Reversible Generation of Labile Secondary Carbocations from

Jun 1, 2017 - It was confirmed that the generation of 2 from 1 was controlled by thermodynamic equilibrium rather than kinetic regulations. The equili...
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Reversible Generation of Labile Secondary Carbocations from Alcohols in the Nanospace of H‑Mordenite and Their Long-Lasting Preservation at Ambient Temperature Yoichi Masui,*,† Taiki Hattori,‡ and Makoto Onaka*,† †

Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

‡

S Supporting Information *

ABSTRACT: Secondary carbocations are rarely observed spectrometrically for prolonged durations at ambient temperatures because of their instability. In this study, when 4,4′-difluorobenzhydrol (1) was mixed with H-mordenite (H-Mor), the 4,4′-difluorodiphenylmethyl cation (2) was generated as the main product, identified by UV−vis and 13C-MAS NMR spectroscopies, and was preserved for over 1 week at ambient temperature. Surprisingly, the polymerization and disproportionation of 1 barely proceeded within the micropores of H-Mor. However, these side reactions prevailed in TfOH and formation of 2 was not observed. Preservation of other secondary carbocations from benzhydrol, 4,4′-dichlorobenzhydrol, and 9-fluorenol was also realized in H-Mor. It was confirmed that the generation of 2 from 1 was controlled by thermodynamic equilibrium rather than kinetic regulations. The equilibrium between 2 and 1 was accompanied by reversible chromism, which could be easily controlled by altering the moisture content in H-Mor. Moreover, novel insights into specific acid catalysis in zeolites densely populated with acid sites on the inner surface of micropores are described herein.

1. INTRODUCTION Carbocations1 are fundamental intermediates in various organic reactions. Whereas tertiary or aromatic carbocations are relatively stable and can be observed using various forms of spectroscopy or can even be isolated in some cases,2 secondary carbocations are generally so unstable that their survival and spectroscopic detection at ambient temperatures is rare. For example, we reported that the substitution of secondary alcohols with certain nucleophiles proceeds through the corresponding carbocation intermediates, such as diphenylmethyl cations and allylic cations, using a solid-acid catalyst composed of tin hydroxide-embedded montmorillonite clay (Sn-Mont). Although the reactions proceeded through an SN1type mechanism under thermodynamic control, the intermediate carbocations were not observed spectroscopically because the intermediates of organic reactions are generally high-energy species that exist only briefly.3 If such carbocations can be stabilized enough to be observed easily, or more desirably, if their stability can be readily controlled, the physicochemical properties of these carbocations can be studied and organic reactions that proceed through carbocations may be able to be regulated. Electron-donating substituents such as methoxy and amino groups on the benzene rings of diarylmethyl cations can afford considerable stability, and such diarylmethyl cations can be isolated.4 However, the 4,4′-difluorodiphenylmethyl cation (2) © 2017 American Chemical Society

and the diphenylmethyl cation are unstable and easily decompose. Specifically, Mayr et al. utilized poor nucleophiles like alkenes and alkylbenzenes to determine the electrophilicity parameter (E) of conjugated carbocations like diarylmethyl cations.4a,5 According to their study, 2 and the diphenylmethyl cation are 106 times more reactive than the 4,4′-dimethoxydiphenylmethyl cation and 1012 times more reactive than the 4,4′-diaminodiphenylmethyl cation (Figure 1). These data clearly indicate that 2 and the diphenylmethyl cation are so electrophilic that they can easily undergo Friedel−Crafts alkylations not only with toluene, but also with precursors such as benzhydrol. Accordingly, the preservation of diphenylmethyl cations without overreactions is difficult because of their high reactivity. To date, spectroscopic observation of diphenylmethyl cations has only been accomplished under limited conditions. For instance, diphenylmethyl cations generated by laser-flash photolysis of suitable precursors were tracked by UV−vis (ultraviolet−visible) spectroscopy over their lifetime, which was on the order of microseconds at room temperature (rt) (Scheme 1 a).6 The NMR spectra of diphenylmethyl cations generated by superacids under dilute conditions were only measured at low temperatures (−78 to −40 °C; Scheme 1 b).1,7 Received: April 3, 2017 Published: June 1, 2017 8612

DOI: 10.1021/jacs.7b03336 J. Am. Chem. Soc. 2017, 139, 8612−8620

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Zeolites have been extensively utilized as catalysts, adsorbents, and separation agents. They are composed of negative aluminosilicate frameworks with intrinsic pore structures and extra framework cations (proton, sodium, etc.) that compensate for the negative charge of the framework. Because the delocalized negative charges on the inner surface of the zeolite framework are considered to be macroanions with little nucleophilicity toward countercations, we postulated that they may be able to stabilize positively charged complexes or organic cations produced by the coordination of organic molecules to metal cations or protons in the zeolite pores. Therefore, the stabilization and long-term storage of labile organic molecules such as formaldehyde, acrolein, and propargyl aldehyde in the supercages of the Na-Y zeolite have been investigated.9 Some unexpected phenomena, in terms of stabilization and reactivity, were rationalized by quantum-chemical calculations.10 As part of our ongoing studies on the stabilization of labile molecules inside zeolites, we envisaged that if we could generate a secondary carbocation from a secondary alcohol using an H-mordenite (H-Mor) zeolite with a relatively strong Brønsted acidity and an adequate pore system (see section 3.2), we would be able to preserve labile cations inside the zeolite cavity and observe them at ambient temperatures by UV−vis as well as 13C-MAS NMR. Moreover, we also describe the mechanistic aspects behind the generation and long-term preservation of secondary carbocations in the H-Mor zeolite pores herein.

Figure 1. Kinetic stability of carbocations in dilute solutions. The electrophilicity parameters (E) and reaction rate constants with water (kw) and toluene (ktol) are collectively summarized from refs 4a and 5b.

Scheme 1. Previous Studies Regarding the Generation of Diarylmethyl Cations and a Schematic of This Study

2. EXPERIMENTAL SECTION 2.1. Instrumentation and Chemicals. NMR spectra were recorded in CDCl3 on a Bruker Avance III 500 USPlus NMR spectrometer operated at 500 MHz for 1H NMR and 126 MHz for 13C NMR. The chemical shift values for 1H and 13C were referenced to Me4Si and residual solvent resonances, respectively. Chemical shifts are reported in δ ppm. The coupling constants (J) are given in Hz. 13 C-MAS NMR spectra were recorded on a Bruker Avance III 400 WB USPlus spectrometer operated at 100 MHz with a 4 mm zirconia rotor at a spinning rate of 10 kHz at 25 °C. An external standard was used to calibrate the chemical shifts. UV−vis adsorption spectra were obtained on a Jasco V-550 spectrometer using a 1 cm or 1 mm cell for the liquid samples or using a Jasco PSH-002 powder cell and an ISV-469 integrating sphere for powder samples. IR spectra were measured on a Jasco FT/IR-6300 spectrophotometer using NaCl discs. Thin-layer chromatography (TLC) was performed using commercially available 60-mesh silica-gel plates containing fluorescent agents visualized with short-wavelength UV light (254 nm). The powder forms of H-Mor (TOSOH Co., Ltd., HSZ-640HOA, Si/Al = 9), H-Y (TOSOH Co., Ltd., HSZ-320HOA, Si/Al = 2.75), H-Beta (TOSOH Co., Ltd., HSZ940HOA, Si/Al = 18.5), H-ZSM-5 (Clariant Catalysts K.K., Si/Al = 45), and silica-alumina (JGC Catalysts and Chemicals, Ltd., JRC-SAH1, Si/Al = 1.85) were activated at 400 °C in a vacuum for 2 h. The organic reagents were obtained from commercial suppliers or prepared according to standard procedures unless otherwise noted. All solvents were distilled prior to use. 2.2. UV−vis Measurements. Solid 4,4′-difluorobenzhydrol (1, 0.09 mmol) was mixed with powdery H-Mor (Si/Al = 9; 250 mg containing 0.42 mmol of aluminum), which was activated in a vacuum at 400 °C for 2 h, under a nitrogen atmosphere at rt. The mixture was stirred with a magnetic stir bar for 1 h with occasional vibration from a hand-held vibrator. The mixture was then placed on a diffuse reflectance cell in a glovebag filled with nitrogen gas and subjected to UV−vis measurements. 2.3. 13C-CP/MAS or DD/MAS NMR. The mixture of 1 and H-Mor (as described in section 2.2) was placed in a zirconia NMR rotor in a glovebag filled with nitrogen gas and analyzed by 13C-CP/MAS (cross-

In regards to the observations of carbocations in acidic zeolites, specifically microporous crystalline aluminosilicates, Garciá et al. investigated the photochemical behaviors of a variety of carbocations in acidic zeolites. For instance, allylic cations conjugated with two phenyl groups were generated in H-ZSM5, and observed with UV−vis and FTIR (Fourier transform infrared) spectroscopies.8 They also attempted the reaction of benzaldehyde with benzene in H-Y to produce a more-stable triphenylmethyl cation and diphenylmethane via the transient formation of a diphenylmethyl cation (Scheme1 c). 8613

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Journal of the American Chemical Society polarization magic-angle spinning) NMR with the parameters listed in Table S4c or by 13C-DD/MAS (dipolar-decoupling magic-angle spinning) NMR using the depth-pulse program listed in Table S4e. 2.4. 13C-DD/MAS NMR of Samples Exposed to Moisture. After the 13C-DD/MAS NMR measurement, the solid sample (250 mg) was removed from the rotor and placed on an evaporating dish. The sample was subsequently placed in a closed vessel containing a small amount of water and kept overnight. The moisturized sample was then subjected to 13C-DD/MAS NMR using the parameters listed in Table S4e. 2.5. Addition of MeOH to the Carbocation in H-Mor, Followed by Extraction and Identification of Products. Solid 4,4′-difluorobenzhydrol (1, 0.45 mmol) was mixed with powdery HMor (Si/Al = 9; 1.5 g containing 2.52 mmol of aluminum), which was activated in a vacuum at 400 °C for 2 h, under a nitrogen atmosphere at rt. The mixture was stirred with a magnetic stir bar for 1 or 12 h with occasional vibration from a hand-held vibrator. Dry MeOH (30 mL) was then added to the mixture, and the mixture was stirred under a nitrogen atmosphere at rt overnight. H-Mor was filtered off with filter paper and washed with CH2Cl2. The combined organic layers were concentrated to afford a mixture of products, which were dissolved in CDCl3 and analyzed by 1H NMR. 2.6. Addition of MeOH to Carbocations Generated by TfOH. TfOH (0.13 mmol) in CDCl3 (0.48 mL) was slowly added to 1 (0.05 mmol) at rt. After stirring for 1 or 12 h at rt, the mixture was dissolved in CDCl3, and analyzed by 1H NMR. 2.7. Cation-Regeneration Experiments. After the first UV−vis measurement (as described in section 2.2), the yellow-colored sample was removed from the cell, and it was exposed to air for several minutes. The sample immediately turned white. The white sample was subjected to UV−vis measurements. Then, the sample was evacuated in a vacuum at rt for 30 min. The yellow color returned, and the sample was placed on a diffuse reflectance cell in a glovebag filled with nitrogen gas, subjected to UV−vis measurements. The exposure/ evacuation/observation cycle was repeated twice.

Figure 2. Comparison of the thermodynamic stabilities of various carbocations based on the dissociation energies (ΔEdis) calculated at the B3LYP/6-31G(d) level.

carbocations in H-Mor is shown in Scheme 2. We selected 4,4′-difluorobenzhydrol (1) as a precursor for the generation of

3. RESULTS AND DISCUSSION 3.1. Estimation of the Instability of Carbocations. First, we assessed the instabilities of a series of carbocations in order to determine the target carbocation. As mentioned above, Mayr et al. quantified the reactivity of a variety of carbocations by defining electrophilicity parameters (E), which were determined on the basis of experimental reaction rate constants of carbocations with nucleophiles in dilute solutions (Figure 1).4a,5 As another index, we tried to evaluate the stabilities of carbocations using the dissociation energy (ΔEdis) of alcohols to form the corresponding carbocations and an OH− anion using B3LYP/6-31G(d) level calculations (Figure 2, SI 9). For the carbocations whose E values were defined by Mayr, a linear relationship between the calculated ΔEdis values of the carbocations and experimental E values was found (Figure S9a). The linear relationship between the thermodynamic and kinetic stabilities of these carbocations is reasonable according to the BEP principle.11 Therefore, the instability of these carbocations can be predicted by simple calculations even for several carbocations without kinetic data. As a result, it was confirmed that H-, F-, or Cl-substituted diphenylmethyl cations and a fluorenyl cation were thermodynamically far more unstable than carbocations such as triphenylmethyl, 1,5-diphenyl-1,3-pentadienyl, and tropylium cations, which were observed previously.2−4,8 Therefore, we tried to generate and preserve these unstable carbocations in this study. Initially, we selected 4,4′-difluorodiphenylmethyl cation (2) as a target carbocation. 3.2. Fundamental Strategy for Capturing Carbocation 2. A schematic of our strategy for the preservation of

Scheme 2. Schematic of the Mechanism for the Preservation of Carbocations in H-Mor

the 4,4′-difluorodiphenylmethyl cation. Although one molecule of 2 would be generated through the protonation and dehydration of 1 in the zeolite pores, an additional proton would be necessary to “deactivate” the produced water as H3O+, in order to prevent recombination with the carbocation. Therefore, we fixed the ratio of Al sites, which were considered almost equivalent to that of the proton sites in the zeolite, to introduced alcohol 1 to >4 for confirmation.12 In addition, we aimed to trap generated H3O+ in a more stable state in order to prevent it from engaging in side reactions. Therefore, we predicted that H-Mor would be the most adequate zeolite because the pore system of mordenite is composed of onedimensional straight main channels and smaller side-pockets directly connected to the main channels (see SI 8 for details). Only the main channels can accommodate large organic molecules, and H3O+ should be stably trapped in the subchannels or side-pockets. Because some solvents can nucleophilically attack the carbocation or deprotonate H3O+ 8614

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Journal of the American Chemical Society in the subchannels or side-pockets of H-Mor to release a reactive water molecule, 1 was mixed with H-Mor without a solvent. 3.3. Generation and UV−vis Observation of Cationic Species. Along with these assumptions, 1 was mixed with typical acidic zeolites or other solid acids for 1 h at rt, and the formation of the corresponding cations was analyzed by diffuse reflectance UV−vis spectroscopy. Molecule 1 exhibited no absorption in the range of 350−550 nm (Figure S3b). No color development was observed on the surface of silica-alumina, solid zinc chloride, or Amberlyst-15. As shown in Figure 3, H-

Figure 4. Changes in the UV−vis spectra of H-Mor upon contact with 1: One hour later (red line), 1 day later (dark blue line), and 8 days later (green line).

Figure 3. UV−vis diffuse reflectance spectra of solid acids upon contact with 1: H-Mor (red), H-ZSM-5 (gray), H-Beta (black), H-Y (yellow-green), Amberlyst-15 (light blue), zinc chloride (dark blue), and silica-alumina (green). Inset shows yellow-color development of H-Mor containing 2 (white object in the yellow powder is a Tefloncoated stir bar).

Figure 5. 13C-CP/MAS NMR spectrum of a mixture of 1 having a natural abundance of 13C and H-Mor. The asterisks show spinning side bands.

Mor (Si/Al = 9) changed color from white to bright yellow, giving a strong peak at 450 nm in the UV−vis spectra, which was assigned to the 4,4′-difluorodiphenylmethyl cation (2).6a Although detailed confirmation of 2 is discussed in sections 3.4−3.7, it is reasonable to consider that 1 was adsorbed into the pores of the zeolite and was protonated and subsequently dehydrated to afford 2. Other zeolites such as H-ZSM-5 (Si/Al = 45), H-Beta (Si/Al = 19), and H-Y (Si/Al = 2.75) similarly turned yellow upon generation of 2 but exhibited much weaker absorptions.13 The absorption intensity at 450 nm followed the acid strength of the zeolites, which was evaluated by the ammonia desorption energy (ΔH): H-Mor (ΔH = 33.8 kcal/mol) > H-ZSM-5 (ΔH = 31.9 kcal/mol) > H-Beta (ΔH = 30.4 kcal/mol) > H-Y (ΔH = 25.7 kcal/mol).14 As shown in Figure 4, the absorption at 450 nm of 2 in H-Mor remained for more than 1 week, although some decrease in its intensity was observed after the first day. 3.4. Identification of 2 by 13C-MAS NMR. In order to determine the structures of 2 as well as other products, a mixture of 1 and H-Mor was analyzed by 13C-CP/MAS NMR at rt (Figure 5).15 The peaks at 122, 138, 147, 176, and 192 ppm were very similar to those of 2 measured at −78 °C, as reported previously.7b The peak at 192 ppm corresponds to the methine carbon of carbocation 2. In addition, the peaks at 78, 80, 114, 116, 130, 137, 138, and 161 ppm belong to bis(1,1′difluorodiphenylmethyl) ether 3 (Figure S4c). Notably, if reactant alcohol 1 remained in the mixture, the peaks at 72 and

74 ppm, corresponding to the C-1 of 1, should be observed (Figure S4d); however, the corresponding peaks were not observed, indicating that 1 was almost completely consumed. 3.5. Confirmation of the Structure and Time-Dependent Changes of 13C-Enriched Alcohol 1′ and Other Species by 13C-MAS NMR. In order to more definitively distinguish molecules 1, 2, and 3 from each other, 1′, which was >98% enriched with a 13C atom at the C-1 position,16 was mixed with H-Mor and analyzed by 13C-DD/MAS NMR. Strong peaks corresponding to the methylene group of 4,4′difluorodiphenylmethane (4′) were observed at 41 ppm, in addition to the carbonyl carbon of 4,4′-difluorobenzophenone (5′) at 204 ppm (for the spectrum of the mixture of H-Mor and 5, see Figure S4e). The carbocation of 2′ appeared at 192 ppm, and the methine carbon of bis(1,1′-difluorodiphenylmethyl) ether (3′) was observed at 78 ppm (Figure 6a). Again, the presence of alcohol 1′ around 75 ppm was hardly detected. To evaluate the time-dependent changes in the amounts of chemical species in H-Mor by 13C-DD/MAS NMR, 1′ was mixed with H-Mor at rt; the mixture was monitored every hour. The individual spectra and time-dependent changes are shown in Figures S4a and 7, respectively. After 2 h, peaks corresponding to 2 (2′) as well as 1 (1′), 3 (3′), 4 (4′), and 5 (5′) were detected by 13C-DD/MAS NMR (Figure S4aA). The peaks corresponding to 1 (1′) nearly disappeared after 8 h, 8615

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was recovered, as noted by the appearance of a peak at 75 ppm (Figure 6b). The result clearly indicated that excess water attacks 2 to regenerate starting alcohol 1. Meanwhile, the peak corresponding to protonated 4,4′-difluorobenzophenone (protonated 5′) at 204 ppm disappeared and a new peak, which was assigned to liberated 5′ (Figure S4e), appeared at 197 ppm. It was reconfirmed that most of 1′ (or 1) was completely converted into 2′ (or 2), 3′ (or 3), and the other products after 24 h. 3.7. Extraction of Chemical Species in H-Mor with MeOH. In general, comparing the integral values of 13C-MAS NMR peaks does not necessarily facilitate the quantitative analysis of chemical species formed in H-Mor pores. Therefore, in order to quantify the amounts of 2, 3, and the other products, excess MeOH was added to the mixture of 1 and HMor after the mixture was stirred for 1 h; the production of 4,4difluorodiphenylmethyl methyl ether (6) from 2 and MeOH (Scheme 3, Table 1 top row) was expected. It was roughly Scheme 3. Plausible Reactions Caused by Adding Excess MeOH to 2 and H3O+ in H-Mor Figure 6. 13C-DD/MAS NMR spectrum of the mixture of 1′ and HMor before (a) and after exposure to moisture (b).

estimated using the 1H NMR in CDCl3 of the filtrated reaction mixture with an internal standard that 2 was present in H-Mor in at least 14% yield, based on the 14% yield of 6. However, the amount of recovered 1 (54%) was much larger than the estimated value from 13C-DD/MAS NMR, which suggested that 1 was almost completely consumed after 1 h. We also performed the same methanol-addition experiment after the mixture of 1 and H-Mor had been stirred for 12 h at rt (full conversion conditions). As a result, the yield of 6 was 9%, and as much as 40% of 1 was recovered in spite of the fact that 1 was hardly observed in the solid-state NMR spectra after 12 h (Figures 5 and 6a). This result can be understood as follows: when 2 was formed from 1 in H-Mor, H2O was released and trapped as H3O+ inside the zeolite pore. MeOH is a small, polar molecule, so as soon as excess MeOH was added, H2O was liberated and promptly reacted with 2 to regenerate 1 (Scheme 3). Therefore, it is reasonable that the 40% yield of 1 originated from 2, and most of 1 was converted into 2, which was maintained in the pores of H-Mor in ca. 49 (9 + 40)% yield after 12 h. In addition, when MeOH was added to the mixture of 1 and H-Mor that was only stirred for 1 h, the total yield of extracted products 1, 3, 4, 5, and 6 exceeded 99% (Table 1, top row). This strongly indicates that almost no side reactions, such as Friedel−Crafts-type reactions and subsequent consecutive Friedel−Crafts polymerization of 2 with aromatic compounds, took place in H-Mor during the first hour. It should be also noted that the actual yields of 4 and 5 were very low (Table 1),

Figure 7. Time-dependent changes in the peak areas of the 13C-DD/ MAS NMR spectra of each chemical species (1′ and 3′, blue diamonds; 2′, red circles; 4′, green triangles; 5′, purple squares) in the mixture of 1′ and H-Mor.

while those of 2 (2′) and 3 (3′) remained almost unchanged after 10 h (Figures S4aI−S4aP). Accordingly, most of 2 was generated within 1 h, and no reactant alcohol 1 remained after 12 h of stirring with H-Mor. Therefore, the two mixing conditions (1 h, high selectivity conditions; 12 h, full conversion conditions) were adopted for the subsequent experiments. 3.6. Additional Confirmation of the Presence of 2 Upon Exposure to Moisture. To further confirm the presence of 2′ in H-Mor after mixing 1′ with H-Mor for 24 h, the mixture was exposed to moisture and the 13C-DD/MAS NMR spectra were obtained. Notably, the color of the sample immediately changed from yellow to white upon exposure to moisture, which strongly supports the assumption that the yellow color is attributable to the generation of some carbocation species. Compared to the spectrum shown in Figure 6a, which was obtained prior to the exposure to moisture, the peak at 192 ppm was lost and starting alcohol 1′ 8616

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Journal of the American Chemical Society Table 1. Comparison of the Yields (%) of 1−6 Using H-Mor or TfOH.a,b

a1

acid

1

6

3

4

5

total yield of 1, 3, 4, 5, and 6 (%)

H-Mor TfOH

54(40) 0(0)

14(9) 0c (0)c

30(38) 0(0)

0.3 (0.1) 45 (45)

0.7 (0.8) 40 (36)

99 (89) 85 (81)

H NMR yields were calculated using mesitylene as the internal standard. bThe yields of 1−6 after 1 and 12 h (in parentheses). cThe yield of 2.

even though the peaks of 4 and 5 looked relatively large in the 13 C-MAS NMR spectra (Figure 6). 3.8. Conventional Explanation for Reaction Regulation in Zeolite Pores. Conventionally, the reaction regulations (or reaction selectivity) in the pores are interpreted to stem from steric obstruction by the zeolite pore walls (Figure 8a). This effect would be categorized as a kinetic regulation. For

Figure 8. Schematic of the reaction regulation in the zeolite pores; (a) conventional steric obstruction and (b) thermodynamic stabilization as discussed in section 3.11.

Figure 9. UV−vis spectra of a mixture of 4,4′-difluorobenzhydrol 1 and H-Mor after being mixed (left), and the apparent color changes of the mixture in a UV−vis cell (right): just after mixing (a), after the first exposure to the air (b), after the first evacuation at rt (c), after the second exposure to the air (d), after the second evacuation at rt (e), and after the third exposure to the air (f).

example, Garciá et al. claimed that α,ω-diphenyl-capped allylic cations in H-ZSM-5 were protected from the approach of water by two terminal phenyl groups, as well as the zeolite framework.2h,8c Although 2 is not only thermodynamically far more labile than α,ω-diphenyl-capped allylic cations (Figure 2), and the pore size of H-Mor is large enough to encapsulate 1, carbocation 2 was successfully preserved without any alcohol 1 in the system. The results of the moisture-exposure experiment (Figure 6b) as well as the MeOH-addition experiment (Table 1) revealed that the pore walls of H-Mor did not kinetically prevent the attack of small molecules, such as MeOH and H2O, on the carbocation. Therefore, in this system, cation 2 should exist as a thermodynamically metastable species, and the longterm preservation of cation 2 in H-Mor cannot be explained by conventional kinetic reasons. 3.9. Reversible Carbocation Formation Between 1 and 2. In order to confirm that 2 is a thermodynamically metastable species, reversible cation-formation experiments were performed. After the generation of 2 from 1 and HMor, the sample was repeatedly exposed to air and became colorless; after evacuation at rt, the yellow color was recovered (Figure 9 a−f). Such reversible color changes indicate that the cation generation of 2 from 1 is in thermodynamic equilibrium, and they indicate that the chromism was easily controlled by the amount of water in the system.17 3.10. Side Reactions Induced by Homogeneous Superacid. According to the discussion in section 3.9, the activation energy of the elementary reaction step of the generation of 2 from 1 is small. Therefore, the generation itself probably proceeds regardless of whether the system is homogeneous or heterogeneous as long as the acid is efficiently strong. Then, the successful preservation of cation 2 is entirely dependent on the suppression of undesired side-reactions with nucleophiles other than H2O. Three types of side reactions can accompany the formation of 2: the reaction of 2 with 1 affording ether 3 (path A in Scheme 4), or 4 and 5 through the

Scheme 4. Three Types of the Side Reactions of Carbocation 2 Promoted by Acid Catalysts, and the Corresponding Calculated Activation Energies (ΔG‡ /kcal·mol−1) for the Forward and Backward Pathsa

a

See SI 10).

disproportional oxidation/reduction of 1 with 2 (path B). The Friedel−Crafts alkylation of 2 with 1, 3, 4, or 5 to 7 and subsequent polymerizations (path C) are also feasible. In fact, when 1 (1′) was mixed with homogeneous superacid CF3SO3H (TfOH) for 1 h at rt, 4 (4′), 5 (5′), and 7 (7′) formed immediately, but 2 (2′) was not detected (Table 1 bottom row). A simple quantum-chemical calculation using a proton as an activator18 revealed that the activation energy for path A (8.7 kcal/mol) was much lower than those for path B (14.2 kcal/ mol) and path C (22.0 kcal/mol) (Scheme 4 right edge). On the basis of the activation energies of the reverse reactions, only path A should be reversible.19 These results can explain the experimental results induced by TfOH as described above. Although path A is the most kinetically favorable for a short 8617

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surfaces of the narrow pores (similar to H-Mor), the number of acid site-adsorbed molecules is equal to or less than that of the acid sites, and there is no room for electrically neutral molecules to attack the adsorbed species; thus, the bimolecular side reactions are hindered. Therefore, unstable cationic species like secondary carbocations can exist as thermodynamically metastable species, and survive in the narrow pores of acidic HMor (Figure 8b). 3.12. Side Reactions in the Presence of Zeolites and Requisites for the Preservation of 2. The quantumchemical calculations also showed that 3 and 7 are too large to be formed in the narrow pores of H-Mor via paths A and C. Likewise, the transition state involved in path B is so large (Figures S10a,c) that reaction B would not or would barely proceed in the pores (Scheme 6). The acid sites on the inner

time, only thermodynamically stable products were observed through path B or C, after the reverse process of path A. Paths A, B, and C involve second-order bimolecular reactions between cation 2 and the electrically neutral reacting counterparts of 1, 3, 4, or 5. Hence, the reaction rate (v) is expressed as the product of three factors: v = k × [2] × [1, 3, 4 or 5], where k is the rate constant determined by the activation energy of the reaction, and the second and third terms are the concentrations of 2 and 1, 3, 4, or 5, respectively. Therefore, removing anionic or electrically neutral molecules that can act as potential nucleophiles from the vicinity cation 2 is the most effective way to realize the long-term preservation of cation 2. However, in homogeneous reactions, this is difficult because the concentrations of all chemical species are uniform throughout the system. 3.11. Mechanism for the Preservation of Secondary Carbocations in H-Mor. In contrast, in a heterogeneous catalytic system, the concentrations of the chemical species depend on their locations around the catalyst (Scheme 5). In

Scheme 6. Schematic Models of the Reactions Inside and Outside the Pores of H-Mor

Scheme 5. Models of Acid Sites on Porous/Nonporous Catalysts

surface of H-Mor are so strong and abundant that the equilibrium between 1 and 2 could be overwhelmingly shifted toward 2 in H-Mor. Therefore, in the pores, there would be almost no electrically neutral molecules, and hence no side reactions would proceed. On the outer surface of H-Mor, the number of acid sites is less than that on the inner surface. However, some side reactions could not be completely suppressed, in the same way as on the nonporous catalyst (Scheme 5a). The activation energy for path A is much lower than B and C, and the corresponding reaction rate constant for A should be much larger than B and C. Consequently, outside the pores, path A would predominate, and 2 would easily be transformed into kinetically favored bulky ether 3. As a result, irreversible side reactions via paths B and C would not proceed or would proceed very slowly, as shown in Scheme 6. In fact, carbocation 2 and ether 3 were observed as major products, and almost no alcohol 1 and only small amounts of 4 and 5 were detected by 13C-CP/MAS NMR (Figure 5) inside or outside the pores. In summary, because H-Mor has adequate pores in terms of size and shape, as well as a dense population of strong acidic sites for the reaction with alcohol 1, labile cation 2 can be generated and conserved for a long time (hours to days), even at ambient temperatures. 3.13. Formation of Other Unstable Carbocations in HMor. To illustrate the generality of the carbocation formation in H-Mor, we tested the formation of other secondary alcohols such as benzhydrol, 4,4′-dichlorobenzhydrol, and 9-fluorenol in H-Mor (Figure 10), and we found that they could also be formed successfully. Notably, these carbocations were considered to be much more unstable than 2 according to the discussion in section 3.1. The colors of the reaction mixtures were bright yellow, bright orange, and pale red,20 all of which

a

Sparse/dense population acid sites on the surface of a nonporous acid catalyst. bSparse population of acid sites on the inner surfaces of a porous acid catalyst. cDense population of acid sites on the inner surfaces of an acid catalyst with narrow pores.

model (a), sparsely or densely populated with acid sites on the surface of the nonporous solid acid catalyst, the adsorbed molecules (pink) on the acid sites (red dots) become cationic. No matter how strong and densely populated the acid sites are, electrically neutral molecules (blue) from the bulk solution can undergo a bimolecular side reaction with the adsorbed cationic molecules. In the case of model (b), sparsely populated with acid sites on the inner surfaces of the narrow pores of the catalyst, the number of acid sites is related to the number of molecules adsorbed on the acid sites. However, if the total molecular volume is much lower than the pore volume of the catalyst, electrically neutral molecules can enter the interstices and attack the cationic molecules, and bimolecular side reactions can occur. Therefore, no matter how strong the acid sites are, the side reactions also cannot be prevented. In model (c), densely populated with acid sites on the inner 8618

DOI: 10.1021/jacs.7b03336 J. Am. Chem. Soc. 2017, 139, 8612−8620

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Journal of the American Chemical Society

2 was found to be the thermodynamically metastable species owing to reasons (1) and (2) described above, thus it was able to be sufficiently generated, observed under moderate conditions, and preserved for at least several days (section 3.3). The use of H-Mor for the generation and preservation of 2 facilitated the observation of a labile secondary carbocation via 13C-MAS NMR for the first time. Moreover, novel insights into specific acid catalysis in zeolites with a dense population of acid sites on the inner surface of micropores were obtained. This study offers a new avenue to clarify the physicochemical properties of unstable carbocations and also elucidate and control organic reaction processes via carbocations in the micropores of zeolites.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03336. Representative experimental procedures, spectral, and analytical data.(PDF)

Figure 10. UV−vis spectra of H-Mor after being mixed with 4,4′difluorobenzhydrol (black line), 4,4′-dichlorobenzhydrol (brown line), benzhydrol (red line), or 9-fluorenol (blue line).

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corresponded to their UV absorption. The corresponding carbocation from benzhydrol was formed in over a 59% yield, but the Friedel−Crafts type alkylation of the diphenylmethyl cation via path C was suppressed inside the pores, even though benzhydrol has no substituents in the para-position where Friedel−Crafts alkylation is prone to occur (Figure S5b).

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Makoto Onaka: 0000-0001-7893-7264

4. CONCLUSIONS By simply mixing benzhydrol analogue 1 with a strong solid acid, H-Mor, the corresponding labile secondary carbocation 2 was able to be produced in a fair yield (i.e. at least 50%) (section 3.8); 2 was able to be preserved for a long time (at least several days) at ambient temperature, as confirmed by UV−vis (section 3.3) and 13C-MAS NMR (section 3.4). It was confirmed by the quenching experiment with MeOH that at least 50% of 1 was converted into 2 (section 3.6). In contrast, 2 was not detected when homogeneous superacid TfOH was applied, because the side reactions proceeded exclusively (Table 1). The generation of 2 from 1 in H-Mor was found to be in equilibrium, and reversible chromism was observed upon exposure to moisture and subsequent vacuum evacuation (section 3.9). The reversibility of the formation of 2 indicates that it exists as a thermodynamically metastable species, and the stability of 2 in H-Mor cannot simply be explained by kinetic confinement effects of the zeolite pores. The reasons why the carbocation can be maintained in HMor are as follows: (1) the equilibrium between 1 and 2 is shifted in favor of the cation formation because of the sufficiently strong acid sites of H-Mor, by which the released water molecules are captured as H3O+, presumably in the subchannels or side-pockets of H-Mor; (2) side reactions are bimolecular reactions between 2 and electrically neutral nucleophiles of 1, 3, 4, 5, and 7. The dense population of acid sites on the inner surfaces of the narrow pores of H-Mor allow the electrically neutral nucleophiles to be separated from 2, thus preventing bimolecular side reactions (section 3.11). In contrast, in a homogeneous acid, TfOH, 2 immediately reacts with 1, 3, 4, 5, and 7 in solution to form side products as soon as 2 is generated from 1. It should be noted that the preservation of 2 was not attributed to conventional kinetic regulations, in other words, to the steric obstruction attributable to the zeolite pore walls. In the pores of H-Mor,

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present study was partially supported by JSPS KAKENHI Grant Numbers JP23105511 and JP16H04562. This paper is dedicated to Professor Teruaki Mukaiyama in celebration of his 90th birthday (Sotsuju).

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REFERENCES

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