Preparation and NMR studies of 3-selenaheptadecanoic and 4-selenaoctadecanoic acid, RSe(CH2)nC(O)OH (R = C14H29, n = 1 and 2), and their methyl esters

The synthesis of the title compounds and their precursor, ditetradecyl diselenide, R₂Se₂, in high yield and of high purity is described. A 600 MHz ¹H NMR study at 298 K in CDCl₃ and CCl₄ has revealed that 4-selenaoctadecanoic acid displays a singlet for the 2- and 3-H₂ protons integrating for four protons, while the corresponding sulfur compound shows the expected doublet of triplets. Only in C₆D₆, and partly in (CD₃)₂SO, are the signals dueto these protons sufficiently separated to give two triplets. No obvious relationship between the form of the spectra and any of the usual solvent parameters could be found. The particular form of the ¹H NMR spectrum in CDCl₃ and CCl₄ was found to be independent of the concentration, i.e. the monomer:dimer ratio. A temperature study has revealed that while the signal dueto the 3-H₂ protons is essentially independent of the temperature, the 2-H-2 proton signal is shifted downfield with decreasing temperature, causingthe close-lying signals to be indistinguishable in the 290-310 K range. NMR spectra of the carboxylic OH group and IR spectra of the carbonyl group of dilute solutions of 3-selena- and 3-thiaheptadecanoic acids in CCl₄ suggest that the monomers of these acids exist predominantly as 4-membered cyclic species with Se(S)... O=C or Se(S)... O(H) intramolecular interactions