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Unambiguous assignment of trifluoromethylpyrazole regioisomers by 19 F– 15 N correlation spectroscopy
Author(s) -
Kline Mike,
Cheatham Steve
Publication year - 2007
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1919
Subject(s) - chemistry , structural isomer , regioselectivity , moiety , trifluoromethyl , pyridine , chemical shift , pyrrole , stereochemistry , computational chemistry , nuclear magnetic resonance spectroscopy , organic chemistry , alkyl , catalysis
Analysis of the 19 F 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 19 F 15 N 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, 19 F 15 N 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. Copyright © 2007 John Wiley & Sons, Ltd.