A new way to study reaction mechanisms

Readers of Anslyn and Dougherty can’t help but come away impressed with how much ingenuity goes into the determination of organic reaction mechanisms.  Here’s an entirely new tool for this — Desorption electrospray ionization (DESI) coupled to high resolution Orbitrap mass spectrometry. [ Proc. Natl. Acad. Sci. vol. 109 pp. 2186 – 2187, 2246 – 2250 ’12 ] 

Quite a mouthful, but here’s how it works.  First put a hydrogenation precatalyst — RuCl2(p-cymeme)2 on a surface, then spray the molecule you want (aminoindanol) in a solution (methanol with or without KOH) onto it.  The surface is within .5 milliMeters from the mass spectrometer inlet.  So the reaction proceeds for only a few milliSeconds as the secondary drops travels through the inlet until the spectromer causes the formation of gas phase ions.  The mass spectrometer can show you what ions are in the drops, giving you some idea as to the intermediates formed.  

       If my transitional metal organic chemistry were stronger, I’d be better able to appreciate what they found, but as is usual in true scientific work, they found all sorts of intermediates they didn’t expect.  Another strength of the technique is that they discovered all sorts of side reactions as well.  As the authors note in studies of reaction mechanisms “as concentration dependence is studied in more detail, observations appear in conflict with initially assumed reaction mechanisms, which of course confuses.  This is almost always because one or several parallel reaction paths have been ignored.”  
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  • luysii  On February 22, 2012 at 12:02 pm

    For an even more incredible technique — effectively using infrared spectroscopy on the 10,000 or so ions of a catalyst (3 amino acids) substrate (a biphenyl) to show conformational change. Actually the absorbance isn’t measured directly, but by the dissociation of D2 ions from the complex. I’m not sure I believe it. It seems pretty indirect. For details see Science vol. 335 pp. 668 – 669 ’12.

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