Lithiation of 1-Methylcyclopropene and Subsequent Reaction with Benzaldehyde

In order to minimize the chance of this isomerization, the deprotonation is carried out at a low temperature (strongly cooled liquid ammonia). The use of the soluble, and therefore kinetically more active, potassium amide, in combination with a large excess of 1 -methylcyclopropene, warrants a sufficiently fast conversion into the cyclopropenyl anion. Ammonia is a particularly suitable solvent for alkylation reactions. As expected from its higher basicity the anion of 1-methylcyclopropene reacts more easily with bromohexane than do acetylides (compare Ref. [6]). 

If more than 100 mg of ferric nitrate is used in the preparation of potassium amide, difficulties may be encountered in the separation of the layers owing to the formation of a gel of ferric hydroxide. 

12 Lithiation of 1-Methylcyclopropene and Subsequent Reaction with Benzaldehyde 

For couplings of metallated cyclopropenes with carbonyl compounds liquid ammonia seems to be generally a less suitable solvent since enolizable carbonyl compounds may easily undergo deprotonation under strongly polar conditions, while aldehydes such as benzaldehyde may react with the solvent. For the lithiation of 1-methylcyclopropene both BuLi THF and LDA THF seem suitable systems. We prefer to use, however, the latter base because of its decreased tendency to add across multiple bonds. The thermodynamic basicity of LDA will be sufficient though in the less polar THF the rate of deprotonation might be lower than that with potassium amide in liquid ammonia. 

We have the impression that the deprotonation is slower than that of 1-alkynes under the same conditions. 

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