Search

• The effect of Eu(hfc) 3 on the hydrogen signal chemical shift of a series of bi-1,2,3-triazoles was studied by 1H NMR. • A signal discrimination in 1H NMR spectra was found, indicating the formation of atropisomers in these molecules. • The rotational barriers around the C5−C5' bond for bi-1,2,3-triazoles were calculated using the DFT (B3LYP) method. • Significant energy barriers (up 34 kcal/mol) a low atropisomer interconversion were found in bi-1,2,3-triazoles. The effect of europium tris 3-(heptafluoropropylhydroxymethylene)-(+)-camphorate on the hydrogen signal chemical shift of a series of bi-1,2,3-triazoles was studied by 1H NMR, displaying a signal discrimination which indicates the formation of atropisomers in these molecules. In addition, the rotational barrier around the C5−C5' bond for bi-1,2,3-triazoles was calculated using the DFT (B3LYP) method with 6-31G** basis set, revealing significant energy barriers (up 34 kcal/mol) at 204–206° dihedral angles and a low atropisomer interconversion. These elements allow a direct method for recognition of chiral bi-1,2,3-triazoles derived from copper-catalyzed azide-alkyne cycloaddition. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The 1,3-dipolar cycloaddition of adamantylidenefulvene (1) with 2equiv of nitrile oxides 2a–d gave 1/1 cycloadducts, 3a–d and 4a–d, as the major products, and four other 1/2 minor cycloadducts 5–8a,b. The ratios of 1/1 cycloadducts 3a–d to 4a–d in THF solution were about 1/1 in the four different nitrile oxides 2a–d studied and microwave was found to accelerate the reactions and enhance their yields. It is noteworthy that the regioselectivity of 3a/4a was enhanced to 71/29 in β-cyclodextrin (β-CD) aqueous solution compared to that of 40/60 in the absence of β-CD. The regioselectivity of 3b/4b was further enhanced to 99/1 when 4-tert-butylphenyl hydroximinoyl chloride (9b) was complexed with β-CD and then proceeded to react with 1; this is in sharp contrast with that of 33/67 in the absence of β-CD. The binding constant of 1·β-CD in acetone-d 6/D2O (1/1) was determined to be 188±9M−1 by 1H NMR titration experiments. The binding mode of 1·β-CD was further determined by ROESY experiment. Furthermore, molecular dynamic simulations were carried out to provide information of the complexation modes of 1·β-CD, 3a·β-CD, 4a·β-CD, 9a·β-CD, and 9b·β-CD. It was found that both steric and electrostatic effects play important roles in determining the regio- and stereochemistry of 1,3-dipolar cycloaddition of 1. Finally, β-CD is shown to serve as a chiral shift reagent to differentiate the enantiomers of 4a in 1H NMR. [Copyright &y& Elsevier]

Abstract: Experimental conditions for determination of enantiomeric composition of 1-substituted 3-aminocarboranes by 1H and 13C NMR spectroscopy using chiral shift reagent Eu(hfc)3 have been found. [Copyright &y& Elsevier]