Ring opening of radicals

Beckwith, A. L. J. Moad, G. The kinetics and mechanism of ring opening of radicals containing the cyclobutylcarbinyl system. J. Chem. Soc. Perkin Trans. 2 1980, 1083—1092. 

Phenyl-substituted radical clocks (Fig. 6) display definite enthalpy effects that one expects for strong radical-stabilizing groups. The result is that unimolecular clock reactions are orders of magnitude less rapid than their non-substituted counterparts as evidenced in the rate constants at ambient temperatures for 5-exo cyclization of radical 9 [39] and ring openings of radicals 10 [4, 40], 11 [4], and 12 [41]. Note that... 

Below — 140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above — 140°C, however, the spectmm of a second species was seen, and above — 100°C, this was the only spectmm observed. This second spectmm could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to — 140°C. The conclusion is that the ring opening of the cyclopropyl radical is a very facile process and that the lifetime of the cyclopropyl radical above — 100°C is very short. Even though the equilibrium favors the 3-butenyl radical, the reversible ring closure can be detected by isotopic labeling experiments, which reveal the occurrence of deuterium migration ... 

Results of studies of the electrocyclic ring opening of 3 fluoro-, 3,3-difluoro-, and 3-tnfluoromethylcyclobutene are consistent with the theoretical predictions of the effect of fluonne on this reaction [142] Surpnsingly, fluonnated analogues of hexa tnene-cyclohexadiene systems undergo complex rearrangements mainly via free radical mechanisms and not by electrocyclic nng operung as expected [143] (equation 35)... 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

Reductive ring opening of epoxides in radical reactions in presence of titanocenes as electron transfer catalysts 98SL801. 

Rate constants for ring-opening of dioxolan-2-yl radicals have been measured by Barclay et a/.241 as 103-104 s 1 at 75 °C (Scheme 4.29). There is also evidence that ring-opening is reversible.24 244 Thus, isomerization of the initially formed product to one more thermodynamically favored is possible if propagation is slow. 

Quinone methides 156 and 157 were generated on irradiation of the cis and trans forms of 155, respectively (Eqs. 1.42 and 1.43).99 100 The mechanism is believed to involve a radical ring opening of the cyclopropane ring, followed by hydrogen or methyl migration. 

Monitored by ESR spectroscopy, the continuous radiolysis of furan derivatives in water leads, in effect, to the addition of the hydroxy group at the 2-position ring opening of the resultant radical is rapid.2Sla... 

Epoxides will also participate in radical reactions and this usually results in ring opening of the epoxide. The addition of a radical derived from xanthate 38 to butadiene monoepoxide provides the addition product 39 in good yields as an E/Z mixture of olefins <06AG(I)6520>. This reaction presumably proceeds through the addition of the xanthate-derived radical to the olefin, which then opens the epoxide. 

The lack of stereospecificity in equation (72) (as compared to equations (70) and (71)) is readily accommodated by the long lifetime of the triplet ion pair produced by the sensitized irradiation method. The latter allows the facile ring opening of DBC+ cation radical followed by isomerization of the resulting xylylene cation radical, i.e.,... 

Using the catalytic system described above, the enantioselective opening of meso epoxides could also be pursued. Although many excellent examples of ring-opening of meso epoxides by Sn2 reactions have recently been reported, the reaction planned here is conceptually different [40]. In the SN2 reaction, the path of the incoming nucleophile has to be controlled. In the titanocene-catalyzed reaction, the intermediate radical has to be formed selectively. If an intermediate similar to that invoked in the Bartmann ring-open-... 

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