Dimerization reaction pathway

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

Schemes 6-29 A plausible reaction pathway for Rh(l)-catalyzed reductive dimerization of DMAD...

Nevertheless, the further mechanistic steps leading to indole dimerization is not defined and a computational investigation could suggest feasible reaction pathways, providing important anticipation about IR and UV absorption spectra, which could be very useful for the assignment of the intermediates involved. It has been experimentally proposed that the semiquinone 1-SQ may decay via disproportionation to... 

The question of the stability of the biomolecules is a vital one. Could they really have survived the tremendous energies which would have been set free (in the form of shock waves and/or heat) on the impact of a meteorite Blank et al. (2000) developed a special technique to try and answer this question. They used an 80-mm cannon to produce the shock waves the shocked solution contained the two amino acids lysine and norvaline, which had been found in the Murchison meteorite. Small amounts of the amino acids survived the bombardment , lysine seeming to be a little more robust. In other experiments, the amino acids aminobutyric acid, proline and phenylalanine were subjected to shock waves the first of the three was most stable, the last the most reactive. The products included amino acid dimers as well as cyclic diketopiperazine. The kinetic behaviour of the amino acids differs pressure seems to have a greater effect on the reaction pathway than temperature. As had been recognized earlier, the effect of pressure would have slowed down certain decomposition reactions, such as pyrolysis and decarboxylation (Blank et al., 2001). 

There are many possible reaction pathways between acrylonitrile and adiponitrile and, in each, there are several possible rate-determining steps. None of the reaction intermediates has yet been detected electrochemically or spectroscopically thus indicating very fast chemical processes with intermediates of half-lives of < 10-5 s. Bard and Feiming Zhou [104a] have recently detected the CH2 = CHCNT radical by Scanning Electrochemical Microscopy (SCEM) using a 2.5 fim radius Au electrode (1.5 mol CH2 = CHCN in MeCN/TBAPF6). The dimerization rate has been determined to 6.107 M-1 S l. 

Fig. 1 illustrates the types of reaction pathway which have been proposed to be of importance in the decomposition reaction. In practice, where additional processes such as reforming of DME may possibly occur, the reaction scheme could be much more complex than that shown. For example, on the basis of the product spectrum over molybdenum oxycarbide dimers reported in NaY,20 the following pathway has... 

Another reaction mechanism explaining the observed enhancement of A 0bs values with increasing [amine]o values in S vAr reactions is the dimer mechanism 276, which involves the self-association of the amines277-279 and which (in some cases) may be considered overlapped with mechanism of Scheme 14. A reaction pathway for dimer mechanism is shown in Scheme 17. Considering the zwitterionic intermediate 114 it is possible to have a catalysis in removing the proton and the leaving group (reaction pathway indicated by fc3). 

Detailed NMR and theoretical studies have identified and characterized a number of the complexes along the proposed reaction pathways for anisole, 1,2-dimethoxybenzene and Al,Al-dimethylaniline ° . For example, anisole deaggregates the BuLi hexamer to form a tetrameric BuLi-anisole complex 4. Adding TMEDA displaces the anisole from the tetramer and breaks it down further to give a BuLi-TMEDA dimer 5, which deprotonates anisole at >0°C yielding 6 (Scheme 4). 

When desired vinylidene-mediated pathways are not sufficiently favorable. Group 9 metal catalysts can access a set of typical side-reaction pathways. Alkyne dimerization to give conjugated enynes or higher oligomers is often observed. Polysubstituted benzenes resulting from [2 + 2 + 2] alkyne cyclotrimerization are also common coproducts. Fortunately, the selectivity of rhodium and iridium catalysts can often be modulated by the variation of spectator ligands. 

A different reaction pathway and a remarkably stable ketene-containing l,3-dioxin-4-one has been prepared by cross dimerization of ketenes 213 and 214, generated in situ by FVP of appropriate precursors. The product, l,3-dioxin-4-one 215, was obtained in 40% yield after recrystallization from hexane <2002J(P1)599> (Equation 74). 

There is as yet insufficient evidence for proper evaluation of electron transfer reaction as the possible key process in the photohydration reactions (and perhaps in some of the dimerization processes). It has also been suggested that there is a distinct charge separation in the (overall) neutral excited pyrimidine molecule, and that the charge is sufficiently localized that reaction of the excited molecule with OH or H+ (or both successively) can become a competitive reaction pathway.116 Such a dipolar reactant species has also been specifically proposed by Wacker et al.60 (Chart 8). This is an electrophilic attack on water similar to that proposed above for uracil photohydration. 

Figure 4 shows the reaction pathway for the isobutene oligomerization. After the dimer formation, the addition of another one butene molecule will depend mainly of Brbnsted acidity to stabilize the formation of the carbocation. The oligomerization to heavier olefins will be favored on catalyst showing a low L/B acids sites ratio [12]. 

Trialkylated borazines R3B3N3H3 are deprotonated by organolithium reagents, but other reaction pathways also occur.For example, the reaction of Me3B3N3H3 with one equivalent of methyllithium produces the solvated monolithium derivative [(Me3B3N3H2)Li(OEt2)]2 (9.9), which is dimeric in the solid state. The formation of di- or trilithiated derivatives. 

From the ethylene results, and similar results on acetylene [306], it is evident that interdimer reactions play an important role in the chemistry of organic molecules on Ge(100)-2 x 1. The simple picture of reaction across a single Ge-Ge dimer, while capturing a number of important reaction pathways, is incomplete. Even small C2 molecules such as ethylene and acetylene can bridge across dimers along a dimer row. Other molecules are found to bridge across the wider trench. Furthermore, these studies indicate that multiple reaction products can form even for simple systems. 

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