Abstraction polar transition states

Evidence for the polar character of the transition state is that electron-withdrawing groups in the para position of toluene (which would destabilize a positive charge) decrease the rate of hydrogen abstraction by bromine while electron-donating groups increase it,10 However, as we might expect, substituents have a smaller effect here (p -1,4) than they do in reactions where a completely ionic intermediate is involved, e.g., the SnI mechanism (see p. 344). Other evidence for polar transition states in radical abstraction reactions is mentioned on p. 685. For abstraction by radicals such as methyl or phenyl, polar effects are... 

If one pursues this picture a step further and tries to apply it to radical abstraction reactions it would suggest that a similar ionicity is present in these cases and that there must appear large contributions of polar forms to the transition states. In the simplest cases of H + D2 - HD + D or H + CH4 - Hs + CH3 we might write for the polar transition states ... 

The final product ArCH ONO is formed in further oxidation of ArCH/ to ArCH/ by CAN and the subsequent reaction with NOj . For toluene derivatives with electron-donating substituents such as the methoxy group, the electron transfer reaction (Equation 4.73) was confirmed by the laser flash photolysis method [44]. For toluene, there is a probability for direct H-atom abstraction (Equation 4.72) with a highly polar transition state. Furthermore, for toluene derivatives with electron-withdrawing substituents, the addition ability of NO3 to phenyl 7t-bonds can be considered on the basis of data for reactions with phenols [41] and furan [45]. To clarify the interchanges in the reaction paths by the substituent in toluene, reaction rate constants for various toluene derivatives were evaluated by flash photolysis [44]. The substituent effect of the rate constants for toluene derivatives was correlated with ionisation energies (lEs) of these substances. The reaction rate for anisole is too fast to obtain accurate rate constants, and only lower limits of the rate constants are obtained (anisole) >310 M -s h For p-nitrotoluene, the rate constant is 2.3T0 M -s IE = 9.5 eV. A deuterium kinetic isotope effect of 1.6 was observed for the reaction of NO3 with toluene and toluene - dg. This indicates that NO3 predominantly abstracts the H atom from methyl groups. In the case of p-xylene, the deuterium isotope effect was not observed [43]. The rate constant forp-xylene (> 2 x 10 M/s) is close to the diffusion-controlled limit in acetonitrile, and consequently selectivity becomes low. 

The metallocene-catalyzed polymerizations of Lewis acidic monomers normally proceed well if the electronic influence of the heteroatom (B, Si, Al) is diminished by a long spacer. However, when the heteroatom locates in the vicinity of the double bond, its behavior at the metallocene cation is altered. This was clearly seen in the studies of Guram et al. [147, 148], where the vinyl-Si(CH3)3 was found to coordinate with the cationic metallocene strongly via a secondary 2,1-insertion mechanism. This was explained as being due to the electropositive nature of silicon, which drives the electron density at the internal olefinic carbon and therefore favors the 2,1-insertion (Scheme 3a) [147,148]. In contrast to vinyl-Si(CH3)3, allyl-Si(CH3)3 was found to coordinate with the metallocene catalyst via the primary 1,2-insertion mechanism [147]. The behavior of allylsilanes was rationalized on the basis of the formed polar transition state (Scheme 3b), which was stabilized by the p-effect of silicon [149]. This was also the reason for the promoted p-hydrogen abstraction after allylsUane insertions in the studies of Guram [148], Byun [150], Brandow [151], and Casey [152]. 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Polar factors can play an extremely important role in determining the overall reactivity and specificity of hotnolytic substitution.97 Theoretical studies on atom abstraction reactions support this view by showing that the transition state has a degree of charge separation.101 10 ... 

Hydrogen abstraction from a position a to the oxygen of alcohols and ethers provides a simple route to a-oxyalkyl radicals. Resonance stabilization and polar factors have been used to explain the ease of radical attack on these substrates. Recent studies appear to exclude the possibility that the oxygen atom in position a to the free C-radical may cause stabilization by resonance. The ease of hydrogen abstraction Avould be determined only by polar factors, arising with electrophilic radicals (X ) in contributions from the polar forms 13-15 to the transition state. 

The understanding of polar effects on free radical reactions arose from studies of free radical polymerization where transition state effects were empha-sized. Further studies involved diacyl peroxide reactions (equation 45), hydrogen abstraction from ring-substituted toluenes, and reactions of peresters involving transition state 38 (equation 57). ... 

The supposedly high cross sections for these reactions again require the existence of very loose transition states, much looser than those which are normally found for H-atom abstractions and it again appears quite reasonable to suppose that ionic states are contributing to the formation of very loose transition states. In the preceding sequence (eq. T) we see that the transition states A and B are very close to the polar states (eq. B) we have already proposed for CH3 recombination. If our suggestions for the latter are correct then they would automatically imply a fairly high cross section for the insertion reactions of CH2 radicals. 

Polar factors will be important when the polarity of the abstracting radical is significantly different from that of the group which comprises the H-donor. A good example is given below, wherein it is seen that although hydrogen-abstraction from HC1 by CFy is more exothermic by 2 kcal/mol than that of CH3- [23], its Ea for H-abstraction is double that of CH3, because CH3- provides the better match-up of polarities in the abstraction transition state [130]. 

As in the case for alkene additions, if the SOMO of the radical is relatively high in energy, such as is the case for alkyl radicals, the principal interaction with the abstractable X-H bond will be with its unoccupied a MO (one-electron-two-orbital type), and such a radical would be considered nucleophilic. If the SOMO is relatively low in energy, such as is the case for perfluoroalkyl radicals, the principal interaction with the abstractable X-H bond will be with its occupied a MO (three-electron-two-orbital type), and the radical is considered electrophilic. Either way, a good match-up in polarities in an H-atom transition state will give rise to beneficial transition state charge-transfer interaction [130,136,137]. 

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