Radical cyclopropylmethyl

EPR spectra have been widely used in the study of reactions to detect fiee-radical intermediates. An interesting example involves the cyclopropylmethyl radical. Much chemical experience has indicated that this radical is unstable, giving rise to 3-butenyl radical rapidly after being generated. 

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 ... 

In 1951 Roberts and Mazur observed that the free radical chlorination of methylcyclopropane gave a mixture of cyclopropylmethyl chloride and 4-chloro-l-butene (equation 74). This reaction was studied furtherand in 1969 Kochi, Krusic, and Eaton observed the cyclopropylmethyl radical 46 by ESR and also monitored its rearrangement. 

Another important rearrangement is that of cyclopropylmethyl radicals to the corresponding homoallyl radicals. This is an exceptionally fast reaction (t1/2 10 8) and has been used as a radical clock to determine the rates of other free-radical reactions.95 Cyclopropylcarbene also undergoes rearrangement, leading to cyclobutene.96... 

The equilibrium between homoallyl and cyclopropylmethyl radicals favors the former (i.e. kf kQ) in the parent as well as in simple, alkylated systems (equation 38). Hence, to... 

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. 

Rate constants for ring-opening of cyclopropylmethyl radicals... 

Radical clock rearrangements can be used to provide evidence for radical intermediates these include the ring opening of cyclopropylmethyl radical and the ring closing of the hexenyl radical (equation 21). 

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