Enhanced basicity of Ag2O by coordination to soft anions

Article abstract of DOI:10.1002/cctc.201403035

In Ag₂O-mediated benzylation, the addition of a catalytic amount of KI can greatly improve reactivity. This is usually attributed to the formation of a more reactive iodo-substituted electrophile. However, our studies show this to be due to the enhanced basicity of Ag₂O through coordination of soft iodide anions to the silver atom, and show KI to be an initiator. A catalytic amount of Ag₂O and NaBr can catalyze transesterification reactions, indicating the enhanced basicity of Ag₂O by bromide. We believe that this is a general effect for metal oxides and soft anions, applicable to a wider range of organic reaction systems. All your base belongs to us: In Ag₂O-mediated benzylation, the addition of a catalytic amount of KI can greatly improve reactivity. Our studies show this to be attributable to the enhanced basicity of Ag₂O through coordination of soft iodide anions to the silver atom, and show KI to be an initiator. Thus, NaBr as an initiator combined with Ag₂O has been successfully used to catalyze transesterification reactions.

Full text of DOI:10.1002/cctc.201403035

DOI: 10.1002/cctc.201403035
Communications
Enhanced Basicity of Ag2O by Coordination to Soft Anions
Bo Ren,[a] Meiyan Wang,[b] Jingyao Liu,[b] Jiantao Ge,[a] and Hai Dong*[a]
In Ag2O-mediated benzylation, the addition of a catalytic
amount of KI can greatly improve reactivity. This is usually at-
tributed to the formation of a more reactive iodo-substituted
electrophile. However, our studies show this to be due to the
enhanced basicity of Ag2O through coordination of soft iodide
anions to the silver atom, and show KI to be an initiator. A cat-
alytic amount of Ag2O and NaBr can catalyze transesterification
reactions, indicating the enhanced basicity of Ag2O by bro-
mide. We believe that this is a general effect for metal oxides
and soft anions, applicable to a wider range of organic reac-
Figure 1. Halide promoted Ag2O-mediated protection of hydroxyl group due
to enhanced basicity of Ag2O by coordination to soft anions. X stands for Cl,
tion systems.
Br and I. M stands for K, Na and TBA.
It is well known that many organic reactions are strongly cata-
lyzed by bases. The bases are usually amines, anions, and met-
allic oxides, chosen as catalysts according to their nucleophili
city and basicity. Silver oxide (Ag2O) is a very weak metallic
oxide base that is not generally used as a base catalyst. It is
more often used as a Lewis acid to donate empty orbital to
electron-rich atoms.[1] Use of Ag2O as a Lewis acid to activate
halides has long been employed in organic synthesis for the
creation of reactive cationic organic/inorganic species.[2] Selec-
tive protections play an important role in organic synthesis
strategies, especially in carbohydrate chemistry.[3] Thus, many
methods have been developed to obtain regioselective benzy-
lation[4] or esterification[5] of carbohydrates. Ag2O-mediated se-
lective protections are appealing owing to their efficiency and
their environmental benefits.[2d–i] Ag2O has also been used as
an indispensable additive in organotin catalyzed glycosyla-
tion[6] and organoboron catalyzed benzylation.[4b] The addition
of iodide salts has often been used to improve the reactivities
of alkylation/acylation reactions. The improved reactivity was
often attributed to the formation of a more reactive electro-
phile RI through the substitution of iodide for the less reactive
electrophile RX (X stands for Cl or Br).[2e,f,7] Thus, a catalytic
amount of potassium iodide (KI) was also added in Ag2O-medi-
ated selective acylation,[2d,e] benzylation[2h,i] or sulfonylation.[2f,g]
However, we recently demonstrated that the improved reactiv-
ity for iodide promoted organotin-mediated carbohydrate ben-
zylation was due to coordination of iodide to tin atom.[4a] In
this study, we showed that the improved reactivity of Ag2O-
mediated benzylation by KI was due to the enhanced basicity
of Ag2O through the coordination of iodide to silver, and KI
was an initiator (Figure 1). Although Ag2O was initially used as
a Lewis acid, donating empty orbitals to halides (XÀ), and thus
catalyzing the leaving of the halide groups, the resulting AgX
led to the formation of AgOÀ, which is a strong base and capa-
ble of deprotonation of the OH group. This principle was fur-
ther supported by X-ray diffraction (XRD) data, theoretical cal-
culations, and experiments on transesterification reactions. To
the best of our knowledge, this is the first demonstration of
a weak metallic oxide base with soft anions being employed
as a strong base to catalyze organic reactions.
Although the Ag2O-mediated method led to good selectivi-
ties for acetylation, benzoylation, and sulfonylation of 2,3-diols
in 4,6-O-benzylidene gluco- and galactopyranosides,[2f] good
selectivities for benzylation have only been reported for non-
carbohydrate substrates.[2h,i] To investigate why this has been
the case, methyl 4,6-O-benzylidene-a-glucoside 1 was tested
using Ag2O-mediated benzylation, where 1.1 equiv. of Ag2O
and 1.5 equiv. of benzyl bromide (BnBr) were employed with
acetonitrile (MeCN) as a solvent. When other methods have
been used, the 2-OH of 1 has usually been regioselectively
benzylated.[3c,4a] Although our results have shown that the
Ag2O-mediated method to be less successful for the regiose-
lective benzylation of glucoside 1. By analysis of the unreacted
starting material, we discovered the reason why a catalytic
amount of potassium iodide (KI) could greatly improve Ag2O-
mediated benzylation (Table 1). Without the addition of KI
(entry 1 in Table 1), the reaction proceeded very slowly. With
the addition of 0.1 equiv. of KI (entry 2 in Table 1), the reaction
proceeded rapidly, and 30% and 8% of the starting material
1 remained after 4 h and 8 h, respectively. The rapid reaction
also occurred in less polar solvent dichloromethane (entry 3 in
Table 1). If the improved reactivity with the addition of KI was
[a] B. Ren, J. Ge, Prof. H. Dong
Key Laboratory for Large-Format Battery Materials and System
Ministry of Education, School of Chemistry & Chemical Engineering
Huazhong University of Science & Technology
Luoyu Road 1037, 430074 Hongshan, Wuhan (P.R. China)
E-mail: hdong@mail.hust.edu.cn
[b] Dr. M. Wang, Prof. J. Liu
Institute of Theoretical Chemistry, State Key Laboratory of
Theoretical and Computational Chemistry, Jilin University
Liutiao Road 2, Changchun, 130023 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201403035.
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ability order: IÀ >BrÀ >ClÀ >FÀ. The remained glucoside 1 in
one hour were 28, 41, 43, and 91% and in four hours were 22,
26, 32, and 84%, respectively, for TBAI, TBABr, TBACl, and TBAF.
The same activation order was obtained when BnCl was used
instead of BnBr (Table S1).
Table 1. Recorded unreacted starting material at various conditions for
Ag2O-mediated benzylation of glucoside 1.[a]
For halide anions, the softness follows the order: IÀ >BrÀ >
ClÀ >FÀ, the order seen above for activation ability. Thus, the
hard–soft acid–base (HSAB) principle may provide an explana-
tion as to how KI activated the Ag2O-mediated protections. For
Ag2O, oxide anion is a hard base, and Ag+ is a soft acid. Thus,
when Ag2O encounters a soft base such as BrÀ or IÀ, the soft
Ag+ preferentially coordinates to the soft base BrÀ or IÀ, lead-
ing to the formation of AgOÀ (Figure 1). AgOÀ, a much stron-
ger base than Ag2O, deprotonates the hydroxyl group, then
followed by benzylation with BnBr to regenerate AgOÀ and
AgBr and restart the cycle. Consequently, a softer anion would
be an initiator and show higher activation ability in these reac-
tions. As can be seen (entry 11 in Table 1), the soft sulfur anion
also appeared activation ability for the reaction. Without con-
sideration of the essential reason (HSAB principle), the positive
effect of catalytic amount of halides (X) could be simply ex-
plained by the precipitation of AgX, with the formation of
basic species AgOÀ. The relative solubility of AgX in acetoni-
trile follows the order: AgI>AgBr>AgCl>AgF, the order seen
above for activation ability. The basicity in the reaction simply
depends on the amount of halides and remains unchanged
until Ag2O is depleted. Without the addition of halides, there is
no AgOÀ formed, thus the benzylation showing low reactivity.
According to this mechanism, we hypothesized that the addi-
tion of a catalytic amount of hydroxyl anion should also trigger
the activation process. As can be seen (entry 12 in Table 1), the
addition of 0.1 equiv. of NaOH proved efficient. The conversion
of Ag2O to AgI and hydroxide in aqueous iodide solution on
open-circuit potential has been reported.[8] The enhanced ba-
sicity can also be confirmed by the measurement of pH values
(Figure S2, S3). The results indicate that Ag2O (6 mg) or TBABr
(8 mg) alone in water (1 mL) appears roughly 6–7 of pH value,
whereas the mixture of Ag2O (6 mg) with TBABr (8 mg) in
water appears roughly 13–14 of pH value. Powder X-ray dif-
fraction (XRD) data indicated the formation of AgBr after Ag2O
and KBr were mixed thoroughly (Figure S4). It was reported
that AgOK were obtained as colorless, tetragonal single crystals
under very harsh condition.[9] Density functional theory (DFT)
calculations for four reactions of Ag2O with KX (X=F, Cl, Br, or
I) further support the proposed principle on the enhanced ba-
sicity of Ag2O by coordination to soft halides.
Entry Various conditions Reaction time [h] Unreacted substrate [%][b]
1
2
Only Ag2O
With KI
1
4
4
8
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
100
95
30
8
3
DCM as solvent
With KBr
48
27
88
59
75
44
28
22
94
92
41
26
43
32
91
84
87
76
49
28
4
5
With NaI
6
With TBAI
With KF
7
8
With TBAB
With TBACl
With TBAF
With Na2S
With NaOH
9
10
11
12
[a] Reagents and conditions: substrates (50 mg), Ag2O (1.1 equiv.), BnBr
(1.5 equiv.), M+XÀ (0.1 equiv.), RT. [b] NMR yield.
due to the formation of more reactive BnI from BnBr, the addi-
tion of 0.1 equiv. of KBr would be ineffective. However, when
KBr was tested (entry 4 in Table 1), the reaction remained
highly reactive, with 88% and 59% of unreacted 1 in 1 h and
4 h respectively, thus excluding the proposal that the im-
proved reactivity is due to the formation of more reactive BnI
(also see Figure S1). When sodium or tetrabutylammonium
(TBA) cations were used instead of the potassium cation, the
reaction remained highly reactive (entry 5, 6 in Table 1), indi-
cating that the cation does not play dominating role on the re-
activity. It was recently found that the acetylation of hydroxyl
groups with acetic anhydride can be activated by the forma-
tion of H-bonds between the hydroxyl groups and anions.[5a,b]
Thus, we firstly hypothesized that the activated benzylation by
KI was due to the formation of H-bonds between the hydroxyl
groups and iodide. If this was the case, the anions that can
form stronger H-bond with hydroxyl group should have higher
activation ability for benzylation, and the activation order
would be FÀ >ClÀ >BrÀ >IÀ. However the addition of KF
proved inefficient (entry 7 in Table 1). Considering about the
better solubility of tetrabutylammonium salts in acetonitrile, in
further experiments, 0.1 equiv. of tetrabutylammonium salts of
Br, Cl and F were used in the benzylations (entries 8–10 in
Table 1) to compare their activation ability with that of TBAI.
These experiments indicated an exactly opposite activation
The relative electronic energies in gas phase (DEgas) and sol-
vation-corrected relative electronic energies (DEsol) of the four
reactions [Eqs. (1–4)] were given. It is seen that both DEsol and
DEgas decrease in the order F>Cl>Br>I, indicating that the
stability of AgX follows the order AgF<AgCl<AgBr<AgI, con-
sistent with the HSAB principle that Ag+ as a soft acid forms
a stronger bond with softer bases such as IÀ. Moreover, ac-
cording to the stability of AgX, the ability to generate AgOK
from the reaction of Ag2O with KX is in the order KF<KCl<
KBr<KI. In other words, a great quantity of AgOK is generated
when Ag2O meets with KI, while little AgOK even without
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Table 2. Intramolecular and intermolecular acyl migration catalyzed by
Ag2O/NaBr.
Entry Substrate
Migrated product
NMR ratio
3a:4a (37:63)
3b:4b (17:83)
1[a]
5a:6a (31:69)
5b:6b (50:50)
2[a]
7a:8a (68:32)
7b:8b (56:44)
Figure 2. The density functional theory (DFT) calculations were performed
for the four reactions [Eqs. (1–4)] of Ag2O with KX (X=F, Cl, Br, or I).
3[a]
9a:10a (35:65)
9b:10b (16:84)
AgOK is generated when Ag2O meets with KF due to the insta-
bility of AgF. The natural bond orbital (NBO) charges of O in
both Ag2O and AgOK were thus analyzed to compare the abili-
ty of each to remove a proton from an alcohol. The NBO
charge of O in AgOK (À1.514) was more negative than that of
O in Ag2O (À1.309), showing the basicity of AgOK to be stron-
ger than Ag2O. Furthermore, by using ethanol (EtOH) as the
model of alcohol, the solvation-corrected relative electronic en-
ergies, DEsol, of the reaction of Ag2O and AgOK with EtOH
[Eqs. (5) and (6)] were calculated. The reaction of Ag2O with
EtOH is endothermic (2.7 kcalmolÀ1) while the reaction of
AgOK with EtOH is exothermic (15.2 kcalmolÀ1), indicating that
the anion EtOÀ is more easily generated from the reaction of
AgOK with EtOH, i.e., AgOK is a stronger base than Ag2O and
so can more easily remove the proton from EtOH, producing
the anion EtOÀ.
4[a]
12a (90%)[b]
12b (85%)[c]
5
14a (85%)[b]
14b (80%)[c]
6
7[d]
Quant.
Quant.
If our proposed principle is valid, the transesterification reac-
tion would be able to be catalyzed by the mixture of Ag2O
with soft halides, such as bromide and iodide. However, Ag2O
or soft halides alone would not catalyze these reactions. Ag2O-
mediated acyl migration has been reported recently.[10] As the
mechanism was unclear at that time, 1 equiv. of Ag2O with the
addition of 0.1 equiv. of TBAI was employed in the reaction. In
our cases, only 0.1 equiv. of Ag2O with the addition of
0.1 equiv. of NaBr was used. We firstly tested intramolecular
acyl migration of substrates 3, 5, 7, and 9 to corresponding
compounds 4, 6, 8, and 10 (entries 1–4 in Table 2). The results
are in accordance with previous reports[10b] where 1 equiv. of
Ag2O and 0.1 equiv. of TBAI were employed. The acylation of
carbohydrate diols 11 and 13 by EtOAc/MeOBz with the addi-
tion of 0.1 equiv. of NaBr and 0.1 equiv. of Ag2O, leading to re-
gioselectively mono-acylated products 12a/12b and 14a/14b
in 90/85% and 85/80% isolation yields respectively (entries 5
and 6 in Table 2). The deprotection of acyl groups for penta-
acetyl/benzoyl-galactose 15 and tetra-acetyl/benzoyl-glucoside
17 is also tested with Ag2O/BrÀ in methanol (entries 7 and 8 in
Table 2), leading to deprotected products 16 and 18 in quanti-
tative yields. All these intermolecular and intramolecular trans-
esterification have been successfully catalyzed by the mixture
8[d]
Reaction conditions: [a] substrates (50 mg), acetonitrile (1 mL), Ag2O
(0.1 equiv.), NaBr (0.1 equiv.), RT–408C, 24 h.; [b] substrates (50 mg), Ag2O
(0.1 equiv.), NaBr (0.1 equiv.), EtOAc (1 mL), RT, 24 h.; [c] substrates
(50 mg), Ag2O (0.1 equiv.), NaBr (0.1 equiv.), MeOBz (0.2 mL), vacuum,
408C, 1 h. [d] substrates (100 mg), Ag2O (0.1 equiv.), NaBr (0.1 equiv.),
MeOH (1 mL), RT, 2–12 h.
of 0.1 equiv. of Ag2O and 0.1 equiv. of soft halide. However,
these reactions cannot proceed with whether 1 equiv. of Ag2O
or 1 equiv. of halide alone. Autocatalysis of reactions of alkyl
halides with Ag2O had been noticed as early as 1937.[11] It was
further found that AgI/Ag2O combinations accelerate transes-
terification reactions of inositol derivatives.[12] However, a large
amounts of Ag2O and AgI were used in those cases (@
1 equiv.). We compared our method with AgI/Ag2O method in
the transesterification of ethyl benzoate in methanol. With
0.1 equiv. of Ag2O and 0.1 equiv. of KI, the transesterification of
ethyl benzoate was completed in 0.5 h. However, with 2 equiv.
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residue by flash column chromatography (2:1 hexane-ethyl ace-
tate) afforded the products.
of Ag2O and 1 equiv. of AgI, the transesterification proceeded
very slow, leading to only 9% conversion rate in 4 days.
Though Ag2O is insoluble in acetonitrile, the mixture turned
into a clear solution with the addition of more than 3 equiv. of
TBAI or TBABr and displayed very strong basicity (Figure S5). It
may indicate the formation of AgOÀ and (AgX2)À. Thus, the re-
action catalyzed by AgOÀ should be homogeneous catalysis.
Through the investigation of the Ag2O-route in the synthesis
of AgI N-heterocyclic carbine complexes by DFT calculations,
the advantage of Ag2O was attributed to its stronger basici-
ty.[13] However, we suggest that it should be also attributed to
the strong basicity of AgOÀ which is formed through coordina-
tion of iodide anion to Ag2O.
General Ag2O-mediated deacylation: Condition A: carbohydrate
substrates (100 mg) in dry methanol (1 mL) were added silver
oxide (0.1 equiv.) and sodium bromide (0.1 equiv.). The mixtures
were stirred at room temperature for 2–12 h. After being filtered,
the filtrate was concentrated in vacuum. Purification of the residue
by flash column chromatography (2:1 methanol-ethyl acetate) af-
forded the products. Condition B: carbohydrate substrates
(100 mg) in dry methanol (1 mL) were added silver oxide
(0.1 equiv.) and bromide or chloride resin (85 mg). The mixtures
were stirred at room temperature for 2–12 h. After being filtered,
the filtrate was concentrated to afford the products.
In conclusion, although Ag2O itself is a weak base and is
usually used as Lewis acid catalyst, its basicity can be greatly
enhanced by the coordination of a soft halide anion to Ag+.
Thus, KI as an initiator activated Ag2O-mediated protections
were found to be due to the enhanced basicity of Ag2O. The
basicity depends on the initial additive amount of KI and re-
mains unchanged until Ag2O is depleted. We believe that this
is a general effect for metal oxides and soft anions, applicable
to a wider range of organic reaction systems. The catalyst will
show the particular advantage in organic reactions related
with substitution of a soft anion and requiring an unchanged
basic condition.
Acknowledgements
This study was supported by the National Nature Science Foun-
dation of China (Nos. 21272083).
Keywords: base catalysis · coordination · hard-soft acid-base ·
silver
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General Ag2O-mediated benzylation: Carbohydrate substrates
(50 mg) in dry acetonitrile (1 mL) were added silver oxide
(1.1 equiv.), halide salts (0.1 equiv.), and benzyl bromide (1.5 equiv.).
The mixtures were stirred at room temperature for 1–4 h. The con-
version rates were calculated through the detection of product
1
compositions by H NMR analysis.
General Ag2O-mediated acyl migration: Carbohydrate substrates
(50 mg) in dry acetonitrile (1 mL) were added silver oxide
(0.1 equiv.) and sodium bromide (0.1 equiv.). The mixtures were
stirred at room temperature or 408C for 24 h. The product distribu-
1
tions were directly detected by H NMR spectrum.
General Ag2O-mediated intermolecular acyl migration:
Condition A: carbohydrate substrates (50 mg) in dry ethyl acetate
(1 mL) were added silver oxide (0.1 equiv.) and sodium bromide
(0.1 equiv.). The mixtures were stirred at room temperature for
24 h. After being filtered, the filtrate was concentrated in vacuum.
Purification of the residue by flash column chromatography (2:1
hexane-ethyl acetate) afforded the products. Condition B: carbohy-
drate substrates (50 mg) in dry methyl benzoate (1 mL) were
added silver oxide (0.1 equiv.) and sodium bromide (0.1 equiv.). The
mixtures were stirred at 408C under vacuum for 24 h. After being
filtered, the filtrate was concentrated in vacuum. Purification of the
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Received: December 18, 2014
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These are not the final page numbers! ÞÞ

COMMUNICATIONS
B. Ren, M. Wang, J. Liu, J. Ge, H. Dong*
All your base belongs to us: In Ag2O-
mediated benzylation, the addition of
a catalytic amount of KI can greatly im-
prove reactivity. Our studies show this
to be attributable to the enhanced
basicity of Ag2O through coordination
of soft iodide anions to the silver atom,
and show KI to be an initiator. Thus,
NaBr as an initiator combined with
Ag2O has been successfully used to cat-
alyze transesterification reactions.
&& – &&
Enhanced Basicity of Ag2O by
Coordination to Soft Anions
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ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!