Radical clocks conformational

An elegant example of the radical clock principle is illustrated in the investigations of Rychnovsky where a conformational radical clock is used [15]. This approach relies upon knowing the rate of racemisation of a radical formed at a centre which is originally configurationally pure and using this process as the clock . The enantiomeric purity of the product from the reaction is, therefore, directly related to the lifetime of the radical. 

Scheme 10.17 Use of a conformational radical clock (LiDBB = lithium di-fert-butylbiphenyl ide).

The biradical benzo-l,2 4,5-bis(l,3,2-dithiazolyl) (BBDTA) is known in the literature but characterization is incomplete. A new study reports the electronic, molecular, and solid-state structure of BBDTA.224 The lifetime of an alkyl phenylglyoxalate-derived 1,4-biradical has been estimated, using the cyclopropylmethyl radical clock , to be in the range 35—40 ns.225 The indanols (88) and their C(3) methyl and trideuteromethyl analogues have been prepared from phenyl benzyl ketone via photo-cyclization of an intermediate 1,5-biradical species.226,227 Selectivity for these products over their C(l) epimers is high but is profoundly effected by substitution in the benzyl ring or the alkyl side-chain. The findings are rationalized in terms of the conformational preference of the intermediate 1,5-biradicals. 

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... 

---