Effects of protonation and co‐anion on the 1H and 13C chemical shifts of 2r,6c‐ diphenylpiperidin‐4‐ones: evidence for the formation of ion pair

Abstract

(1)H and (13)C NMR spectra have been recorded for 2r,6c-diphenylpiperidin-4-one (1a), 3t-alkyl-2r,6c-diphenylpiperidin-4-ones 1b-d, 3t-alkyl-2r,6c-diphenyl-4-oxopiperidinium nitrates 2b and 2d, 3t-alkyl-2r,6c-diphenylpiperidin-4-one hydrochlorides 3a-c and 3t-methyl-2r,6c-diphenyl-4-oxopiperidinium picrate 4b in DMSO-d(6). For 1b, 2b and 3b, (1)H and (13)C NMR spectra have been recorded in CH(3)OD also. For 1b, 1d, 2b, 2d, 3b and 4b, 2D spectra have also been recorded. In DMSO-d(6) the protons of the piperidine ring and the ortho protons of the phenyl groups are markedly deshielded due to protonation. Protonation shields all the carbons of the piperidine ring and the ipso carbons of the phenyl groups markedly but deshields the other aromatic carbons slightly. The deshieldings on H-3a, H-5a and the ortho protons of the phenyl groups are less in the nitrate and picrate than in the corresponding hydrochloride. The effects on (13)C chemical shifts are not influenced by the co-anion. These observations suggest that the nitrates and pictrate exist as ion pairs in DMSO-d(6) and the nitrate and picrate ions shield, H-3a, H-5a and the ortho protons by magnetic anistropic effect. In CH(3)OD for 2b and 3b in addition to the ion-pair containing free ions two ion-pairs containing solvated ions are also present. The effects of protonation in the ion pairs containing the solvated ions are significantly different from those in the ion-pair containing free ions.