Dimerization Processes

Substituents on the methine chain can stabilize the dye radical cation if the substituent (like methyl) is located on the high electron density carbons. However, no significant stabilization occurs when alkyl groups are on the alternate positions (like 9, 11 for the dication in Fig. 9). Current results for several dyes including die arbo cyanines and carbocyanines indicate that electronic stabilization of the dication radical lengthens the radical lifetime and also enhances the reversibiUty of the dimerization process (37). 

When high purity isobutylene is not required, the acid extract from the rich stage may be heated for a few minutes to 250-300°F, and then quickly cooled. Under these conditions the isobutylene dimerizes to form largely 2,4,4, trimethyl pentene-1. This is known as the dimer process and may be used to concentrate i-butenes for dehydrogenation feed, the isobutylene dimer being added to the motor gasoline pool. Trimers, as well as codimers with normal butenes are also produced. 

The Alphabutol process (Figure 7-8) operates at low temperatures (50-55°C) and relatively low pressures (22-27 atm). The reaction occurs in the liquid phase without a solvent. The process scheme includes four sections the reactor, the co-catalyst injection, catalyst removal, and distillation. The continuous co-catalyst injection of an organo-hasic compound deactivates the catalyst downstream of the reactor withdrawal valve to limit isomerization of 1-hutene to 2-hutene. Table 7-2 shows the feed and product quality from the dimerization process. 

Table 9.2b). Increasing also is the list of phenotypes associated with these dimerization processes. With the emergence of receptor dimers as possible therapeutic targets have come parallel ideas with dimerized ligands (see Section 9.5). 

H-Azepines 1 undergo a temperature-dependent dimerization process. At low temperatures a kinetically controlled, thermally allowed [6 + 4] 7t-cycloaddition takes place to give the un-symmetrical e.w-adducts, e.g. 2.231-248-249 At higher temperatures (100-200°C) the symmetrical, thermodynamically favored [6 + 6] rc-adducts, e.g. 3, are produced. These [6 + 6] adducts probably arise by a radical process, since a concerted [6 + 6] tt-cycloaddition is forbidden on orbital symmetry grounds, as is a thermal [l,3]-sigmatropic C2 —CIO shift of the unsym-metrical [6 + 4] 7t-dimer. 

Under different conditions (in aqueous electrolyte) the selectivity of the cleavage reaction may be perturbed by the occurrence51-53 of a dimerization process. Thus, while the major process remains the two-electron reductive pathway, 20% of a dimer (y diketone) may be isolated from the cathodic reduction of PhC0CH2S02CH3. The absence of crosscoupling products when pairs of / -ketosulphones with different reduction potentials are reduced in a mixture may indicate that the dimerization is mainly a simple radical-radical coupling53 and not a nucleophilic substitution. 

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Ti2. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

The dimerization process was carried out in a basic aqueous medium, below reflux temperatures. Usually the reaction was complete in less than one hour. 

As noted, the alkaloid yield from the Beocin plants was low, which the authors suggested might be caused by the poor soil in which the plants were growing (Popovic et ah, 1992). One could ask whether the soil conditions to which they refer might be influential in the overall alkaloid biosynthetic processes in this species. It would be of interest to see experimental studies aimed at determining the effect of soil components on these processes. In the present case, it may be a lack of, or reduction in the activity of, the oxidase(s) necessary for the dimerization process (required to form the bibenzyldihydroisoquinolines) to occur. It is also possible that the lack of dimeric alkaloids may simply reflect a concentration effect caused by the edaphic conditions. These questions should be accessible to experiment. 

Ionic liquid solvents are non-volatile and non-toxic and are liquids at ambient temperature. Originally, work was concerned with battery electrolytes. These ionic liquids (IL) show excellent extraction capabilities and allow catalysts to be used in a biphasic system for convenient recycling (Holbrey and Seddon, 1999). IFP France has commercialized a dimerization process for butenes using (LNiCH2R ) (AlCU) (where L is PRj) as an IL and here the products of the reaction are not soluble in IL and hence separate out. The catalyst is very active and gives high selectivity for the dimers. 

Alkylation reactions by the iminium methide species are well known in the mitomycin and mitosene literature 4,49,51-53 and are largely responsible for the cytotoxicity/antitumor activity of these compounds. As illustrated in Scheme 7.8, the electron-rich hydroquinone intermediate can also be attacked by the iminium ion resulting in either head-to-head or head-to-tail coupling. The head-to-head coupling illustrated in Scheme 7.8 is followed by a loss of formaldehyde to afford the coupled hydroquinone species that oxidizes to the head-to-head dimer upon aerobic workup. Analogous dimerization processes have been documented in the indole literature, 54-56 while the head-to-tail mechanism is unreported. In order to... 

The mechanisms by which silenes dimerize, both head-to-tail and head-to-head, have been the subject of intense interest to theoreticians for as long as the dimerization process has been known. Siedl, Grev, and Schaefer167 calculated that the barrier between the parent H2Si=CH2 68 and its head-to-tail dimer 69 was 5.2 kcal mol-1, with the product head-to-tail dimer 69 being 79 kcal mol-1 more stable than reactants. The head-to-head dimer 70 was found to be 19.8 kcal mol-1 less stable than 69. 

Other indications for the generation of phosphasilenes involved compounds 520 and 6.21 Both derivatives were generated by thermally induced elimination of LiF/THF from corresponding lithium(fluorosilyl)phospha-nylides. Whereas 5 was directly observed by means of 3,P-NMR spectroscopy, surprisingly, the sterically more hindered phosphasilene 6 was not detected due to its lower thermostability. Therefore, evidence for the existence of 6 was achieved solely by trapping experiments. Due to the more crowded substitution in 5 compared with 2, the dimerization process to give 7 is relatively slow (Eq. 1) but is complete within a few hours. 

The dimerization process of silylene Mes2Si to 1 was studied using matrices with varying viscosity. The dimerization rate was found to be dependent on the viscosity. Interestingly, dimerization took place even in the matrix at 77 K without annealing when a soft matrix (e.g., isopentane/3-methylpentane = 9/1) was used.13... 

These calculations indicated that the dimerization process la 3a is exothermic... 

Derivatives 47-51 were prepared from the iV-oxide 46 using zinc as the deoxygenating agent (Scheme 11). The use of Zn in NH4CI solution led to the deoxy derivatives 47 and 48 in moderate yields. The reduction was clearly observed by NMR HETCOR experiments (HMQC and HMBC). When the reduction of the derivative 46 (R = CH2C1) was carried out under the same conditions, compound 51 was generated via a Zn-promoted reductive dimerization process <2001EJM771>. 

Pyran-4-ones bear an obvious structural similarity to the all-carbon cyclohexadienones discussed above. However, the original studies of their photochemical behavior revealed only dimerization processes to produce a cage product resulting from two successive head-to-tail [2 + 2]-photocycloadditions (Scheme 29)54. Much later, small amounts of substituted furfural 121 were observed during the irradiation of 11955a. It was speculated that 121 could arise from bicyclic epoxide 120, an intermediate analogous to those formed in cyclohexadienone photochemistry. Subsequent reports noted that further irradiation of... 

The Pd °-catalyzed dimerization of 2 leads to the Diels-Alder adduct 314 as the main product (81%), for whose formation only one allene group has participated in the dimerization process, in addition to the dimer 315 (14%), in which no allene group has survived in addition, several other dimers are produced in trace amounts [131]. 

Second-order irreversible chemical reaction following a reversible electron transfer dimerization. It is quite common in chemical reactions that newly formed radicals couple to each other. This also often happens in the electrochemical generation of radicals according to a dimerization process that can be written as ... 

Development of the industrial process for electrochemical conversion of acrylonitrile to adiponitrile led to extensive investigation into the mechanism of the dimerization process. Reactions of acrylonitrile radical-anion are too fast for investigation but the dimerization step, for a number of more amenable substrates, has been investigated in aprotic solvents by electrochemical techniques. Pulse-radiolysis methods have also been used to study reactions in aqueous media. 

The intermolecular coupling of phenols is used extensively in what are believed to be biomimetic alkaloid syntheses. Aqueous solutions of iron(lll) salts are most frequently used as the oxidising agent and the dimerization process must involve phenoxy radicals. Examples are the dimerization of orcinol 21 [114] and the formation of bis-benzyltetrahydroisoquinolines 22 [115],... 

Electron donating substituents favour further oxidation to the carbonium ion and thus suppress the Kolbe dimerization process [62],... 

Anodic oxidation of a-cyanocarboxylates generates a delocalised radical. Dimerization occurs by both carbon-carbon and carbon-nittogen coupling [79], Oxidation of p-ketocarboxylates using a platinum anode and a mercury pool cafliode, to prevent the solution becoming alkaline, causes the dimerization process. The alternative non-Kolbe process, giving an a,P-unsaturated ketone, is followed when die solution is strongly alkaline [80]. 

As mentioned above, an especially interesting possibility for some of the semi-buckminsterfullerenes is their possible dimerization to buckminsterfullerene. Molecule 28 is chiral, and while bowl-to-bowl inversion would produce racemiza-tion, the barrier is expected to be sufficiently high (see below) that both enantiomers must be considered for the dimerization process that is, d-CjoHjj + d-C 2 d-CjoHiz + /-C3QH12. 

The dimerization process of [(C5H4Me)(dmpe)Mn=C=CHR]+ to [(C5H4Me) (dmpe)Mn=CCHRCHRC=Mn(dmpe)(C5H4Me)], studied both experimentally and theoretically, was briefly discussed. Density functional theory calculations showed that the dimerization process was favorable when R=H, Me, Ph and C6H4Me) but unfavorable when R = silyl. The calculations also showed that the LUMO of a given dimer consisted of Tc -antibonding character between Mn and Co,... 

The various methods that convert N-vinylcarbazole to the cyclodimer all depend on the generation of a radical cation one resonance contributor (102) to which shows the radical character essential to explain the head-to-head linking and the role of nitrogen in stabilizing the cationic center. This much-studied dimerization process, including a consideration of the role of oxygen, has been discussed elsewhere. 

The main feature of such a paramagnetic quenching is that not only an active 02 molecule is formed during this process, but that the second partner, i.e., the aromatic molecule, has also acquired chemical reactivity, having been transformed into the biradical A (triplet). We expect, therefore, that paramagnetic quenching will be accompanied by association or dimerization processes, induced... 

Classical precedents for these hydration reactions are far fewer than those of the dimerization process, and reveal more complexity in the reaction. Water has been observed to add photochemically to crotonic acid21a>b and to ergotamine.21b Methanol, ethanol, ammonia, and aniline were also found to add photochemically to crotonic acid.21a More recently, methanol and propanol-2 have been observed to add... 

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. 

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