Cyclization rate constants

Based on a unimolecular cyclization rate constant of 5 x 105s" 1 for A5-hexenyl radical at 318 K (cf. Reference 34). 

Relatively few kinetic data are available for the carbon-carbon bond forming reactions of alkene radical cations. Nevertheless, rate constants for the cyclization illustrated in Scheme 9, with generation of the alkene radical cation by the fragmentation method, have been measured. These cyclization rate constants are significantly faster than those of the corresponding neutral radicals [89]. 

Newcomb and Curran, 1988). It is indeed difficult to reconcile the two observations in (156) (Ashby, 1988 Kuivila and Alnajjar, 1982) and (157) (Ashby and DePriest, 1982 Ashby et al., 1984 Kitching et al., 1981, 1982) a minimal requirement for inversion to occur is that R- and Nu couple before leaving the solvent cage but, in view of the cyclization rate constants, cyclized products can only be formed outside the solvent cage. 

The work of Herington and Rideal (H4) showed that a good prediction of the relative rates of cyclization of various paraffins could be made based on considerations of the number of ways in which ring closure could be effected, assuming that cyclization involves two-point adsorption in the manner illustrated in Fig. 8. For example, in the dehydro-cyclization of re-hexane, five two-point contacts with the surface are possible, but only two can lead to formation of a six-membered ring. By assuming the cyclization rate constant to be given by... 

First-order cyclization rate constants n-Butylbenzene n-Pentylbenzene 2-Phenylpentane... 

Photoionization of probe 3 in acetonitrile generates a styrene radical cation that decays with a rate constant of 3 x iO s. This decay is not accompanied by the formation of a 500-nm signal, as is observed for the other two probes. Additional cationic signals in the 300-400-nm region are observed but have not been conclusively identified. The rate constant for cyclization of this probe is 2-3 orders of magnitude faster than the intermolecular additions of the p-aIkyl-4-methoxystyrene radical cation to 2,4-hexadiene and 1,3-cycIohexadiene (6 x 10 and 6 x 10 M" s, respectively). Interestingly, the cyclization rate constant for 3 agrees well with the earlier estimate from product studies, after correction of an error in one of the rate constants used for calibration purposes. " ... 

These results provide the first detailed calibration for a series of intramolecular radical cation probes based on cycloaddition chemistry. The cyclization rate constants cover several orders of magnitude in timescale, an ideal case for using 1—3 as probes for radical cations of different lifetimes. However, the time-resolved experiments demonstrate that the application of radical cation probes, at least those based on aryl alkene cycloaddition chemistry, may be considerably less straightforward than similar experiments with free radical probes or clocks. Some of the problems that need to be addressed include the variation of products with the reaction conditions and method of radical cation generation, and the possibility of reversibility of the initial adduct formation. Furthermore, at least some radical cation reactions are quite sensitive to solvent and this may mean that calibrations for radical cation cycloadditions will have to be done in a variety of solvents. 

Thus, theory states that the cyclization rate constant Kc decreases with 2.5-power of the degree of polymerization, in the case of strainless rings, and this has actually been observed for higher homologs of cyclo-octenes (Figure... 

Baran et analyzed a series of initiators for polymerization of s-caprolactone and found that for bulky active species in systems initiated with triisopropoxyaluminum the selectivity ratio, defined as the ratio of the propagation rate constant to the sum of cyclization rate constants (depropagation neglected), was equal to 3 x 10 while for anionic polymerization (sodium ethanolate) this ratio was equal only to 1.6 x 10. ... 

The observed dependence of the cyclization rate constants on the number of monomer units was often steeper than... 

Experimental methods which permit precise measurements to obtain cyclization equilibrium constants Key and cyclization rate constants key all depend upon spectroscopic studies of polymers covalently labelled with appropriate dye molecules (3). The various spectroscopic methods... 

WF theory, which neglects excluded volume, predicts that the diffusion controlled cyclization rate constant key decreased as n" (or N 2) for free-draining chains, or, more realistically, as non-draining chains (5). 

The controversial aspect of the original WF theory which elicited such attention was the way the sink term denoting cyclization was introduced into the model. This aspect and the theory in general are described in detail in a marvelous review by Cuniberti and Perico (6). These authors have taken great pains to make the WF theory accessible to experimentalists (12). They extended the theory to the case of finite chains, under partial draining conditions, and showed how one could use intrinsic viscosity data to obtain the parameters b and Cr necessary to calculate theoretical values of key , the diffusion controlled cyclization rate constant, for specific polymers. Their equations are summarized in Table 1. 

In the first expression, ki is the experimental cyclization rate constant. If the reaction between A and Q is less than diffusion controlled, ki < kcy, and these terms are related by the expression... 

For Py polystyrene Py and DMA-polystyrene-Py samples of M < 30000 in cyclohexane at 34.5, fluorescence decay and steady-state studies of pyrene excimer formation give essentially identical values of the cyclization rate constant . For higher molecular weights, is nearly equal to kM eq 9 gives values with a large uncertainty. Under these circumstances, eq. 11 provides more reliable data. Measurements of Ie/Im become progressively more difficult for samples for which the values are less than 2 or 3 x 10 sec . ... 

Indeed in toluene at 22 , the cyclization rate constant for the exciplex forming polymer is substantially larger than that for the excimer forming polymer, figure 9. I find it remarkable that such a subtle feature of the experiment is effectively predicted by the WF model. While the difference in the slopes of the two lines in the log-... 

Radical C -C Myers-Saito cyclization of enyne-allene, as well as the reaction of C -C cyclization do not depend on the donor properties of the solvent [427]. For enyne carbodiimides the situation is different, because the nitrogen atom is a potential donor center and is well known for the high electrophilicity of the central carbon atom of the car-bodiimide group. Indeed, the study of the thermolysis of carbodiimide 3.971a showed a strong dependence of the cyclization rate constant on the solvent properties. At 85°C, the reaction was seven times faster in dioxane k = 5.3 x 10 s ) and was nine times faster in acetonitrile K = 6.93 X 10 s ) than in benzene (k = 7.83 x 10 - s ). The rate constant of the thermal C -C cyclization of enyne-carbodiimides correlates better with the donor properties of the solvent rather than its dielectric constant, which is different from the reactions of enyne-allenes. Therefore, any discussion of the mechanism requires the consideration of alternative routes (Scheme 3.147) [424]. 

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