It’s been a while since I blogged about the use of computed spectra to determine the structure or configuration of a compound. Well, here’s a nice example of the use of computed electronic circular dichroism to determine the configuration of a coumarin that displays axial chirality.

Mazzanti and coworkers have synthesized a series of coumarins,1 obtained their ECD and computed their structures, stero-interconversion barriers (at B3LYP/6-31G(d)) and ECD (at TD-DFT/B3LYP/6-311++G(2d,p)//B3LYP/6-31G(d)). I will mention explicitly here just one example, compound 1, which elutes off a chiral column in two mirror image forms, both of which do not stereomutate over time.


1

The computed structure of 1 is shown in Figure 1 and the barrier for steromutation is predicted to be quite large, 35.7 kcal mol-1. This explains the lack of stereomutation. The computed ECD of 1M matches very well with the experimental ECD of the first eluted isomer, making the second eluted isomer 1P.

1

Figure 1. B3LYP/6-31G(d) optimized structure of 1.

References

(1) Lunazzi, L.; Mancinelli, M.; Mazzanti, A.; Pierini, M., "Stereomutation of Axially Chiral Aryl Coumarins," J. Org. Chem., 2010, ASAP, DOI: 10.1021/jo101261k

InChIs

1 (6-isopropyl-4-(2-methyl-1-naphthyl)chromen-2-one):
InChI=1/C23H20O2/c1-14(2)17-10-11-21-19(12-17)20(13-22(24)25-21)23-15(3)8-9-16-6-4-5-7-18(16)23/h4-14H,1-3H3
InChIKey=OEQFRNPVUJVJFO-UHFFFAOYAS