Alkylations of delocalized nucleophiles with alkyl trifluoromethanesulfonates and other electrophiles.

Abstract

Reactions of alkyl trifluoromethanesulfonates (triflates) and other alkylating agents with various nucleophiles were studied in a search for new synthetic methods. Alkyl triflates alkylated the potassium enolates of cyclohexanone and 2,6-dimethylcyclohexanone in high yields (86 and 96%, respectively) without significant proton scrambling and concomitant exchange products which often occur with alkyl halides. Further evidence was obtained for the existence of α,α'-dianions from ketones. 1°,1°,1°,2° and 1°,3° α,α'-Dianions were dialkylated for the first time in a new one-pot ketone synthesis, but the generation of 2°,3° and 3°,3° α,α'-dianions did not proceed in synthetically useful yields. Excess alkyl triflate gave α,α'-dialkylated ketones in 49-78% yield. Alkylation with methyl iodide followed by an alkyl triflate gave α,α'-dialkylation with different alkyl groups in 24-58% yield. Intramolecular dialkylation of acetone dianion gave cyclohexanone in low yield. C,O-Dialkylation of acetic acid dianion was not obtained in synthetically useful yields under a variety of conditions, but instead gave C- or O-alkylation products. Carboxylates were esterified by alkyl triflates, but yields were not as good as for other methods of esterification. Benzyl anions and phenoxides reacted smoothly with alkyl triflates, but these electrophiles offered no particular advantage over alkyl halides and dialkyl sulfates to justify the extra expense and precautions necessary for alkyl triflates. 1,2-Ethaneditriflate showed promise as a bis-alkylating agent by reacting with the dianion from o-xylene to give a better yield of tetralin than was obtained with other bis-electrophiles. Other alkylations using α,ω-bis-triflates went in lower yields than those using α,ω-dihaloalkanes. Ethylene sulfite reacted as a monoalkylating agent with benzylic anions and phenoxide, whereas ethylene sulfate gave monoalkylation with phenoxide and mono- and bis-alkylated products with benzylic anions. Methylene sulfate did not give identified mono- or bis-alkylated products with benzylic anions or phenoxide.