Trimethylene radical anion

Electron transfer to cyclopropane should lead to the cyclopropane radical anion which, in principle, can isomerize to the ring-opened trimethylene radical anion. Further reduction of the trimethylene radical anion should give a 1,3-dianion. A less likely two-electron transfer to cyclopropane could conceivably give the ring-opened 1,3-dianion via the corresponding cyclopropane dianion. 

From these results it is quite understandable that the parent cyclopropyl radical anion is not observable by ESR (vide supra), and that ring-opening to give the parent trimethylene radical anion, the parent 1,3-dianion or any follow-up products also does not occur " ... 

The role of the phenyl group is to accept an initial electron to form the short-lived radical anion 49 ESR experiments, however, failed to demonstrate the existence of species such as 49, or the trimethylene radical anions 139 and 140 . This means that if 49 is formed it must readily open to 139 and 140 which themselves must quickly add another electron to form a dianion which is protonated by the solvent to give the anions 141 and 142 , respectively. 

From the very different reduction products from cis- and trans-145, respectively, with sodium in NH3 one can conclude that a possible trimethylene radical anion intermediate is not reversibly formed. Otherwise, both stereoisomers cis- and trans-145 should lead to the same reduction products. An irreversible ring-opening has similarly been observed in the Na/NH3 reduction of (+ )-(R)-49 as shown earlier. Comparable results to those of cis-... 

The suggestion that the stereoisomerization is an ET-catalyzed reaction occurring via the stereoisomeric trimethylene radical anions 180 (Scheme 18) is confirmed by the following three results ... 

SCHEME 20. Reactions of the trimethylene radical anion 186 after 60 h at 20°C, followed by workup with water... 

In Section III.C.2.b. above clear evidence has been presented that the trimethylene radical anions 180 are intermediates in the stereoisomerization of the cyclopropanes 178 under ET catalysis (see also Scheme 18). This is also true for other cyclopropanes. 

The formation of the ring-opened propene (179) and propane (181) also seems straightforward from Scheme 18, pathway A the trimethylene radical anion 180 is further reduced to the dianion 180" which, as a function of time (Table 23), loses p-hydride H (if the j5-carbon atom bears hydrogen atoms) protonation of the reaction mixture gives propene (179) and propane (181). Thus, the structural isomer propene (179) does not result from an ET-catalyzed rearrangement reaction (pathway B m Scheme 18) ... 

The literature offers an alternative pathway for the formation of propenes from cyclopropanes in the presence of electron sources as mentioned earlier rearrangement of the trimethylene radical anion 175a to give 176a is the important step, see also pathway B, Scheme 17 . ... 

A similar rearrangement has been proposed to occur in the trimethylene radical anion 193 to give 194 . ... 

Finally, it should be noted that geometrical and structural isomerizations of substituted cyclopropanes by means of ET-catalyzed reactions, via intermediate trimethylene radical anions, is only one pathway to perform these reactions. Other possibilites are the thermal reaction via trimethylenes , the light induced reaction the photosensitized reaction via trimethylene radical cations the Pd/C-catalyzed reaction S and the base-catalyzed reaction ... 

The predominant formation of 43 from 42 via 1,2-bond cleavage of 45 to give 46 is expected on the basis that the trimethylene radical anion 46 would be predicted to be more stable than the trimethylene radical anion 47. This argument is based on the reasonable assumption that in 46 there is more negative charge on the carbon atom bearing the two phenyl groups otherwise the isomer 47 should be more stable. 

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). 

The cyclopropane stereoisomerization cis-78a trans-79a and the transformation of the cyclopropanes into the ring-opened products 82 a and 84 a are explained by the reversible formation of the trimethylene radical anion 80 a and pathway A in Scheme 5 ... 

Stereoisomerization cis-78a and trans-79a accept an electron to give the diphenyl-cyclopropane radical anions cis-78a and trans-79a which rearrange reversibly into the trimethylene radical anion 80 a. The stereoisomerization takes place at the trimethylene radical anion stage. The reversible formation of 80a in THF is noteworthy because the trimethylene radical anions 46 and 47 have been formed irreversibly, however in this case in NH3, see Scheme 3, page 12. 

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. 

In summary, there is no doubt that the reversible stereoisomerizations of the cyclopropanes cis-78,. trans-79, cis,cis-88, trans,trans-89, and cis,trans-90 in the presence of Na/K alloy occur via the trimethylene radical anions 80 (Scheme 5) and 92. These ET-catalyzed reactions do not occur in a thermal or base-catalyzed or dianion fashion. They differ greatly from the irreversible ET reactions of the cyclopropanes 42, cis-50 and trans-51 (pages 12 and 13) with Na in NH3 49 50). Undoubtedly, in the latter reactions trimethylene radical anions are also intermediates which, however, are quickly protonated by the rather acidic NH3. Spectroscopic evidence for trimethylene radical anions is not available to date. 

The mechanism of the formation of the ring-opened species 1,3-diphenylpropene 84a and 1,3-diphenylpropane 82a from the cyclopropanes cis-78a and trans-79a has been outlined in Scheme 5, route A, page 18. Common intermediate is the 1,3- dianion 81 a which in the case of the formation of 84 a first loses hydride and then is protonated. The l,3- dianion 81a is formed by electron transfer from the trimethylene radical anion 80a which thus is the key intermediate 1) in the ET-catalyzed-stereoisomeriza-tion and 2) in the structural isomerization of the cyclopropanes cis-78 and trans-79. Importantly, the structural isomerization is not an ET-catalyzed reaction. 

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