On the Mechanism of 1,3-Prototropic Shifts in Acetylene-Allene Isomerizations

Article abstract of DOI:10.1021/jo00385a033

The mechanism of 1,3-proton transfers in allene-acetylene rearrangements mediated by alkali metal amides of 1,3-diaminopropane has been investigated with the object of determining the value of multipositional acetylene isomerizations in syntheses of long chain compounds containing a chiral center with an alkyl branch.The lithium amide of 1,3-diaminopropane-mediated isomerization of two diastereomeric allenes 4 and 5 differing only in the relationship of a methyl group on a cyclohexane ring to the allene afforded identical mixtures containing both terminal acetylenes 8 and 9.Also with use of a mixed potassium/lithium reagent, rearrangement of an acetylenic alcohol 15 with defined relative stereochemistry gave two products 16 and 18 containing terminal acetylenes.These results demonstrate that 1,3-prototropic shifts effected by alkali metal amides of diamines proceed with some loss of stereochemical integrity, which is most likely caused by discrete anionic intermediates.A cyclic concerted bimolecular mechanism may be operating in part, but cannot be the exclusive mode of proton transfers.