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. Author manuscript; available in PMC: 2024 Mar 22.
Published in final edited form as: J Am Soc Mass Spectrom. 2022 Oct 21;33(11):2120–2128. doi: 10.1021/jasms.2c00210

Unexpected Gas-Phase Nitrogen-Oxygen Smiles Rearrangement: Collision-induced Dissociation of Deprotonated 2-(N-Methylanilino)ethanol and Morpholinylbenzoic Acid Derivatives

Yuxue Liang 1, Yamil Simón-Manso 1, Pedatsur Neta 1, Stephen E Stein 1
PMCID: PMC10959088  NIHMSID: NIHMS1856726  PMID: 36269933

Abstract

A nitrogen-oxygen Smiles rearrangement was reported to occur after collisional activation of the PhN(R)CH2CH2O- (R = alkyl) anion, which undergoes a five-membered ring rearrangement to form a phenoxideion C6H5O-. When R = H, such a Smiles rearrangement is unlikely since the negative charge is more favorably located on the nitrogen atom than the oxygen atom, hence alternative neutral losses dominate the fragmentation. For example, collisional activation of deprotonated 2-anilinoethanol (PhN-CH2CH2OH) leads to the formation of an anilide anion (C6H5NH-, m/z 92) rather than a phenoxideion (C6H5O-, m/z 93.0343). However, when the amino hydrogen of 2-anilinoethanol is substituted by a methyl group, i.e., 2-(N-methylanilino)ethanol, a Smiles rearrangement does occur, leading to the phenoxide ion, as the negative charge can only reside on the oxygen atom. To confirm the Smiles rearrangement mechanism, 2-(N-methylanilino)ethanol-18O was synthesized and subjected to collisional activation, leading to an intense peak at m/z 95.0385, which corresponds to the 18O phenoxideion ([C6H518O]-). The abundance of the phenoxideion is sensitive to substituents on the N atom, as demonstrated by the observation that an ethyl substituent results in the rearrangemention with a much lower abundance. The nitrogen-oxygen Smiles rearrangement also occurs for various morpholinylbenzoic acid derivatives with a multistep mechanism, where the phenoxideion is found to be predominantly formed after loss of CO2, proton transfers, breaking of the morpholine ring, and Smiles rearrangement. The Smiles mechanism is also supported by density functional theory calculations and other observations.

For Table of Contents Use Only

Nitrogen-oxygen Smiles rearrangement for fragmentation of deprotonated 4-(4-morpholinyl)benzoic acid.

graphic file with name nihms-1856726-f0013.jpg

Introduction:

High performance liquid chromatography in conjunction with electrospray tandem mass spectrometry (HPLC-MS/MS) is an excellent analytical technique for detection and quantification of chemical species. Mass spectral libraries are employed for fast and reliable identification of unknown compounds in complex samples, and they are constantly updated by adding new spectra from a variety of fragmentation techniques. For example, the NIST MS/MS library includes reference mass spectral data for the identification of various chemical species by electrospray ionization (ESI). The latest version (2020) includes 1.3 × 106 spectra from 31 × 103 chemical compounds [1]. We have been expanding the library by acquiring the tandem mass spectra of authentic compounds with higher-energy collision dissociation (HCD) andion trap (IT) fragmentation techniques. During the spectrum evaluation process, the Smiles rearrangement caught our attention since NIST MS interpreter software [2] that applies general fragmentation rules to interpret spectra cannot explain the production of the Smiles rearrangement. Our lab has been studying the unexpected peaks in the reference spectrum to ensure the peaks originated from the authentic precursor with a reasonable mechanism, thus enhancing the spectrum quality. Additionally, new fragmentation rules may also help to improve the performance of the NIST MS interpreter software.

The gas phase Smiles rearrangement is an intramolecular nucleophilic aromatic substitution reaction and has been well documented over the past decades [3, 4], a classic Smiles rearrangement is shown in Scheme 1, where X represents a good leaving group and Y represents a strong nucleophile. However, there are only a dozen papers published on gas-phase Smiles reaction in mass spectrometry. Bowie and collaborators employed a heavy atom labeling approach to study the collision-induced dissociation (CID) mass spectra of the anion PhX(CH2CH2)nY-, where X = Y = O or X = S, Y = O [3,4, 5–7]. When n = 2, Smiles intermediates are exclusively involved in the formation of PhO- or PhS- fragmention and the ratio of Smiles rearrangement to SNi reaction decreases as n increases. Smiles rearrangement does not occur with PhO(CH2)nS- (n = 2 to 5) since the sulfur atom is not a good nucleophile and the effect of ring substitution has been also studied in a subsequent published paper [3]. Recently, we studied the CID fragmentation of deprotonated N-acyl aromatic sulfonamides and found it to occur via nitrogen-oxygen and Smiles-type rearrangements, in which the amide oxygen atom of the N-acyl aromatic sulfonamide attacks the aromatic ring at the ipso position to the sulfonyl to form a Smiles intermediate. The mechanism was supported by a heavy atom labeling study and density functional theory calculations [8]. In electron ionization, a fragmentation mechanism of acylarylsulfonamide radical cations has been studied by Irikura and Todua and concluded that it follows a Smiles-type rearrangement reaction [9].

Scheme 1.

Scheme 1.

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Classic Smiles rearrangement.

As mentioned above, during the Smiles rearrangement an oxygen atom can replace another oxygen atom or sulfur which is attached to benzene ring. However, to the best of our knowledge, there are no reports showing that the oxygen atom can replace a nitrogen atom via a Smiles rearrangement to form phenoxide ion, such as X = NR and Y = O in the anion PhX(CH2CH2)nY-. In this paper, we show that deprotonated 2-(N-methylanilino)ethanol 2 and its derivatives undergo nitrogen-oxygen Smiles rearrangement to yield phenoxide ion, which is not observed from CID spectra of deprotonated 2-anilinoethanol 1 (Scheme 2). 2-(N-Methylanilino)ethanol-18O 2a was synthesized and subjected to measurement on an Orbitrap Lumos mass spectrometer to confirm the Smiles rearrangement mechanism. This novel finding also applies to fragmentation of morpholinylbenzoic acid derivatives where a Smiles rearrangement occurs after loss of CO2, proton transfers and breaking of the morpholine ring.

Scheme 2.

Scheme 2.

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Structures of 2-anilinoethanol, 4-(4-morpholinyl)benzoic acid and their derivatives

Experimental

Materials

Water and acetonitrile (HPLC grade) were purchased from Honeywell - Burdick & Jackson (Muskegon, MI, USA). Formic acid solution (50 % in water, v/v) was purchased from Fluka (Charlotte, NC, USA). Thionyl chloride, lithium aluminum hydride solution (1.0 mol/L in tetrahydrofuran), water-18O, 2-(methyl(phenyl)amino)acetic acid hydrochloride, 2-anilinoethanol, 2-(N-methylanilino)ethanol, 2-(N-ethylanilino)ethanol, 3-[methyl(phenyl)amino]propan-1-ol, 4-[methyl(phenyl)amino]butan-1-ol, 4-[(2-hydroxyethyl)(methyl)amino]benzoic acid, 4-(4-morpholinyl)benzoic acid, 3-bromo-4-(4-morpholinyl)benzoic acid and 2-fluoro-4-(4-morpholinyl)benzoic acid were purchased from Sigma-Aldrich (St. Louis, MO, USA). 2-(N-Methylanilino)ethanol-18O was synthesized from 2-(N-methylanilino)acetic acid (Scheme 3).

Scheme 3.

Scheme 3.

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Synthesis of 2-(N-methylanilino)ethanol-18O

Preparation of 2-(N-methylanilino)ethanol-18O

2-(N-Methylanilino)acetic acid hydrochloride (16 mg) was added to thionyl chloride (100 µL) and stirred at 40 °C for 4 h, the solution was cooled down to room temperature and the excess thionyl chloride was removed in vacuo. Anhydrous tetrahydrofuran (20 µL) and H218O (10 µl) was added to the residue and the mixture was stirred at room temperature for 2 h. Removal of the solvent yielded a residue which was dried in vacuo. The resulting yellow oil was dissolved in anhydrous tetrahydrofuran (40 µL) and added dropwise at 0 ℃ over a 5-min period to a stirring suspension of lithium aluminum hydride (8.0 mg) in anhydrous tetrahydrofuran (200 µL). The mixture was heated at 50 ℃ for 3 h and cooled to 10 °C, aqueous sodium sulfate (saturated, 20 µL) was added, and the mixture was dried and diluted prior to analysis on an Orbitrap mass spectrometer.

Mass Spectrometry

2-(N-methylanilino)ethanol 2 and analogous compounds were dissolved in water/acetonitrile/ammonium hydroxide (50:50:0.1, v:v:v) at a concentration of about 0.1 g/L and measured on an Orbitrap Fusion Lumos mass spectrometer via a nano electrospray source. HCD and FT-IT (ion trap) spectra were acquired in the negative mode. All morpholinyl benzoic acid solutions were prepared by dissolving them into acetonitrile/water/formic acid (50:50:0.1, v:v:v) at a concentration of about 0.1 g/L. The HCD spectra were analyzed by using variable collision energies andion trap spectra were acquired at a normalized collision energy (NCE) of 35 [8].

Computational methods

The potential energy surface (PES) of deprotonated 1, 2, 3 and 4-(4-morpholinyl)benzoic acid 7, was explored first performing relaxed scans using the semiempirical method PM3 [10–12] regarding potential fragmentation mechanisms for the Smiles rearrangement reaction. Although this information was not explicitly used in the article, it provided a fast way for identifying stationary points, particularly minima. Density functional theory (DFT) calculations were performed on the mechanisms consistent with the experimental observations. The hybrid density functional method B3LYP [13, 14] was used in conjunction with Pople’s basis sets [6–311++g(d,p)] [15] as implemented in Gaussian 09 [16]. All optimized structures, minima and transition states were confirmed by frequency analysis. The connection between transition states and local minima was verified by using intrinsic reaction coordinate (IRC) calculations [17]. This type of calculations has been used in previous studies ofion fragmentation of small molecules under CID conditions [8, 18–20].

Results and discussion

Comparison of mass spectra of 2-anilinoethanol 1 with 2-(N-methylanilino)ethanol 2

As shown in the Scheme 1, Smiles rearrangement proceeds with a five-membered ring intermediate, generally X is a good leaving group and Y is a strong nucleophile. In comparison with good leaving groups such as oxygen and sulfur, nitrogen is a relatively poor one. Therefore, the displacement reactions reported in this paper are somewhat unexpected. For example, deprotonated 2-anilinoethanol 1 (Scheme 2), at m/z 136, undergoes losses of H2O and H2 to form anion at m/z 116 (Figure 1a). Alternatively, the precursor may lose formaldehyde to yield the N-methylanilide [C6H5NCH3]- at m/z 106, which is the most abundant peak. In the lower mass range, a pronounced peak at m/z 92, which corresponds to the anilide anion ([C6H5NH]-) was observed, along with a barely observable peak at m/z 93.0343 that represents a phenoxide ion([C6H5O]-) as confirmed by high accuracy spectrum. This scenario indicates that the neutral losses and the NH-CH2 bond breaking dominate in the CID of deprotonated 1, whereas the Smiles rearrangement has a very minor contribution, that is because the negative charge is more favorably located on the nitrogen atom than the oxygen atom, thus alternative neutral losses dominate the fragmentation. When the hydrogen atom in the NH group of aniline is substituted by a methyl group, i.e., 2-(N-methylanilino)ethanol 2, the high resolution CID spectrum shows the phenoxideion of m/z 93.0343 as a highly intense peak while there is only a minor anilide peak at m/z 92 (Figure 1b). The abundances of the phenoxide and anilide anions are approximately 40 % and 1 %, respectively.

Figure 1.

Figure 1.

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a) Tandem mass spectrum of deprotonated 2-anilinoethanol 1 (ion trap, precursor m/z 136, top), b) Tandem mass spectrum of deprotonated 2-(N-methylanilino)ethanol 2 (ion trap, precursor m/z 150, bottom).

Apparently, the Smiles rearrangement pathway predominates in the collisional activation of the deprotonate 2. In addition to those two peaks, other distinctive production peaks were also observed in the ESI tandem mass spectrum. The most intense peak is at m/z 106 ([C6H5NCH3]-) after loss of C2H4O. Small peaks are observed at m/z 122 ([C7H8NO]-) after loss of C2H4 and at m/z 133 ([C8H7NO]-) after loss of H2 and CH3 radical.

The proposed Smiles rearrangement mechanism for the formation of the phenoxideion is exhibited in Scheme 4, the alkyl oxygen anion attacks the aniline group at the ipso position via a five-membered ring transition state, displacing a molecule of C3H7N to generate the phenoxideion at m/z 93.

Scheme 4.

Scheme 4.

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Proposed nitrogen-oxygen Smiles rearrangement mechanism for the formation of the phenoxideion at m/z 93.

Tandem mass spectrum of 2-(N-methylanilino)ethanol-18O

In order to corroborate the Smiles mechanism that the oxygen atom of the observed phenoxideion is originated from the alkoxide group and not from solvent or collision gas [21–22], we synthesized 2-(N-methylanilino)ethanol-18O 2a. High resolution spectrum of the deprotonated 2a shows an intense peak at m/z 95.0384 which corresponds to an 18O-phenoxideion with a relative abundance of 41 % (Figure 2). The peak intensity of 18O-phenoxideion is comparable to that of the phenoxideion in the spectrum of deprotonated 2. The isotopic labeling experiment confirms that the oxygen atom in the phenoxide production originates from the oxygen atom of the precursor anion, thus supporting the Smiles rearrangement mechanism. The peak at m/z 135.0537 was also observed with relatively low intensity and its m/z value is shifted two mass units upward compared with theion of m/z 133 in the spectrum of deprotonated 2. High resolution spectrum confirms the formula of theion of m/z 135.0537 as [C8H7N18O]-, which is an isotopic form of the m/z 133ion corresponding to loss of H2 and CH3 radical. The peak at m/z 106.0659 present in the 18O labeling spectrum is identical to that observed with non-labeled compound since it is assigned as N-methylanilide [C6H5NCH3]- containing no oxygen atom.

Figure 2.

Figure 2.

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Tandem mass spectrum of deprotonated 2-(N-methylanilino)ethanol-18O 2a (ion trap, precursor m/z 152)

Tandem mass spectra of other analogs

As discussed above, a methyl group that is attached to the nitrogen atom in 2 is critical for the Smiles rearrangement to occur. We also investigated the spectra of 2-(N-ethylanilino)ethanol 3. The deprotonated compound 3 undergoes the Smiles rearrangement under collisional activation to form the phenoxideion at m/z 93.0343, but with a much lower abundance (6 %) than that of compound 2 (40 %). Non-Smiles rearrangement peak at m/z 92.0503 was also observed with an intensity of 3 %. The low contribution of the Smiles rearrangement in compound 3 is due to a competing process, where the loss of ethylene to form the peak at m/z 136.0762 ([C8H10NO]-) is the dominant pathway. Additionally, the precursor undergoes losses of H2O and H2 to produce theion of m/z 144.0813 ([C10H10N]-), and loss of formaldehyde to form theion of m/z 134.0969 ([C9H12N]-).

The effect of alcohol chain length on the Smiles rearrangement has also been studied. 2-(N-methylanilino)propanol 4 and 2-(N-methylanilino)butanol 5 were chosen for this study. It is assumed that if the Smiles rearrangement could take place on those two compounds, the former compound 4 should proceed through a six-membered ring and the latter 5 via a seven-membered ring. However, both of CID spectra show two major peaks at m/z 106 and 92, which correspond to a N-methylanilide [C6H5NCH3]- and an anilide anion, respectively. Since no phenoxideion of m/z 93 was observed, we conclude that the Smiles rearrangement does not occur for compounds which have a longer alcohol chain (>2 carbons) than the compound 2.

A carboxyl group was introduced to the benzene ring of 2 to investigate the substituent effect. It is facile for 4-[(2-hydroxyethyl)(methyl)amino]benzoic acid 6 to lose a proton to generate gaseous anions under ESI condition. The spectrum of deprotonated compound 6 exhibits the most intense peak at m/z 150 which represents the loss of CO2, which is a very common fragmentation pathway for benzoic acids in negative mode. The Smiles rearrangement takes place as a phenoxideion peak at m/z 93 was observed with a relative intensity of 7 %. The peak at m/z 106, which can be found in the spectrum of 2 as the most intense peak and assigned as [C6H5NCH3]-, was also seen with a relative intensity of 13 %. Since loss of CO2 is facile, we presumed that the peaks at m/z 106 and 93 originated from the CO2 loss peak at m/z 150. MS3 experiment was conducted to validate the assumption. The anion of m/z 150 was selected to further fragment inion trap to generate a MS3 spectrum, which displayed the peak at m/z 106 as the base peak and the phenoxideion peak with a relative intensity of 27 %.

Tandem mass spectra of various 4-(4-morpholinyl)benzoic acid derivatives

Motivated by the fact that the deprotonated 4-[(2-hydroxyethyl)(methyl)amino]benzoic acid 6 which bears an open chain is able to fragment via the Smiles rearrangement to form the phenoxide ion, we considered whether a closed ring aminobenzoic acid could achieve the Smiles rearrangement as well. 4-(4-Morpholinyl)benzoic acid 7 was selected as a model. HCD mass spectrum of deprotonated 7 was recorded at a normalized collision energy (NCE) of 27. We were surprised to find a very strong Smiles rearrangement occurrence, the phenoxideion of m/z 93 appears as the base peak as shown in Figure 3. The peak at m/z 162 corresponds to loss of CO2 and the peak at m/z 118 arises from losses of CO2 and C2H4O.

Figure 3.

Figure 3.

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Tandem mass spectrum of deprotonated 4-(4-morpholinyl)benzoic acid (HCD, normalized collision energy (NCE) at 27, precursor m/z 206).

In order to investigate the detailed fragmentation mechanism of deprotonated 4-(4-morpholinyl)benzoic acid, we plottedion abundance as a function of collision energy based on the HCD tandem mass spectra. 2-Fluoro-4-(4-morpholinyl)benzoic acid 8 is selected to study since its spectrum provides the highest abundance of Smiles rearrangement anion (98 %) and its HCD spectra afford sufficient product ions at higher energies.

Figure 4 shows the dependence of the relative intensities of the main peaks in the spectrum of deprotonated 2-fluoro-4-(4-morpholinyl)benzoic acid 8 on HCD collision energies. When normalized collision energies (NCE) are raised, the intensity of the precursor peak at m/z 224 drops quickly until it practically disappears at NCE = 27. It seems the first production to appear at low collision energies is the Smiles rearrangemention at m/z 111 with a formula [FC6H4O]-, its abundance quickly reaches its maximum level of 88 % at NCE = 20 and remains the most abundantion across most of the energy range. This phenomenon that the 3-fluorophenoxideion at m/z 111 dominates in the spectrum of 8 indicates that the Smiles rearrangement proceeds rapidly and is energetically favorable. A non-Smiles rearrangement peak at m/z 136 which is ascribed to losses of CO2 and C2H4O from the precursorion and assigned as a formula [C8H7FN]- appears at NCE = 9 and its intensity reaches its maximum level of 11 % at NCE = 35. Another non-Smiles rearrangemention at m/z 124 ascribed to [C7H7 FN]- appears at NCE = 20 and reaches the apex of 6 % at NCE = 54. When the collision energy reaches 54, the abundance of 3-fluorophenoxideion at m/z 111 decreases to 67 % and two lower mass peaks appear. The peak at m/z 91 is ascribed to [C6H3O]- formed by loss of HF from the 3-fluorophenoxideion (m/z 111), and the peak at m/z 75 is ascribed to the radical anion [C6H3]-. It is noteworthy that loss of CO2 from the deprotonated 8 remains a minor fragmentation pathway across low collision energy range, its peak at m/z 180 reaches its maximum level of only 0.2 % at NCE = 9. However, as shown in Fig. 4 and as will see later, the loss of CO2 precedes the Smiles rearrangement.

Figure 4.

Figure 4.

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Relative peak intensities as a function of normalized collision energy in the HCD tandem mass spectrum of deprotonated 2-fluoro-4-(4-morpholinyl)benzoic acid 8 on the orbitrap instrument. Intensities are normalized to totalion current. Each curve is identified by the m/z value of the corresponding production (m/z 224 is the precursor ion). Notice that the y-axis is truncated and the scale from 0.0 to 0.2 units of relative intensity is exaggerated on purpose to show the maximum of the peak at m/z 180 (loss of CO2).

A proposed Smiles rearrangement mechanism for fragmentation of deprotonated 4-(4-morpholinyl)benzoic acid is shown in Scheme 5. The precursor anion undergoes loss of CO2, 1,2- proton transfers, morpholine ring opening and Smiles rearrangement to generate the phenoxide ion. This mechanism is supported by theoretical calculations and other experimental evidence (see section Theoretical Calculations).

Scheme 5.

Scheme 5.

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Proposed nitrogen-oxygen Smiles rearrangement mechanism for fragmentation of deprotonated 4-(4-morpholinyl)benzoic acid.

To extend the application of the Smiles rearrangement mechanism, we examined some morpholine substituted benzoic acid derivatives and their production abundances normalized to totalion current are listed in Table 1.

Table 1.

Various benzoic acids which exhibit the nitrogen-oxygen Smiles-type rearrangements underion trap collision at NCE = 35 and their production abundances.

Compound Structure Observed rearrangement production Abundance normalized to totalion current (100%)
4-[(2-Hydroxyethyl)(methyl)amino]benzoic acid, 6 graphic file with name nihms-1856726-t0014.jpg graphic file with name nihms-1856726-t0015.jpg 7
4-(4-Morpholinyl)benzoic acid, 7 graphic file with name nihms-1856726-t0016.jpg graphic file with name nihms-1856726-t0017.jpg 89
2-Fluoro-4-(4-morpholinyl)benzoic acid, 8 graphic file with name nihms-1856726-t0018.jpg graphic file with name nihms-1856726-t0019.jpg 98
2-Chloro-4-(4-morpholinyl)benzoic acid, 9 graphic file with name nihms-1856726-t0020.jpg graphic file with name nihms-1856726-t0021.jpg 77
3-Bromo-4-(4-morpholinyl)benzoic acid, 10 graphic file with name nihms-1856726-t0022.jpg graphic file with name nihms-1856726-t0023.jpg 98
3-Methyl-4-(4-morpholinyl)benzoic acid, 11 graphic file with name nihms-1856726-t0024.jpg graphic file with name nihms-1856726-t0025.jpg 63
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Via a Smiles rearrangement, open chain benzoic acid 6 undergoes fragmentation to generate the phenoxideion with a low abundance of 7 %. The Smiles rearrangement predominates in the fragmentation of a morpholine substituted benzoic acid, 4-(4-morpholinyl)benzoic acid 7, showing a huge increase in the extent of the rearrangement to form the phenoxideion with an abundance of 89 %. Furthermore, fluorine substituted compound, 2-fluoro-4-(4-morpholinyl)benzoic acid 8 provides the corresponding rearrangemention with the highest abundance of 98 %. That means its rearrangement production is almost the only peak observed in the CID spectrum of deprotonated compound 8 at a collision energy of 35. Substitution effects on the benzene ring of 4-(4-morpholinyl)benzoic acid 7 were also investigated. For example, electron-withdrawing bromine substituted compound, 3-bromo-4-(4-morpholinyl)benzoic acid 10, affords a same extent of Smiles rearrangement as 8 with the production abundance of 98 %. Fragmentation of deprotonated 2-chloro-4-(4-morpholinyl)benzoic acid 9 generates the corresponding phenoxideion with a lower abundance of 77 % as loss of CO2 peak was observed with an intensity of 24 %. Electron-donating substituent on the benzene ring has also been studied, 3-methyl-4-(4-morpholinyl)benzoic acid 11 undergoes the Smiles rearrangement to form the 2-methylphenoxideion with a relatively low abundance of 63 %, loss of CO2 production at m/z 107 accounts for an abundance of 31 %.

It is worth mentioning that 4-(4-morpholinyl)benzoic acid 7 is a water-based additive used as a heat-sealing and anti-blocking agent and is also an important intermediate in organic synthesis.

Theoretical calculations

Theoretical calculations on deprotonated 2-(anilino)ethanol 1 show that the amine deprotonation is slightly favored over the alcohol deprotonation (≈ 15 kJ/mol). Also, the C-H bond lengths in the methylene group next to the hydroxyl group for the optimized structure of the deprotonated alcohol are larger than usual, 1.14 Å (0.114 nm) vs. 1.09 Å (0.109 nm). These facts tend to favor the loss of water from the deprotonated amine and of a hydrogen molecule from the deprotonated alcohol. This was not further explored as compound 1 barely exhibits the Smiles rearrangement reaction. Replacing the amine hydrogen with a methyl group, i.e., 2, prevents the previous neutral losses and favors the Smiles rearrangement. In fact, the activation energy for the Smiles rearrangement reaction for compound 2 is almost half of that for compound 1 (128 kJ/mol). The calculations also show that the reaction proceeds through a transition state that resembles the oxazolidine ring shown in scheme 4. An IRC path calculation connecting the transition state heterocyclic compound and the product ions also shows that the chemical structure of the neutral loss C3H7N is likely ethylidene(methyl)amine (Inline graphic). On the other hand, the optimization of transition states producing heterocyclic rings containing six or more carbon atoms fails indicating that these structures are not quite stable in agreement with the experimental observations.

On the other hand, replacing the methyl group with bulky substituent groups larger than methyl decreases the intensity of the Smiles rearrangement product ions. Although the reaction proceeds in a similar fashion by forming an oxazolidine ring, the ring itself becomes less stable. For example, the barrier for the formation of the oxazolidine ring for (2-(N-ethylanilino)ethanol), 3, is barely 30 kJ/mol, which is significantly lower than that observed for the methyl-substituent compound. However, the oxazolidine ring does not form a stable intermediate compound and undergoes decomposition producing deprotonated 1,3-dihydro-1-ethyl-2H-azepin-2-one plus ethene through a transition state with elongated C8-N7 and C9-O10 bonds as shown in Figure 5.

Figure 5.

Figure 5.

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Oxazolidine-type transition state structure of compound 3 leading to the formation of de-protonated 1,3-dihydro-1-ethyl-2H-azepin-2-one and ethene.

Regarding the 4-(4-morpholinyl)benzoic acid derivatives as described above, the Smiles rearrangement reaction is accompanied by the neutral loss of carbon dioxide. A careful analysis of the tandem mass spectra reveals that although the neutral loss intensity remains low (<1%) and it is barely observed at low collision energies the reaction precedes the Smiles rearrangement reaction. In fact, for 2-fluoro-4-(4-morpholinyl)benzoic acid 8, the neutral loss peak at m/z 180 is intense enough to allow recording the MS3 spectrum. It shows the base peak at m/z 111 corresponding to the Smiles rearrangement reaction production (see Figure S1 of supporting information).

Based on the premise that the neutral loss of CO2 precedes the Smiles rearrangement reaction the phenyl carbanion derived from 4-(4-morpholinyl)benzoic acid 7, was used as a model compound for the study of the reaction mechanism of 4-(4-morpholinyl)benzoic acid derivatives. The experimental observations and theoretical calculations show the rearrangement reaction involves two-consecutive proton transfers followed by the morpholine ring-opening reaction. DFT calculations on the minima and saddle-point structures followed by IRC calculations reveal a potential mechanism for the Smiles rearrangement reaction for this family of compounds. The reaction proceeds as follows; the initial phenyl carbanion with the excess electron in para-position is stabilized by two consecutive 1,2-proton transfers until the charge reaches position ortho. This is reasonable because the excess electron is in a sigma orbital (sp2), and it is not stabilized by the π-system and the electron loses potential energy when it moves toward a more electronegative atom. There are a variety of reports regarding the chemistry of the 1,2-proton-shifted isomers of several aryl cations and aryl anions [23–25]. These reports show that most proton-shifted isomers are kinetically stable with high energetic barriers to unimolecular isomerization in the gas phase. Finally, the abstraction of a nearby hydrogen atom from the morpholine ring followed by ring opening of morpholine produce a structure that resembles that of deprotonated 2 as shown in Scheme 4. Thus, the subsequent steps show no significant differences with the previous discussion. Regarding the ring opening reaction of morpholine, Florio et al. reported that treatment of benzomorpholine or morpholine derivatives with lithium diisopropylamide leads to enamides by base-promoted reverse 6-endo-trig elimination of the phenoxide anion [26].

A more detailed analysis of the energetic and reaction mechanism for the Smiles rearrangement reaction of deprotonated 4-(4-morpholinyl)benzoic acid derivatives, based on the above mentioned DFT calculations, is shown in Figure 6. This figure shows the relationship between energy and reaction progress for the mechanism of the Smiles rearrangement reaction of 4-(4-morpholinyl)benzoic acid, 7. All energies are relative values referred to the deepest minimum and obtained from the electronic energies without zero point energy correction and expressed in kJ/mol. The two first steps with similar activation energies correspond to two consecutive 1,2-proton shifts. The third step corresponds to the abstraction of a proton from the morpholine ring with a small barrier of 69 kJ/mol. Finally, the intermediate structure, min 4, overcomes a barrier of 62 kJ/mol generating the precursor of the phenoxide ion. It is also worth mentioning that kinetic isotopic effect (tunneling) may be occurring in this reaction regarding the 1,2-proton transfers; however, this is beyond the purposes of this article.

Figure 6.

Figure 6.

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Proposed reaction mechanism for the Smiles rearrangement reaction of 4-(4-morpholinyl)benzoic acid 7 based on DFT calculations. The loss of carbon dioxide is not shown. The plot was drawn using Angnes, R. A. mechaSVG, GitHub repository, 2020, doi: 10.5281/zenodo.4065333.

Subsequent observations on other morpholine derivatives such as compounds 12 and 13 show several interesting patterns that are consistent with the theoretical calculations. For example, compound 12 shows the base peak in the spectrum of this compound at m/z 190 corresponding to neutral loss of formaldehyde from morpholine ring-opening [see Figure S2 of supporting information]. The second most intense peak appears at m/z 151 corresponding to the Smiles rearrangement reaction. It means the Smiles rearrangement reaction occurs in this morpholine derivative without previous neutral losses of substituents.

On the other hand, the most intense peak in the spectrum of 2-(4-morpholinyl)-5,6,7,8-tetrahydro-4-quinazolinol, 13, appears at m/z 165 corresponding to a Smiles rearrangement. Two other major peaks are the loss of water at m/z 216 and the loss of acetaldehyde at m/z 190 from morpholine ring-opening (Figure S3).

In general, the calculations indicate that the Smiles rearrangement reaction for these compounds is significantly affected by substituents on the phenyl ring and on the nitrogen. If the phenyl ring bears one or more ionizable groups that favor a neutral loss such as the loss of carbon dioxide from carboxyl, this fragmentation pathway is favored or in competition with the Smiles rearrangement. The neutral loss leaves behind a negativeion in which the negative charge resides on the ring decreasing the nucleophilic character of the oxygen atom. On the other hand, the formation and stability of the oxazolidine ring is very susceptible to ring opening reactions in various ways depending on the nitrogen substituents.

Conclusions

Deprotonated 2-(N-methylanilino)ethanol 2 undergoes a nitrogen-oxygen Smiles rearrangement to form the phenoxideion as a major product. The Smiles rearrangement proceeds with the attack of oxygen atom at the ipso position of aniline ring to form a five-membered ring transition state, followed by loss of a neutral ethylidene(methyl)amine molecule to generate the phenoxideion at m/z 93. 18O isotopic labeling experiments shows an abundant 18O-phenoxideion at m/z 95 is generated, confirming the Smiles rearrangement mechanism. The occurrence of the novel rearrangement is determined by the substituents on the nitrogen atom of aniline and the length of alcohol chain. The Smiles rearrangement can extend to various morpholine substituted benzoic acids and other compounds, which undergo fragmentation to significantly form the phenoxide ion. The key step involves proton transfers, morpholine ring opening, followed by the Smiles rearrangement via an oxazolidine transition stateion similar to that of the 2. The mechanism is supported by density functional theory calculations.

Supplementary Material

Supporting info

Scheme 6.

Scheme 6.

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Structures of 1-(2-hydroxy-4-morpholinophenyl)ethenone, 12 and 2-morpholino-5,6,7,8-tetrahydroquinazolin-4(3H)-one, 13

Acknowledgments

The author would like to thank Dr. Karl K. Irikura of the NIST for his advice and comments. Certain commercial equipment, instruments, or materials are identified in this document. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products identified are necessarily the best available for the purpose.

Footnotes

Associated content

B3LYP/6–311g(d, p)-optimized structures of O-deprotonated and N-deprotonated 2-anilinoethanol 1, a MS3 spectrum of theion from the deprotonated form of compound 8, a MS2 spectrum of compound 12, and a MS2 spectrum of compound 13 showing the Smiles rearrangement reactions are included. This material is available free of charge via the internet at http://pubs.acs.org.

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Supplementary Materials

Supporting info
NIHMS1856726-supplement-Supporting_info.pdf (167.1KB, pdf)

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