Solution Conformations of Chiral Mannich Bases
Molecular conformation can have significant effects on the properties of a molecule such as its solubility and reactivity. Variable temperature 1H NMR spectroscopy was used to study the solution conformations of two enantiomeric pairs of chiral Mannich bases: N-(2-hydroxylbenzyl)-methylbenzylamine (1-R and 1-S) and N-(2-hydroxylbenzyl)-naphthylethylamine (2-R and 2-S).
These molecules are capable of intramolecular hydrogen bonding under appropriate solvent and temperature conditions. We observed significant chemical shift and line-width changes in the NMR spectra of these molecules as a function of different solvents (CD2Cl2, DMSO-d6, DMF-d7, CDCl3, CD3OD, and D2O), temperature (a range from -50°C to 130°C, depending on the freezing and boiling points of each solvent), and most surprisingly on whether the molecule was the R- or S-enantiomer. These NMR spectrum changes can be attributed to conformational changes and molecular bond rotation that can be hindered by intramolecular hydrogen bonding. The NMR spectrum changes were more extreme for the heavier naphthyl compounds (2-R, 2-S) consistent with more hindered rotation. Preliminary computational chemistry results suggest different molecular conformations for the R- and S-enantiomers in some solvents. It is possible that these results may lead to a method of determining the absolute configuration of chiral amines.
Mannich bases are compounds capable of intramolecular hydrogen bonding between amino and hydroxyl groups forming a six-membered ring under appropriate solvent and temperature conditions. These molecules can undergo conformational change in solution that can have significant effect on their reactivity and chemical properties. There are several factors that can affect molecular conformation. In solution, the solvent forms a cage around a solute that results from favorable interactions. The choice of solvent can play large roles in a molecule’s preferred conformation. These intramolecular and intermolecular interactions are certain to be temperature dependent and may show the greatest effect on molecular conformation at lower temperature where less energy is available for molecular bond rotation. In addition to solvent and temperature effects, stereochemistry may also have significant effect on a molecule’s preferred conformation. We propose to compare variable temperature 1H NMR spectra of enantiomeric pairs of N-(2-hydroxylbenzyl)-methylbenzylamine (1-R and 1-S) and N-(2-hydroxylbenzyl)-naphthylethylamine (2-R and 2-S) under a variety of solvent and temperature conditions to study the conformational changes of these chiral Mannich bases in solution.
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Much of this work has focused on the enantiomeric pairs of N-(2-hydroxylbenzyl)-methylbenzylamine (1-R and 1-S) and N-(2-hydroxylbenzyl)-naphthylethylamine (2-R and 2-S). Based on a secondary amine synthesis method developed by Carlson, et. al, a chiral amine (3 mmol) and aldehyde reactant (3 mmol) were dissolved in 20 mL dichloromethane and 2 equivalents of a weaker reducing agent, sodium triacetoxyborohydride, were added to selectively reduce the imine to the secondary amine.
6. Carlson, M.; Ciszewski, J.; Bhatti, M.; Swanson, W.; Wilson, A. A Simple Secondary Amine Synthesis: Reductive Amination Using Sodium Triacetoxyborohydride. J. Chem. Ed. 2000, 77(2), 270-271.
After neutralizing the reducing agent with 25 mL of 5% sodium bicarbonate aqueous solution, the mixture was washed with water in a separatory funnel three times before using a rotovap to evaporate the solvent. A vacuum pump was used to dry the amines further before collecting 1H NMR spectra of the product.
1H NMR spectra of the chiral amines were collected with an Agilent DD2 500 MHz NMR Spectrometer in the Integrated Molecular Structure Education and Research Center (IMSERC). Room temperature spectra in CDCl3 were used to confirm synthesis of the desired product, but spectra also were collected under a variety of solvent and temperature conditions to study changes in the solution conformations of these molecules. A closed-cycle refrigeration unit and appropriate 1H NMR probe allowed temperature variation from -50 °C to 130 °C depending on the solvent used. Solvents and the temperature ranges of the solvents that were studied were benzene-d6 (26°C), CD2Cl2 (-50°C to 40°C), DMSO-d6 (26°C to 130°C), CDCl3 (-50°C to 40°C), DMF- d7 (-50°C to 130°C), CD3OD (-50°C to 40°C), and D2O (15°C to 80°C). The instrument was manually re-shimmed at each temperature and all spectra are referenced to TMS for chemical shift and linewidth.
Room Temperature 1H NMR Spectroscopy
Although it is expected that enantiomers should have the same NMR spectra because of their identical chemical and physical properties, there were significant differences in the 1H NMR chemical shifts and line-widths for the 1,2-R and 1,2-S pairs of compounds as a function of different solvents and temperatures.
Chemical shift differences and differences in the separation of the diastereotopic methylene proton signals between enantiomers were observed only in the polar protic solvents CD3OD and D2O. There was a consistent trend in which the aromatic and alkyl protons of 1-S were further downfield than the corresponding signals for 1-R. However, the chemical shifts were more downfield for 2-R than for 2-S. Differences in the separation of the diastereotopic methylene (-CH2-) signals, which are observed as two doublets with different chemical shifts in the NMR spectrum, with chirality were also observed at room temperature for1-R and 1-S in CD3OD and D2O as shown in Table 1 below.
The separation of the diastereotopic methylene signals varied with solvent. In CD3OD, the difference in the diastereotopic doublet proton separation was smaller for the 1-S enantiomer than for 1-R. There was an opposite effect in the more polar solvent (D2O) in which the separation between the diastereotopic doublets was larger for 1-S than for 1-R. The separation of the diastereotopic doublet proton signals also was greater for the 2-S enantiomer in CD3OD than for 2-R. The data suggest that both polar protic solvents interact heavily with these molecules and give different solvent cage effects that result in different preferred conformations in solution for the R- and S-enantiomers. There was even greater difference in the behavior of the separation of the diastereotopic methylene signals for the 1-R and 1-S enantiomers in D2O as a function of temperature as described in the variable temperature section of this paper.
Differences in signal line-width and in the behavior of the OH and NH proton signals also were observed for the R- and S- enantiomers at room temperature but only for non-polar solvents. In CDCl3, the alkyl proton peaks of the S-enantiomers were broader and more asymmetric (broader downfield methylene doublet than upfield methylene doublet) than those for the R-enantiomers. These effects were greater for the naphthyl compounds. In addition, the combined OH/NH signal was much broader for the R-enantiomers than for the S-enantiomers with a larger effect for the naphthyl compounds. These effects are consistent with conformational changes that could be caused by a more hindered rotation with the heavier naphthyl substituent.
Significant changes in the NMR behavior of the chiral benzyl and naphthyl compounds in protic and aprotic solvents as a function of temperature are described in the next section.
Variable Temperature NMR Spectroscopy
The observed NMR chemical shift differences between the enantiomers in CD3OD and D2O and the line-width and OH-NH effects observed in CDCl3 at room temperature suggested significant molecular conformational changes dependent on the polarity of the solvents. In this section, variable temperature 1H NMR spectroscopy of 1-R and 1-S and 2-R and 2-S is discussed. Similar trends in these spectroscopic features were observed as a function of temperature for the benzyl and naphthyl products in non-polar solvent CDCl3 and the polar-protic solvents CD3OD and D2O.
1-R and 1-S
Variable temperature 1H NMR spectra are shown for the 1-R and 1-S in CDCl3 in Figures 1-2. Significant changes in chemical shifts and line-widths are seen as temperature changes.
For the diastereotopic methylene protons, the downfield doublet shifted upfield and the upfield doublet shifted downfield with increasing temperature resulting in decreased separation between the doublets. The chemical shift differences for the 1-S diastereotopic protons were greater than those for 1-R at lower temperature but they converged to equal separation at temperatures above 5°C as shown in Figures 1-2. In addition, the methine and methyl shifts of the 1-R and 1-S benzyl products in CDCl3 were different at low temperature and converged with temperature. Chemical shifts for methine and methyl shifted upfield as temperature increased with a slightly larger effect for 1-S than 1-R. The convergence of the chemical shifts of 1-R and 1-S suggests that the two enantiomers have different solution conformations at lower temperature in CDCl3 but similar conformations at room temperature and above.
Significant line-width differences between the NMR spectra of 1-R and 1-S benzyl products were observed in CDCl3 as shown in Figures 1-4.
While slight line-broadening was observed for the methine and methylene proton signals for 1-R at lower temperatures, all alkyl NMR signals were significantly broader for 1-S especially at temperatures lower than -30°C where the methine quartet and methylene doublets are almost unresolved in Figure 2. The line-widths also broaden again at temperatures above 5°C for 1-S, but not for 1-R. Figures 3 and 4 also show asymmetric line broadening for the diastereotopic methylene peaks for both enantiomers at room temperature. The downfield methylene peaks are broader than the upfield methylene peaks at high temperature for both 1-R and 1-S (2.38 Hz versus 1.95 Hz for 1-R at 26°C and 3.15 Hz versus 2.50 Hz for 1-S at 26°C) and at low temperature for only the 1-S enantiomer (5.60 Hz versus 3.85 Hz for 1-S at -30°C). These line-widths suggest differences in the solution conformations of the 1-R and 1-S enantiomers.
Variable temperature 1H NMR spectra of the OH and NH signals for 1-R and 1-S in CDCl3 are shown in Figures 5-6. Significant changes in the behavior of the OH and NH peaks occurred as temperature changed.
At higher temperature, a single very broad peak is observed for the exchanging OH/NH protons with the peak broader and centered more upfield for 1-R than 1-S as described at room temperature. At temperature lower than -30°C, Figure 5 shows a clear separation of the OH (11.8 ppm) and NH (2.1 ppm) proton signals for 1-R as the exchange slows dramatically. This effect is smaller for the 1-S enantiomer in Figure 6 where the OH and NH peaks may separate at -50 °C, but the peaks are not as distinct as those seen in the 1-R spectrum.
2-R and 2-S
The chemical shifts for 2-R and 2-S show smaller differences at low temperature but show similar convergence at higher temperature. The methine and methyl chemical shifts for 2-R and 2-S shifted upfield slightly and the separation of the diastereotopic methylene signals decreased with temperature, similar to behavior observed for 1-R and 1-S. The convergence of the chemical shifts with temperature suggests that 2-R and 2-S also have different solution conformations at lower temperatures in CDCl3 but with smaller differences compared to 1-R and 1-S.
Similar line-broadening behavior was observed for the 2-R and 2-S but with greater effects than 1-R and 1-S. There was greater line broadening of the methine, methylene, and methyl proton peaks for the 2-S enantiomer than the 2-R enantiomer. While the methine and methylene protons for 2-R only broadened with decreasing temperature, the corresponding protons for 2-S also showed broadening effects at temperatures greater than 5°C. The 2-R and 2-S enantiomers also showed different asymmetric line-broadening trends as a function of temperature. While there was no asymmetric line-broadening for 2-R at any temperature, 2-S showed greater broadening of the downfield methylene doublet than the upfield methylene doublet (3.62 Hz versus 2.75 Hz for 2-S at 26°C) at higher temperatures. While these line-width differences suggest solution conformation difference for the chiral naphthyl compounds as a function of temperature, the effects are different from those observed for the benzyl compounds 1-R and 1-S.
The OH/NH signal behaviors for the 2-R and 2-S naphthyl products are qualitatively similar to but larger than those of the benzyl products. At higher temperature, the OH-NH peak is much broader for 2-R than for 2-S. The OH-NH peak for 2-R separated at a slightly higher temperature than for 1-R and sharpened from -30°C to -50°C. The difference in the broadness of the 2-R and 2-S OH-NH peaks as a function of temperature suggests different exchange rates that are likely due to hydrogen bonding. Both the 2-R and 2-S OH-NH peaks were much broader than the 1-R and 1-S signals; this is consistent with greater hindered rotation for the 2-R and 2-S compounds enhancing the conformational differences.
At low temperature, the sharpness of the separated OH and NH peaks suggests that the 2-R compound is in a conformation that does not allow intramolecular hydrogen bonding. A conformation that allows proton exchange requires molecular bond rotation, and the increased mass of the naphthyl substituent in 2-R compared to the benzyl substituent in 1-R could inhibit bond rotation at low temperature. The differences in the sharpness of the separated OH and NH peaks between the 2-R and 2-S enantiomers also suggest different solution conformations at low temperature.
1-R and 1-S
Figures 7-8 show the methine and methyl proton chemical shift temperature behavior for 1-R and 1-S in CD3OD.
Unlike the spectra acquired in CDCl3, chemical shifts for the 1-R and 1-S alkyl protons were different in CD3OD even at room temperature and this difference becomes larger at lower temperature. The observed trends suggest the alkyl proton chemical shifts might converge, but at temperatures above the CD3OD boiling point.
There are significant differences between 1-R and 1-S for the spectra acquired in CD3OD as shown in Figures 9-10.
The chemical shift separation of the diastereotopic methylene peaks for 1-R and 1-S in CD3OD increased with temperature compared to a decrease in this separation with temperature in CDCl3. In addition, the chemical shift separation for the diastereotopic methylene peaks for 1-S is considerably smaller than that for 1-R in CD3OD, whereas it was slightly larger in CDCl3. Even greater differences in the chemical shift trends with temperature were observed in D2O, which will be discussed later.
Line broadening effects are even more different for spectra acquired in CD3OD as shown in Figure 11.
While there is greater line-broadening of the methine and methylene proton signals of 1-S than of 1-R at low temperature, an opposite asymmetric line broadening effect is observed for the methylene protons of the 1-S enantiomer in CD3OD compared to those in CDCl3. In CD3OD, the upfield methylene peaks for 1-S are significantly broader than the downfield methylene peaks (8.90 Hz versus 6.65 Hz at -50°C) at low temperatures as shown in Figure 11. For 1-R, no significant asymmetry in the methylene line broadening is observed.
These results suggest that while CD3OD solvent effects still create different molecular conformation effects for the two enantiomers, these effects seem to be very different for the polar protic solvent CD3OD compared to the effects of the nonpolar solvent CDCl3.
2-R and 2-S
Figures 12-13 show the observed alkyl proton chemical shift trends for 2-R and 2-S in CD3OD.
While the direction of chemical shift changes with temperature is similar to that for 1-R and 1-S, other chemical shift trends are considerably different. As reported previously, alkyl chemical shifts for 2-R are downfield from those of 2-S and this difference becomes larger with decreasing temperature for the methine and methyl protons.
The diastereotopic methylene proton behavior is quite different between the benzyl and naphthyl compounds as shown in Figures 14-15.
Unlike the behavior observed for 1-R and 1-S, the spectral separation of the methylene proton signals for 2-S is larger than that for 2-R at all temperatures and this effect increases as temperature decreases. For 2-R, Figure 14 shows the methylene peaks have merged almost completely at -50°C whereas Figure 15 shows 2-S with clearly separated signals for the methylene protons.
Line-broadening behavior for 2-R and 2-S in CD3OD is similar to that for 1-R and 1-S as shown in Figure 16.
All alkyl peaks broaden with decreasing temperature and this effect is greater for 2-S than 2-R. In addition, the upfield methylene peaks for 2-S are considerably broader than the downfield methylene peaks (4.90 Hz versus 4.77 Hz for 2-R at -50°C and 9.50 Hz versus 9.07 Hz for 2-S at -50°C) and this effect is even greater for the naphthyl compounds than the benzyl compounds. At -50°C, the ratio of the upfield line-widths to the downfield line-widths is 1.74 for the 2-S naphthyl enantiomer compared to a ratio of 1.34 for the 1-S benzyl enantiomer. This asymmetric broadening also seems to be evident for the 2-R enantiomer at low temperatures, but it is difficult to quantify due to significant second order coupling effects as the two methylene peaks merge with decreasing temperature. These line-width differences imply differences in conformation with the heavier naphthyl substituent responsible for more extreme line-broadening effects.
1-R and 1-S
1H NMR spectra also were recorded for 1-R and 1-S in D2O at temperatures from 10 °C to 80 °C. Shimming of these samples was difficult because these compounds were not very soluble in D2O. Consequently, observation of the line-width effects is less reliable for these spectra.
The chemical shift trends for the chiral benzyl products in D2O are significantly different from those in CD3OD and other solvents as shown in Figures 17-18.
While the alkyl proton chemical shifts for 1-S are downfield of those for 1-R as in CD3OD, the differences in these chemical shifts decrease with decreasing temperatures, an effect opposite to that observed in other solvents. While alkyl proton chemical shifts for 1-S in other solvents showed larger temperature effects than those for 1-R, the D2O chemical shift effects as a function of temperature were larger for the 1-R enantiomer. In addition, the 1-R and 1-S enantiomers showed opposite effects for the separation of the methylene protons, a behavior not seen in any other solvent, as shown in Figures 19-20.
For 1-R, the two methylene doublets are almost completely merged at 15°C and their separation increases with increasing temperature. However, for 1-S, the two methylene doublets are separated at 15 °C and merged with increasing temperature. This very different spectral behavior for 1-R and 1-S in D2O as a function of temperature suggests very different conformational changes for these molecules in D2O compared to molecules in other solvents.
Results have shown significant differences in the 1H NMR spectra of the pairs of enantiomeric molecules 1-R, 1-S and 2-R, 2-S as a function of solvent, temperature, and stereochemistry. Significant differences have included changes in the chemical shifts of the alkyl and aromatic protons with chirality, different line-broadening effects with chirality, different OH/NH exchange rates, and the chemical shift separation of the diastereotopic methylene protons. We have postulated that the spectral differences between these molecules probably can be attributed to different molecular conformations of the two enantiomers as a function of temperature and solvent. A more detailed explanation is necessary.
In future work, electronic structure calculations will be used to help understand the solvent and temperature effects on the molecular conformations of the chiral benzyl and naphthyl enantiomers. In addition, the corresponding tertiary amines for 1-R, 1-S, 2-R, and 2-S will be synthesized to provide additional information about the preferred conformations of these chiral amine molecules as a function of solvent and temperature without the possibility of OH/NH exchange. Our results indicate solvent and temperature effects play significant roles in determining the solution conformations of chiral Mannich bases.
Paul Lee (’15) is the recipient of the 2015 Chemistry Department Scholar Award for his senior honors thesis (2015) and placed second place in the best poster competition for the Science and Engineering category at the 2015 Northwestern University Undergraduate Research and Arts Exposition. He plans on pursuing a master’s degree in biomedical engineering at Northwestern University where he will continue to develop his research experiences.