Radical-clock technique

The reaction of azide ions with carbocations is the basis of the azide clock method for estimating carbocation lifetimes in hydroxylic solvents (lifetime = 1 lkiy where lq, is the first-order rate constant for attack of water on the carbocation) this is analogous to the radical clock technique discussed in Chapter 10. In the present case, a rate-product correlation is assumed for the very rapid competing product-forming steps of SN1 reactions (Scheme 2.24). Because the slow step of an SN1 reaction is formation of a carbocation, typical kinetic data do not provide information about this step. Furthermore, the rate constant for the reaction of azide ion with a carbocation (kaz) is assumed to be diffusion controlled (ca. 5 x 109 M 1 s 1). The rate constant for attack by water can then be obtained from the mole ratio of azide product/solvolysis product, and the molar concentrations of azide (Equation 2.18, equivalent to Equation 2.14) [48]. The reliability of the estimated lifetimes was later... 

Recently the ultrafast radical-clock technique (kr = 5xl012 -1013 s 1) has been developed (Newcomb et al, 2000 and references therein). Two probes, trans, trans-2-methoxy-3-phenylmethyl cyclopropane and methyl cubane were used to study the... 

The kinetic data reported in this chapter have been determined either by direct measurements, using for example kinetic EPR spectroscopy and laser flash photolysis techniques or by competitive kinetics like the radical clock methodology (see below). The method for each given rate constant will be indicated as well as the solvent used. An extensive compilation of the kinetics of reaction of Group 14 hydrides (RsSiH, RsGeH and RsSnH) with radicals is available [1]. 

The flash photolysis technique for generating radicals with fast spectroscopic observation was pioneered by Porter and Wright, " who observed benzyl, anilino, and phenoxyl radicals in the gas phase. Further applications of this technique include diradicals from diazenes, " nucleoside-derived radicals, and ultrafast radical clocks. ... 

The elucidation of mechanisms of reactions of Sml2 have involved polarography, kinetics, radical clocks and trapping techniques (radical cyclisation) [19, 20]. The reagent is able to reduce alkyl halides and ketones/aldehydes, as shown in Scheme 10.25, in non-chain radical reactions. 

The reaction of triethyl phosphite with CC14 has been studied from a mechanistic point of view by Chanon and co workers using a variety of techniques from radical clocks to electrochemical methods99b. They isolated diethyl trichloromethanephosphonate and chloro-ethane as major products and diethyl chlorophosphate and 1,1,1-trichloropropane as minor products (equation 80). Under certain circumstances they also observed radical... 

This approach has been used, for example, to find whether the intramolecular photocycloaddition reaction of the triplet excited cyclopropyl-substituted 4-(buteny-loxy)acetophenone 220 proceeds via the 1,4-biradical 221 (Scheme 6.87).827 This presumption was confirmed by identifying the three rearrangement cyclization products 222 224. Because the rate constant of the cyclopropylcarbinyl radical opening to the allylcarbinyl radical is known to be 7 x 107 s 1,828 it was suggested that the rate constant for the formation of the (not observed) or// o-photocycloaddition adduct (225) must be less than 3 x 106s This technique comparing the rate constants of two parallel processes, of which one is known is often referred to as a kinetic (or radical) clock 29... 

The evolution of kinetic scales has been highly dependent on radical clock and, more generally, indirect competition kinetic studies [6], These types of studies provide ratios of rate constants as discussed above. One can build an extensive series of relative rate constants for unimolecular clocks and bimolecular reactions, and the relative rate constants often are determined with very good to excellent precision. At some point, however, absolute rate constants are necessary to provide real values for the entire kinetic scale. These absolute kinetic values are the major source of error in the kinetics, but the absolute values are becoming more precise and, one certainly hopes, more accurate as increasingly refined techniques are introduced and multiple methods are applied in studies of specific reactions. 

The pioneering work on the calibration of intramolecular cy-clization of the 5-hexenyl radical by Ingold and co-workers provided the basis for the development of a large number of radical clocks." These are now used both for the calibration of rate constants for intermolecular radical reactions and as mechanistic probes to test for the intermediacy of radical intermediates in a variety of processes. Furthermore, the ready availability of bimolecular rate constants from competitive product studies using free radical clocks without the use of time-resolved experiments has greatly enhanced the synthetic utility of free radical chemistry. The same concept has recently been extended to radical ion chemistry. For example, rate constants for carbon—carbon bond cleavage reactions of a variety of radical cations and anions derived from substituted diarylethanes have been measured by direct time-resolved techniques. " ... 

Radical clocks are one experimental technique that has received considerable use in the analysis of radical reactions. Most radical clocks involve an intramolecular free radical rearrangement that proceeds with a well-defined rate constant. The prototype is the rearrangement of 5-hexenyl radical to cyclopentylmethyl radical, which occurs with a unimo-lecular rate constant of 1.0 X 10 s" at 25 °C (Eq. 8.75). The clock strategy is to embed a 5-hexenyl unit into the reactive system of interest. If a radical forms, and if its lifetime is comparable to or greater than 10 s, cyclopentylmethyl-derived products should form. 

The extension of equilibrium measurements to normally reactive carbocations in solution followed two experimental developments. One was the stoichiometric generation of cations by flash photolysis or radiolysis under conditions that their subsequent reactions could be monitored by rapid recording spectroscopic techniques.3,4,18 20 The second was the identification of nucleophiles reacting with carbocations under diffusion control, which could be used as clocks for competing reactions in analogy with similar measurements of the lifetimes of radicals.21,22 The combination of rate constants for reactions of carbocations determined by these methods with rate constants for their formation in the reverse solvolytic (or other) reactions furnished the desired equilibrium constants. 

The technique has been used to determine rate constants for a number of radical reactions in solution, notably ring closure and ring fission processes which serve as clock reactions in conventional radical kinetics [45]. As an example, the bimolecular reaction of the cyclohexadienyl radical with molecular iodine is shown in Figure 11. The straight line behavior demonstrates a pseudo-first order... 

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