.95 Convergent disproportionation reaction under a iuthenium/Br0nsted acid relay catalysis system. 

The disproportionation reaction proceeds both in aqueous and nonaqueous solvents and is catalyzed by hydroxonium ions (specific acid catalysis) and by molecules of nondissociated acids (general acid... 

Antelo, J.M., Arce, F., Calvo, R, Crugeiras, J., Rios, A. General acid-base catalysis in the reversible disproportionation reaction of N-chlorotaurine. J. Chem. Soc. Perkin Trans. 2. 2000, 2109-2114. 

Of course, other reaction types have been also investigated more recently, such as the Beckmann rearrangement [247,277,278] or ethylbenzene disproportionation [279, 280], just to name a couple. In situ NMR methods are expected to play a vital role in the future science of heterogeneous catalysis. 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

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