Unimolecular ring opening

K. It too gives the characteristic adducts obtained from the singlet, but the addition does not occur directly between the bicyclic hydrocarbon (28) and the alkene. Instead, the reaction occurs in two steps, first the reversible unimolecular ring opening of 28 to singlet biradical 14b, followed by a bimolecular capture of the latter (Scheme 5.5). Another hydrocarbon isomer 29 can be prepared as a transient intermediate. Its thermal conversion to the biradical 14b apparently occurs at even lower temperature. 

A significantly more important tool to decipher the reaction mechanism is probe reactions (Fig. 8). Most commonly used are cyclopropylcarbinyl radical ring opening reactions and radical 5-exo cyclizations to intercept coupling reactions with metal centers. Cyclopropylmethyl bromide 20 is reduced by a metal complex and generates cyclopropylcarbinyl radical 20A. Unimolecular ring opening to... 

In the oligomerization of 47 e, the neighboring group participation of the equatorially located bromine atom is impossible, unless the oxonium ion is unimolecularly ring-opened to form an oxacarbenium ion. The electron withdrawing bromine atom disfavors such an unimoleeular ring-opening reaction. Therefore, the cyclic dimer cannot be formed by a mechanism similar to that described for the cyclodimerization of 47 a. 

On the other hand, when highly stabilized carbenium ion results from unimolecular ring opening, the carbenium active species may be present in the system in detectable concentrations. Indeed, in the polymerization of cyclic acetals —O -Ct + active species exist in equilibrium with tertiary oxonium ion active species (cf., Section lI.B.6.b.). 

Termination by Reactions of More Reactive Species Existing in Equilibrium with Stable Onium Species As already discussed, in the systems, in which unimolecular ring-opening of cyclic onium ion leads to highly stabilized carbocationic species, a concentration of the latter species in equilibrium with onium ions may be significant. This is, for example, the case of cationic polymerization of cyclic acetals, where carboxonium ions exist in equilibrium with their oxonium counterpart ... 

This is an analog of the 1 reaction (of Aj in acid hydrolysis). Thus, the rate of the overall reaction would be controlled by a slow unimolecular ring-opening within an oxonium ion which, in turn, is formed in the fast reaction with the next monomer molecule. 

DBO approaches the terminal unit from the opposite side of the positively charged oxygen (a) because approach from the other side (b) is sterically hindered. If cyclic structure (8-42) were in equilibrium with the product of the unimolecular ring opening, i.e. carbenium ion, then the situation should be different as the carboxonium ion is nearly flat. 

The pH-independent epimerisation of penicilloic acid at C(5) probably occurs by unimolecular ring opening of the thiazolidine to form the iminium ion tautomer of penamaldic acid [74]. There are two possible explanations for the base-catalysed reaction. Above pH 12 deprotonation of the iminium ion could become faster than ring closure, and the rate of epimerisation would increase because the steady state concentration of the ring-opened thiazolidine is increased. Alternatively, epimerisation could occur by the penamaldate mechanism followed by penicilloyl esters (Davis and Page, 1985). 

Preliminary results on the enantioselective formation of sulfur and nitrogen mediumsized heterocycles by base-induced ring opening of hetero-oxabicyclic [3.2.1] and [3.3.1] systems have been reported.91 The reaction involves a deprotonation-C—O bond elimination sequence. The kinetics and mechanism of gas-phase unimolecular elimination reactions of some substituted aminoazoles have been studied as an aid to heterocycle synthesis.92... 

Photoisomerization of c/.y-stilbene 191 Ionic fragmentation reaction 191 Cyclopropyl radical ring-opening 192 Ionic molecular rearrangement 193 Ene reaction 196 Thermal denitrogenation 198 Unimolecular dissociation 199 Sn2 reaction 200... 

Vieira AJSC, Steenken S (1987a) Pattern of OH radical reaction with 6- and 9-substituted purines. Effect of substituents on the rates and activation parameters of unimolecular transformation reactions of two isomericOH adducts. J PhysChem 91 4138-4144 Vieira AJSC, Steenken S (1987b) Pattern of OH radical reaction with N6,N6-dimethyladenosine. Production of three isomeric OH adducts and their dehydration and ring opening reactions. J Am Chem Soc 109 7441-7448... 

Vieira AJSC, Steenken S (1990) Pattern of OH radical reaction with adenine and its nucleosides and nucleotides. Characterization of two types of isomeric OH adduct and their unimolecular transformation reactions. J Am Chem Soc 112 6986-6994 Vieira AJSC, Steenken S (1991) Pattern of OH radical reaction with N6,N6,9-trimethyladenine. Dehy-droxylation and ring-opening of isomeric OH-adducts. J PhysChem 95 9340-9346 Vieira AJSC, Candeias LP, Steenken S (1993) Hydroxyl radical induced damage to the purine bases of DNA in vitro studies. J Chim Phys 90 881-897... 

The acid-catalyzed ring openings of ethylenimine and 2-ethyl-ethylenimine have also been characterized as A-2 reactions (Earley et al., 1958). The entropies of activation are —9-4 and —10.0 e.u., respectively. However, AS for the reaction of 2,2-dimethylethylenimine is —1.9 e.u., suggesting incursion of the unimolecular mechanism, a conclusion which is supported on other grounds (Earley et al., 1958). The volume changes of activation have also been measured (Earley et al., 1958). These results, if interpreted in terms of Whalley s (1959) criterion, would indicate the A-l mechanism for ethylenimine and the A-2 for the ethyl and dimethyl derivatives, a conclusion which seems unacceptable. 

Smith, D. M. Nicolaides, A. Golding, B. T. Radom, L. Ring opening of the cyclopropylcarbinyl radical and its N- and O-substituted analogues a theoretical examination of very fast unimolecular reactions./. Am. Chem. Soc. 1998, 120,10223-10233. 

Another common scenario in competition kinetics utilizes unimolecular radical reactions as a clock against which other reactions can be timed. Among the most commonly used free radical clocks are the cyclization of 1 -hexenyl and other radicals with double or triple bonds in the chain,33 ring opening,34 and p-elimination from alkoxyl radicals.35... 

Although some interesting synthetic processes of cyclic polymers have been addressed, the cyclization principle can be classified into two main methods as indicated in Fig. 2. One is the utilization of the ring-chain equilibrium that occurs in many polycondensation and ring opening polymerization. Another is the end-to-end cyclization method that can be used for synthesizing cyclic polymers from a/o-difunclional linear precursors. The ring-closure reaction by the end-to-end cyclization is further divided into intermolecular reaction and intramolecular reaction, i.e., bimolecular process and unimolecular process, respectively. 

The (high-pressure) pre-exponential factor for the ring-opening of cyclo-butene into butadiene is 1013 4 s 1, and the activation energy is 137.6 kJ/mol. Using the RRK theory calculate the unimolecular rate constant for the reaction at an excitation energy of 200 kJ/mol. 

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