Br?nsted Basicities and Nucleophilicities of N-Heterocyclic Olefins in Solution: N-Heterocyclic Carbene versus N-Heterocyclic Olefin. Which Is More Basic, and Which Is More Nucleophilic?

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nsted Basicities and Nucleophilicities of N‑Heterocyclic Oleﬁns in

Solution: N‑Heterocyclic Carbene versus N‑Heterocyclic Oleﬁn.

Which Is More Basic, and Which Is More Nucleophilic?

Zhen Li, Pengju Ji,* and Jin-Pei Cheng*

Cite This: J. Org. Chem. 2021, 86, 2974 −2985

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sı Supporting Information

ABSTRACT: A Bronsted basicity scale comprising nine repre-

sentative N-heterocyclic oleﬁns (NHOs) was established by

measuring the equilibrium acidities of their corresponding

precursors in DMSO using an ultraviolet−visible spectroscopic

method. The basicities (pK s) of the investigated NHOs cover a

range from 14.7 to 24.1. The basicities of unsaturated NHOs are

stronger than those of their N-heterocyclic carbene (NHC)

analogues; however, the basicities for the saturated ones are much

weaker than those of their NHC analogues, which is largely due to

the aromatization eﬀect that intrinsically inﬂuences the acidic

dissociations of NHC and NHO precursors. The nucleophilicities

of four NHOs were measured photometrically by monitoring the kinetics of reactions of these NHOs with common reference

electrophiles for quantifying nucleophilic reactivities. In general, the nucleophilicity of the NHOs is much stronger than that of

commonly used Lewis bases such as Ph P or DMAP [4-(dimethylamino)pyridine] but weaker than that of their NHC analogues;

however, caution should be taken when generalizing this conclusion to a wide range of electrophiles with distinctively electronic and

structural properties.

INTRODUCTION

From a preparative point of view, NHOs can be readily

obtained by deprotonating the corresponding precursors using

strong bases such as potassium hydride (KH). However, such

a strong base is not compatible with many functional groups,

which greatly reduces the practicality of this convenient method.

Therefore, the knowledge of the precise basicities (hereafter

■

As an emerging type of organocatalyst and ligand for metal

complexes, N-heterocyclic oleﬁns (NHOs), also known as

heterocyclic ketene aminals (HKAs), deoxy Breslow inter-

mediates, or ylidic oleﬁns, play an increasingly important role

in modern organic synthesis. Characterized by their electron-

referring to the Bronsted basicities) of NHOs is crucial for the

rich exocyclic CC bond, NHOs were found to have relatively

strong basicity and nucleophilicity, which can be understood on

the basis of their mesomeric structures. As a consequence of

resonance stabilization, a partial positive charge and a partial

negative charge of NHO are located on the N-heterocyclic

moiety and the exocyclic CC bond, respectively; therefore,

the oleﬁnic double bond is highly polarized, thus rendering the

C2 atom even more nucleophilic than the nitrogen centers of the

selection of appropriate bases as well as substrates to achieve the

desired synthetic goals. NHOs can be considered as the

alkylidene derivatives of N-heterocyclic carbenes (NHCs) and

as the structural analogues to a new type of superbase N-

heterocyclic imines (NHIs); in this context, how basic and

nucleophilic NHOs are compared with NHCs and NHIs is one

of the most fundamental questions regarding the properties and

reactivities of NHOs that greatly interests the chemical

community.

Surprisingly, although NHOs have found extensive applica-

tions in modern organic synthesis in recent years (vide supra),

studies of the fundamental aspects, such as the thermodynamic

ring (Scheme 1). In view of their interesting features, NHOs

have been recognized as promising organocatalysts and found

applications in a broad range of chemical transformations,

including activation of small molecules (such as CO

sequestration), base-catalyzed alkylation, hydroborylation,

silylation, transesteriﬁcation reactions, ring-opening polymer-

Received: December 4, 2020

Published: January 19, 2021

ization of epoxides and lactones, C−F bond activation, and

formation of super electron donors. In addition, NHOs have

also been combined with Lewis acids to form a series of novel

frustrated Lewis pairs (FLPs) that have been successfully applied

4b,14

to polymerization, main group chemistry,

and metal-

mediated catalysis (Scheme 1).

2021 American Chemical Society

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Scheme 1. Mesomeric Structures of NHOs and Their Applications in Modern Organic Synthesis

Figure 1. Indicators (HIns) used to determine the pK and NHOs and their precursors studied in this work. FH = ﬂuorene. HZF = 9-

ﬂuorenonephenylhydrazone. HZFP2 = 9-ﬂuorenone(4-chlorophenyl)hydrazone.

properties and the kinetic behaviors, i.e., the basicities and

nucleophilicities, of NHOs are rare; previously, there have been

only a few sporadic reports that involved a rather limited number

of NHOs in this respect. For example, Bordwell and co-workers

which may potentially facilitate the choice of suitable bases for

the deprotonation of NHO precursors and act as the references

for the calibration of theoretical methods. Second, because the

knowledge of thermodynamic and kinetic properties of

structurally similar NHCs and NHOs was often required to

rationalize the experimental observations in the reactions

involving NHO or NHC, in this regard, the results generated

from this work provide precisely measured thermodynamic and

kinetic parameters that facilitate comparisons of basicity and

nucleophilicity between NHCs and NHOs, which may shed

light on the underlying principles for their selection and

application in organic transformations.

reported the pK scale of a series of thiazole-based NHO

precursors in DMSO. Nguyen and co-workers reported that

the pK of an NHO precursor, i.e., 1,2,3,4,5-pentamethylimida-

zolium iodide, falls between 24.3 and 28.4 in acetonitrile-d by

semiquantitative NMR titration experiments. Mayr and co-

workers reported the nucleophilicities of ﬁve C2-aryl-

substituted NHOs in THF and uncovered the eﬀect of

aromaticity on the catalytic activities of N-heterocyclic

carbenes. Alternatively, theoretical calculations were also

RESULTS AND DISCUSSION

employed to predict the proton aﬃnities (PAs) and the

■

nucleophilic indices of NHOs in the gas phase; in addition,

very recently, the basicities of a series of NHOs and the

Basicities in DMSO. As shown in Figure 1, nine NHO

precursors commonly used in organic synthesis, including

imidazoline (1a), hexahydropyrimidine (1b), imidazole (1c−

1f), and triazole-based precursors (1g−1i), were studied in this

work. NHCs with bicyclic scaﬀolds have signiﬁcantly promoted

the development of NHC-enabled asymmetric transforma-

stabilities of NHO−CO adducts were also calculated in

solution. To the best of our knowledge, so far there has

been no systematic experimental investigation of the basicities

and nucleophilicities of NHOs in solution; herein, we report the

basicities of several commonly used NHOs in DMSO and their

nucleophilicities in THF.

tions; following this logic, NHOs 1h and 1i containing bicyclic

scaﬀolds were synthesized and studied. Because DMSO has

been proven to be a suitable solvent for the measurement of

The purpose of this work is twofold. The ﬁrst is to provide a

necessary experimental basicity scale for NHOs in solution,

acidities of nonpaired ions for a wide range of weak C−H

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f (Figure 2 d), as indicated by the follow-up DEPT-135

Article

4,25

acids,

the basicities of NHOs, namely pK_a_Hvalues of

experiment (Figure 2e). The NMR experiments showed that the

deprotonation of the NHO precursor and protonation of NHO

in DMSO solution were quick and reversible, and no other

undesired complication was observed under the experimental

conditions.

NHOs, which equal the pK values of their corresponding

precursors, were measured by the indicator overlapping method

IOM) via ultraviolet−visible (UV−vis) spectrophotometric

titration experiments in DMSO. Eight ﬂuorene-derived

carbon and nitrogen acids with known pK values in DMSO

were chosen as the indicators [HIns (Figure 1)]. These

indicators are suitable for the precise acidity determination in

DMSO as shown previously,

ranges of the corresponding indicator anions were extended to

50 nm, thus avoiding potential interference caused by the

(

pK_a^H^A= pK_a− log₁₀K_e_q= pK_a− log [HIn][A ]

HIn

−

23,25

−

and the UV−vis absorption

/([In ][HA])

(1)

To accurately measure the pK s of the NHO precursors in

NHOs that have an absorbance of normally <450 nm (Figure

S2). Typically, the IOM requires establishing an equilibrium

between the indicator anion (In ) and the NHO precursor in

DMSO, a working calibration curve was needed. This was done

by stepwise titrating a DMSO solution of a suitable indicator

−

into a K DIMSYL solution prepared from KH and DMSO

DMSO solution (Scheme 2). To verify this, deprotonation of

the NHO precursor and protonation of NHO were performed

and monitored by NMR spectroscopy.

(Scheme S1) and plotting the absorbance versus the

concentration (Figure 3a). After the complete consumption of

K DIMSYL , as indicated by the absorbance of the solution not

increasing with a further addition of an indicator solution

(Figure 3a, purple curve), several aliquots of the NHO precursor

1f solution were then added to the indicator anion solution. The

−

Scheme 2. Equilibrium between the Indicator Anion (In⁻)

and NHO Precursor

equilibrium constant (K ) could be derived because the

concentration of each species in the equilibrium should be

determined through a Beer’s law plot in combination with the

law of conservation of mass and charge (Figure 3b; see the

Supporting Information for details). As long as the pK value of

HIn

the indicator (pK_a) is known, the pK of NHO precursor 1f

can be calculated according eq 1. To make sure a steady

equilibrium was achieved, UV−vis scans were performed

repeatedly until the obtained spectra agreed well for each

addition (Figure 3b). For the purpose of accuracy, at least two

independent runs were carried out for each NHO precursor,

whereby the standard deviations (SD) were calculated. It was

With NHO precursor 1f as an example, as shown in Figure 2,

upon deprotonation by potassium hydride (KH) in DMSO

solution, NHO precursor 1f was converted into NHO 2f cleanly,

as indicated by the C{1H} NMR spectrum (Figure 2b) and a

negative DEPT-135 signal at 47.6 ppm (Figure 2c). Then a

25b

proton donor NH BF , whose pK in DMSO is 10.4, was

found that the obtained pK values of NHO precursors have a

added, and NHO 2f was completely recovered as its precursor,

very small uncertainty (SD ≤ ±0.05 pK units), and the results

are listed in Table 1 .

Figure 2. C{ H} NMR and DEPT-135 spectra of the deprotonation of NHO precursor 1f (b and c) and protonation of its conjugated base NHO 2f

d and e) in DMSO.

(

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−

−4

Figure 3. (a) UV−vis spectra of a K DIMSYL solution (6−8 × 10 M) upon addition of an indicator solution. The black arrow shows the increase in

absorbance upon addition of an indicator. (b) UV−vis spectra of the indicator anion solution upon addition of an NHO precursor solution. The black

arrow shows the decrease in absorbance upon stepwise addition of NHO precursor 1f.

Table 1. Measured pK Values of NHO Precursors in DMSO

As shown in Table 1, the established basicity scale for NHOs

with diﬀerent backbone structures and ﬂanking substituents

NHO

precursor

pK of

measured

calculated

covers a range from 14.70 to 24.10. The pK values of 1a and 1b

indicator

are <20 in DMSO, comparable to that of phenol (pK = 18.0 in

9-(m-ClC H )-FH

16.85

18.78

24.35

19.90

21.34

16.85

14.90

14.15

17.90

16.85

17.00

19.0

19.2

22.1

24.5

21.4

23.5

17.2

16.5

25b,d

DMSO).

The size of ring brings about 2 pK units diﬀerence

9-(o-MeC H )-FH

(ﬁve-membered 1a vs six-membered 1b), and this is consistent

9- Bu-FH

24.10

19.95

20.90

16.45

14.70

with the results from the theoretical pK prediction in DMSO.

carbazole

9-Bn-FH

Table 1 also shows that the basicity of imidazole-based NHOs is

obviously greater than those of imidazoline-based ones by as

much as 7.1 pK units (1c vs 1a). The experimental results from

9-(m-ClC H )-FH

HZF

HZFP2

9-Ph-FH

this work conﬁrmed previous theoretical predictions of PA

17.60

19.2

and pK_a_Hvalues, in which the large basicity diﬀerence

originated from aromatization: the protonation of unsaturated

imidazole-based NHOs generates stable aromatic imidazolium

cations and thus increased aromaticity, while the reverse is true

for the saturated imidazoline-based NHOs, as suggested by the

-(m-ClC H )-FH

6 4

9-(m-ClC H )-FH

16.65

−

The structure and abbreviation are shown in Figure 1. From ref 25.

The mean value of at least two independent runs with SD ≤ ±0.05

pK units. From ref 21. SD ≤ ±0.10 pK units.

NICS(1) calculation from a recent work by Wang et al.

Interestingly, NHO precursor 1f is only slightly more acidic than

Figure 4. Comparisons of pK values of NHC and NHO precursors with similar backbone structures and ﬂanking substituents in DMSO. The data for

NHC precursors are from refs 29 and 30.

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Table 2. Quinone Methides Employed as the Reference Electrophiles in This Work

saturated 1a but much more acidic than the rest of the

imidazole-based NHO precursors (1c−1e), which is most

probably due to the fused benzene ring that signiﬁcantly

stabilizes the mesomeric form with partial charges of the

representative combinations of an electrophile 3d with diﬀerent

NHOs were studied to conﬁrm this. As demonstrated in Scheme

3, the reactions proceeded successfully in THF and gave the

corresponding NHO 2f.

Scheme 3. Reactions of Electrophile 3d with Various NHOs

in THF

The basicities of triazole-based NHOs are weaker than those

of imidazole-based NHOs as expected, with the presence of the

additional electronegative nitrogen in triazole-based NHOs, and

electrons are withdrawn from the center of basicity of NHOs,

which accounts for their lower basicities compared to those of

the corresponding imidazole derivatives. For triazole-based

NHOs, the bicyclic scaﬀold signiﬁcantly enhances their basicity,

and the order of basicities follows a trend similar to that for the

triazole-based NHCs; i.e., the pK_a_Hvalue of pyrrolidine-derived

NHO (1h) is larger than those of morpholine-derived (1i) and

acyclic-derived ones (1g). Because the side chain at position 5

of the morpholine ring does not cause an appreciable change in

the pK values of chiral triazolium salts, a similar scenario

could be expected for the analogous chiral NHO precursors,

which lays the foundation for the design of novel chiral NHOs in

future asymmetric syntheses.

Moreover, as one of the objectives of this work (vide supra),

the pK_a_Hvalues of NHCs and NHOs with similar backbone

structures and ﬂanking substituents in DMSO were examined.

As shown in Figure 4, two opposing trends exist; i.e., for those

NHCs and NHOs that contain unsaturated aromatic rings

(

Figure 4, entries 3−5 and 7−9), the basicity of NHOs is slightly

expected Michael adducts 4 as the only isolated products in 56−

stronger than that of NHCs, whereas the saturated NHCs are

more basic than their NHO analogues (Figure 4, entries 1 and

94% yields. The structures of products 4a−4f were determined

by general spectroscopic methods (Experimental Section).

Having conﬁrmed the formation of adducts as expected, we

paid attention to the rates of these reactions.

The kinetics for the reactions of NHOs with the reference

electrophiles were measured photometrically in THF at 20 °C

by monitoring the decrease in absorbance of 3a−3e (350 nm ≤

). Again, aromatization plays as a “regulator” that intrinsically

dictates their basicities because the aromaticity of NHCs (both

saturated and unsaturated) remains almost unchanged upon

protonation, while that for unsaturated NHOs increases

signiﬁcantly and thus renders the unsaturated NHOs as more

20a,21

basic than their NHC analogues.

≤ 380 nm) over time. To achieve pseudo-ﬁrst-order

max

Nucleophilicities in THF. To quantify the nucleophilic

conditions, a large excess of NHO (>10 equiv) over the

electrophile was used in all of the kinetic measurements. As a

result, the decay function A = A exp(−k t) + C was ﬁtted to

parameters of NHOs, a linear free energy relationship developed

by Mayr et al. was employed, in which nucleophiles are

characterized by a solvent-dependent parameter N and a

sensitivity parameter s , and electrophiles are characterized by

a solvent-independent parameter E (eq 2). Because a single

reference electrophile is inadequate for comparing the

nucleophilicities of various NHOs, a series of p-quinone

methides (Table 2) that have been served as reference

electrophiles were used in this work. These quinone methides

diﬀer widely in reactivity, while the steric congestion of the

reaction center is almost the same.

obs

the observed time-dependent absorbance (A) in a least-squares

sense to provide ﬁrst-order rate constant k_o_b_s(Figure 5). A linear

correlation was observed by plotting k_o_b_sversus the concen-

tration of NHO, and the slope corresponds to second-order rate

constant k . This way, second-order rate constants k for the

reactions of four NHOs (2a, 2b, 2d, and 2f) with reference

electrophiles 3a−3e were measured and are summarized in

Table 3. It worth noting that the colored THF solution of

triazole-based NHO 2g absorbs in the region of 300−450 nm,

which strongly interfered with the characteristic absorbances of

most reference electrophiles; therefore, unfortunately, the

nucleophilicity parameter for 2g could not be determined.

Only the kinetic data for the reaction of 2g with a single

log₁₀k (20 °C) = s (N + E)

(2)

The kinetics associated with NHOs and quinone methides

should give rise to analogous Michael adducts; a few

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NHOs. Figure 6 illustrates that the logarithm of second-order

Article

to yield the product or alternatively could undergo a general

base-catalyzed deprotonation (k , NHO as the base) to give

deoxy Breslow intermediate G followed by a fast protonation to

form the product. However, whether intermediate F would

undergo a base-catalyzed deprotonation process strongly

depends on the basicities of NHO and G, as well as the steric

hindrance from the ﬂanking substituents of the NHO. The

introduction of an alkyl chain to the exocyclic C2 atom of NHO

normally increases its basicity, which suggests that G would be

more basic than F. Consequently, in most cases, the

deprotonation of intermediate F would not occur even with a

large excess of NHO, and the second-order kinetics were

observed, i.e., ﬁrst-order in both NHO and E. However, for

NHO 2c, a relatively small steric hindrance and additional

aromatization may promote the NHO-catalyzed deprotonation

of F; as a result, deviations from the second-order kinetic are

thus encountered. As a result, the formation of intermediate G

consumes 2c and G turns to product upon a fast protonation,

which also explains why the reaction has a relatively low yield of

4c (Scheme 3) and the product analysis failed to isolate any

byproduct. However, described above is a tentative ration-

alization of the unusual kinetic behavior of 2c toward the

reference electrophiles, and further investigations are needed to

elucidate the detailed mechanism.

Figure 5. Decay of the absorbance (gray square) of electrophile 3e at

00 nm and exponential ﬁt (red line) for the pseudo-ﬁrst-order reaction

−

−1

−5

−1

of NHO 2f (7.86 × 10 mol L ) with 3e (3.41 × 10 mol L ) at 20

−1

C in THF (k_o_b_s= 0.103 s ). The inset shows the determination of the

second-order rate constant (k = 1.43 × 10 L mol s ) from the

−1 −1

^d^e^p^eⁿ^d^eⁿ^c^e^o^f^kobs on the concentration of NHO 2f.

Table 3. Second-Order Rate Constants k for the Reactions of

NHOs with Reference Electrophiles 3a−3e in THF at 20 °C

With the available second-order rate constants k in hand, we

then sought to explore the nucleophilic parameters for the

reference electrophile [3h (Table 2)] were obtained (Table

S25 ). NHOs 2e, 2h, and 2i encountered a similar issue;

therefore, their nucleophilicity parameters could not be

determined by this approach either.

To our surprise, the k_o_b_svalues for the reactions of NHO 2c

with the reference electrophiles [3e−3g (Table 2)] increased

exponentially, instead of linearly like the others, with NHO

concentration (Tables S11 −S13 ), indicating a deviation from

the ﬁrst-order kinetics with respect to NHO 2c. To understand

this unusual phenomenon, let us ﬁrst consider the possible

mechanism for the reaction of NHOs with quinone methides as

shown in Scheme 4. The nucleophilic attack of NHO on

electrophile E generates zwitterion intermediate F, which may

Figure 6. Plot of log₁₀k₂vs E for the reactions of NHOs with the

reference electrophiles in THF at 20 °C.

rate constants (log₁₀k₂) for the reactions of NHOs with

electrophiles 3a−3e correlates linearly with the corresponding E

parameters according to eq 2, whereby the slope corresponds to

be directly protonated by the proton donor (HBF in this case)

nucleophile-speciﬁc parameter s and nucleophilicity parameter

Scheme 4. Possible Mechanism for the Reaction of NHO with Quinone Methides in THF

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Figure 7. Bronsted basicities and nucleophilicities for NHOs.

N equals the ratio of the intercept to the respective slope. These

nucleophilic parameters are listed in Table 3.

The correlation lines of NHOs 2a, 2b, and 2f are almost

parallel with each other, which suggests that their nucleophilic-

ities are practically independent of the reference electrophile.

Introducing N-Mes groups into the NHO structure results in

steric congestion, thus rendering a greater sensitivity of NHO 2d

studied in this work exhibited nucleophilic reactivities much

higher than those of Ph P, DMAP, and DBU (Figure 7).

Nevertheless, the nucleophilicities of NHCs and NHOs

should be compared with caution because their sensitivity

parameters (s ) in THF are quite diﬀerent; this is because the

reaction center of NHC is hindered by the N substituents, while

that of the NHO (exocyclic C2 atom) stretches out, thus

avoiding a strong steric perturbation from the ﬂanking bulky

groups. Taking imidazole-derived NHC 2d′ and NHO 2d as

to the electrophilicities of reaction partners (s = 0.79 for 2d).

As shown in Table 3, the nucleophilicities for four NHOs

studied in this work are in a narrow range, i.e., 17 < N < 20,

indicating that these NHOs are slightly more nucleophilic than

aryl-substituted NHOs (N values in a range of 14−17 in THF)

examples (Table 4), we ﬁnd sensitivity parameter s (0.45) of

Table 4. Comparisons of Imidazole-Derived NHC 2d′ and

with similar s values. This could be rationalized by the

NHO 2d

resonance eﬀect of the aryl ring in the latter that decreases the

electronic density of the exocyclic C2 atom and consequently

attenuates their nucleophilicity. Ring size has a very small eﬀect

on the nucleophilicity of the saturated NHO (2a vs 2b);

however, a fusion of the benzene ring signiﬁcantly increases the

nucleophilicity from 17.80 to 19.84 (2d vs 2f).

In 2011, Mayr reported the nucleophilicities of three

representative NHCs together with three Lewis bases, i.e.,

k in liters per mole per second, with reference electrophile 3d,

whose E = −15.83. k in liters per mole per second, with reference

Ph P, DMAP, and DBU (diazabicyclo[5.4.0]undecene).

electrophile 3h, whose E = −12.18. Data taken from ref 30. Data

follow-up report revealed the inﬂuence of N substituents on the

taken from ref 33. Data taken from ref 34 and k derived from eq 2.

nucleophilicity and Lewis basicity of NHCs. Fortunately, the

reactivities of these NHCs and Lewis bases toward reference

electrophiles were also measured in a THF solution at 20 °C,

which enables us to compare the kinetic properties of these

commonly used organocatalysts under the same conditions. The

sensitivity parameter s values for Ph P, DMAP, and DBU are

k derived from eq 2.

2d′ is much smaller than that of its structural analogue, 2d

(0.79) and a direct comparison of N values for 2d′ and 2d

suggests that the former is more nucleophilic than the latter

(Table 4); however, the kinetic measurements showed that

NHC 2d′ reacts with reference electrophile 3d 11.4 times faster

than does NHO 2d, whereas 2d is more reactive toward the

electrophile 3h than 2d′ (Table 4). It is obvious that the

nearly the same (0.66, 0.66, and 0.67, respectively), which are on

the same level as those of NHOs involved in this work; however,

nucleophilicity parameters N for Ph P, DMAP, and DBU are

3.59, 15.90, and 16.12, respectively. Obviously, the NHOs

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nucleophilicities of the NHC and NHO with bulky N

substituents are dependent on the reaction partners. It is also

worth noting that basicity fails to act as a useful guide for

predicting the nucleophilic reactivity in this case because the

steric demand for a proton is much weaker than that of quinone

methide reference electrophiles such as 3d and 3h.

EXPERIMENTAL SECTION

General Information. All of the reagents and solvents were

■

purchased from commercial sources and used as received unless

otherwise noted. H and C{ H} NMR spectra were recorded on a

Bruker Avance III HD 400 MHz spectrometer. Chemical shifts (δ) are

reported in parts per million with the residual protio solvent as a

reference for H NMR spectra in deuterated solvent samples (CDCl , δ

Interestingly, NHO 2f reacts faster than NHOs 2a and 2b

Figure 6 and Table 5) with all of the reference electrophiles

.26; DMSO-d , δ 2.50; toluene-d , δ 2.09, 6.98, 7.00). H NMR data

(

are reported as follows: chemical shift, multiplicity (s, singlet; d,

doublet; t, triplet; q, quartet; br, broad; m, multiplet), and coupling

constant (hertz). HRMS spectra were recorded on a Thermo-Scientiﬁc

Q-Excative Orbitrap analyzer. UV−vis spectra were recorded on a

Hitachi U-3900H spectrometer. Melting points were uncorrected. The

kinetics were measured by a HI-TECH KinetAsyst Cryo Stopped-Flow

System with a UV−vis spectrophotometer as the detector.

^T^a^b^l^e⁵^.^C^o^m^p^a^rⁱ^s^oⁿ^s^o^f^p^KaH Values and Nucleophilicities

of NHO

Preparation of NHO Precursors. 1,2,3-Trimethyl-4,5-dihydro-

H-imidazol-3-ium Tetraﬂuoroborate (1a). A 100 mL round-

bottom ﬂask charged with triethyl orthoacetate (6.48 g, 40 mmol, 1.0

equiv), NH BF (4.20 g, 40 mmol, 1.0 equiv), and N ,N -dimethyl-

ethane-1,2-diamine (3.52 g, 40 mmol, 1.0 equiv) was heated in an oil

bath at 120 °C for 3 h. After the reaction mixture had been cooled to

In liters per mole per second, with reference electrophile 3d, whose

room temperature, the crude product was recrystallized from ethanol to

E = −15.83.

yield white needles: 7.94 g, 97% yield; H NMR (400 MHz, DMSO-d )

δ 3.76 (s, 4H), 3.03 (s, 6H), 2.20 (s, 3H); C{ H} NMR (101 MHz,

DMSO-d ) δ 166.2, 49.3, 33.3, 10.0; HRMS (ESI) m/z [M ] calcd for

C H N 113.1073, found 113.1075.

a−3d, though the basicities of 2a and 2b are stronger than that

,2,3-Trimethyl-3,4,5,6-tetrahydropyrimidin-1-ium Tetraﬂuoro-

of 2f. Because the steric congestion and electronic nature of the

reaction center are similar, presumably, again this is due to the

aromatization eﬀect playing a role in unsaturated NHO 2f (vide

supra), which stabilizes not only the ground state in its

protonation but also the transition states in the nucleophilic

attack of reference electrophiles. It should be noted that the

measurement of the reaction rates and basicities of these NHOs

was performed in a diﬀerent solvent, i.e., in THF and DMSO,

respectively; however, it is known that the acidity order of a

borate (1b). NHO precursor 1b was prepared according to the

same procedure as 1a from N ,N -dimethylpropane-1,3-diamine (4.08

g, 40 mmol, 1.0 equiv), triethyl orthoacetate (6.48 g, 40 mmol, 1.0

equiv), and NH BF (4.20 g, 40 mmol, 1.0 equiv): white solid; 6.94 g,

81% yield; H NMR (400 MHz, DMSO-d

.15 (s, 6H), 2.27 (s, 3H), 1.96 (p, J = 6.0 Hz, 2H); C{ H} NMR (101

MHz, DMSO-d ) δ 161.7, 48.0, 40.5, 19.0, 16.0; HRMS (ESI) m/z

) δ 3.39 (t, J = 6.0 Hz, 4H),

[

M ] calcd for C H N 127.1230, found 127.1234.

7 15 2

10a

,2,3,4,5-Pentamethyl-1H-imidazol-3-ium Iodide (1c). 1,2,4,5-

Tetramethylimidazole (5.00 g, 40 mmol, 1.0 equiv), MeI (17.0 g, 120

mmol, 3.0 equiv), and 60 mL of acetonitrile was mixed in a round-

bottom ﬂask and heated to reﬂux in an oil bath for ∼8 h. The reaction

mixture was cooled to room temperature, and the solvent was removed

in vacuo. The residue was washed with ethyl acetate (3 × 100 mL) and

series carbon acids in THF is consistent with that in DMSO.

CONCLUSIONS

■

In summary, we have established the ﬁrst experimental Bro

nsted

dried under high vacuum to yield the title compound as an oﬀ-white

basicity and nucleophilic reactivity scale of a series of commonly

used NHOs in solution. Aromatization, as an intrinsic factor,

plays an important role in dictating the basicity and

nucleophilicity of NHOs, which closely depends on the

backbone structure of NHOs. Collectively, the basicities of

unsaturated NHOs are stronger than those of their parent

NHCs; however, in contrast, the basicities for the saturated ones

are much weaker than those of their NHC analogues in DMSO.

This is largely due to the aromaticity eﬀect that works as an

intrinsic basicity regulator during the protonation of NHO or

NHC. The nucleophilicity of the NHOs involved in this work is

much stronger than those of commonly used Lewis bases, such

solid: 8.6 g, 81% yield; H NMR (400 MHz, DMSO-d ) δ 3.62 (s, 6H),

.58 (s, 3H), 2.21 (s, 6H); C{ H} NMR (101 MHz, DMSO-d ) δ

42.6, 124.9, 31.8, 9.9, 8.1; HRMS (ESI) m/z [M ] calcd for C H N

39.1230, found 139.1234.

,3-Dimesityl-2-methyl-1H-imidazol-3-ium Iodide (1d). NHO

13c

precursor 1d was prepared according to the same procedure as 1g (vide

infra) from 1,3-dimesitylimidazolium chloride (2.0 g, 5.9 mmol),

Na[N(SiMe ) ] (2 M in THF, 3.2 mL), and MeI (1.2 mL): light yellow

solid; 1.85 g, 70% yield; H NMR (400 MHz, DMSO-d

H), 7.24 (s, 4H), 2.36 (s, 6H), 2.18 (s, 3H), 2.06 (s, 12H); C{ H}

NMR (101 MHz, DMSO-d ) δ 145.3, 140.9, 134.5, 130.0, 129.6, 123.8,

) δ 8.19 (s,

0.7, 16.8, 9.3; HRMS (ESI) m/z [M ] calcd for C H N 319.2169,

22 27 2

found 319.2168.

as Ph P, DMAP, and DBU, and slightly stronger than those of

,3-Dimethyl-1-phenyl-1H-imidazol-3-ium Iodide (1e). NHO

precursor 1e was prepared according to the same procedure as 1c from

-methyl-1-phenyl-1H-imidazole (4.75 g, 30 mmol, 1.0 equiv) and MeI

C2-aryl-substituted NHOs. In general, the nucleophilicities of

the NHOs are weaker than those of their NHC analogues;

however, caution should be taken when generalizing this

conclusion to a wide spectrum of reaction partners with

distinctively electronic and structural characters. Figure 7

summarizes the main results of this work.

We hope the accurately measured data in this work could act

as a guide for the future development of computational methods

for evaluating (and predicting) the bond energetic data and

reactivities of NHOs in solution and also deepen our

understanding of the structure−reactivity relationships, as well

as promote a rational design of novel (chiral) NHOs in future

synthetic applications.

(8.40 g, 60 mmol, 2.0 equiv): beige solid; 7.20 g, 80% yield; H NMR

(400 MHz, DMSO-d ) δ 8.00−7.79 (m, 2H), 7.77−7.53 (m, 5H), 3.87

(d, J = 1.6 Hz, 3H), 2.52 (d, J = 1.7 Hz, 3H); C{ H} NMR (101 MHz,

DMSO-d ) δ 144.9, 134.7, 130.3, 130.0, 125.9, 122.7, 121.9, 35.2, 10.7;

HRMS (ESI) m/z [M ] calcd for C H N 173.1073, found

73.1072.

,2,3-Trimethyl-1H-benzo[d]imidazol-3-ium Iodide (1f). NHO

precursor 1f was prepared in a manner similar to that of 1c from 2-

methyl-1H-benzo[d]imidazole (5.00 g, 37.8 mmol), MeI (21.48 g, 151

mmol), and K CO (5.2 g, 38 mmol). The crude product was

recrystallized from methanol to give the title compound as a white solid:

8.2 g, 75% yield; H NMR (400 MHz, DMSO-d ) δ 8.03−7.93 (m,

981

J. Org. Chem. 2021, 86, 2974−2985

The Journal of Organic Chemistry

Article

(

H), 7.69−7.59 (m, 2H), 4.00 (s, 6H), 2.87 (s, 3H); C{ H} NMR

Et O (120 mL) for 8 h: light yellow solid; 0.6 g, 53% yield; H NMR

101 MHz, DMSO-d ) δ 152.3, 131.3, 125.8, 112.7, 31.7, 10.6; HRMS

ESI) m/z [M ] calcd for C H N 161.1073, found 161.1072.

-Methyl-1,3,4-triphenyl-4H-1,2,4-triazol-1-ium Iodide (1g). To

(400 MHz, toluene-d ) δ 6.81 (s, 4H), 5.75 (s, 2H), 2.48 (s, 2H), 2.31

(s, 12H), 2.16 (s, 6H); C{ H} NMR (101 MHz, toluene-d ) δ 148.2,

137.3, 134.4, 129.2, 112.7, 41.4, 20.7, 17.9.

an oven-dried Schlenk ﬂask were added 1,3,4-triphenyl-1,2,4-triazolium

perchlorate, the “Enders Triazole” (2.0 g, 5 mmol), and dry THF (20

mL) under argon protection. The ﬂask was immersed in an acetone/

liquid nitrogen bath for 15 min and cooled to −78 °C. Then

Na[N(SiMe ) ] (2 M in THF, 2.8 mL) was added dropwise. The

1-Methyl-2-methylene-3-phenyl-2,3-dihydro-1H-imidazole

(2e). Prepared from NHO precursor 1e (1.2 g) and KH (0.2 g) in dry

MHz, toluene-d

Hz, 2H), 6.89−6.82 (m, 1H), 5.88−5.77 (m, 1H), 5.55−5.47 (m, 1H),

O (80 mL) for 16 h: brown solid; 0.40 g, 58% yield; H NMR (400

) δ 7.32 (dt, J = 7.8, 1.2 Hz, 2H), 7.07 (dd, J = 8.5, 7.4

reaction mixture was slowly warmed to room temperature over 2 h and

cooled to −78 °C again. MeI (1.0 mL) was added dropwise, and the

mixture was gradually warmed to room temperature and stirred until

the starting material had been completely converted. The reaction

mixture was diluted with dry diethyl ether (30 mL) and ﬁltered. Then

the ﬁlter cake was washed three times with diethyl ether, dissolved in

dichloromethane, and ﬁltered again. Evaporation of the solvent yielded

3.57 (d, J = 2.9 Hz, 1H), 2.75 (d, J = 2.9 Hz, 1H), 2.49 (s, 3H); C{ H}

NMR (101 MHz, toluene-d ) δ 150.6, 141.4, 129.3, 124.4, 122.9, 116.0,

111.4, 44.5, 32.6.

1,3-Dimethyl-2-methylene-2,3-dihydro-1H-benzo[d]imidazole

(2f). Prepared from NHO precursor 1f (2.7 g) and KH (0.5 g) in dry

MHz, toluene-d

O (200 mL) for 24 h: pink solid; 1.1 g, 73% yield; H NMR (400

) δ 6.84−6.73 (m, 2H), 6.35−6.23 (m, 2H), 2.96 (s,

2H), 2.63 (s, 6H); C{ H} NMR (101 MHz, toluene-d

13 1

the title compound as a white solid: 1.87 g, 85% yield; H NMR (400

) δ 152.3,

MHz, DMSO-d ) δ 7.95−7.85 (m, 2H), 7.84−7.76 (m, 3H), 7.75−7.66

135.2, 118.8, 103.5, 46.5, 27.8.

5-Methylene-1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazole (2g).

Prepared from NHO precursor 1g (1.4 g) and KH (0.2 g) in dry Et

(

m, 4H), 7.63−7.56 (m, 1H), 7.53−7.44 (m, 4H), 2.59 (s, 3H);

C{ H} NMR (101 MHz, DMSO-d ) δ 154.0, 152.9, 135.2, 132.6,

(

120 mL) for 24 h: green to light yellow solid; 0.7 g, 74% yield; H

32.3, 132.2, 131.9, 131.1, 130.7, 129.7, 129.4, 128.0, 125.7, 123.3,

NMR (400 MHz, toluene-d ) δ 7.62 (d, J = 8.0 Hz, 2H), 7.55−7.42 (m,

2.4; HRMS (ESI) m/z [M ] calcd for C H N 312.1495, found

(

H), 7.35 (ddd, J = 12.8, 9.3, 4.8 Hz, 7H), 7.17 (t, J = 7.4 Hz, 1H), 3.47

12.1492.

-Mesityl-3-methyl-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-2-

d, J = 3.4 Hz, 1H), 2.69 (d, J = 3.4 Hz, 1H); C{ H} NMR (101 MHz,

ium (1h). NHO precursor 1h was prepared according to the same

procedure as 1g from 2-mesityl-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]-

triazol-2-ium tetraﬂuoroborate (2.0 g, 6.3 mmol), Na[N(SiMe ) ] (2

toluene-d ) δ 149.8, 147.7, 144.5, 140.6, 136.2, 135.3, 130.6, 130.5,

130.4, 129.7, 129.4, 129.3, 128.9, 128.9, 128.7, 128.6, 128.4, 128.2,

128.0, 127.5, 126.6, 124.3, 120.4, 119.9, 119.2, 114.5, 49.7.

2-Mesityl-3-methylene-2,5,6,7-tetrahydro-3H-pyrrolo[2,1-c]-

1,2,4]triazole (2h). Prepared from NHO precursor 1h (0.8 g) and KH

M in THF, 3.2 mL), and MeI (1.2 mL): pale yellow solid; 1.82 g, 78%

[

yield; H NMR (400 MHz, DMSO-d ) δ 7.16 (s, 2H), 4.38 (t, J = 7.2

(

0.1 g) in dry Et O (80 mL) for 6 h: light yellow solid; 0.3 g, 57% yield;

Hz, 2H), 3.19 (t, J = 7.7 Hz, 2H), 2.76 (p, J = 7.5 Hz, 2H), 2.50 (s, 3H),

.34 (s, 3H), 2.03 (s, 6H); C{ H} NMR (101 MHz, DMSO-d ) δ

H NMR (400 MHz, DMSO-d ) δ 6.92 (s, 2H), 3.41 (t, J = 6.9 Hz, 2H),

61.8, 149.9, 141.3, 135.4, 130.4, 129.4, 46.2, 46.2, 26.3, 21.9, 20.7,

6.9, 16.9, 9.8, 9.7; HRMS (ESI) m/z [M ] calcd for C H N

42.1652, found 242.1655.

-Mesityl-3-methyl-5,6-dihydro-8H-[1,2,4]triazolo[3,4-c][1,4]-

2.57−2.43 (m, 6H), 2.40 (s, 1H), 2.27−2.18 (m, 2H), 2.08−2.00 (m,

5H), 1.97 (s, 1H); C{ H} NMR (101 MHz, DMSO-d ) δ 154.8,

147.6, 137.9, 137.5, 137.5, 135.2, 129.3, 42.1, 41.9, 26.2, 20.6, 18.2;

HRMS (ESI) m/z [M + H] calcd for C15

242.1652, found

oxazin-2-ium Iodide (1i). NHO precursor 1i was prepared according

to the same procedure as 1g from 2-mesityl-5,6-dihydro-8H-[1,2,4]-

triazolo[3,4-c][1,4]oxazin-2-ium tetraﬂuoroborate (1.70 g, 5 mmol),

Na[N(SiMe ) ] (2 M in THF, 2.8 mL), and MeI (1.2 mL): white solid;

242.1655.

2-Mesityl-3-methylene-2,5,6,8-tetrahydro-3H-[1,2,4]triazolo[3,4-

c][1,4]oxazine (2i). Prepared from NHO precursor 1i (0.8 g) and KH

(0.1 g) in dry Et O (80 mL) for 6 h: light yellow solid; 0.4 g, 75% yield;

.57 g, 79% yield; H NMR (400 MHz, DMSO-d ) δ 7.18 (s, 2H), 5.12

H NMR (400 MHz, toluene-d ) δ 6.79 (s, 2H), 4.21 (s, 2H), 3.21 (t, J

(

s, 2H), 4.35 (t, J = 5.2 Hz, 2H), 4.23 (t, J = 5.3 Hz, 2H), 2.52 (s, 3H),

= 5.5 Hz, 2H), 2.71 (t, J = 5.5 Hz, 3H), 2.32 (s, 6H), 2.17−2.09 (m,

.35 (s, 3H), 2.02 (s, 6H); C{ H} NMR (101 MHz, DMSO-d ) δ

9H); C{ H} NMR (101 MHz, toluene-d ) δ 137.6, 137.4, 136.8,

52.5, 149.5, 141.5, 135.4, 129.7, 129.5, 62.3, 61.3, 43.7, 20.7, 16.8, 9.3;

129.2, 63.4, 62.5, 42.1, 40.7, 17.9; HRMS (ESI) m/z [M + H] calcd for

HRMS (ESI) m/z [M ] calcd for C H N O 258.1601, found

C H N O 258.1601, found 258.1601.

15 20

58.1601.

Reactions of Electrophile 3d with NHOs. General Procedure.

To an oven-dried round-bottom ﬂask were added electrophile 3d (67.9

mg, 0.2 mmol, 1.0 equiv) and dry THF (10 mL) in a glovebox. The

NHO (2a−2d and 2f) (0.24 mmol, 1.2 equiv) in 10 mL of dry THF was

added, and the mixture was stirred for 10−30 min. An excess (2.5

General Procedure for the Preparation of NHOs. The NHO

precursor (1a−1g) and potassium hydride (KH) were suspended in dry

diethyl ether in a Schlenk ﬂask under an argon atmosphere. The

reaction mixture was stirred for 6−48 h in the dark. Then the solvent

was evaporated at 0 °C or room temperature in vacuo, and the residue

was extracted with dry pentane and ﬁltered to yield a clear solution. The

evaporation pentane gave the corresponding NHO (2a−2g).

equiv) of HBF in diethyl ether (50−55% mole fraction) was added to

quench the reaction, and the solvent was removed under a decreased

pressure. The residue was puriﬁed by ﬂash column chromatography on

silica gel with a dichloromethane/acetone solvent [5:1 (v/v)] to give

the corresponding adduct (4a−4d and 4f).

,3-Dimethyl-2-methyleneimidazolidine (2a).

Prepared from

NHO precursor 1a (6.0 g) and KH (1.2 g) in dry Et O (200 mL) for 48

h: colorless to light yellow oil; 2.1 g, 62% yield; H NMR (400 MHz,

toluene-d ) δ 3.07 (s, 2H), 2.66 (s, 4H), 2.46 (s, 6H); C{ H} NMR

(

2-[2-(3,5-Di-tert-butyl-4-hydroxyphenyl)-2-(p-tolyl)ethyl]-1,3-di-

methyl-4,5-dihydro-1H-imidazol-3-ium Tetraﬂuoroborate (4a). Pre-

pared from electrophile 3d (61.5 mg) with NHO 2a (46.0 mg): white

101 MHz, toluene-d ) δ 159.0, 51.9, 50.0, 35.1.

,3-Dimethyl-2-methylenehexahydropyrimidine (2b).

Pre-

foaming solid; 72.3 mg, 78% yield; mp 96−99 °C; H NMR (400 MHz,

pared from NHO precursor 1b (4.3 g) and KH (0.8 g) in dry Et O

chloroform-d) δ 7.13 (q, J = 8.0 Hz, 4H), 6.93 (s, 2H), 4.01 (t, J = 7.6

Hz, 1H), 3.80 (s, 4H), 3.19 (d, J = 7.8 Hz, 2H), 2.75 (s, 6H), 2.33 (s,

(200 mL) for 48 h: colorless to light yellow oil; 2.0 g, 79% yield; H

NMR (400 MHz, toluene-d ) δ 3.23 (s, 2H), 2.59 (t, J = 6.1 Hz, 4H),

3H), 1.80 (brs, 1H), 1.39 (s, 18H); C{ H} NMR (101 MHz,

.50 (s, 6H), 1.72−1.63 (m, 2H); C{ H} NMR (101 MHz, toluene-

chloroform-d) δ 168.1, 153.3, 138.6, 137.4, 136.6, 132.1, 129.7, 127.5,

d ) δ 158.4, 62.7, 50.2, 39.8, 24.6.

124.1, 50.0, 47.4, 34.5, 33.9, 31.5, 30.4, 21.2; HRMS (ESI) m/z [M ]

,3,4,5-Tetramethyl-2-methylene-2,3-dihydro-1H-imidazole

calcd for C H N O 421.3213, found 421.3206.

(

2c). Prepared from NHO precursor 1c (2.2 g) and KH (0.4 g) in dry

-[2-(3,5-Di-tert-butyl-4-hydroxyphenyl)-2-(p-tolyl)ethyl]-1,3-di-

Et O (200 mL) for 24 h: light yellow solid; 0.8 g, 70% yield; H NMR

methyl-3,4,5,6-tetrahydropyrimidin-1-ium Tetraﬂuoroborate (4b).

Prepared from electrophile 3d (75.6 mg) with NHO 2b (46.6 mg):

(

400 MHz, toluene-d ) δ 2.71 (s, 2H), 2.61 (s, 6H), 1.51 (s, 6H);

C{1H} NMR (101 MHz, toluene-d ) δ 153.5, 114.2, 40.6, 29.2, 8.5.

white solid; 79.6 mg, 68% yield; mp 97−101 °C; H NMR (400 MHz,

,3-Dimesityl-2-methylene-2,3-dihydro-1H-imidazole (2d).

chloroform-d) δ 7.18−7.09 (m, 4H), 6.95 (s, 2H), 5.19 (s, 1H), 4.05−

3.99 (m, 1H), 3.51−3.39 (m, 4H), 3.34−3.31 (m, 2H), 2.90 (s, 6H),

Prepared from NHO precursor 1d (1.6 g) and KH (0.3 g) in dry

982

J. Org. Chem. 2021, 86, 2974−2985

The Journal of Organic Chemistry

https://pubs.acs.org/10.1021/acs.joc.0c02838

Article

.33 (s, 3H), 1.97−1.86 (m, 2H), 1.39 (s, 18H); C{ H} NMR (101

MHz, chloroform-d) δ 164.5, 153.2, 138.9, 137.3, 136.6, 132.3, 129.7,

27.6, 124.2, 49.0, 48.2, 41.3, 35.8, 34.5, 30.4, 21.2, 19.5; HRMS (ESI)

Notes

m/z [M ] calcd for C H N O 435.3370, found 435.3364.

The authors declare no competing ﬁnancial interest.

-[2-(3,5-Di-tert-butyl-4-hydroxyphenyl)-2-(p-tolyl)ethyl]-1,3,4,5-

tetramethyl-1H-imidazol-3-ium Tetraﬂuoroborate (4c). Prepared

from electrophile 3d (105.9 mg) with NHO 2c (78.0 mg): white

ACKNOWLEDGMENTS

The authors are grateful for the ﬁnancial grants from the

National Natural Science Foundation of China (21672124 and

■

solid; 93.4 mg, 56% yield; mp 113−116 °C; H NMR (400 MHz,

chloroform-d) δ 7.16−7.08 (m, 4H), 6.75 (s, 2H), 5.18 (s, 1H), 4.04 (s,

H), 3.63 (s, 2H), 3.23 (s, 6H), 2.33 (s, 3H), 2.18 (s, 6H), 1.34 (s,

8H); ¹³C{1H} NMR (101 MHz, chloroform-d) δ 153.1, 144.8, 138.6,

37.2, 136.6, 132.5, 129.7, 127.7, 125.9, 124.3, 48.7, 34.5, 32.0, 31.8,

1933008). The authors thank Ms. Jing-Jing Zhang for the

preparation of quinone methides used in this work.

0.3, 21.2, 9.0, 0.1; HRMS (ESI) m/z [M ] calcd for C H N O

DEDICATION

47.3370, found 447.3362.

■

-[2-(3,5-Di-tert-butyl-4-hydroxyphenyl)-2-(p-tolyl)ethyl]-1,3-di-

mesityl-1H-imidazol-3-ium Tetraﬂuoroborate (4d). Prepared from

electrophile 3d (83.9 mg) with NHO 2d (85.8 mg): pale yellow solid;

65.7 mg, 94% yield; mp 127−130 °C; H NMR (400 MHz,

chloroform-d) δ 7.68 (s, 2H), 7.11 (d, J = 6.9 Hz, 4H), 6.92 (d, J =

.8 Hz, 2H), 6.47 (d, J = 7.8 Hz, 2H), 6.35 (s, 2H), 5.12 (brs, 1H),

.82−3.74 (m, 1H), 3.31−3.12 (m, 2H), 2.44 (s, 6H), 2.30 (s, 3H),

.98 (s, 6H), 1.84 (s, 6H), 1.24 (s, 18H); ¹³C{ H} NMR (101 MHz,

Dedicated to the 100th anniversary of chemistry at Nankai

University.

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■

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3.0, 30.1, 26.6, 21.4, 21.1, 17.6, 17.6; HRMS (ESI) m/z [M ] calcd for

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13 1

H), 1.20 (s, 18H); C{ H} NMR (101 MHz, chloroform-d) δ 153.3,

4) (a) Crocker, R. D.; Nguyen, T. V. The Resurgence of the Highly

38.1, 137.3, 136.8, 132.2, 131.6, 129.7, 127.7, 127.0, 124.2, 112.4, 70.8,

8.8, 34.3, 31.7, 30.2, 26.6, 21.2; HRMS (ESI) m/z [M ] calcd for

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Eur. J. 2016 , 22 , 2208 −2213. (b) Roy, M. M. D.; Rivard, E. Pushing

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Catalysis to Main Group Element Chemistry. Acc. Chem. Res. 2017 , 50 ,

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.joc.0c02838.

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Sequestration, Activation, and Catalytic Transformation Using N-

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HRMS raw data (PDF)

Kinetic raw data (ZIP)

FAIR data, including the primary NMR FID ﬁles, for

compounds 1a−1i, 2a−2i, and 4a−4f (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

Pengju Ji − Center of Basic Molecular Science (CBMS),

Department of Chemistry, Tsinghua University, Beijing

■

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Jin-Pei Cheng − Center of Basic Molecular Science (CBMS),

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