Trimethylene radical

Early extended HUckel calculations by Hoffmann on the stereomutation of cyclopropane invoked the intermediacy of the trimethylene radical 27. The single-rotation TS connecting 26 with 27 was estimated to be much higher than the double-rotation TS. Of the two possible double-rotation pathways. 

Electron transfer and ring-opening—trimethylene radical cation... 

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. 

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