Ring opening of the cyclopropylmethyl radical (1) gives the but-3-enyl radical (2) with a rate constant at 37 "C of 1.2 x lo8 sl .The cyclopropylmethyl carbanion also undergoes ring opening giving the but-3-enyl anion. The rate of this process is very much slower than that of the corresponding radical and has been estimated to be about 9.5 x l o p 5 s -l at -24"C.2 Unlike the corresponding cations, cyclopropylmethyl radicals do not rearrange to cyclobutane derivatives.The rapidity of the ring opening of the cyclopropylmethyl radical has resulted in the widespread use of cyclopropane rings as a probe of reaction mechanism both in chemical and enzymecatalysed reactions. This is only a valid probe if the ring opening of the cyclopropylmethyl radical is more rapid than any competing reactions. For this reason it is essential to have a sound appreciation of the factors which control rates of ring opening of substituted cyclopropylmethyl radicals. As will be demonstrated, hypersensitive radical probes can be used, i.e. substituted cyclopropylmethyl radicals which ring open much faster than the parent cyclopropylmethyl radical.The variation in the rate of ring opening with substituents either at the radical centre or on the cyclopropane ring will be discussed in this review along with the regioselectivity of the ring opening process. Examples of the application of cyclopropylmethyl radical ring opening to the study of enzyme mechanisms and to synthetic methodology will also be discussed.
Characteristics of the RearrangementThe parent cyclopropylmethyl radical contains ca. 1 15 kJ molof strain and hence homolytic cleavage of the C,-C, bond to give a but-3-enyl radical is highly exothermic. EPR studies show that the preferred conformation of the radical is bisected, (3).3(3) Derek Nonhebel received his B.Sc. degree from King's College, University of London in 1954 and his D. Phil.jrom the University of Oxford where he worked under the supervision o j the late Prof. W. A . Waters, F.R.S. He was elected a Fellow of the Royal Society of Edinburgh in 1982. His research interests centre on various aspects of radical chemistry. He has co-authored two books on radical chemistry.
