I am a fledgling chemist, and today I performed some Grubb's catalyzed (1st generation) alkene metathesis. I wanted a cross product between camphene (4.87 mmol) and 2-methyl-3-buten-1-ol (4.78 mmol). Upon mixing these together in a 1.5 mL centrifuge tube (camphene is soluble in the 2-methyl-3-buten-1-ol) and adding 5 mol% (0.20 mmol) of the catalyst, there was nothing discernible about the reaction, and this seemed to be far too much catalyst.
So I repeated the experiment with much less catalyst (a tiny un-weighed spatula full) and my colorless solution of alkenes turned a brown/tan color. I thought that I had made the cross metathesis product, but I also performed a control experiment in which I only used 2-methyl-3-buten-1-ol and a small amount of the catalyst and the solution still turned the brown/tan color.
I believe as a result of this that I am forming the homodimer (2,5-dimethyl-3-hexene-2,5-diol), and not the cross metathesis product. I tried different ratios of camphene and 2-methyl-3-buten-1-ol, but got the same result. Any and all advice/tips/tricks on how to perform this reaction would be greatly appreciated.
Mr. Foley,
I would not be able to recommend you some approaches to this reaction, mechanisms and s.o., but if you are interested, particularly of the interaction product 2,5-dimethyl-3-hexene-2,5-diol, then please find as attaachment few synthetic schemes and related refs.
I am going to assume you meant 2-methyl-3-buten-2-ol otherwise you may be using the incorrect chemical as 2-methyl-3-buten-1-ol does not dimerise to give 2,5-dimethyl-3-hexene-2,5-diol
Observation of a colour change is insufficient evidence to draw conclusions from. Apart from the indication of a chemical change it is otherwise meaningless, nor does the absence of perceptible change indicate that no reaction has occurred. The most likely candidate is catalyst decomposition.
So, first, obtain more data. Monitor with a more indicative method (GCMS, TLC, NMR etc.).
General notes for the olefin cross metathesis reaction:
It should be rigorously air and moisture free; freeze/pump/thaw your freshly distilled solvents and reagents, flame dry your glassware under vacuum.
When carrying out a cross metathesis reaction there is a lot of precedent and existing data. Often selectivity in cross metathesis is ensured by using a large excess of one of the corresponding reagents. However it is worth knowing that the metathesis reaction is active in equilibrium and that the products of the reaction are also capable of interacting with the active catalyst.
In your particular case you are using two fairly sterically hindered alkenes. Camphene cannot and will not easily dimerise but should react with the catalyst, your trisubstituted product is unlikely to react with the catalyst. Comparatively your methylbutenol will dimerise slowly; however the dimer that is formed will still be somewhat active in the metathesis reaction.
Given the steric hindrance of your reaction, you should switch to Grubb's 2nd generation catalyst and be patient, it may require some heat.
Mr. Foley,
I just saw this post. Hope the advice mentioned has helped you already. However, if it has not and you know spectroscopically that you have a dimer, then there are other ways to reduce the formation of the dimer.
One way is to make the solution extremely dilute. The reactions I carry out with Grubbs' I catalyst is usually in mM concentration or even more dilute sometimes.
Again,if the reaction is slow and needs a night or two for completion, you might also want to add the catalyst in two additions. That has helped our group in the past.
Just a side note on the olefin metathesis using Grubbs' I, you want to run the reaction under static vaccuum to eliminate any ethylene gas that is formed in the reaction.
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