Reversible Generation of Labile Secondary Carbocations from Alcohols in the Nanospace of H-Mordenite and Their Long-Lasting Preservation at Ambient Temperature

Article abstract of DOI:10.1021/jacs.7b03336

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.

Full text of DOI:10.1021/jacs.7b03336

Article
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
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017
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Journal of the American Chemical Society
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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, 3ꢀ8ꢀ1 Komaba, Meguroꢀku, Tokyo 153ꢀ8902
‡Graduate School of Science, The University of Tokyo, 7ꢀ3ꢀ1 Hongo, Bunkyoꢀku, Tokyo 113ꢀ0033
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ABSTRACT: Secondary carbocations are rarely observed spectrometrically for prolonged durations at ambient temperatures beꢀ
cause of their instability. In this study, when 4,4'ꢀdifluorobenzhydrol (1) was mixed with Hꢀmordenite (H-Mor), the 4,4'ꢀ
difluorodiphenylmethyl cation (2) was generated as the main product, identified by UVꢀVis and 13CꢀMAS NMR spectroscopies,
and was preserved for over one week at ambient temperature. Surprisingly, the polymerization and disproportionation of 1 hardly
proceeded within themicropores of H-Mor. On the other hand, these side reactions prevailed in TfOH and formation of 2 was not
observed. Preservation of other secondary carbocations from benzhydrol, 4,4'ꢀdichlorobenzhydrol, and 9ꢀfluorenol was also realꢀ
ized 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 alterꢀ
ing the moisture content in H-Mor. Moreover, novel insights into specific acid catalysis in zeolites with dense acid sites on the
inner surface of micropores are described herein.
1. INTRODUCTION
To date, spectroscopic observation of diphenylmethyl cations
has only been accomplished under limited conditions. For
instance, diphenylmethyl cations generated by laser flash phoꢀ
tolysis of suitable precursors were tracked by UVꢀVis (ultraviꢀ
oletꢀvisible) spectroscopy for the 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 Concernꢀ
ing the observations of carbocations in acidic zeolites—
microporous crystalline aluminosilicates—García et al. invesꢀ
tigated the photochemical behaviors of a variety of carboꢀ
cations in acidic zeolites. For instance, allylic cations conjuꢀ
gated with two phenyl groups were generated in H-ZSM-5,
and observed with UVꢀVis and FTꢀIR (Fourier transform inꢀ
frared) spectroscopies.8 They also attempted the reaction of
benzaldehyde
Carbocations1 are fundamental intermediates in various organꢀ
ic reactions. While tertiary or aromatic carbocations are relaꢀ
tively stable and can be observed using various forms of specꢀ
troscopy 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 alcoꢀ
hols with certain nucleophiles proceeds through the correꢀ
sponding carbocation intermediates, such as diphenylmethyl
cations and allylic cations, using a solid acid catalyst comꢀ
prised of tin hydroxideꢀembedded montmorillonite clay (Snꢀ
Mont). Although the reactions proceeded through an SN1ꢀtype
mechanism under thermodynamic control, the intermediate
carbocations were not observed spectroscopically because the
intermediates of organic reactions are generally highꢀenergy
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.
Electronꢀdonating 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 On the other hand, the 4,4'ꢀdifluorodiphenylmethyl
cation (2) and diphenylmethyl cation are unstable and easily
decompose. Specifically, Mayr et al. utilized poor nucleoꢀ
philes like alkenes and alkylbenzenes to determine the electroꢀ
philicity parameter (E) of conjugated carbocations like diaꢀ
rylmethyl cations.4(a),5 According to their study, 2 and dipheꢀ
nylmethyl cation are 106 times more reactive than 4,4’ꢀ
dimethoxydiphenylmethyl cation and 1012 times more reactive
than 4,4’ꢀdiaminodiphenylmethyl cation (Figure 1). These data
clearly indicate that 2 and diphenylmethyl cation are so elecꢀ
trophilic that they can easily undergo FriedelꢀCrafts alkylaꢀ
tions not only with toluene, but also with precursors such as
benzhydrol. Accordingly, the preservation of diphenylmethyl
cations without overreactions is difficult due to their high reꢀ
activity.
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
4(a) and 5(b).
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with benzene in H-Y to produce a more stable triphenylmethyl
cation and diphenylmethane via the transient formation of a
diphenylmethyl cation (Scheme1 (c)).
2. EXPERIMENTAL
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2.1 Instrumentation and chemicals
NMR spectra were recorded in CDCl3 on a Bruker Avance III
1
Zeolites have been extensively utilized as catalysts, adsorbents,
and separation agents. They are composed of negative alumiꢀ
nosilicate frameworks with intrinsic pore structures and extra
framework cations (proton, sodium, etc.) that compensate for
the negative charge of the framework. Since the delocalized
negative charges on the inner surface of the zeolite framework
are considered to be macroanions with little nucleophilicity
towards countercations, we postulated that they may be able to
stabilize positively charged complexes or organic cations proꢀ
duced by the coordination of organic molecules to metal catiꢀ
ons or protons in the zeolite pores. Therefore, the stabilization
and longꢀterm storage of labile organic molecules, such as
formaldehyde, acrolein, and propargyl aldehyde, in the superꢀ
cages of Na-Y zeolite have been investigated,9 and some unꢀ
expected phenomena, in terms of stabilization and reactivity,
were rationalized by quantum chemical calculations.10
500 USPlus NMR spectrometer operated at 500 MHz for H
NMR and 126 MHz for 13C NMR. The chemical shift values
1
for H 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. 13CꢀMAS
NMR spectra were recorded on a Bruker Avance III 400 WB
USPlus spectrometer operated at 100 MHz with a 4 mm zircoꢀ
nia rotor, at a spinning rate of 10 kHz at 25 °C. An external
standard was used to calibrate the chemical shifts. UVꢀVis
adsorption spectra were obtained on a Jasco Vꢀ550 spectromeꢀ
ter using a 1 cm or 1 mm cell for the liquid samples or using a
Jasco PSHꢀ002 powder cell and ISVꢀ469 integrating sphere
equipment for powder samples. Infrared spectra were measꢀ
ured on a Jasco FT/IRꢀ6300 spectrophotometer using NaCl
discs. Thinꢀlayer chromatography (TLC) was performed using
commercially available 60 mesh silica gel plates containing
fluorescent agents visualized with shortꢀwavelength UV light
(254 nm). The powder forms of H-Mor (TOSOH Co., Ltd.,
HSZꢀ640HOA, Si/Al=9), H-Y (TOSOH Co., Ltd., HSZꢀ
320HOA, Si/Al=2.75), H-Beta (TOSOH Co., Ltd., HSZꢀ
940HOA, Si/Al=18.5), H-ZSM-5 (Clariant Catalysts K.K.,
Si/Al=45), and silicaꢀalumina (JGC Catalysts and Chemicals,
Ltd., JRCꢀSAHꢀ1, Si/Al=1.85) were activated at 400 °C in a
vacuum for 2 h.
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As part of our ongoing studies on the stabilization of labile
molecules inside zeolites, we envisaged that if we could genꢀ
erate a secondary carbocation from a secondary alcohol using
an Hꢀmordenite (H-Mor) zeolite with 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 UVꢀVis
as well as 13CꢀMAS NMR. Moreover, we also describe the
mechanistic aspects behind the generation and longꢀterm
preservation of secondary carbocations in H-Mor zeolite
pores herein.
The organic reagents were obtained from commercial suppliꢀ
ers or prepared according to standard procedures unless othꢀ
erwise noted. All solvents were distilled prior to use.
2.2 UVꢀVis measurements
Previous studies
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 handꢀheld vibrator. The mixture was then placed on a diffuse
reflectance cell in a glove bag filled with nitrogen gas, and
subjected to UVꢀVis measurements.
Cl
(a)
LFP
rt
F
F
F
F
Lifetime on the
order of µs
Cl
SO2ClF/SbF5
-78oC
(b)
2.3 13CꢀCP/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 glove bag filled with niꢀ
trogen gas and analyzed by 13CꢀCP/MAS (Cross Polarization
Magic Angle Spinning) NMR with the parameters listed in
Table S4c or by 13CꢀDD/MAS (Dipolar Decoupling Magic
Angle Spinning) NMR using the depth pulse program listed in
Table S4e.
O
Ph
H-Y
+
(c)
Ph
Ph
Ph
Ph
Ph
H
Ph-H
Ph
Ph
This study
OH
2.4 13CꢀDD/MAS NMR of samples exposed to moisture
H-Mor
rt
After the 13CꢀDD/MAS NMR measurement, the solid sample
(250 mg) was removed from the rotor and placed on an evapoꢀ
rating dish. The sample was subsequently placed in a closed
vessel containing a small amount of water overnight. The
moisturized sample was then subjected to 13CꢀDD/MAS NMR
using the parameters listed in Table S4e.
X
X
X
X
Lifetime on the
order of days
X= F, Cl, H
Scheme 1. Previous studies regarding the generation of diarylmeꢀ
thyl cations and a schematic of this study.
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2.5 Addition of MeOH to the carbocation in H-Mor, followed
by extraction and identification of products
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Solid 4,4'ꢀdifluorobenzhydrol (1, 0.45 mmol) was mixed with
powdery H-Mor (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 stirrer bar for 1 or 12 h with occasional vibraꢀ
tion from a handꢀheld vibrator. Dry MeOH (30 mL) was then
added to the mixture, and the mixture was stirred under a niꢀ
trogen 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.
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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 UVꢀVis measurement (as described in section
2.2), the yellowꢀcolored sample was removed from the cell,
and exposed to air for several minutes; the sample immediateꢀ
ly turned white. The white sample was subjected to UVꢀVis
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 glove bag filled with
nitrogen gas, and subjected to UVꢀVis measurements. The
exposure/evacuation/observation cycle was repeated twice.
Figure 2. Comparison of the thermodynamic stabilitiesof various
carbocations based on the dissociation energies (∆Edis) calculated
at the B3LYP/6ꢀ31G(d) level.
3.2 Fundamental strategy for capturing carbocation 2
A schematic of our strategy for the preservation of carboꢀ
cations in H-Mor is shown in Scheme 2. We selected 4,4'ꢀ
difluorobenzhydrol (1) as a precursor for the generation of
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 necesꢀ
sary 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 introꢀ
duced alcohol 1 to > 4 for confirmation.12 In addition, we
aimed to trap generated H3O+ in a more stable state in order to
prevent H3O+ from engaging in side reactions. Therefore, we
predicted that H-Mor would be the most adequate zeolite beꢀ
cause the pore system of mordenite is composed of oneꢀ
dimensional straight mainꢀchannels and smaller sideꢀpockets
directly connected to the mainꢀchannels (see SI 8 for details).
Only the mainꢀchannels can accommodate large organic moleꢀ
cules, and H3O+ should be stably trapped in the subꢀchannels.
Because some solvents can nucleophilically attack the carboꢀ
cation or deprotonate H3O+ in the subꢀchannels of H-Mor to
release a reactive water molecule, 1 was mixed with H-Mor
without a solvent.
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 carboꢀ
cations by defining electrophilicity parameters (E), which
were determined based on experimental reaction rate constants
of carbocations with nucleophiles in dilute solutions (Figure
1).4(a),5 As another index, we tried to evaluate the stabilities of
carbocations using the dissociation energy (∆Edis) of alcohols
to form the corresponding carbocations and an OHꢀ anion usꢀ
ing B3LYP/6ꢀ31G(d) level calculations (Figure 2, SI 9). For
the carbocations whose E values were defined by Mayr, a linꢀ
ear relationship between the calculated ∆Edis 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 carꢀ
bocations can be predicted by simple calculations even for
several carbocations without kinetic data.
As a result, it was confirmed that Hꢀ, Fꢀ, or Clꢀsubstituted
diphenylmethyl cations and a fluorenyl cation were thermodyꢀ
namically far more unstable than other carbocations such as
triphenylmethyl, 1,5ꢀdiphenylꢀ1,3ꢀpentadienyl, and tropylium
cations, which were observed previously.2ꢀ4,8 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.
H-Mor
H3O+
trapped &
deactivated
OH
+ 2H+
H3O+
+
F
side reactions
F
F
F
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Scheme 2. Schematic of the mechanism for the preservation of
carbocations in H-Mor.
1
2
3
4
5
3.3 Generation and UVꢀVis 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
UVꢀVis spectroscopy. 1 exhibited no absorption in the range
of 350ꢀ550 nm (Figure S3b). No color development was obꢀ
served on the surface of silicaꢀalumina, solid zinc chloride, or
Amberlystꢀ15. As shown in Figure 3, H-Mor (Si/Al = 9)
changed color from white to bright yellow, giving a strong
peak at 450 nm in the UVꢀVis, which was assigned to the 4,4'ꢀ
difluorodiphenylmethyl cation (2).6a Although detailed conꢀ
firmation of 2 is discussed in sections 3.4ꢀ3.7, it is reasonable
to consider that 1 was adsorbed into the pores of the zeolite
and was protonated and subsequently dehydrated to afford 2.
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Figure 4. Changes in the UVꢀVis spectra of H-Mor upon
contact with 1: One hour later (red line), one day later (dark
blue line), and eight days later (green line).
3.4 Identification of 2 by 13CꢀMAS NMR
In order to determine the structures of 2 as well as other prodꢀ
ucts, a mixture of 1 and H-Mor was analyzed by 13CꢀCP/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). Notaꢀ
bly, if reactant alcohol 1 remained in the mixture, the peaks at
72 and 74 ppm corresponding to the Cꢀ1 of 1 should be obꢀ
served (Figure S4d); however, the corresponding peaks were
not observed, indicating that 1 was almost completely conꢀ
sumed.
3.5 Confirmation of the structure and timeꢀdependent changes
of 13Cꢀenriched alcohol 1’ and other species by 13CꢀMAS
NMR
Figure 3. UVꢀVis diffuse reflectance spectra of solid acids upon
contact with 1: H-Mor (red), H-ZSM-5 (gray), H-Beta (black),
H-Y (yellowꢀgreen), Amberlystꢀ15 (light blue), zinc chloride
(dark blue), and silicaꢀalumina (green). Inset shows yellow color
development of H-Mor containing 2 (white object in the yellow
powder is a Teflonꢀcoated stir bar).
In order to more definitely distinguish molecules 1, 2, and 3,
1’, which was >98% enriched with a 13C atom at the Cꢀ1 posiꢀ
tion,16 was mixed with H-Mor and analyzed by 13CꢀDD/MAS
NMR. Strong peaks corresponding to the methylene group of
4,4’ꢀdifluorodiphenylmethane (4’) were observed at
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 (ꢁH): H-Mor (ꢁH = 33.8 kcal/mol) > H-ZSM-5 (ꢁH
= 31.9 kcal/mol) > H-Beta (ꢁH = 30.4 kcal/mol) > H-Y (ꢁH
= 25.7 kcal/mol).14 As shown in Figure 4, the absorption at 450
nm of 2 in H-Mor remained for more than one week, although
some decrease in its intensity was observed after the first day.
Figure 5. 13CꢀCP/MAS NMR spectrum of a mixture of 1 having a
natural abundance of 13C and H-Mor. The asterisks show spinꢀ
ning side bands.
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41 ppm in addition to the carbonyl carbon of 4,4'ꢀ Accordingly, most of 2 was generated within 1 h, and no reacꢀ
1
2
3
4
5
6
7
8
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 6(a)). Again, the presence of alcohol 1’ around 75 ppm
was hardly detected.
tant alcohol 1 remained after 12 h of stirring with H-Mor.
Therefore, the two mixing conditions (1 h, high selectivity
condition; 12 h, full conversion condition) were adopted for
the subsequent experiments.
3.6 Additional confirmation of the presence of 2 upon expoꢀ
To evaluate the timeꢀdependent changes in the amounts of
chemical species in H-Mor by 13CꢀDD/MAS NMR, 1’ was
mixed with H-Mor at rt; the mixture was monitored every
hour. The individual spectra and timeꢀdependent 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 13CꢀDD/MAS NMR (Figure S4aA). The
peaks corresponding to 1 (1’) nearly disappeared after 8 h,
while those of 2 (2’) and 3 (3’) remained almost unchanged
after 10 h (Figures S4aI ꢀ S4aP).
sure 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 13CꢀDD/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 due to the generation of
some carbocation species. Compared to the spectrum shown in
Figure 6(a), which was obtained prior to the exposure to moisꢀ
ture, the peak at 192 ppm was lost and starting alcohol 1’ was
recovered, as noted by the appearance of a peak at 75 ppm
(Figure 6(b)). The result clearly indicated that excess water
attacks 2 to regenerate starting alcohol 1. Meanwhile, the peak
corresponding to protonated 4,4'ꢀdifluorobenzophenone (proꢀ
tonated 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.
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3.7 Extraction of chemical species in H-Mor with MeOH
In general, comparing the integral values of 13Cꢀ 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 H-Mor after
the mixture was stirred for 1 h; the production of 4,4ꢀ
difluorodiphenylmethyl methyl ether (6) from 2 and MeOH
(Scheme 3, Table 1 top row) was expected. It was roughly
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 13CꢀDD/MAS NMR, which suggested
that 1 was almost completely consumed after 1 h.
Figure 6. 13CꢀDD/MAS NMR spectrum of the mixture of 1’ and
H-Mor before (a) and after exposure to moisture (b).
4’
5’
1’+3’
2’
OH
1
Ar Ar
+ 2H+
MeOH attack
OMe
+
+ H3O+
2
to 2
+ H2O + 2MeOH2
Ar Ar
Ar
Ar
6: 9%
MeOH attack
to H3O+
H2O attack
OH
Ar Ar
to 2
+
+
+ 2MeOH
+ MeOH
2
2+ H2O + MeOH2
1: 40%
0
5
10
15
20
25
Scheme 3. Plausible reactions caused by adding excess MeOH
to 2 and H3O+ in H-Mor.
Time (h)
Figure 7. Timeꢀdependent changes in the peak areas of the
13CꢀDD/MAS NMR spectra of each chemical species (1’ and
3’, blue diamonds; 2’, red circles; 4’, green triangles; 5’, purꢀ
ple squares) in the mixture of 1’ and H-Mor.
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Table 1. Comparison of the yields (%) of 1ꢀ6 using H-Mor or
TfOH.[a],[b]
(a)
(b)
1
2
H2O
Total yield of 1,
3, 4, 5, and 6
(%)
3
4
5
6
7
8
9
Acid
1
6
3
4
5
H2O
F
H+
F
F
F
54 14
(40) (9)
30 0.3
0.7
99
H-Mor
TfOH
(38) (0.1) (0.8) (89)
45 40 85
(0) (0)[c] (0) (45) (36)
(81)
Figure 8. Schematic of the reaction regulation in the zeolite
pores; (a) conventional steric obstruction and (b) thermodyꢀ
namic stabilization as discussed in section 3.11.
0
0[c]
0
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[a] 1H NMR yields were calculated using mesitylene as the internal standard.
[b] The yields of 1-6 after 1 h and 12 h (in parentheses).
[c] The yield of 2.
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 perꢀ
formed. After the generation of 2 from 1 and H-Mor, the samꢀ
ple was repeatedly exposed to air and became colorless; after
evacuation at rt, the yellow color was recovered (Figure 9 aꢀf).
Such reversible color changes indicate that the cation generaꢀ
tion of 2 from 1 is in thermodynamic equilibrium, and that the
chromism was easily controlled by the amount of water in the
system.17
We also performed the same methanolꢀaddition experiment
after the mixture of 1 and H-Mor had been stirred for 12 h at
rt (full conversion condition). 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 solidꢀstate NMR spectra
after 12 h (Figures 5 and 6(a)). 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. Because
MeOH is a small, polar molecule, 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), strongly indicating that almost no side
reactions, such as FriedelꢀCrafts type reactions and subsequent
consecutive FriedelꢀCrafts 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), even though the peaks of 4 and 5 looked relaꢀ
tively large in the 13CꢀMASꢀNMR spectra (Figure 6).
Figure 9. The UVꢀVis 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 UVꢀVis cell
(right): just after mixing (a), after the first exposure to the air
(b), after the first evacuation at rt (c), after the second expoꢀ
sure to the air (d), after the second evacuation at rt (e), and
after the third exposure to the air (f).
3.8 Conventional explanation for reaction regulation in zeolite
pores
Conventionally, the reaction regulations (or reaction selectiviꢀ
ty) in the pores are interpreted to stem from steric obstruction
by the zeolite pore walls (Figure 8(a)). This effect would be
categorized as a kinetic regulation. For example, García et al.
claimed that α,ωꢀdiphenylꢀcapped 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,ωꢀ
diphenylꢀcapped 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 moistureꢀexposure experiment (Figure 6(b))
as well as the MeOHꢀaddition 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 longꢀterm
preservation of cation 2 in H-Mor cannot be explained by
conventional kinetic reasons.
3.10 Side reactions induced by homogeneous superacid
According to the discussion in section 3.9, the activation enerꢀ
gy 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 heterogeꢀ
neous as long as the acid is efficiently strong. Then, the sucꢀ
cessful preservation of cation 2 is entirely dependent on the
suppression of undesired sideꢀreactions with nucleophiles othꢀ
er 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 disproportional oxidaꢀ
tion/reduction of 1 with 2 (path B). The FriedelꢀCrafts alkylaꢀ
tion 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
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homogeneous superacid, CF3SO3H (TfOH), for 1 h at rt, 4 (4’), side reactions also cannot be prevented. In model (c) with
1
2
3
4
5
6
7
8
5 (5’), and 7 (7’) formed immediately, but 2 (2’) was not deꢀ
tected (Table 1 bottom row).
dense acid sites on the inner surfaces of the narrow pores (simꢀ
ilar to H-Mor), the number of acid siteꢀadsorbed 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 speꢀ
cies; thus, the bimolecular side reactions are hindered. Thereꢀ
fore, unstable cationic species like secondary carbocations can
exist as thermodynamically metastable species, and survive in
the narrow pores of acidic H-Mor (Figure 8(b)).
(a) sparse/dense acid sites on a nonporous catalyst
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+
(b) sparse acid sites on a porous catalyst
Scheme 4. Three types of the side reactions of carbocation 2
promoted by acid catalysts, and the corresponding calculated
activation energies (∆G‡ /kcal·molꢀ1) for the forward and
backward paths (see SI 10).
+
(c) dense acid sites on a porous catalyst
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).
Based on 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
time, only thermodynamically stable products were observed
through path B or C, after the reverse process of path A.
+
+
electroneutral
molecule
acid site
cationic
molecule
Scheme 5. Models of (a) sparse/dense acid sites on the surface
of a nonporous acid catalyst, (b) sparse acid sites on the inner
surfaces of a porous acid catalyst, and (c) dense acid sites on
the inner surfaces of an acid catalyst with narrow pores.
Paths A, B, and C involve secondꢀorder bimolecular reactions
between cation 2 and the electrically neutral reacting counterꢀ
parts of 1, 3, 4, or 5, and hence the reaction rate (v) is exꢀ
pressed as the product of three factors: v = k x [2] x [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 longꢀterm preservation of cation 2.
However, in homogeneous reactions, this is difficult because
the concentrations of all chemical species are uniform
throughout the system.
3.12 Side reactions in the presence of zeolites and requisites
for the preservation of 2
The quantum chemical 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 and c) that reaction B would not or
would hardly proceed in the pores (Scheme 6). The acid sites
on the inner surface of H-Mor are so strong and abundant that
the equilibrium between 1 and 2 could be overwhelmingly
shifted towards 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 completeꢀ
ly suppressed, in the same way as on the nonporous catalyst
(Scheme 5(a)). 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 proꢀ
ceed or would proceed very slowly, as shown in Scheme 6. In
fact, carbocation 2 and ether 3 were observed as major prodꢀ
ucts, and almost no alcohol 1 and small amounts of 4 and 5
were detected by 13CꢀCP/MAS NMR (Figure 5) inside or outꢀ
side the pores.
3.11 Mechanism for the preservation of secondary carboꢀ
cations in H-Mor
In contrast, in a heterogeneous catalytic system, the concentraꢀ
tions of the chemical species depend on their locations around
the catalyst (Scheme 5). In model (a) with sparse or dense 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 dense 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) with sparse acid sites on
the inner surfaces of the narrow pores of the catalyst, the numꢀ
ber of acid sites is related to the number of molecules adꢀ
sorbed on the acid sites. However, if the total molecular volꢀ
ume is much lower than the pore volume of the catalyst, elecꢀ
trically 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
In summary, because H-Mor has adequate pores in terms of
size and shape, as well as dense, strong acidic sites for the
reaction with alcohol 1, labile cation 2 can be generated and
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conserved for a long time (hours to days) even at ambient al days) at ambient temperature, as confirmed by UVꢀVis (secꢀ
tion 3.3) and 13CꢀMAS NMR (section 3.4). It was confirmed
that at least 50% of 1 was converted into 2 by the quenching
experiment with MeOH (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 equiꢀ
librium 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.
1
2
3
4
5
6
7
8
temperatures.
Inside pore
A: etherification
OH
B: disproportionation
C: polymerization
Ar
Ar
Ar
Ar
2
2
1
Outside pore
9
A: etherification
OH
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B: disproportionation
C: polymerization
Ar
Ar
Ar
Ar
1
The reasons why the carbocation can be maintained in H-Mor
are as follows: (1) the equilibrium between 1 and 2 is shifted
in favor of the cation formation due to sufficiently strong acid
sites of H-Mor, by which the released water molecules are
captured as H3O+ presumably in the subꢀchannels of H-Mor
directly connected to the mainꢀchannel; (2) side reactions are
bimolecular reactions between 2 and electrically neutral nuꢀ
cleophiles of 1, 3, 4, 5, and 7. The dense 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 hoꢀ
mogeneous acid, TfOH, 2 immediately reacts with 1, 3, 4, 5,
and 7 in solution to form side products as soon as 2 is generatꢀ
ed 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 due to the zeolite pore walls. In the
pores of H-Mor, 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 HꢀMor for the generation and
preservation of 2 facilitated the observation of a labile secondꢀ
ary carbocation via 13CꢀMAS NMR for the first time. Moreoꢀ
ver, novel insights into specific acid catalysis in zeolites with
dense acid sites on the inner surface of micropores were obꢀ
tained. This study offers a new avenue to clarify the physicoꢀ
chemical properties of unstable carbocations, and also eluciꢀ
date and control organic reaction processes via carbocations in
the micropores of zeolites.
Scheme 6. Schematic models of the reactions inside and outꢀ
side the pores of H-Mor.
3.13 The formation of other unstable carbocations in H-Mor
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, as well as 9ꢀfluorenol,
in H-Mor (Figure 10), and found that they could also be
formed successfully; notably, these carbocations were considꢀ
ered to be much more unstable than 2 according to the discusꢀ
sion in section 3.1. The colors of the reaction mixtures were
bright yellow, bright orange, and pale red,20) which correꢀ
sponded to their UV absorption. Notably, the corresponding
carbocation from benzhydrol formed in over 59% yield, but
the FriedelꢀCrafts type alkylation of the diphenylmethyl cation
via path C was suppressed inside the pores, even though benꢀ
zhydrol has no substituents in the paraꢀposition where Friedelꢀ
Crafts alkylation is prone to occur (Figure S5b).
This paper is dedicated to Professor Teruaki Mukaiyama in
celebration of his 90th birthday (Sotsuju).
ASSOCIATED CONTENT
SUPPORTING INFORMATION
Representative experimental procedures, spectral and analytical
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 10. UVꢀVis spectra of H-Mor after being mixed with 4,4'ꢀ
difluorobenzhydrol (black line), 4,4'ꢀdichlorobenzhydrol (brown
line), benzhydrol (red line), or 9ꢀfluorenol (blue line).
AUTHOR INFORMATION
Corresponding Author
cmasui@mail.ecc.uꢀtokyo.ac.jp
conaka@mail.ecc.uꢀtokyo.ac.jp
4. CONCLUSIONS
ACKNOWLEDGMENT
The present study was partially supported by JSPS KAKENHI
Grant Numbers JP23105511 and JP16H04562.
By simply mixing benzhydrol analog 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 severꢀ
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15) We preliminarily optimized the experimental conditions,
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such as the Si/Al ratios of H-Mor, molar ratio of 1 to the acid
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17) Indeed, the hydrated white sample gradually turned yellow
under a flow of dry nitrogen, indicating that the dehydration of
the hydrated sample proceeded easily.
18) The activation energy for the reaction of 2 with 1 was imꢀ
possible to calculate because the paraꢀposition on a benzene
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lated the activation energy for the reaction of benzhydrol (1’’)
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See SI 6 for details.
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o
ered by warming (at 80 C even in the air) the “white sample”
that had been exposed to moisture the red color of the fluoꢀ
renyl cation cannot be regenerated by heating (at over 200 oC)
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ered instead of the number of the Al sites. See SI 7.
13) Although the absorption peak at 480 nm on H-ZSM-5 was
slightly different in wavelength from that of the other zeolites,
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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
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Yoichi Masui,†,*Taiki Hattori,‡ and Makoto Onaka†,*
H+
H+
H+
H+
OH
H
O
H
H
F
F
F
F
H+
H+
H+
H+
H-Mordenite
–H2O
Reversible Generation &
Preservation of Carbocations
at Ambient Temperature
+H2O
ACS Paragon Plus Environment