Metallation of Cyclopentene and Subsequent Methylthiolation

If BuLi r-BuOK and a large excess of cyclopentene are allowed to interact under conditions similar to those used in the case of norbornene (Exp. 7), low to moderate yields of derivatization products are obtained after subsequent reaction with electrophilic reagents. Obviously, cyclopentene is metallated less easily than the strained olefin, and a considerable part of the base is consumed by reaction with THF. If this solvent is omitted, the conversion is very incomplete because of the slight solubility of the base couple in apolar solvents. The system BuLi r-BuOK TMEDA (equimolar amounts) has a good solubility in alkanes and is stable at temperatures up to about 0 °C [22] at higher temperatures TMEDA is attacked. The procedure below shows that cyclopentene can be metallated with satisfactory results using this ternary system. 

Cyclobutene presumably can be metallated under similar conditions (compare Ref. [198]). It is expected that this deprotonation will proceed much faster than that of cylopentene, and thus only a slight excess of cyclobutene needs to be used and the reaction time may be shortened considerably. 

Abstraction of the vinylic proton with formation of a cyclopropenylide. 

Anionic dimerization initiated by addition of a cyclopropenyl anion to the double bond of the free cycloalkene [16]. 

Alkyllithium has been reported to add to cyclopropene [54]. In the presence of alkali amides this hydrocarbon can form oligomers. The chance of these additions, however, will be less great if the double bond bears an alkyl group. The kineti-cally favoured reaction between 1-methylcyclopropene and potassium amide undoubtedly will be formation of the cyclopropenyl anion. Although the deprotonation equilibrium is expected to be strongly on the side of the anion, (compare Ref. [8]) in the protic solvent ammonia it may easily shift to the other direction to give via the allylic anion) methylenecyclopropane  

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