Dimerisations of cyclopentadiene

Table 1.1. Second-order rate constants kj for the dimerisation of cyclopentadiene in solution and in the gas pha.se at 25°C. ...

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

Another way of looking at the bonds in the transition structure for the dimerisation of cyclopentadiene is to see that they develop from the best frontier orbital overlap the leading bond comes from overlap between the large lobes on C-1 and C-1 in both the HOMO/LUMO interaction marked in bold in the drawing 6.269 and the equally effective LUMO/HOMO interaction marked in bold in the drawing 6.270. The two partly formed bonds, marked with dashed lines, come from overlap between a large lobe on C-4 and a small lobe on C-2 and between a large lobe on C-4 and a small lobe on C-2,822 either of which can develop into the full bond of the product. 

Several additions have been kinetically studied by more than one research group, but most of them were studied in solution and different workers used different solvents, so that the published data cannot be compared for reproducibility, Such a difficulty does not arise for reactions in the gas and in the pure liquid phases. Two popular Diels-Alder reactions, the dimerisation of cyclopentadiene to endo-dicyclopentadiene and that of butadiene to 4-vinyl-cyclohexene are suitable for a comparison of experimental results. Rate coefficients from 8 different sources for the former reaction in the pure liquid phase at various temperatures (in the range where comparison is possible) are given in Table 2. The coefficients are all extrapolated to zero time, as both secondary reactions and variation of the environment during the reaction cause a drift in the observed values of k. Rate coefficients for butadiene dimerisation in the gas phase, from different sources, are collected in Table 3. In this case also the temperature range has been limited to that where comparison is possible. In both Tables 2 and 3 activation parameters, as given by the authors (unless otherwise indicated) are also listed. 

A large amount of experimental evidence has been cited to show that the rates of several Diels-Alder additions are little dependent on the medium. Comparison of rates in the gas and liquid phases can only occasionally be made. The dimerisation of cyclopentadiene has the following coefficients (l.mole .sec ) at 20°C ... 

However, it may be anticipated that the portion of photochemical product, which structurally corresponds to the Diels-Alder adduct, does not do so stereochemically. In the photosensitised dimerisation of cyclopentadiene, CTo-dicyclopentadiene is obtained in about the same amount as the endo-isomer, together with an equal amount of tro/ji-1,2-cycloadduct , while the kinetic product of the thermal reaction is exclusively e/i(/o-dicyclopentadiene. 

Recent work on the dimerisation of 1,1-diphenylethylene by aluminium chloride produced conclusive evidence that direct initiation does not lead to the total ctmsump-tion of the catalyst. This excellent piece of research diowed that about 2.5 aluminium atoms are needed to give rise to one carbenium ion. Similar indications were reported by Kennedy and Squires for the low temperature polymerisation of isobutene by aluminium chloride. They underlined the peculiar feature of limited yields obtained in flash polymerisations with small amounts of catalyst. The low conversions could be increased by further or continuous additions of the Lewis acid. Equal catalyst increments produced equal yield increments It was also shown that introductions of small amounts of moisture or hydrogen chloride in the quiescent system did not reactivate the polymerisation. This work was carried out in pentane and different purification procedures for this solvent resulted in the same proportionality between polymer yield and catalyst concentration. Experiments were also performed in which other monomers (styrene, a-methylstyrene, cyclopentadiene) were added to the quiescent isobutene mixture. The polymerisation of these olefins was initiated but limited yields were again obtained. Althou the full implications of these observations must await more precise data, we agree with the authors interpretation that allylic cations formed in the isobutene polymerisation, while incapable of activating that monomer, are initiators for the polymerisation of the more basic monomers added to the quiescent mixture. The low temperature polymerisation of isobutene by aluminium chloride was also studied... 

Similar variations are found for the dimerisation of gaseous cyclopentadiene. Two sources give rate coefficients having a ratio of 1.7 (at 150°C), but activation energies as different as 14.9 and 16.7 kcal.mole", and log/1 equal to 4.93 and 6.1, respectively. 

A less-common activation parameter, the volume of activation (AF ), has been determined for a few Diels-Alder reactions carried out under pressure in liquid phase. The processes are cyclopentadiene dimerisation , isoprene dimerisation , addition of 2,3-dimethylbutadiene to butyl acrylate , addition of cyclopentadiene to dimethyl acetylene dicarboxylate , and addition of maleic anhydride to 1,3-cyclohexadiene, /rans-l-methoxybutadiene and isoprene . Activation volumes are negative, i.e. the reacting systems contract on passing from the initial to the transition state. In some cases the transition state appears to be even smaller than the adduct, independently of the solvent . Some of these experimental results gave rise to controversial interpretations however, the most recent ones favour a concerted four-center mechanism for the reaction. 

It has recently been confirmed by differential thermal analysis that dimerisation of pure liquid cyclopentadiene is accelerated by hydrochloric acid . ... 

The HDAR was also studied on reactive dienes such as cyclopenta-diene. In spite of the propensity of tyclopentadiene to dimerise, its reactivity with carbonyl compounds was observed in water. The capacity of water to accelerate the hetero cycloaddition was sufficient compared to the rate of the dimerisation in these conditions. As mentioned recently by Chisholm, the cycloaddition of cyclopentadiene with aldehydes are uncommon, but can be observed in water. 

Thus the rate of both cyclopentadiene and butadiene dimerisations are remarkably little affected by a change of phase. 

In the previous, and almost in every calculation on Diels-Alder reactions, it has been assumed that diene and dienophile lie on top of each other in parallel (or roughly parallel) planes. Under this condition, the endo approach is theoretically better than the exo approach only because of secondary interactions. However, it has been shown that, for cyclopentadiene dimerisation, if the two molecules are allowed to approach in a non-parallel way, the endo preference can be mainly attributed to a more favourable primary interaction, due to an approach at an angle of ca. 60°, which could be allowed only to the c/ido-oriented dienophile because of steric reasons . The pmo method has also been applied to a simplified treatment of some competing 1,4 and 1.2 thermal cycloadditions involving diradical intermediates . 

Diazocyclopentadienes lose nitrogen by the action of light or heat, generating carbene species, whose reactions with alkenes and with other caibene acceptors have been extensively studied. Cyclo-pentadienylidene has been directly observed in a photolysis of diazo-cyclopentadiene at very low temperature in a matrix [83,84]. IVhen warmed it dimerised or reacted with carbon monoxide present in the matrix to give a ketene ... 

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