para- undortho-Chinodimethan-Zwischenstufen mit cumulierten Doppelbindungen

Heuristic chemistry--elimination reactions

Besides additions and substitutions, elimination reactions play a fundamental role in organic synthesis. However, conceptual reviews of known 1,x-elimination patterns that go beyond the typical olefin-forming 1,2-eliminations are scarce. To develop a broader understanding of elimination reactions, we follow a heuristic approach and deduce recurrent reaction patterns from traditional and specific elimination reactions. Our work demonstrates that 1,x-elimination reactions and their outcomes can be easily rationalized by defined mnemonic categories.

Direct transformation of silyl enol ethers into functionalized allenes

The first elimination reactions of silyl enol ethers to lithiated allenes are reported. These reactions allow a direct transformation of readily available silyl enol ethers into functionalized allenes. The action of three to four equivalents of lithium diisopropylamide (LDA) on silyl enol ethers results in the formation of lithiated allenes by initial allylic lithiation, subsequent elimination of a lithium silanolate, and finally, lithiation of the allene thus formed. Starting with amide-derived silyl imino ethers, lithiated ketenimines are obtained. A variety of reactions of the lithiated allenes with electrophiles (chlorosilanes, trimethylchlorostannane, dimethyl sulfate and ethanol) were carried out. Elimination of silanolate is observed only for substrates that contain the hindered SiMe2tBu or Si(iPr)3 moiety, but not for the SiMe3 group. The reaction of 1,1-dilithio-3,3-diphenylallene with ketones provides a convenient access to novel 1,1-di(hydroxymethyl)allenes which undergo a domino Nazarov-Friedel-Crafts reaction upon treatment with p-toluenesulfonic acid.