Unambiguous assignment of trifluoromethylpyrazole regioisomers by 19F-15N correlation spectroscopy.
Analysis of the 19F chemical shifts of trifluoromethylpyrazole regioisomers has shown that while chemical shift is in general a reliable predictor of regiochemistry in this series, there is a narrow chemical shift range in which the two isomers overlap and the regiochemistry cannot be assigned with certainty. We have examined the usage of 19F--15N correlation spectroscopy as a method to provide a second unambiguous confirmation of regiochemistry of 3- and 5-trifluoromethylpyrazole regioisomers. In the case of 3-trifluoromethyl analogs, one expects a three-bond coupling to the pyridine type nitrogen (N-2). In the case of the 5-trifluoromethyl pyrazole the situation is exactly reversed, with the fluorines of the trifluoromethyl moiety being 3-bonds from the pyrrole type nitrogen (N-1). We have observed that 3-trifluromethyl analogs follow the expected pattern of readily observable 3-bond coupling. However, 19F--15N couplings in 5-trifluromethyl analogs do not follow the normal pattern of 3-bond coupling. Using this information we have been able to develop unambiguous methods to distinguish the isomers.
Kline
Mike
M
DuPont Crop Protection, Stine-Haskell Research Center, Building S315/1311, Newark, DE 19714, USA.
Cheatham
Steve
S
D005462:Fluorine Radioisotopes; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D015394:Molecular Structure; D009587:Nitrogen Isotopes; D011720:Pyrazoles / Q000737:chemistry*; D013237:Stereoisomerism
D005462:Fluorine Radioisotopes; D009587:Nitrogen Isotopes; D011720:Pyrazoles