Coupled homogeneous reactions Dimerization

The methoxycarbonylmethyl radical I is obtained by Kolbe electrolysis from methyl-malonate [Eq. (4, path a)], and in homogeneous solution by reductive fragmentation of the hydroperoxide of dimethyl acetone dicarboxylate [Eq. (4, path b)]. Radical I forms with styrene the adduct II, which reacts by disproportionation to III and IV, by coupling with I to V and by dimerization to VI. The yields and product ratios for the electrolysis and the reductive fragmentation correspond surprisingly well, and no extensive polymerization was found for the homogeneous reaction. 

Simulations that have been carried out for a vast number of different rate parameters and mechanistic variants show that only the slow second-order coupling steps between dimers and the subsequent slow elimination of protons can explain the trace-crossing (Fig. 2). In any case, the so-called nucleation loop in cyclic voltammetry is not the result of a nucleation step, but of slow, homogeneous follow-up reactions of soluble oligomers in the diffusion layer. 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

BINOL and its derivatives have been utilized as versatile chiral sources for asymmetric catalysis, and efficient catalysts for their syntheses are, ultimately, required in many chemical fields [39-42]. The oxidative coupling of 2-naphthols is a direct synthesis of BINOL derivatives [43, 44], and some transition metals such as copper [45, 46], iron [46, 47] and manganese [48] are known as active metals for the reaction. However, few studies on homogeneous metal complexes have been reported for the asymmetric coupling of 2-naphthols [49-56]. The chiral self-dimerized V dimers on Si02 is the first heterogeneous catalyst for the asymmetric oxidative coupling of 2-naphthol. 

Once formed, the dihydrofuryl radicals may undergo a variety of reactions as known from homogeneous chemistry including disproportionation, addition to double bonds, electron transfer and dimerization. Surprisingly, the latter pathway is followed to about 90 % as indicated by the complete material balance [145]. It is therefore likely that C-C coupling occurs between radicals within the water-2,5-DHF surface layer. This is corroborated by the quadratic dependence of the initial... 

The IFP-SABIC Alphabutol process utilizes a proprietary homogeneous titanium based catalyst which demonstrates high dimerization activity coupled with excellent selectivity to butene-1 at moderate pressures and temperatures. This performance is influenced by the catalyst composition and reaction parameters. 

In future the aqueous biphasic processes will grow in importance because of the great advantages of this homogeneously catalyzed variant. Dimerizations, telomerizations, hydrocyanations, hydrosilylations, aldolizations, Claisen condensations, and a wide variety of C-C coupling reactions will be objectives above and beyond the currently used syntheses. Only the future will tell what importance asymmetric/enantiomeric conversions, especially hydroformyla-tions, will have here, although initial experience is encouraging. 

SECM theory has been developed for lour mechanisms with homogeneous chemical reactions coupled with electron transfer, i.e., a first-order irreversible reaction (ErQ mechanism) (5), a second-order irreversible dimerization (ErC2i mechanism) (36), ECE and DISP1 reactions (38). [The solution obtained for a EqCr mechanism in terms of multidimensional integral equations (2) has not been utilized in any calculations.] While for ErC, and ErC2i mechanisms analytical approximations are available (39), only numerical solutions have been reported for more complicated ECE and DISP1 reactions (38). 

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