Oxygen-17 NMR spectroscopy: evaluation of intra- and intermolecular hydrogen bonding of phenols to carbonyl groups

Abstract

NMR spectroscopy1 is an important method for the assessment of structural problems in organic chemistry,2 especially hydrogen-bonding phenomena.3 45Previous studies have shown that 170 NMR carbonyl data are sensitive to torsion-angle variations in aryl systems.2,4 In rigid, planar systems, 170 NMR methodology has been employed to detect lone-pair distortions (in-plane structural deformations).2,8 170 NMR data for intramolecular hydrogen-bonded aminoand amido-substituted acetophenones have been factored into electronic, torsional, and hydrogen-bonding components.6 170 NMR methodology has been shown7 8to be sensitive to the geometry as well as the relative p/Cas of intramolecular hydrogen-bonded N-H groups in rigid carbonyl systems. We report here an 170 NMR study of intraand intermolecular hydrogen bonding of phenols to ketone carbonyl groups.The 170 NMR chemical shift data for a series of intramolecular hydrogen-bonded phenols containing carbonyl groups, 1-10, in acetonitrile at 75 °C are listed in Table I. The signals for the intramolecular hydrogen-bonded carbonyl groups in 1-10 are shifted upfield.3,6,8 Data for several of the compounds listed in Table I have been reported previously.8,9 However, the 170 data were obtained under varied conditions such that quantitative analysis was difficult. In addition, the influence of electronic and torsional effects were not included in the determinations of the hydrogen-bonding components of the observed shielding effects.8,9 The contributions of intramolecular hydrogen bonding ( 6 ß) to the chemical shifts for the carbonyl groups in 1-10 ranged from 39 to 67 ppm after correction6 for electronic and torsional factors. The average 5 ß for one intramolecular hydrogen bond in these systems was determined to be 51 ± 7 ppm based upon the assumptions that the pKas of all the phenols are similar and that the basicities of the carbonyl groups are comparable.Proton coupling to the phenol oxygen is observed directly in six of the examples listed in Table I. Representative 170 NMR spectra for the OH signals ( -coupled and -decoupled) for 2,-hydroxyacetophenone are shown in Figure 1. This type of coupling can only occur when intramolecular proton transfer takes place without spin (1) (a)

A study of the 17O carbonyl chemical shifts in substituted benzaldehydes

“…Based on an analysis of 17 O carbonyl chemical shifts for substituted benzaldehydes, the following equation is proposed. …”

“…According to the study by Boykin et al, 17 in a polar solvent such as acetonitrile there are strong intermolecular hydrogen-bonding effects between solvent and solute molecules, which substantially inhibit hydrogen-bonding effects between solute molecules. Hence the shielding effects of intermolecular hydrogen bonds between solute molecules on the 17 O carbonyl chemical shifts are reduced or eliminated. However, in a non-polar solvent such as toluene, the intermolecular hydrogen-bonding effects between solute molecules will produce upfield effects on the 17 O carbonyl chemical shifts.…”

“…Substituted benzaldehydes as typical aromatic aldehydes have been extensively explored by 17 O NMR techniques. 1 -13 Steric effects and hydrogen-bonding effects play important roles in determining 17 O chemical shifts.…”

“…Methods used for methyl phenyl ether and chained aliphatic ethers, etc., are extended to substituted benzaldehydes. 14 -16 In this work, literature data for 17 O carbonyl chemical shifts of substituted benzaldehydes were used to investigate the roles of substituent, steric and hydrogen-bonding effects. An empirical equation is developed to estimate 17 O chemical shifts.…”

17O NMR spectroscopy: Proton–oxygen coupling in simple alcohols at natural abundance

“…A recent review describes the factors that influence the I7O NMR chemical shifts of alcohols [5]. Prior to the first direct observation of proton to oxygen coupling [6] in intramolecular hydrogen-bonded phenolic ketones, the only previous recognition of proton to oxygen coupling resulted from studies on "0-enriched water "71 and line shape analysis of I7O…”