This week’s choice is a contribution from Casey R. Larsen, Gulin Erdogan, and Douglas B. Grotjahn at the San Diego State University and deals with a long standing problem; the controlled terminal alkene isomerisation to trans-2-alkenes without forming the cis alkene and preventing further isomerisation along the hydrocarbon chain. This new method appeared in JACS.  It employs a a mixture of ruthenium catalysts A and B  in a ratio of  1:5 – 1:2 at the 1mol% level.

This catalyst system achieves this conversion at 95% yield or better and ratios of E/Z >99. As one may imagine it took considerable effort to devise this catalyst system, however this group built upon their previous experience and eventually discovered this system. The fact that it is a mixture is interesting. The authors were not able to isolate pure A but depending upon the conditions always obtained a mixture of A & B.

This startling control of isomerisation is due to the steric properties of the two catalysts and is derived, not from the groups attached to phosphorus, but that of the other ligand to ruthenium as well as the substituents on the 1-alkene.

The authors show a list of results which suggest that this system is well tolerated by functionalised alkenes although I would have liked to see a bit more variety in the functional groups used. Pent-4-en-1-ol is converted to pent-3-en-1ol in 24 hours, 94% yield at 40°C. Once again we have NMR yields quoted, so the real world yields may well be lower, quantitative GC measurements would have been more appropriate to measure yields. Never the less this is another tool in the chemists catalyst tool box.

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