APPLICATIONS OF COMPUTATIONAL AND NMR METHODOLOGIES TO STUDY OF HOMOALLYLIC ALCOHOLS DIASTEREOMERS

Through the reducing system InCl₃-Li-DTBB(cat.) in THF at room temperature and in the absence of any additives or anti-caking ligand we have synthesized indium nanoparticles (InNPs) of about 4 nm. The catalyst was employed in the allylation of carbonyl compounds, giving excellent yields of the corresponding homoallylic alcohols. We have established that the reaction products come from a γ-coupling, via a six members cyclic transition state, type Zimmerman-Traxler.¹ Relative the selectivity, the allylation with crotyl bromide of ortho substituted benzaldehydes (e.g. o-NO₂, o-OMe, o-Cl, o-CF₃) showed syn selectivity. With the aim to improve the mentioned selectivity, we synthetized o-ⁱPrO-benzaldehyde, and evaluated the reaction with crotyl bromide and InNPs. The homoallylic alcohol 1-(2-isopropoxyphenyl)-2-methylbut-3-en-1-ol was obtained almost quantitatively after 1h as a mixture of the syn- and anti- isomers. The relationship observed by 1H-RMN was 76:24, but we did not know if the syn-isomer was the dominant because the product has not been reported in the scientific literature. Based on this, and in order to determinate which ones 1H-NMR signals corresponding to each isomer, we started computational theoretical and NMR studies.

The initial conformational analysis was performed using the semiempirical PM3 method, then we work with the B3LYP functional, applying the 6-31+G* basis set and the solvent effect (chloroform) was evaluated with the PCM model as implemented in Gaussian09. So, we found thirteen low-energy conformations for the syn-diastereomer and six low-energy conformations for the anti-diastereomer.

On the other hand, we have carried out NMR experiments such as 1H, 13C, HSQC, to assign the signals of each diastereomer; and experiments such as NOESY, selective NOE, JRes, homo- and hetero-nuclear J-coupled and J-decoupling, to be able to measure coupling constants and establish the structure of each diastereomer.

We appreciate the support received from SGCyT-UNS, CONICET and ANPCyT.

¹ Dorn, V., Chopa. A., Radivoy, G. RSC Advances, 2016, 6, 23798-23803.