Tonight I am happy to host Adam Zajdlik, a second year PhD student in my lab, on my blog. Over the past year or so, Adam has been tirelessly pushing the frontiers of boron chemistry. I was hoping that he would share some of the “tricks of the trade” on this page. Because TLC and its significance came up on this blog recently, it is fitting that Adam has his favorite spot visualization method to share. Here is how Adam develops boron-containing molecules in his TLC experiments.
“Curcumin is a diarylheptanoid found in the popular spice turmeric. It is a highly conjugated diketone and its enol form is thermodynamically favored (see inset A below) as it allows electron delocalization across the entire molecule. An interesting feature of Curcumin is its ability to coordinate with boronic acids to form brightly colored boronate conjugates (inset B). During the coordination process, electron density is donated from the curcuminoid pi system to fill boron’s valence shell. This lowers the LUMO of the pi system and leads to a red-shift of its UV/vis absorbance. Boron-coordinated curcuminoids are bright red in color, while curcumin is yellow in the absence of boron.
This visible change in color has enabled some interesting applications. The reaction of curcumin with boric acid to form the corresponding boronate complex (Rosacyanine) is a remarkably favorable process and can be harnessed to detect trace amounts of boric acid. This is a highly sensitive analytical tool and allows quantification of boric acid in slightly alkaline solutions at levels as low as 3 ppm (!). Curcumin is also used as a TLC stain that is selective for boronic acids and their derivatives. Most boronic acid analogues appear as bright red spots on TLC when stained with curcumin including free boronic acids, boronates esters, MIDA boronates and BF3K salts. This is an extremely useful extension of TLC reaction monitoring in the development of novel boron-containing molecules.”