Absolute and Relative Reactivities

Absolute and relative reactivity data obtained using the various metliods (Table 3.6) are in broad general agreement. 

Entries 4 and 5 point to another important aspect of free-radical reactivity. The data given illustrate that the observed reactivity of the chlorine atom is strongly influenced by the presence of benzene. Evidently, a complex is formed which attenuates the reactivity of the chlorine atom. This is probably a general feature of radical chemistry, but there are relatively few data available on solvent effects on either absolute or relative reactivity of radical intermediates. 

There are few addition reactions to a,/J-disubstituted enoyl systems 151 that proceed in good yield and are able to control the absolute and relative stereochemistry of both new stereocenters. This is a consequence of problematic A1,3 interactions in either rotamer when traditional templates such as oxazolidinone are used to relieve A1,3 strain the C - C bond of the enoyl group twists, breaking conjugation which results in diminished reactivity and selectivity [111-124], Sibi et al. recently demonstrated that intermolecular radical addition to a,/J-disubstituted substrates followed by hydrogen atom transfer proceeds with high diastereo- and enantioselectivity (151 -> 152 or 153, Scheme 40). 

For A-B/oxide systems, and more generally non-reactive liquid A-B/solid systems, the model predicts three main types of Wa(XB) and 0(XB) isotherms, depending on the absolute and relative values of the adsorption energies E v and E L (Figure 6.29). In the particular case of isotherms (b), two different... 

Until recently, knowledge about absolute and relative rates of reaction of alkenes with carbocations was very limited and came almost exclusively from studies of carbocationic polymerizations [119-125]. The situation changed, when it became obvious that reactions of carbocations with alkenes do not necessarily yield polymers, but terminate at the 1 1 product stage under appropriately selected conditions (see Section III.A). Three main sources for kinetic data are now available Relative alkene and carbo-cation reactivities from competition experiments, absolute rates for reactions of stable carbocation salts with alkenes, and absolute rates for the reactions of Laser-photolytically generated carbocations with alkenes. All three sets of data are in perfect mutual agreement, i.e., each of these sets of data is supported by two independent data sets. 

Cyclization of the more reactive o-geranylphenol with (/ )-BINOL-SnCLt gives the frani-fused tricyclic compound as a major diastereomer (36% ee, 84% ds) in good yield (eq 11). The enantioselectivity is improved to 50% ee by using (/ )-BINOL-Me-SnCU. The monobenzoyl ester of (/ )-BINOL [(/ )-BINOL-Bzj-SnCU complex is the most effective for controlling the absolute and relative stereochemistries (54% ee, 95% ds). 

Cyclization of the more reactive o-geranylphenol with the (f )-BINOL-SnCl4 complex in dichloromethane at -78 °C was complete within 1 day, and the tran -fused tricyclic compound was obtained as a major diastereomer (84 % ds) in good yield (Eq. 99). The optical yield, however, was only 36 % ee. The enantioselectivity was improved to 50 % ee by using the (f )-BINOL-Me-SnCl4 complex. Finally, we found that the monobenzoyl ester of the (i )-BINOL ((f )-BINOL-Bz)-SnCl4 complex enabled the most effective control of the absolute and relative stereochemistries (54 % ee, 95 % ds). It seems that the stereoselectivity depends on the activity of LBA, which decreased in the order BINOL-SnCU, BINOL-Me-SnCl4, and BINOL-Bz-SnCU. 

Both competitive methods give fairly reliable relative rate coefficients in most cases. However, discrepancies between them have been found when t-butyl hypochlorite was used as the source of alkoxy radicals and when aralkanes (e.g. toluene) were the substrates because of the incursion of a chlorine atom chain, and relative reactivities to Cl- rather than to t-BuO- were determined. Absolute rate coefficients reported in this review do not include this suspect-data. 

What about the classic ambiphile, MeOCCl In Table 8, we summarize values for MeOCCl, determined by a combination of absolute and relative rate measurements. [101] Also included are analogous data for PhCOMe, [102] MeCOMe, [73] and MeCOCH2CF3. [103] For MeOCCl, we note that the ambi-philic reactivity pattern emerges from the absolute rate constants of Table 8 as clearly as it does from the relative rate constants of Table 4 high reactivity toward electron-rich or electron-poor alkenes, but low reactivity toward aUcenes of intermediate electron density. However, whereas the relative rate data can only inform us about the carbene s selectivity pattern, the absolute rate data reveals the carbene s true reactivity. In fact, for the addition of MeOCCl to trans-butene (3.3 x 10 M s ) is the lowest bimolecular rate constant yet measured for a carbene/aUcene addition in solution. [101] And with 1-hexene, only 5% of MeOCCl addition was observed this reaction is so slow that other competitive processes prevail. [101]... 

Although it is not possible to predict the absolute rates for the reduction of organic chemicals in environmental systems, recent laboratory studies have established relationships between substrate properties and relative reactivities (Tratnyek et al., 1991). For example, Schwarzenbach et al. (1990) studied the quinone and iron porphyrin mediated reduction of a series of nitrobenzenes and nitrophenols (see Section 3.C.2 for further discussion of electron mediated reductions). From the experimentally determined rate law, it was determined that the transfer of the first electron from the monophenolate species of the hydroquinone, lawsone (HLAW ), to the nitroaromatic was rate determining (Equation 3.50). 

A number of esters [10], ethers [11, 12] and alcohols [13] were investigated with respect to reactivity with nitrate radicals. Both absolute and relative rate methods were employed. Rate coefficients for the reaction of NO3 are given in Table 1. The rate coefficients for aliphatic esters may be predicted from available group reactivity factors for alkanes provided that formate carbonyl hydrogen atoms are treated as primary hydrogen atoms. The rate coefficients with temperature dependence for ethers and alcohols are valid between 268 to 363 K. 

Amatore, C., Oturan, M. A., Pinson, J., Saveant, J. M., and Thiebault, A., Nucleophile and aryl radical reactivity in Sjjjjl reactions absolute and relative electrochemical determination, /. Am. Chem. Soc., 107, 3451, 1985. 

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