Homolytic rearrangements of bicyclo[2.2.0]hexane and bicyclo[3.2.0]heptane

Homolytic Substitution: A Molecular Ménage à Trois

“…Trifluoromethyl radicals displace methyl radicals from 2,2-dimethylpropane, and although radical attack on monocyclobutanes occurs exclusively by hydrogen abstraction, ring cleavage supervenes for C−C bonds shared by two four-membered rings. For example, the brominations of bicyclo[2.2.0]hexane ( 48 ) and [ n .2.2]propellanes (50) (but, interestingly, not bicyclo[3.2.0]heptane ( 49 )) cleanly gave the corresponding dibromides. Photobromination of cubane 51 launched a cascade which produced a single stereoisomer of tricyclo tetrabromide 52 as the sole product (Scheme ).…”

Bridgehead radicals

“…showed that free radical addition of CC1, or CBr, to diene (1 8) gave adamantane derivatives, formed via bridgehead radical (I 9), and nor-adamantane derivatives, derived from (20), in the ratio ( 19):(20) = 1:3. In all these examples the bridgehead species formed quite easily, considering that endo cyclizations were involved, and any destabilization of the bridgehead radical seems to have played no part.…”

Comparison of hydrogen abstraction and homolytic substitution in pentacyclo[4.n.0.02,5.03,8.04,7]alkanesElectronic supplementary information (ESI) available: experimental details (GC-MS) for photobrominations of basketane and homocubane. See http://www.rsc.org/suppdata/p2/b2/b200699e/

“…The most spectacular example involved photobromination of cubane (pentacyclo[4.2.0.0 2,5 .0 3,8 .0 4,7 ]octane, 4). 6 Substitution by a bromine atom launched a cascade that produced a single stereoisomer of syn-tetrabromotricyclo[4.2.0.0 2,5 ]octane (5) as the sole product (Scheme 2). Trihalomethyl radicals including ؒ CCl 3 , ؒ CBr 3 and ؒ CI 3 , on the other hand, abstracted a hydrogen atom leading to the formation of monohalocubanes.…”

“…For example, photobromination of bicyclo-[3.2.0]heptane (3) led to a mixture of monobromides from H-abstractions (Scheme 1). 5 It is evident that there is a direct competition between H-abstraction and homolytic substitution during radical attack on polycycles containing 4-membered rings. To probe the structural factors controlling this competition, we examined the reactions of pentacyclo[4.4.0.0 2,5 .0 3,8 .0 4,7 ]decane (basketane, 6) and pentacyclo[4.3.0.0 2,5 .0 3,8 .0 4,7 ]nonane (homocubane, 14) with tert-butoxyl radicals and bromine atoms.…”