Polar transition state rearrangement

Much of the kinetics and products work already described has been due to Banthorpe et al. who have produced a mechanism for the benzidine rearrangement42 which adequately explains the known facts. This has been called the Polar-Transition-State Mechanism and is currently accepted as being the most satisfactory description of the rearrangement. Other mechanisms have been proposed over the years and their limitations discussed (for detailed account see ref. 48). 

Since ditriptoyl peroxide is electrically symmetrical, and since benzene is not outstanding in its ability to solvate polar transition states, it seems probable that the inversion reaction in this case is due to the rearrangement of an acyloxy radical rather than cation. It may be that failure to isolate comparable products from other peroxides under free radical conditions is due to competition from very fast substitution... 

Regarding electronic effects exercised by substituents as a function of solvent, the results show that in all the solvents used the reactivity of the piperidine-catalyzed reaction is increased either by electron-withdrawing or by electron-releasing substituents, but with different efficiency for each solvent. Moreover, in accordance with a polar transition state in the rearrangement, substituent effects on the global reactivity increase with increasing solvent polarity [86JCS(P2)1183]. 

Cope rearrangement of arylazo-t-allylmalononitriles [236] shows rate acceleration by acceptor substituents in the phenyl ring. Again, a more polar transition state favor the reorganization. 

In another study, 25 was found to rearrange to a mixture of 135 and 136 (Scheme 45).84 In the thermal reaction, 136 increased at the expense of 135 with increasing solvent polarity, showing that migration to C involved a more polar transition state. With increasing acid concentration [trifluoro-acetic acid (TFA) in dioxane] the rate of reaction increased rapidly, and the isomer ratio 135 136 changed from 78 22 in pure dioxane to 10 90 in 5 AT... 

According to the data reported in the literature up to 1972, the gas-phase pyrolysis of alkyl halides was to be described in terms of a discrete polar transition state which yields products directly without the formation of an ion-pair intermediate. The only apparent exception to this consideration is the Wagner-Meerwein rearrangement of neopentyl chloride8,9. 

Allyl esters rearrange to isomeric allyl esters. Reactions are concerted and undoubtedly involve 6-center polar transition states, viz. 

Miscellaneous. The kinetics of the thermal rearrangement of the A -aryl-iV -cyanodiazetidinone (232) to imidazo[l,2-a]benzimidazole (233) have been studied and the rearrangement rate has been shown to be independent of solvent polarity. The moderate p value (—2.9) obtained suggests a polar transition state, but not purely ionic intermediates a number of mechanisms... 

Scheme 14. Resonance structures for the polar transition state during Claisen rearrangement [24,25]...

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

---