Propellanes radical attack

The central bond of [l.l.l]propellanes (1) is the center of their reactivity, and in many ways, it is useful to think of it as somewhat akin to the n bond in an alkene. The strengths of the two are comparable, and both are susceptible to electrophilic and radical attack. The main difference is that the central bond in la is apparently somewhat susceptible to nucleophilic attack as well, whereas the n bond in unsubstituted ethylene is not. In both cases, introduction of electron-withdrawing groups enhances reactivity towards nucleophiles. 

Radical attack on the central bond in [1.1.1 [propellane 1 occurs 2-3 times faster than attack on styrene96 and yields bridgehead bicyclo[l.l.l]pent-l-yl radicals6. Laser flash photolysis techniques were used to measure the rate constants for the reactions of la with five different radicals (Table 12)96"99. The addition of the phenylthiyl radical to la is... 

Although the synthesis and reactivity of propellanes, recently reviewed, is discussed later, it may be noted that symmetry considerations and calculations have been used to rationalize the observed high reactivity of strained propellanes to acid and to radical attack but their stability to nucleophilic attack. EH calculations have been used to probe the stabilization of bridged carbonium ions by through-bond interactions. 

The reaction of EtsSiH with [l.l.l]propellane under photolytical decomposition of di-tert-butyl peroxide afforded products 17 and 18 in 1 3 ratio (Reaction 5.15) [36]. A rate constant of 6.0 x 10 M s at 19 °C for the addition EtsSi radical to [l.l.l]propellane was determined by laser flash photolysis [37]. Thus, it would appear that [l.l.l]propellane is slightly more reactive toward attack by EtsSi radicals than is styrene, and significantly more reactive than 1-hexene (cf. Table 5.1). 

Cyclopropanes without electron-withdrawing substituents usually resist the attack of nucleophiles. Exceptions are strained systems and a-halo-substituted derivatives which undergo homoallylic substitution reactions.[l.l.l]Propellane structures, though usually formed only as intermediates, underwent addition of benzenethiol with cleavage of the central cyclopropyl bond. This addition possibly follows a radical chain mechanism. 

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