Absolute configurations of chiral herbicides determined from vibrational circular dichroism

✍ Scribed by Jiangtao He; Feng Wang; Prasad L. Polavarapu

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 502 KB
• Volume: 17
• Category: Article
• ISSN: 0899-0042
• DOI: 10.1002/chir.20096

✦ Synopsis

Enantiopure herbicides (+)-2-(4-chloro-2-methylphenoxy) propanoic acid, (+)-1 and (+)-2-(2,4-dichlorophenoxy) propanoic acid, (+)-2 were investigated using vibrational circular dichroism ( VCD). Experimental absorption and VCD spectra of (+)-1 and (+)-2 in CDCl 3 solution in the 2000 -900 cm À1 region were compared with the ab initio predictions of absorption and VCD spectra obtained with density functional theory using the B3LYP/6 -31G* basis set for different conformers of (R)-1 and (R)-2. Due to the intermolecular hydrogen bonding, this comparison did not provide unambiguous conclusions. To eliminate intermolecular hydrogen bonding influence, the two acids 1 and 2 were converted to the corresponding methyl esters, namely, (+)methyl 2-(4-chloro-2-methylphenoxy) propanoate, (+)-3 and (+)-methyl 2-(2,4-dichlorophenoxy) propanoate, (+)-4. The experimental VCD spectra were measured for these esters and ab initio calculations for different conformers of (R)-3 and (R)-4 were carried out. The experimental VCD spectra and corresponding population-weighted theoretical VCD spectra were found to be in excellent agreement, which allowed unambiguous determination of absolute configuration of 3 and 4 as (+)-(R). Since esterification does not invert the configuration, the absolute configuration of the parent acids 1 and 2 is the same as that of corresponding methyl esters. Chirality 17:S1 -S8, 2005.