Cyclotrimerization rates

The observation of negative apparent activation energy can most simply be interpreted in terms of the competition between the adsorption and desorption of methylacetylene on the surface. This qualitative explanation is illustrated in Figure 3, where the steady-state production of trimethylbenzene is compared with the TPD trace of methylacetylene. The fall off in steady state cyclotrimerization rate matches the tail of the desorption spectrum and illustrates the role of reactant desorption at higher temperatiu-es controlling the availability of alkyne monomers and thus the overall cyclotrimerization rate in this temperatime/pressure regime. 

Key to the success of this co-cyclotrimerization procedure is the selection of the appropriate monomers. A co-cyclotrimerization in which one monomer reacts much more rapidly than the other will result in a heterogeneous product as the monomer ratio changes. Moreover, if the mono-ethynyl capping agent reacts much more slowly than the multi-ethynyl monomer, gel formation can occur early in the reaction. Alternatively, if the mono-ethynyl material reacts much more rapidly, it can be exhausted early in the reaction, having produced a non-reactive trimer, and the multi-ethynyl monomers will gel later in the reaction. These problems can be avoided by using monomers of comparable reactivity or by adjusting the feed ratio to compensate for unequal reactivities. With either approach, it is necessary to determine the relative cyclotrimerization rates of each ethynyl monomer. In this paper, we report the initial results of our measurements of these rates. 

TABLE I. Cyclotrimerization Rates for p-Substituted Phenylacetylenes (Sub) and Phenylacetylene (PA)... 

Because the exact mechanism of the cyclotrimerization reaction is not adequately understood, it is useless to conjecture on exactly how the substituent influences the reaction rate. However, it is useful to know that spectroscopic data correlates with the observed rates and this may prove advantageous in the prediction of cyclotrimerization rates for other substituted phenylacetylenes. 

The solvents used in the cyclotrimerization of isocyanate had a very strong effect on the reaction rate with the increase of the relative permitivity of the solvent system, the cyclotrimerization rate constants increased. The experimental data... 

Similar effects of relative permitivity D on the cyclotrimerization were also observed in the case of substituted ammonium carboxylate catalysts. It was also observed that, besides relative permitivity, the specific solvation of reactants by aprotic dipolar solvents had a considerable effect on the rate constant of cyclotrimerization of isocyanates (see Table II). 

The deactivation reaction transfers an active catalyst into the inert (non-reactive) polymer. This phenomenon, when cyclic sulfonium zwitterions act as anionic initiators, can be utilized for the control of the cyclotrimerization of difunctional isocyanates. Therefore the degree of oligomerization of difunctional isocyanates can be controlled by the concentration of the initiator, rate of addition of the initiator, as well as by the temperature of the reaction system. 

The effect of the concentration of the initiator and temperature on the final conversion of cyclotrimerization of 80/20 2,4- and 2,6-tolylene diisocyanate is depicted in Figs. 2 and 3. As can be seen, the extent of the reaction increases with an increase of the concentration of initiator and with decrease of temperature. The rate of addition of the initiator also plays an important role, as can be seen from Fig. 4. A typical distribution of oligomers is shown on the GP chromatogram shown in Fig. 5. The resulting NCO-term-inated oligomers are stable because the polymer formed from the initiator does not contain ionic or other catalytically active centers. 

Transformations to the cyclotrimeric boiazines and cyclotetrameric tetraza-2,4,6,8,l,3,5,7-tetraboracanes also occur. The rate of dimerization for amino iminoboranes has been shown to be stabilized by bulky substituents (76,79,83). This stabilization through dimerization is essentially a [2 + 2] cycloaddition. There are a number of examples of these compounds forming cycloadducts with other unsaturated polar molecules (78). Iminoboranes can add to electron-deficient carbene complexes of titanium such as (C5H5)2Ti(CH2) [84601-70-7] by [2 + 2] cyclo addition, yielding the metallacycle shown in equation 26 (84). 

The rate of cyclotrimerization of PhC=CMc catalysed by the camphor-derived complexes of PdCl2 has been found to be highly dependent on the R group, decreasing in the order R = Me2N, i-Pr, C6H5. The effects of geometric and/or electronic parameters on the catalytic activity of the complexes were evaluated on the basis of X-ray and electrochemical data.91... 

Deng and Martin (1996) also showed the necessity of including a diffusional resistance in the rate equation for the cyclotrimerization of dicyanates, well before vitrification. They observed a significant decrease in the diffusion coefficient from conversions of about 0.40, using dynamic dielectric analysis. They could fit experimental kinetic data in the whole conversion range using Eq. (5.50). Experimental values of the decrease in the diffusion coefficient with conversion were used to estimate kd for different cure temperatures. 

Cyclotrimerization of polyfunctional aryl acetylenes offers a unique route to a class of highly aromatic polymers of potential value to the micro-electronics industry. These polymers have high thermal stability and improved melt planarization as well as decreased water absorption and dielectric constant, relative to polyimides. Copolymerization of two or more monomers is often necessary to achieve the proper combination of polymer properties. Use of this type of condensation polymerization reaction with monomers of different reactivity can lead to a heterogeneous polymer. Accordingly, the relative rates of cyclotrimerization of six para-substituted aryl acetylenes were determined. These relative rates were found to closely follow both the Hammett values and the spectroscopic constants A h and AfiCp for the para substituents. With this information, production of such heterogeneous materials can be either avoided or controlled. 

As a rule, each phenylacetylene derivative was evaluated at an initial concentration of 125 mM reactions using phenylacetylene alone were conducted at concentrations of 125, 250, and 500 mM in order to establish the effect of eth myl concentration on the measured rate. Because more than one material is produced in even the simplest cyclotrimerization reaction, all reactions were followed only by measuring the disappearance of the starting material(s). Although attempts were made to fit the resulting data into the expected second- or third-order kinetics plots, it was finally concluded that the reactions were better described as zero-order. Accordingly, data were plotted on linear concentration and time scales. 

Because preparative cyclotrimerization reactions are usually conducted at high concentration, the initial, faster rates in this study were considered more important. For each run, the rates of disappearance of the substituted aryl acetylene and phenylacetylene, along with the reaction ratio, are listed in Table I. 

An examination of the photodecoloration of bis-[4-(dimethylamino)dithio-benzyl]nickel has shown it to proceed in two stages, the first of which increases with increasing concentration of dissolved oxygen.Rate enhancements of thirty- to forty-fold are reported for the cyclotrimerization of acetylene under u.v. irradiation in the presence of a Ni"-Si02 catalyst pretreated with... 

These results indicate that the origin of the acceleration in the rate of acetylene cyclotrimerization due to the addition of hydrogen measured above (Fig. 1.4) arises from a combination of the formation of a more open ethylidyne-covered surface, and possibly also the removal of the ethylidyne once it has been formed to produce regions of relatively clean palladium. 

The proportion of the surface covered by ethylidyne or vinylidene species, depends on the hydrogen pressure due to the reactions depicted in Scheme 1.2. Both the coverage of carbon monoxide (Fig. 1.5b) and the rate of acetylene cyclotrimerization (Fig. 1.4) vary linearly with hydrogen pressure, as a result of the first-order hydrogen pressure dependence of vinylidene-to-ethylidyne conversion (Fig. 1.8) and ethylidyne titration from the surface (Fig. 1.7). This suggests that the number of surface sites available for reaction varies with hydrogen pressure, / (H ) and can be expressed as ... 

The cyclooligomerization reaction is not confined to BD as the monomer. Activated or monosubstituted 1,3-dienes also react, but reaction rates are usually slow, and selectivity and turnover numbers (TONs) are low. Cyclotrimerization and cyclodimerization of substituted 1,3-dienes - either alone or in admixture with BD - give numerous isomers of substituted CDT, COD, VCH and divinyl-cyclobutane (DVCB). For example, isoprene [34], 1,3-pentadiene [35], 2,3-dimethylbutadiene [36], 1,3-hexadiene [37], and even 1-vinyl-1-cyclopentene [38] do react (eqs. (2)-(6)). 2,4-Hexadiene is inert. 

Butadiene is cyclodimerized by these catalysts predominantly to cod and vch, but dvcb and vmcp are also formed under some conditions ". The rate of conversion and the selectivity for cyclodimerization are highest with a Ni L ratio of 1 1, lower Ni L ratios favoring cyclotrimerization. 

It was previously reported by us that the reaction rate constants for cyclotrimerization increased with the increase of a nucleophilicity of the catalyst (catalytic center). (8,9)... 

Besides the relative permitivity, the specific solvation of reactants by solvents affected the cyclotrimerization reaction. It was observed that in the solvent systems with the constant relative permitivity, the reaction rate decreased with the increasing solvation of reactive groups. (8)... 

The cyclotrimerization of acetylene to benzene has been studied by Rucker et al. (8J) over Pd(lll), (100), and (110) at pressure near 1 atm. The (110) surface was four-fold less active than Pd(lll) or (100), which contrasts with their relative selectivities during TDS under UHV conditions. The activity at high pressures was correlated with the fraction of the various surfaces that exposed clean Pd atoms, as probed by postreaction CO adsorption-desorption. In all cases, most of the surface was covered with a carbonaceous residue. The authors stated that the reaction rate is first-order in acetylene pressure for all three surfaces. The extensive data on the Pd(lll) surface clearly indicate first-order kinetics in that case. However, the limited data presented for Pd(110) seem (to the present author) to be better fitted by an order of —2.5, which is closer to the value of three suggested by the overall stoichiometry. 

Table 7 shows the effect of varying the L Ni ratio for P(OoC6H4Ph)3. In this table, the selectivity to COD is very high at all ratios of 1 1 or greater, but the high rate of COD formation (415 cycles/hr) decreases as more L is added above 1 1. With the phosphite, the L is so strongly bonded to Ni that the trimerization reaction is effectively cut off. The at first puzzling result that CDT is only a minor product when the L Ni ratio is 0.5 1 (when only half the nickel can be tied up by L) is explained by the fact that the other half is tied up as Ni(COD)2 at the temperature of the experiment and in the presence of excess COD, the BD cannot effectively compete for coordination. Note that the BD consumption rate in Table 7 in this case is only half the rate at a 1 1 ratio. COD is a more severe inhibitor of cyclotrimerization than is CDT, because Ni(COD)2 is considerably more stable than Ni(CDT). ... 

With a catalytic system as complex as this, no one has attempted to write a rate law. Using the 16- and 18-Electron Rule one can, however, write a plausible mechanism for cyclotrimerization, shown in Figure 2.9. Here S is used to represent substrate BD, bound as a simple olefin (t/ ) when attached to Ni. A bis-7r-dX y intermediate with an 8-carbon chain (as in 58) is written as r/ -Cg and the one with a twelve carbon chain (as in 69) as Ty -Ci2. No attempt is made to show the detailed stereochemistries of the intermediates or the paths leading to isomeric cyclotrimers. 

Lt-Dpm)2Rh2(CO)2 is a catalyst for the reduction of acetylene to ethane by dihydrogen. The rate of acetylene reduction exceeds that of ethylene reduction and no cyclotrimerization of acetylene is observed. Treatment of (/x-dpm)2Rh2(CO)2 with carbon monoxide and NaBH(OMe)3... 

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