Cyclopropylmethyl radical rearrangement

When (519) is reduced with tri(n-butyl)tin hydride, one product is that of an allylmethyl-cyclopropylmethyl radical rearrangement. Reduction of (520) and (521) clearly shows that a bicyclic cyclopropylmethyl radical undergoes preferential opening of an external bond to give the (nominally) primary allylmethyl radical (Scheme 71). This is in agreement with earlier results. 

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... 

Evidently, Pi is obtained from BRi by a straightforward combination of the two radical centres, but an extensive skeleton rearrangement must occur on the route to the product P2. Because it is natural to assume that no carbon-hydrogen bonds are broken in that process, the polarization of each proton serves as a label of the carbon atom it is attached to. The mechanism displayed in Chart 9.4 sums up the result, and identifies the structural changes as a cyclopropylmethyl-homoallyl rearrangement of the quadricylcane-derived moiety. 

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). 

Cyclopropylmethyl radicals have been generated by thermolytic, photolytic or electron transfer reactions of a wide variety of precursor molecules. The parent cyclopropylmethyl radical 1 contains approximately 27.5 kcal mol ring strain and therefore homolytic cleavage of a C -C bond (j -scission) to give the but-3-enyl radical 2 is exothermic, and occurs very rapidly, even though the rearranged radical is primary. 

Isotope labeling experiments and EPR spectroscopic studies have shown that the cyclopropylmethyl radical is a discrete chemical species with a finite lifetime = 7 x 10 s at 25°C in methylcyclopropane solution). Unlike the corresponding cyclopropylmethyl cation, 1 has no nonclassical or fluxional characteristics, and it does not rearrange to cyclobutane derivatives. Its rate of formation from diazenes, peroxides, and other precursors is slightly greater than that of model primary acyclic radicals, which indicates that it has a small thermodynamic stabilization. EPR spectroscopic studies have shown that rotation of the methylene group carrying the unpaired electron is not free and that the preferred conformation is bisected rather than perpendicular. 

Table 1. Kinetic Data for the Cyclopropylmethyl to But-3-enyl Radical Rearrangement...

An important factor which influences the rate of rearrangement is the extent of overlap between the SOMO and the bond to be cleaved. In conformationally mobile cyclopropylmethyl radicals rotation can occur to maximize this overlap. Substituents at the radical center which withdraw electron density, such as terf-butoxycarbonyl or nitro, dramatically decrease the rate of ring cleavage (Table 1), presumably because they reduce the overlap. Similarly, the rate of rearrangement is much slower in l-(cyclopropyl)allyl (3), l-(cyclopropyl)prop-2-ynyl (4), and a-cyanocyclopropylmethyl (5) radicals where resonance delocalization removes electron density from the a-carbon atom. 

Rearrangements of a-substituted cyclopropylmethyl radicals afford mixtures of the (E)-and (Z)-butenyl radicals usually with a preponderance of the Z-isomer. This stereoselectivity is probably a consequence of the higher proportion of the a /-conformer compared to the j n-conformer, the former being lower in energy for steric reasons i.e. nonbonded interactions between the substituent and ring hydrogens are less important. The -alkene is also thermodynamically more stable than the Z-alkene. ... 

A number of intermediates related to the cyclopropylmethyl radical undergo rapid ring opening. 1-Cyclopropylvinyl radicals 35 rearrange to 2-(allenyl)ethyl radicals 36, while radical anions 37, formed by one-electron reduction of cyclopropyl ketones, ring open to give enolate radicals... 

Free-radical addition to the terminal methylene group of a vinylcyclopropane 1 affords an a-substituted cyclopropylmethyl radical which rearranges to produce a 1 -substituted pent-2-ene. The most frequently used radical precursors are thiols, additions of which lead to alkenyl sulfides 2. ... 

---