Allylcarbinyl radical

Reaction of 711 with terminal alkynes occurs regioselectively living 3-thiahexa-l,5-dienyl radicals 715 which also undergo selective 1,5-ring closure to allylcarbinyl radicals 716. Depending on the substitution pattern, radicals 716 rearrange to cyclopropylcarbinyl radicals 717. From 716 and 717,2,3-dihydrothiophene-3-acetate 718 and 3-thiabicyclo[3.1.0]hexane-6-carboxylates 719 are formed, respectively. 

Cyclopropylcarbinyl radical is indicated by its ESR spectrum to prefer the bisected conformation 142 , and while this species is stable below — 140°C, rearrangement occurs above this temperature and at — 100°C only the allylcarbinyl radical (143) is observed (equation 25). The bisected form (142) is calculated using the STO-3G basis set to be only 1.4 kcal mol more stable than the perpendicular form, whereas ring-opening has been calculated to be exothermic by STO-3G and MINDO-3 methods and by experiment 143 is favored by 5.1 kcalmol" . The ring-opening of equation 25 has a... 

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 cycloproplcarbinyl radical undergoes fast ring-opening to the allylcarbinyl radical, see Griller, D. Ingold, K. U. Acc. Chem. Res. 1980,13, 317. 

A recent study of photoelectron spectra of isomeric C4H7 radicals has provided heats of formation of the allylcarbinyl cation (231 3 kcal mol" ) and the cyclobutyl cation (225.1 1.1 kcalmol" ) but the failure of attempts to generate the cyclopropylcarbinyl radical meant that no heat of formation of the corresponding cation could be obtained Similarly, the heats of formation of geometrically reorganized structures such as puckered cyclobutyl cations and bicyclobutonium ions could not be assigned. Further experiments along these lines are clearly indicated, as well as further calculations to resolve the current dichotomy in the theoretical studies. 

The cyclopropyl moiety is often utilized as a mechanistic probe to trap a radical formed on the vicinal carbon (Scheme 6.86). In such a case, the ring is subsequently opened to give a relaxed but-3-en-l-yl (allylcarbinyl) intermediate, which undergoes various reactions. 

Extensive studies have been devoted to these radicals in connection with their structure (classical or nonclassical) and their reactivity (homoallylic rearrangement and 1,2-vinyl migration) and the main results have been reviewed.For instance, the classical nature of the allylcarbinyl, cyclopropyl carbinyl, and cyclobutyl radicals now seems well established. 

The 1,2-vinyl shift shown in Eq. 11.72 proceeds via a familiar structure, the cyclopro-pylcarbinyl radical we introduced in the context of free radical clocks (Table 8.7). In this case, the two species involved, the allylcarbinyl and cyclopropylcarbinyl radicals, are both discrete chemical entities that have been thoroughly characterized by EPR spectroscopy (Eq. 11.74). The equilibrium very strongly favors the ring-opened form, making the clock reaction, the opening of cyclopropylcarbinyl, essentially irreversible. 

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