Metallated Allenic Ethers, -Thioethers and -Amines

Alkoxypropadienes are easily obtainable by isomerization of propargyl ethers using /-BuOK [6,7]  

Metallation of methoxyallene with butyllithium in Et20 or THF is complete in a few minutes at temperatures in the region of —25 °C, and gives exclusively the a-metallated intermediate. It is obvious that alkoxyallenes are deprotonated more easily than propadiene [97] and vinyl ethers H2C=CH—OR [9]. All types of electrophiles react with perfect regiospecificity at the a-carbon atom (compare the reactions with metallated allenic hydro carbons discussed in Chap. Ill) 

However, the reaction with carbonyl compounds of the a-Grignard or a-zinc halide derivatives (prepared by exchange) affords mixtures of the allenic and acetylenic derivatives [9]  

The predominant y-metallation of f-butoxyallene with lithium dicyclohexyl-amide (LCHA) must be the consequence of a steric repulsion between the two bulky groups with ra-butyllithium a-deprotonation is the only reaction, while methoxyallem is lithiated also at the a-carbon atom by LCHA [98]  

Subsequent interaction with electrophilic reagents in general yields a mixture of the allenic and the acetylenic derivative trimethylsilylation and alkylation, however, seem to afford the y-functionalized allenic ether [99]. Under strongly polar conditions, i.e. if lithium is replaced by potassium (by addition of t-BuOK) and HMPT is added as co-solvent, the y-metallated allenic ethers rearrange into the a-metallic derivatives [100]. This remarkable isomerization may take place via a head-to-tail arrangement of two metallated molecules  

---