kilomentor | 26 November, 2009 19:04
It has long been known that some chemical reactions that proceed slowly or not at all in solution can become predominant when the reactants are adsorbed on solid adsorbants. This can be dramatically demonstrated if we perform preparative chromatography and leave the material on the prep plate or in the column overnight. Very often you would discover the next day that the anticipated product had at least partially decomposed. Fieser & Fieserís Reagents for Organic Chemistry has numerous entries for reactions on alumina or silica. For alumina, these are Vol.1 pg. 19-20; Vol. 2 pg. 17; Vol.3 pg. 6; Vol. 4 pg. 8;Vol. 6 pg. 16-17; Vol7, pg. 5-7; Vol. 8, pg. 9-13; Vol. 9, pg. 8-11; Vol.14, pg. 20-21;Vol. 16, pg. 16-17; Vol. 18 pg 16-17. For Silica, (also alternatively indexed as silic acid), the entries are Vol. 6, pg. 510; Vol. 9, pg 4`10; Vol. 15, pg 282; and Vol. 18, pg 319. Closer inspection of these references, however, would reveal that they deal mostly with reactions that donít occur in the absence of the solid phase. This it seems to me is misleading for the true potential of the methodology. If presence of a catalytic surface can activate new reaction pathways, logically, it might improve the energetics for processes that already proceed to some extent. One can significantly ask whether a useful augmentation of the product can be delivered through this pathway. Adding alumina or silica gel to an otherwise homogeneous reaction system has the potential to selectively catalyze one out of several competing reactions and lead to an optimization of a process step. Even if the possibility is a long shot iit is easy to test this at the process investigation stage. Supposing that a required transformation is not proceeding either at all or fast enough and if raising the reaction temperature is not an option,( perhaps because the bp of the solvent is limiting,) rather than tossing the trial reaction into the waste, we could try adding alumina or silica and observe. There is no down-side because you are working with what will be discarded. In the test, add as much solid as possible consistent with maintaining stirring. The goal is to see something desirable happen. Economics and material handling problems, which are real issues for such a solid catalyzed protocol, are not an issue unless something promising happens. All that needs to occur is that the rate limiting mechanistic step have its transition state energy reduced by adsorbing an intermediary on the solid; but there must be enough solid to provide adequate sites. Another possible mechanism that would lead to a success is for a reversible reaction to be driven forward by the removal through adsorbtion of the product or a co-product. Filtration and washing are easy isolation steps that completely removed the added adsorbant. The ease of work-up in the event of success are the potential advantages.It might be that one should try adding silica gel and alumina together instead of one or the other. Because they are solids they cannot intermingle on the molecular scale so any catalytic sites on one solid are isolated so that if either or both adsorbants produce any beneficial or deleterious effects, their influences will be additive and one will see that something is occurring. One can then sort out whether one or the other or both are having an influence. If you see nothing then both have been removed from consideration with a single experiment!