Allyl sulphides metathesis

Using an equimolar quantity of allyl methyl sulphide and ds-pent-2-ene resulted in incomplete reaction of the allyl sulphide and some self-metathesis of the sulphide substrate. When an excess (4 equiv) of but-2-ene was used, however, the desired but-2-enyl sulphide was formed in a good yield at ambient temperature. In this case, the large quantities of unwanted hydrocarbon starting material and self-metathesis products were gaseous alkenes and therefore easily removed. Using a large excess of one alkene to improve the yield of the desired cross-metathesis product in this way is obviously only viable if this alkene is inexpensive and both it and its self-metathesis product are easily removed. 

Unfortunately, this product was isolated as a mixture with diene 36, formed from cross-metathesis with a second equivalent of the allyl sulphide, and was contaminated with some polymeric residues. It is also important to note that an excess of the sulphide was required to suppress competing ROMP of the norbornene. A similar result was obtained for the reaction of allyl methyl sulphide with cyclop entene. 

Scheme 1. Tungsten catalysed cross-metathesis of allyl methyl sulphide with pent-2-ene and but-2-ene...

The successful cross-metathesis of allyl methyl sulphide with ris-pent-2-ene and czs-but-2-ene, catalysed by the tungsten alkylidene 5, was reported by Basset and co-workers in 1993 [20] (Scheme 1). 

The report by Basset and co-workers on the metathesis of sulphur-containing alkenes using a tungsten alkylidene complex, mentioned previously for the acyclic cross-metathesis reaction (see Sect. 2.2), also contained early examples of ring-opening cross-metathesis of functionalised alkenes [20]. Allyl methyl sulphide was reacted with norbornene in the presence of the tungsten catalyst 5, to yield the desired ring-opened diene 35 (Eq. 29). 

In the presence of 12, allyl methyl sulphide undergoes self-metathesis leading to MeSCH2CH=CHCH2SMe (90% trans), and cometathesis with cw-but-2-ene leading to MeCH=CHCH2SMe (75% trans)m. 

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