Electron transfer catalyzed stereoisomerization

Although it seems appropriate to formulate rearrangements of cyclopropanes induced by electron transfer reactions via trimethylene radical anions (see Sect.t 2.3.2) the existence of such species so far had not been proven unambigously. Especially noteworthy in this context is the observation of Walborsky 49) that in the reaction of optically active (+)-(R)-42 with sodium in ammonia the three-membered (+)-(R)-42 is recovered without loss of optical activity. Thus, the formation of the trimethylene radical anions 46 and 47, if it occurs, is irreversible (see page 12 and 13). Therefore, electron transfer reactions of various cyclopropanes with special attention to the question of the formation of radical anion intermediates have been studied by Boche and coworkers 59). 

A typical example is given by the reaction of cz s-1,2-diphenylcyclopropane cis-78a with Na/K alloy at 0 °C to yield on protonation trans-1,2-diphenylcyclopropane trans-79a, /rans-l,3-diphenylpropene 84a (together with some m-l,3-diphenyl-propene), and 1,3-diphenylpropane 82 a. Of special importance is the time dependence of the yields of cis-78a, trans-79a, 84a and 82a. The results are given in Table 4. Table 4 can be summarized as follows  

trans-79a and the propane 82a pass through a maximum. This demonstrates that the stereoisomer trans-79a is formed as an intermediate likewise, the ring-opened 

Ring-opened products A second electron transfer transforms the trimethylene radical anion 80a into the l,3- dianion 81a. In the absence of a proton (deuterium) source 81a eliminates hydride as a function of time (Table 4) to give the allyl anion 83a. Protonation of 83 a leads to 84a. Protonation of 81 a gives 82 a, as mentioned earlier. 

The following results support the stereoisomerization as an electron transfer catalyzed reaction. 

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Although hole-catalyzed (cycloaddilions involving radical cation intermediates) and PET (photochemical electron transfer) mixed [2 -I- 2] cycloadditions have been reported from electron-rich alkenes, the only report of a cyclodimerization is that of (E)-4-(prop-l-enyl)anisole which gives stereoisomeric mixtures of the head-to-head dimers 1 and 2.12... 

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