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46.3.5 Method 5: Cross Metathesis of Alkenes Followed by Elimination

DOI: 10.1055/sos-SD-046-00046

Diver, S. T.Science of Synthesis, (200946140.

A cross alkene metathesisβ-elimination sequence can provide access to 1,3-dienes as an alternative to direct metathesis between a 1,3-diene and alkene. Initially there was difficulty performing a successful dienealkene cross metathesis, with less than 10% of the desired cross product being obtained, regardless of the catalyst used (Scheme 70).[‌125‌] It was reasoned that the diene had poor reactivity (this may be encountered in other 1,3-dienes; in the present case, the low product yield might also be a result of the stoichiometry used in the cross metathesis with 36 equivalents of the diene being used). To overcome this difficulty, the diene component was replaced by a terminal alkene with a β-pivaloyloxy [(2,2-dimethylpropanoyl)oxy] ester. With this substrate, under microwave irradiation, the alkenealkene cross metathesis takes place smoothly using the second-generation Grubbs carbene complex 2. To obtain the desired conjugated 1,3-diene, a subsequent β-elimination is induced by base, giving a reasonable 46% yield over the two-step sequence (Scheme 70). This tandem sequence provides the 1,3-diene product through use of a cross alkenealkene metathesis.

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