Ring opening in the reaction of diphenylcyclopropane derivatives with strong base: evidence for a proton transfer and against an electron transfer reaction. Facile disrotatory ring opening of a cyclopropyl anion at -75°C
✍ Scribed by Gernot Boche; Michael Marsch
- Publisher
- Elsevier Science
- Year
- 1983
- Tongue
- French
- Weight
- 238 KB
- Volume
- 24
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
The reaction of cyclopropanes like 2,3,4-triphenyl-endo-tricyclo[3.2.1.02'0]octane (la) with base is initiated by proton and not by ele$trov transfer. The facile disrotatory cyclopropyl anion ring opening reaction of 2JR2=CN, M =Ll ) at -75°C does not occur synchronously. 2,3,4-Triphenyl-endo-tricyclo[3.2.1.02" Ioctane (la) isomerizes in the presence of potassium tert-butoxide (KO-t-Bu) in dimethyl sulfoxide (DMSO) (7O"C, 20 h) to 2,3,4_triphenylbicyclo= [3.2.lloct-2-ene (&)I. The reaction was originally' interpreted to proceed via forbidden disrotatory ring opening of the cyclopropyl anion c(R*=Ph, M+=K') to the ally1 anion z (R*=Ph, M+=K+) (Scheme I). Other authors concluded later2 that "the reaction of &with KO-t-Bu in DMSO or hexamethylphosphoricacid amide (HMPA) (25"C, 24 h), or with dimsylpotassium in DMSO (70X, 24 h) appears to proceed by a radical pathway". It was proposed that an initial single electron transfer (SET) from base to kaffords s(M+=K+) which opens to d(M+=K+). The latter was envisaged to rearrange to i(M+=K+) which loses an electron, "possibly to &', to give g3.