Kwon, Y., Lee, S., Oh, D.-C. and Kim, S. (2011), Simple Determination of Double-Bond Positions in Long-Chain Olefins by Cross-Metathesis. Angewandte Chemie International Edition, 50: 8275–8278. doi: 10.1002/anie.201102634
The accurate determination of the double-bond position in unsaturated long-chain compounds remains a challenging task. Current analytical techniques rely on analyzing fragment ions by mass spectrometry, but conventional methods of ionization such as EI and CI are not very reliable. More detailed and reliable information can be obtained after derivatization of the double bond. Transformations used include ozonolysis, epoxidation, dihydroxylation, alkylthiolation and methoxymercuration. With the products of these reactions, usually a second derivatization is necessary to make them suitable for GC- or LC-MS. In addition, these reactions are not very tolerant of the presence of other functional groups.
Cross Metathesis (CM) of long chain olefins with a simple cross partner offers a straightforward way to determine the double bond position. Instead of looking at the fragment ions of the parent molecule by GC-MS, the double bond is reacted with another olefin. The position of the double bond then can be simply deduced from the mass of the cross products. In addition, the CM reaction is very mild and can tolerate different functional groups in the molecule.
In one set of reactions, using methyl acrylate as the cross partner and the second generation Hoveyda-Grubbs catalyst was found to be optimal for LC-MS analysis. The starting materials for these reactions are easily available, can be handled on the bench, they don’t yield a mixture of double bond isomers and the α,β-conjugated double bond is perfect for detection with the common diode-array UV detectors. The procedure is pretty simple: just dissolve the target molecule in DCM/ethyl acrylate, add catalyst, stir for 2-3h, inject the mixture into a LC-MS and analyze the data. The example with elaidic acid (trans-9-Octadecenoic acid) clearly shows that the double bond is at the C-9 position by looking at the mass of the cross product.
Acids with two double bonds gave only the cross product with the first double bond (from the carboxylic terminus) with the second generation Hoveyda-Grubbs catalyst, but with the second generation Grubbs catalyst a second major peak was observed corresponding to the product of the second double bond.
The second set of reactions, using 2-methyl-2-butene as the cross partner and the second generation Grubbs catalyst, was used for GC-MS analysis. The procedure is similar to the one used for the LC-MS. In this case the catalyst has to be removed before injection into the GC-MS. Only one of the cross products, the 2-methyl-2-undecene is seen by GC-MS without derivatization, but it’s enough to determine the position of the double bond. The second product, the 9-decenoic acid, can be seen after conversion into the methyl ester.
This set of conditions wasn’t tested with molecules with more than one double bond; it’s interesting to see if the position of a second double bond can be determined. This method can be useful to check for double bond isomerization as well.
The method is very simple and can be applied to numerous double bond containing compounds, and can become a valuable tool in analytical chemistry.