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 H-1 NMR study at 298 K in CDCl3 and CCl4 has revealed that 4-selenaoctadecanoic acid displays a singlet for the 2- and 3-H-2 protons integrating for four protons, while the corresponding sulfur compound shows the expected doublet of triplets. Only in C6D6, and partly in (CD3)(2)SO, are the signals due to 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 H-1 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 due to the 3-H-2 protons is essentially independent of the temperature, the 2-H-2 proton signal is shifted downfield with decreasing temperature, causing the 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.