Boron Trifluoride?Diethyl Ether-Catalyzed Etherification of Alcohols: A Metal-Free Pathway to Diphenylmethyl Ethers

Article abstract of DOI:10.1002/adsc.201500663

A novel boron trifluoride?diethyl ether (BF₃?OEt₂)-catalyzed etherification procedure has been developed in which primary and secondary alcohols are easily converted into diphenylmethyl ethers with yields of up to 99%.

Full text of DOI:10.1002/adsc.201500663

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DOI: 10.1002/adsc.201500663
Boron Trifluoride·Diethyl Ether-Catalyzed Etherification of
Alcohols: A Metal-Free Pathway to Diphenylmethyl Ethers
Jiaqiang Li,a Xiaohui Zhang,a Hang Shen,a Qing Liu,a Jing Pan,a Wen Hu,a
Yan Xiong,a,b,* and Changguo Chena,*
a
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, Peopleꢀs Republic of China
Fax: (+86)-023-6511-1748; phone: (+86)-023-6511-1748; e-mail: xiong@cqu.edu.cn or cgchen@cqu.edu.cn
Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Science, Chongqing 400714, Peopleꢀs
b
Republic of China
Received: July 9, 2015; Revised: August 6, 2015; Published online: October 14, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500663.
Abstract: A novel boron trifluoride·diethyl ether
(BF3·OEt2)-catalyzed etherification procedure has
been developed in which primary and secondary al-
cohols are easily converted into diphenylmethyl
ethers with yields of up to 99%.
catalyst.[10g] Regarding the environmental concerns as
well as from an economical point of view, the use of
metal-free catalysts is helpful for therapeutic applica-
tions.[10f] Therefore, the discovery of metal-free cata-
lyzed methods is highly desired.
It is well-known that the mixing of two alcohols
under acidic conditions would result in a mixture of
two mono ethers and one mixed ether. Williamsonꢀs
method gives an efficient means for the synthesis of
mixed ether products employing alcohol and halides
Keywords: alcohols; boron trifluoride·diethyl ether;
diphenylmethyl ethers; etherification; metal-free
procedure
or phosphonates in the presence of
a
base.[11]
BF3·OEt2 is regarded as a common Lewis acid cata-
lyst. Our previous studies have demonstrated
Diphenylmethyl ethers are found as an integral part BF3·OEt2 participated in the benzylation reaction via
of pharmacologically active compounds such as benz- a carbonium cation intermediate,[12] the cyanation of
tropine, diphenylpyraline and vanoxerine analogues silyl enol ethers,[13] and the BF3·OEt2-catalyzed syn-
that have potential in pharmacotherapy for cocaine thesis of bis(indolyl)methanes from indoles and car-
abuse (Figure 1).[1] The preparation of diphenylmethyl bonyl compounds.[14] To the best of our knowledge,
ethers has been extensively studied due to their
common utilization in the protection of alcohols be-
cause of the inexpensive nature, stability and easy de-
protection properties of diphenylmethyl moieties.[2]
Some classical synthetic methods towards diphenyl-
methyl ethers are carried out using diphenylmethyl
chloride or bromide in the presence of a base,[3] di-
phenylmethyl phosphate in the presence of trifluoro-
acetic acid,[4] diphenyldiazomethane and diphenylme-
thanol in the presence of p-toluenesulfonic acid or
concentrated sulfuric acid[5,6,7] and other methods cat-
alyzed by sodium-exchanged faujasites[8] and on irra-
diation by microwave.[9] Other catalytic protocols
have also been developed employing metals {e.g., yt-
terbium(III) trifluoromethanesulfonate [Yb(OTf)3]/
ferric chloride (FeCl3),[10a] ferric nitrate nonahydrate
[Fe(NO3)3·9H2O],[10b]
niobium(V)
chloride
(NbCl5),[10c] palladium chloride (PdCl2),[10d] bis(aceto-
nitrile)dichloropalladium [PdCl2(MeCN)2][10e] and
Figure 1. Pharmaceutical structures containing diphenyl-
cupric bromide (CuBr2)[10f]} and silica gel-supported methyl ether moieties.
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Table 1. Optimization of reaction conditions.[a]
Table 2. Formation of diphenylmethyl ethers in alcohols.[a]
Entry
BF3·OEt2 [equiv.]
T [oC][b]
t [h]
Yield [%][c]
Entry Alcohols (1)
Ethers (3)
R=Me
Yield [%][b]
1
2
3
4
5
6
7
8
1.2
1.2
1.2
1.2
1.2
0.8
0.2
0.05
0.05
0.03
0.01
0.01
r.t.
60
80
1
1
1
1
1
1
1
1
7
7
7
1
16
86
89
96
90
99
91
85
91
88
82
18
1
2
3
MeOH
EtOH
i-PrOH
1a
91, 3aa
90, 3ba
92, 3ca
1b R=Et
1c
R=i-Pr
100
120
100
100
100
100
100
100
100
4
5
1d
65, 3da
1e
1f
94, 3ea
76, 3fa
9
10
11
12
6
[a]
[b]
Conditions: 1 (1.0 mL), 2a (0.5 mmol), and BF3·OEt2
(5 mol%) at 1008C for 7 h.
Isolated yields.
[a]
Conditions: 2a (0.5 mmol) in MeOH 1a (1.0 mL) and
BF3·OEt2 (as specified) at temperature (as specified) for
t hours.
Oil bath temperature.
Isolated yields.
[b]
[c]
(Table 1, entries 10–12). Therefore, the optimal reac-
tion conditions were established as: 0.5 mmol diphe-
nylmethanol (2a), 5 mol% BF3·OEt2 in 1.0 mL of
there is no report involving BF3·OEt2-catalyzed syn- methanol (1a) under reflux for 7 h.
thesis of mixed ethers from two different alcohols. In
With the optimal reaction conditions in hand, dif-
this paper, we report our progress in the preparation ferent classes of alcohols (1) were examined with di-
of diphenylmethyl ethers catalyzed by BF3·OEt2, in phenylmethanol in the presence of BF3·OEt2
which various diphenylmethyl alkyl, allyl, benzyl and (Table 2). In this etherification procedure, the alco-
trifluoroethyl ethers were synthesized with high effi- hols (1) acted as both reagents and solvents. Aliphatic
ciency and a catalyst loading reduced to 1–5 mol%.
primary alchohols like methanol and ethanol (EtOH)
We began our investigations by treating diphenyl- and secondary alcohols like isopropyl alcohol (i-
methanol (2a) with 1.0 mL of methanol (MeOH, 1a) PrOH) were readily converted into diphenylmethyl
in the presence of BF3·OEt2 at different temperatures, ethers with excellent yields ranging from 90% to 92%
amounts of BF3·OEt2 and reaction times (Table 1). (Table 2, entries 1–3). However, cyclohexanol exhibit-
We found that running the reaction at room tempera- ed a relatively low reactivity and only a 65% yield
ture led to 3aa with 16% yield (Table 1, entry 1). A was obtained, which might result from large steric
continuous increase of yield was made when the tem- hindrance (Table 2, entry 4). a,b-Unsaturated primary
perature was elevated to 808C (Table 1, entries 2 and alcohols exhibited a higher reactivity than saturated
3). The temperature of 1008C gave the best result ones, for example, a 94% yield of diphenylmethyl
while higher temperatures turned out to be detrimen- allyl ether was obtained (Table 2, entry 5). Ethanediol
tal to the reaction (Table 1, entry 4 vs. 5). The amount showed moderate reactivity and only the monopro-
of BF3·OEt2 was then tested (Table 1, entries 6–9). Al- tected product was afforded with 76% yield (Table 2,
though 0.8 equivalent of BF3·OEt2 resulted in the best entry 6).
yield (99%) compared with other loadings of
The above-mentioned procedures are regarded as
BF3·OEt2 (Table 1, entry 6 vs. 4, 7–11), a catalytic environmental friendly pathways because the residual
amount of BF3·OEt2 (20 mol%) still gave an excellent alcohols with low boiling points can be easily separat-
yield of 91% (Table 1, entry 7). Moreover, a satisfacto- ed by evaporation. However, it is difficult to remove
ry 85% yield was obtained when the loading of alcohols with high boiling points such as benzyl, dode-
BF3·OEt2 was reduced to 5 mol% (Table 1, entry 8). cyl and phenethyl alcohols. In order to solve this
To our delight, a 91% yield was obtained with problem, we therefore developed stoichiometric con-
5 mol% of BF3·OEt2 loading when the reaction time ditions that involved diphenylmethanol and benzyl al-
was prolonged to 7 h which presented the same level cohol in commonly used solvents (Table 3). The ether-
of reactivity compared to 20 mol% of BF3·OEt2 ification procedure showed low reactivity in dichloro-
(Table 1, entry 9 vs. 7). However, further decreasing ethane (DCE) and dichloromethane (DCM), only
the amount of BF3·OEt2 led to reduced yields 18% and 25% yields of diphenylmethyl benzyl ether
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Boron Trifluoride·Diethyl Ether-Catalyzed Etherification of Alcohols
Table 3. Investigation of solvents.[a]
Table 4. Scope of alcohols.[a]
Entry
Benzyl alcohol [equiv.]
Solvents
Yield [%][b]
Entry Alcohols (1)
Ethers (3)
Yield
[%][b]
1
2
3
4
5
6
7
1.2
1.2
1.2
1.2
1.0
1.4
1.2
DCE
18
25
58
98
96
98
97[c]
DCM
CH3CN
toluene
toluene
toluene
–
1
2
3
MeOH
1a
1g
1h
3aa 94[c]
3ga 98 (97)[c]
3ha 85
[a]
Conditions: 2a (0.5 mmol), 1g (as specified), BF3·OEt2
(5 mol%) and solvent (1.0 mL) at 1008C for 7 h.
Isolated yields.
[b]
[c]
4
1i
3ia 94
No solvent for 1 h.
5
6
7
1j
3ja 97 (76)[c]
3ea 97 (96)[c]
3da 94 (93)[c]
were obtained, respectively (Table 3, entries 1 and 2).
A moderate 58% yield was obtained when acetoni-
trile was used as solvent (Table 3, entry 3). Fortunate-
ly, a 98% yield of diphenylmethyl benzyl ether was
obtained in toluene without the benzylation product
(Table 3, entry 4).[12b] The loading of benzyl alcohol
(1g) was also tested, and as a result, 1.2 equivalents of
benzyl alcohol (1g) gave the best yield (Table 3, en-
tries 4–6). We treated benzyl alcohol and diphenylme-
thanol under solvent-free conditions as well and, to
our delight, a 97% yield of diphenylmethyl benzyl
ether was obtained after 1 hour (Table 3, entry 7).
A series of stoichiometric alcohols including aro-
matic, alkylic, allylic and trifluoroethylic alcohols, was
then examined for diphenylmethylation in the pres-
ence of 5 mol% BF3·OEt2 under solvent or solvent-
free conditions (Table 4). Under solvent-free condi-
tions, methanol showed higher reactivity than when
used as solvent (Table 4, entry 1 vs. Table 1, entry 8).
Under solvent conditions, a 98% yield of diphenyl-
methyl benzyl ether and an 85% of diphenylmethyl
phenethyl ether were obtained (Table 4, entries 2 and
3). Irrespective of whether linear, a,b-unsaturated or
cyclic alcohols were employed, the diphenylmethyl
1e
1d
8
9
1k
3ka 70
1l
3la 38 (9)[c]
3ma 0 (23)[d]
10
1m
[a]
Conditions:
1 (0.6 mmol), 2a (0.5 mmol), BF3·OEt2
(0.025 mmol) and toluene (1.0 mL) at 1008C for 7 h.
[b]
[c]
[d]
Isolated yields.
No solvent for 1 h.
Isolated yield of 4-benzhydrylphenol.
ethers were formed in excellent yields (Table 4, en- cyl and trifluoroethyl alcohols as substrates under sol-
tries 4–7). It is noteworthy that cyclohexanol under vent-free conditions (Table 4, entries 5 and 9). Phenol
stoichiometric conditions gave a higher yield of 94% did not give the corresponding ether product under
than when used as solvent (Table 4, entry 7 vs. our standard conditions, however 4-benzhydrylphenol
Table 2, entry 4). Diphenylmethyl menthyl ether was was generated via a benzylation of the arene and iso-
prepared effectively with a moderate 70% yield lated in 23% yield (Table 4, entry 10).[12b] Other terti-
(Table 4, entry 8). A low yield of 38% was obtained ary alcohols such as adamantanol, 1-ethynyl-1-cyclo-
when an alcohol bearing a strong electron-withdraw- hexanol, terpineol were employed as starting materi-
ing group was used as substrate (Table 4, entry 9). als, but complex reactions were observed.
The etherification procedure was terminated by em-
In order to evaluate the compatibility of the etheri-
ploying phenol as substrate (Table 4, entry 10). More- fication procedure toward different diphenylmethyl
over, benzylic, allylic and cyclohexyl alcohols led to groups, diverse substituted diphenylmethanols bearing
excellent results (Table 4, entries 2, 6 and 7) and a de- mono-/dihalo, methyl and cyano substitions were in-
crease of yields was observed when employing dode- vestigated, with the results listed in Table 5. We firstly
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Table 5. Scope of diphenylmethanols.[a]
with yields of 91% (3ed) and 95% (3ee), respectively.
It is noteworthy that ortho-substitution gave lower
yields, due to steric hindrance. When compounds
bearing halogen groups on ortho-positions such as 2f
and 2g were used as substrates, moderate yields of
52% (3ef) and 59% (3eg) were obtained. However,
the etherification process did not take place in 2eh
with a diphenylmethanol bearing two halo groups at
the ortho positions. Diphenylmethanol bearing an
electron-donating methyl group gave a good result
(83% yield, 3ei), however a dramatic decrease yield
to 36% was observed by using diphenylmethanol sub-
stituted by an electron-withdrawing cyano group (3ej)
which probably results in an instability of the carboni-
um intermediate. Triphenylmethanol can also be con-
verted into triphenylmethyl allyl ether (3ek), but only
a 25% yield of ether was obtained by 37% of triphe-
nylmethane.[16] Interestingly, substrates containing
a heterocyclic thiophene also gave the etherification
product with 61% yield (3el).
For such a reactive procedure, a decrease of the
catalyst loading was taken into account in agreement
with green chemistry. Under solvent-free conditions,
a detailed investigation of catalyst loading was con-
ducted using benzylic alcohol (1g) and diphenylme-
thanol (2a) as starting materials (Table 6). 3 mol%
and 5 mol% of BF3·OEt2 gave similar yields of 3ga
(Table 6, entries 1 and 2). An 86% yield of diphenyl-
methyl benzyl ether was obtained with 1 mol% of
BF3·OEt2 (Table 6, entry 3). Even reducing the
amount of BF3·OEt2 to 0.5 mol%, gave an 84% yield
of 3ga after 4 h (Table 6, entry 4). Although the ether-
ification proceed slowly (30 h) with 0.1 mol% of
BF3·OEt2, a moderate yield of 35% was obtained for
3ga accompanied with 9% diphenyl ketone as a by-
3ea, 97%
3ed, 91%
3eg, 59%
3eb, 90%
3ee, 95%
3eh, 0%
3ec, 90%
3ef, 52%
3ei, 83%
3ej, 36%
3ek, 25% (37%)[b]
3el, 61%
[a]
[b]
Conditions: 1e (0.6 mmol),
(0.025 mmol) and toluene (1.0 mL) at 1008C for 7 h. Iso-
lated yields.
2
(0.5 mmol), BF3·OEt2
Yield of triphenylmethane as by-product.
tested the solvent-free conditions for this transforma-
tion, however the reactions of 4-fluorodiphenylmetha-
nol (2b), 4,4’-difluorodiphenylmethanol (2c), 4-chloro-
diphenylmethanol (2d) and 4-methyldiphenylmetha-
nol (2i) gave complex mixtures. So we turned our at-
tention to the studies on stoichiometric alcohols in
toluene and as a result moderate to excellent yields of
diphenylmethyl allyl ethers were attained. Diphenyl-
methanol gave the diphenylmethyl allyl ether with
97% yield (3ea). Considering the potential utilization
of fluorine-containing compounds in pharmaceuticals
and functionalized materials,[15] 4-fluoro- and 4, 4’-di-
fluoro- substituted diphenylmenthanols were chosen
as substrates and as a result both of them gave 4-
fluoro- and 4,4’-difluorodiphenylmethyl allyl ether
(3eb and 3ec) with 90% yield. The chloro- and
bromo-substituted diphenylmethyl alcohols are capa-
ble of being further functionalized by SNAr reactions,
Grignard reactions and coupling reactions, therefore
4-chloro- and 4-bromo- substituted substrates were
utilized as starting materials and gave the products
Table 6. Investigation of catalyst loading.
Entry
BF3·OEt2 [equiv.]
t [h]
Yield [%][a]
1
2
3
4
5
0.05
0.03
0.01
0.005
0.001
1
1
1
4
97[b]
99[c]
86[d]
84[e]
35[f]
30
[a]
Isolated yields.
[b]
1g
(0.6 mmol),
2a
(0.5 mmol)
and
BF3·OEt2
(0.025 mmol).
[c]
[d]
[e]
[f]
1g (1.2 mmol), 2a (1 mmol) and BF3·OEt2 (0.03 mmol).
1g (3.6 mmol), 2a (3 mmol) and BF3·OEt2 (0.03 mmol).
1g (7.2 mmol), 2a (6 mmol) and BF3·OEt2 (0.03 mmol).
1g (24 mmol), 2a (20 mmol) and BF3·OEt2 (0.02 mmol);
9% of diphenyl ketone as by-product and 50% of 2a re-
covered.
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Boron Trifluoride·Diethyl Ether-Catalyzed Etherification of Alcohols
stirred at 1008C (oil bath temperature) for 1 hour. The sol-
vent was evaporated, and the residue was chromatographed
on silica gel (eluent: petroleum ether) to give the colourless
diphenylmethyl ether 3.
product and 50% of diphenylmethanol (2a) recovered
(Table 6, entry 5). The generation of diphenyl ketone
might result from oxidation by oxygen under open-
flask and heating conditions.
In our previous work, BF3·OEt2 has been found to
be insoluble in toluene before and after the benzyla-
tion.[12b] In 2015, Sumihiro presented the recovery and
reusage of boron trifluoride complex by using a hydro-
carbon solvent.[17] This encouraged us to test the recy-
clability of BF3·OEt2. In our execution, diphenylme-
Acknowledgements
We are grateful for funding from the National Natural Sci-
ence Foundation of China (No. 21372265 and No. 61271059),
thanol (8 mmol) and benzyl alcohol (9.6 mmol) were the Natural Science Foundation Project of CQ CSTC
(cstc2013jcyjA10037), the Fundamental Research Funds for
the Central Universities (No. CQDXWL-2013-Z012 and No.
CDJZR14225502) and Chongqing University Postgraduatesꢀ
Innovation Project to J. P.
employed as starting materials, and etherification
then proceeded under optimal solvent-free conditions
in the presence of 5 mol% of BF3·OEt2. After the end
of the reaction, hexane, that is incapable of dissolving
BF3·OEt2 but is capable of dissolving the reaction
product, was loaded into reactor. Then the resulting
reaction mixture was allowed to separate into two
liquid layers. BF3·OEt2 (lower liquid layer) was recov-
ered and reused after removing the mixture of prod-
uct and hexane (upper liquid layer). As a result, the
first run gave the ether product in 97% and the recov-
ered BF3·OEt2 resulted in the similar yield of 94%.
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BF3·OEt2-catalyzed cross etherification of alcohols. In
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and for most cases with low catalyst loading.
Experimental Section
General Procedure: Alcohols as Both of Starting
Materials and Solvents (Table 2)
To a round-bottom flask, diphenylmethanol (2a, 92.1 mg,
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Adv. Synth. Catal. 2015, 357, 3115 – 3120