Cyclopropylcarbinyl radicals radical clocks

By incorporating a cyclopropylcarbinyl radical clock, Cheng and Wagner have confirmed the intermediacy of a 1,4-biradical 222, which cyclized very slowly [274] (Scheme 64). 

In the hydroxylahon of cyclopropyl probes, the cyclopropylcarbinyl radical clock can either rearrange to ring-opened 3-buten-l-yl radical before being trapped or rebound with the hydroxyl carrier to yield the alcohol product direchy (Scheme... 

The rate of the ring-opening reaction of 5, " and other substrates have been determined using an indirect method for the calibration of fast radical reactions, applicable for radicals with lifetimes as short as 1 ps/ This radical clock method is based on the use of Barton s use of pyridine-2-thione-Al-oxycarbonyl esters as radical precursors and radical trapping by the highly reactive thiophenol and benzeneselenol/ A number of radical clock substrates are known/ Other radical clock processes include racemization of radicals with chiral conformations, one-carbon ring expansion in cyclopentanones, norcarane and sprro[2,5]octane, a-and p-thujone radical rearrangements, and cyclopropylcarbinyl radicals or... 

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... 

Bowry, V. W., Lusztyk, J., Ingold, K. U., Calibration of a New Horologery of Fast Radical Clocks Ring opening Rates for Ring Alkyl substituted and a Alkyl substituted Cyclopropylcarbinyl Radicals and for the Bicyclo[2.1.0]pent 2 yl Radical, J. Am. Chem. Soc. 1991,113, 5687 5698. 

V. W. Dowry, J. Lusztyk, and K. U. Ingold. Calibration of a new horologery of fast radical "clocks". Ring-opening rates for ring- and a-alkyl-substituted cyclopropylcarbinyl radicals and for the bicyclo[2.1.0]pent-2-yl radical. /. Am. Chem. Soc., 113 5687 (1991). 

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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