In Situ Alcohol Oxidation-Wittig Reactions Using Non-stabilised Phosphoranes

Abstract

New procedures have been developed which allow the in situ alcohol oxidation-Wittig reaction using manganese dioxide to be employed with non-stabilised phosphoranes and stabilised phosphonate anions. A number of examples are described utilising benzylic, allylic and propargylic alcohols.

Key words

alkenes - Horner-Wadsworth-Emmons - oxidation - tandem - Wittig

References

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7

Manganese dioxide (activated, Aldrich 21, 764-6) was dried in an oven overnight (ca. 75 °C). Dried manganese dioxide (217 mg, 2.5 mmol) was then added to a solution of alcohol (0.5 mmol), phosphonium salt (0.55 mmol), guanidine 4 (1.1 mmol) and Ti(i-PrO)₄ (0.5 mmol, if specified in Table [2] ) in THF (12 mL) which was heated to reflux under nitrogen. A second portion of MnO₂ (217 mg, 2.5 mmol) was added after 1 h and the reaction mixture heated at reflux for the time stated. The reaction mixture was allowed to cool and then filtered through Celite washing well with Et₂O. The combined organic washings were neutralised with aq sat. NH₄Cl solution. The organic layer was separated and the aq layer extracted with Et₂O (3 × 15 mL). The combined organic layers were dried (MgSO₄), filtered and concentrated in vacuo. The crude product was purified by column chromatography. Known products gave data consistent with those published; novel compounds were fully characterised.

10

Addition of Ti(i-PrO)₄ did appear to accelerate these reactions but some transesterification was observed and the procedure was not explored further. We were unable to utilise unactivated alcohols such as decanol in this sequence.