Direct AlCl3-catalyzed transformation of benzyl THP ethers and allyl benzyl ethers

Article abstract of DOI:10.1080/00397911.2020.1829644

THP and allyl groups are frequently used for the protection of alcohols. In this study, novel direct transformations of benzyl THP ethers and allyl benzyl ethers, protected forms of alcohols, are reported. TMSN₃ and AlCl₃ were employ

Full text of DOI:10.1080/00397911.2020.1829644

Synthetic Communications
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Chemistry
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Direct AlCl3-catalyzed transformation of benzyl
THP ethers and allyl benzyl ethers
Tien Tan Bui & Hee-Kwon Kim
To cite this article: Tien Tan Bui & Hee-Kwon Kim (2020): Direct AlCl3-catalyzed
transformation of benzyl THP ethers and allyl benzyl ethers, Synthetic Communications, DOI:
10.1080/00397911.2020.1829644
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https://doi.org/10.1080/00397911.2020.1829644
Direct AlCl3-catalyzed transformation of benzyl THP ethers
and allyl benzyl ethers
Tien Tan Buia,b and Hee-Kwon Kima,b
aDepartment of Nuclear Medicine, Molecular Imaging & Therapeutic Medicine Research Center,
b
Chonbuk National University Medical School and Hospital, Jeonju, Republic of Korea; Research
Institute of Clinical Medicine of Chonbuk National University-Biomedical Research Institute of Chonbuk
National University Hospital, Jeonju, Republic of Korea
ABSTRACT
ARTICLE HISTORY
Received 14 July 2020
THP and allyl groups are frequently used for the protection of alco-
hols. In this study, novel direct transformations of benzyl THP ethers
and allyl benzyl ethers, protected forms of alcohols, are reported.
TMSN3 and AlCl3 were employed as key azide reagent and catalyst
to perform direct conversion of benzyl THP ethers and allyl benzyl
ethers to azido compounds. In addition, direct nucleophile additions
of carbon nucleophiles to benzyl THP ethers and allyl benzyl ethers
were successfully accomplished in the presence of catalytic AlCl3.
The results indicate that this novel method could be applied to vari-
ous direct transformations of benzyl THP ethers and allyl benzyl
ethers such as azidation, allylation, and alkynylation.
KEYWORDS
Allyl ethers; Aluminum(III)
chloride; catalysts; DPM
azides; THP ethers
GRAPHICAL ABSTRACT
Introduction
Alcohols are widely distributed moieties in organic chemistry, and various protecting
groups for alcohols are employed to prevent the generation of by-products during
organic procedures. In particular, tetrahydropyranyl (THP) and allyl groups are com-
monly utilized as protecting groups of alcohols. To achieve nucleophilic substitution of
THP- and allyl- protected alcohols, several reaction steps are required: elimination of
the protecting group and transformation to an active leaving group such as halide,
triflate, and mesylate.[1–3] Thus, the development of an efficient direct transformation of
CONTACT Hee-Kwon Kim
hkkim717@jbnu.ac.kr
Department of Nuclear Medicine, Molecular Imaging &
Therapeutic Medicine Research Center, Chonbuk National University Medical School and Hospital, Jeonju, 54907,
Republic of Korea; Research Institute of Clinical Medicine of Chonbuk National University-Biomedical Research Institute
of Chonbuk National University Hospital, Jeonju, 54907, Republic of Korea.
ß 2020 Taylor & Francis Group, LLC

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T. T. BUI AND H.-K. KIM
THP- and allyl-protected alcohols is an appealing approach with benefits including
shortened reaction steps and a decrease of waste material.
Azides are identified as precious organic synthesis intermediates leading to nitrenes,[4]
amines,[5,6] and heterocycles.[7,8] Furthermore, utilization of azides in click chemistry is
an important protocol for the attachment of various biomarkers to macromolecules.[9,10]
In particular, the formations of azides from benzhydryl alcohols were used for the syn-
thesis of (diarylmethyl)amine precursors, which are components of commercial drugs
such as meclizine[11] and levocetirizine.[12] In additions, azides also contribute important
biological activity in some pharmaceuticals such as the HIV/AIDS drug AZT, known as
zidovudine.[13] Therefore, various methods have been used for the preparation of azides.
The popular method to synthesize azides involves two separate steps: first, conversion of
the hydroxyl group to a halogen[14] or acetal,[15] and then azide formation by a substitution
reaction with sodium azide (NaN3). However, these protocols have drawbacks such as the
toxicity of sodium azide.[16] In addition, several methods for direct azidation from a free
hydroxyl group using various reagents such as sodium tetrachloroaurate,[17] copper tri-
flate,[18] and indium (III) bromide[19] have been reported. We were attracted to discover an
efficient direct conversion of protected alcohols using a novel catalyst. Herein, we describe
novel conversion of benzyl THP ethers and allyl benzyl ethers directly to azido compounds
using a catalyst. Moreover, the direct alkynylation and allylation of benzyl THP ethers and
allyl benzyl ethers could also be achieved using this catalytic procedure (Scheme 1).
Result and discussion
First, various reagents were examined to react with 2-(benzhydryloxy)tetrahydro-2H-pyran
(1a) in the presence of TMSN3 to discover optimal catalysts. Employment of MnCl2, SnCl2,
Scheme 1. Direct transformations of protected benzyl alcohols.

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3
Table 1. Screening of reaction conditions for the conversion of THP benzyl ether to azidea.
Entry
Azide agent
Catalyst
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
NaN3
MnCl2
SnCl2
CuCl2
AlMe3
ZrCl4
ZnCl2
BiCl3
TiCl4
AlCl3
CuI
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
No
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
NRc
NRc
NRc
NRc
NRc
5
7
8
9
95
10
11
12
13
14
15
16
17
18
19
NRc
5
DPPA
NRc
NRc
NRc
NRc
51
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
1,4-dioxane
DMF
Toluene
MeCN
DCE
CH2Cl2
93
12
NRc
aReaction conditions: compound 1a (1.0 mmol), azide agent (1.3 mmol), catalyst (0.2 mmol), room temperature, 5 min.
c
bIsolated yield after column purification. No reaction.
CuCl2, AlMe3, and ZrCl4 as catalyst did not provide any desired product. Reactions using
ZnCl2, BiCl3, and TiCl4 did not provide the corresponding product in satisfactory yield:
synthetic yields were less than 10% (Table 1, entries 6–8). Next, AlCl3 was investigated as a
catalyst for the reaction, and the target product was obtained in excellent yield (Table 1,
entry 9). Utilization of CuI was also ineffective for generation of the corresponding
product. NaN3 and DPPA, the other azide agents, were also examined, but they proved to
be less suitable for the direct reaction of benzyl THP ether to give the azido product.
(Table 1, entries 11 and 12).
Solvent effect on the direct synthesis of azido product was investigated (Table 1, entries
13–18). Utilization of THF, 1,2-dioxane, and DMF as solvent did not generate the corre-
sponding product, while toluene enhanced the synthetic yield of the target product (55%
for toluene). When MeCN was used as solvent, the reaction readily generated the corre-
sponding product in high yield (93%). Dichloroethane (DCE, C2H4Cl2) was also tested;
however, the reaction in DCE produced the target compound in low yield.
The efficiency of azide reagent (TMSN3) and Lewis acid (AlCl3) were investigated to
find optimal amounts of TMSN3 and AlCl3 for the reaction. A series of reactions vary-
ing the amount of TMSN3 with 2-(benzhydryloxy)tetrahydro-2H-pyran in the presence
of a catalytic amount of AlCl3 were performed. Employment of 3.0, 2.0, and 1.3 equiv.
of TMSN3 for the reaction provided the desired product in high yield, as shown in
Table 2. Several different amounts of AlCl3 were also tested to find a suitable amount
for the reaction. Utilization of 1.0, 0.5, 0.3, and 0.2 equiv. of AlCl3 in the reaction pro-
duced the target product in high yield (95%).
With optimized reaction conditions in hand, the scope of the direct conversion
of various benzyl THP ethers to azido compounds using AlCl3 was examined. First,

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T. T. BUI AND H.-K. KIM
Table 2. Screening of amount of reagent and catalyst for the preparation of azidea .
Entry
TMSN3 (equiv)
AlCl3 (equiv)
Time(min)
Yieldb (%)
1
2
3
4
5
6
7
8
3
2
1.3
1
0.5
1.3
1.3
1.3
1.3
1.3
1.3
1.3
0.2
0.2
0.2
0.2
0.2
1.0
0.5
0.3
0.2
0.1
0.05
0.01
5
5
5
5
5
5
5
5
5
5
5
5
95
95
95
71
32
95
95
95
95
37
8
9
10
11
12
1
aReaction conditions: compound 1a (1.0 mmol), TMSN3 (1.3 mmol), AlCl3 (0.2 mmol), room temperature, 5 min. bIsolated
yield after column purification.
mono-substituted benzhydrol THP ethers [2-(benzhydryloxy)tetrahydro-2H-pyrans)]
were used to prepare azido products. 2-(Benzhydryloxy)tetrahydro-2H-pyrans bearing
electron-donating groups (methyl, and methoxy) and an electron-withdrawing group
(chloro) on the aromatic ring were treated with TMSN3 in the presence of AlCl3 to
yield the corresponding azido products in high yields (Table 3, entries 1–4). Di-substi-
tuted benzhydrol THP ethers were also employed for the reaction with TMSN3, and the
target products such as 4,40-(azidomethylene)bis(chlorobenzene) and 4,40-(azidomethyle-
ne)bis(methoxybenzene) were successfully prepared in high yields (90% for 2e, and 95%
for 2f). In addition, benzyl THP ethers containing various alkyl groups such as 1-phe-
nylethan-1-ol THP ether containing a methyl group, 3-methyl-1-phenylbutan-1-ol THP
ether containing an isobutyl group, and cyclohexyl(phenyl)methanol THP ether contain-
ing a cyclohexyl group were treated with TMSN3 under the same reaction conditions to
provide the corresponding azido products in the range of 88-93% (Table 3, entries 7–9).
Reaction of phenyl(thiophen-2-yl)methanol THP ether containing a heterocyclic moiety
was also tested, and 2-(azido(phenyl)methyl)thiophene was successfully synthesized
(Table 3, entry10). Benzyl THP ethers derived from 1-indanol and 4-phenyl-3-buten-2-
ol were also tested, affording the corresponding products in 93% and 90% yield, respect-
ively (Table 3, entries 11 and 12). Reactions of tertiary benzyl THP ethers, which are
more sterically hindered, were carried out, and they provided the corresponding azido
products in high yields (Table 3, entries 13 and 14). Notably, the reaction of 2-(tritylox-
y)tetrahydro-2H-pyran (1n) readily provided the target azido product in 89% yield.
These results suggest that the novel method using TMSN3 and catalytic AlCl3 could be
effectively used to prepare azido compounds from a wide variety of benzyl THP ethers.
The allyl group is a widely utilized protecting group for alcohols in many multi-
step syntheses. However, to the best of our knowledge, there is no report of the dir-
ect synthesis of azido compounds from allyl benzyl ethers. Thus, this novel protocol
using AlCl3 was applied to the direct conversion of allyl benzyl ethers to azido com-
pounds, as shown in Table 4. Reactions of the mono-/di- substituted ((allyloxy)me-
thylene)dibenzene bearing electron-withdrawing substituents (bromo, and fluoro) or

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Table 3. Scope of synthesis of azides from various benzyl THP ethersa.
aReaction conditions: compound 1 (1.0 mmol), TMSN3 (1.3 mmol), AlCl3 (0.2 mmol), room temperature, 5 min. bIsolated
yield after purification by flash column chromatography.
Table 4. Scope of synthesis of azides from various allyl benzyl ethersa.
aReaction conditions: compound 3 (1.0 mmol), TMSN3 (1.3 mmol), AlCl3 (0.2 mmol), room temperature, 5 min. bIsolated
yield after column purification.

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T. T. BUI AND H.-K. KIM
electron-donating substituents (methyl, and methoxy) with TMSN3 successfully gen-
erated the target azido compounds in high yields (Table 4, entries 1–6).
Allyl benzyl ethers containing an alkyl group such as propyl and cyclohexyl were
treated with TMSN3 to provide the corresponding products in high yield (Table 4,
entries 7 and 8). Furthermore, reactions of allyl benzyl ethers derived from various types
of alcohol including 1-indanol, 4-phenyl-3-buten-2-ol, and triphenylmethanol were con-
ducted under the same reaction conditions, and the desired products were prepared in
the range of 92% to 94% yield (Table 4, entries 9-11).
Next, the scope of this synthetic method was extended to direct nucleophilic substitu-
tion by carbon nucleophiles. As shown in Table 5, the benzyl THP ether or allyl benzyl
ether was reacted with allyl-TMS or TMS-phenylacetylene in the presence of AlCl3, and
the target allylated and alkynylated products were successfully prepared in high yields.
These results indicate that this AlCl3 catalyzed protocol could be successfully used to
form new C-C bonds directly from THP protected alcohols and allyl protected alcohols.
Table 5. Reaction of carbon nucleophiles with THP/allyl benzyl ethersa.
aReaction conditions: compound 1 or 3 (1.0 mmol), TMSN3 (1.3 mmol), AlCl3 (0.2 mmol), room temperature, 30 min.
bIsolated yield after column purification.

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Scheme 2. Proposed reaction pathway for generation of azido compounds from THP and allyl ethers.
A possible reaction pathway can be proposed, as shown in Scheme 2. The reaction
begins with activation of the oxygen atom of the benzyl THP ether or allyl benzyl ether
by AlCl3 to give the secondary benzyl cation intermediate 7, and then attack by N3
affords the corresponding azido product 3.
Conclusion
In conclusion, a novel facile method for the direct conversion of benzyl THP ethers and
allyl benzyl ethers to azido compounds has been reported. In this study, TMSN3 and
AlCl3 were used as the key azide reagent and catalyst for direct synthesis of azido com-
pounds from benzyl THP ethers and allyl benzyl ethers. In addition, nucleophilic addi-
tions by carbon nucleophiles on benzyl THP ethers or allyl benzyl ethers were achieved
via utilization of this catalytic protocol. Moreover, this method employed an inexpensive
catalyst and was operationally simple. The method provides a novel approach to the dir-
ect transformation of benzyl THP ethers and allyl benzyl ethers.
Experimental
Reagents and solvents were commercially available and used as received. Column chroma-
tography was performed using silica gel 60 (0.040–0.063 mm) and eluted with proper mix-
ture (DCM/hexane). All of the new compounds were identified by 400 MHz 1H and
100 MHz 13C NMR spectra in deuterated chloroform (CDCl3) with tetramethylsilane(TMS)
as an internal reference, and high resolution mass spectroscopy. The identity of the known
1
compounds was established by the comparison of their H and 13C NMR peaks with the
authentic values.
General experimental procedure for the synthesis of azido compounds from THP
protected alcohols (2a–2n)
To a mixture of THP-protected alcohol (1a) (0.269 g, 1.00 mmol) and AlCl3 (0.027 g,
0.2 mmol) in CH2Cl2 (2 mL) was added dropwise TMSN3 (0.149 g, 1.3 mmol). The

8
T. T. BUI AND H.-K. KIM
reaction mixture was stirred at room temperature for 5 min. After adding water (5 mL)
to the mixture, the reaction mixture was extracted with EtOAc (2 ꢀ 5 mL), then the
organic layer was dried over sodium sulfate and subsequently concentrated under
reduced pressure. The crude product was purified by flash column chromatography on
silica gel with 5% CH2Cl2 in hexane as eluent to afford the desired product 2a
(0.196 g, 94%).
General experimental procedure for the synthesis of azido compounds from allyl
protected alcohols (4a–4k)
To a mixture of allyl-protected alcohol (3a) (0.224 g, 1.00 mmol) and AlCl3 (0.027 g,
0.2 mmol) in CH2Cl2 (2 mL) was added dropwise TMSN3 (0.149 g, 1.3 mmol). The reac-
tion mixture was stirred at room temperature for 5 min. After adding water (5 mL) to
the mixture, the reaction mixture was extracted with EtOAc (2 ꢀ 5 mL), then the
organic layer was dried over sodium sulfate and subsequently concentrated under
reduced pressure. The crude product was purified by flash column chromatography on
silica gel with 5% CH2Cl2 in hexane as eluent to afford the desired product 4a
(0.197 g, 94%).
General experimental procedure for the synthesis of carbon-carbon compounds
from allyl/THP protected alcohols (5a–5b)
To a mixture of THP-protected alcohol (1a) (0.269 g, 1.00 mmol) and AlCl3 (0.027 g,
0.2 mmol) in CH2Cl2 (2 mL) was added dropwise TMS-Allyl (0.149 g, 1.3 mmol). The
reaction mixture was stirred at room temperature for 30 min. After adding water (5 mL)
to the mixture, the reaction mixture was extracted with EtOAc (2 ꢀ 5 mL), then the
organic layer was dried over sodium sulfate and subsequently concentrated under
reduced pressure. The crude product was purified by flash column chromatography on
silica gel with 5% CH2Cl2 in hexane as eluent to afford the desired product 5a
(0.190 g, 91%).
Selected spectroscopic data
1
(Azidomethylene)dibenzene (2a): Light yellow oil (94%); H NMR (400 MHz, CDCl3)
d 7.41 ꢁ 7.32 (m, 10H), 5.75 (s, 1H); 13C NMR (100 MHz, CDCl3) d 139.59 (2 C),
128.72 (4 C), 128.06 (2 C), 127.40 (4 C), 65.54; HRMS (ESI) m/z (M þ H)þ calcd for
C13H12N3 ¼ 210.1031, found 210.1035.
1
(1-Azido-3-methylbutyl)benzene (2h): Light yellow oil (89%); H NMR (400 MHz,
CDCl3) d 7.43 ꢁ 7.32 (m, 5H), 4.51 (dd, J ¼ 14.4 Hz, J ¼ 14.4 Hz,1H), 1.80 ꢁ 1.75 (m,
1H), 1.69 ꢁ 1.56 (m, 2H), 0.97 (s, 3H), 0.95 (s, 3H); 13C NMR (100 MHz, CDCl3) d
140.02, 128.78 (2 C), 128.19, 126.90 (2 C), 64.52, 45.06, 25.01, 22.63, 22.22; HRMS (ESI)
m/z (M þ H)þ calcd for C11H16N3 ¼ 190.1344, found 190.1347.
(Azidomethanetriyl)tribenzene (2n): Light yellow oil (89%); 1H NMR (400 MHz,
CDCl3) d 7.38 ꢁ 7.30 (m, 15H); 13C NMR (100 MHz, CDCl3) d 143.14 (3 C), 128.48

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9
(6 C), 128.20 (6 C), 127.70 (3 C), 77.15 (1 C); HRMS (ESI) m/z (M þ H)þ calcd for
C19H16N3 ¼ 286.1344, found 286.1348.
4,40-(Azidomethylene)bis(fluorobenzene) (4e): Light yellow oil (93%); 1H NMR
(400 MHz, CDCl3) d 7.30 ꢁ 7.27 (m, 4H), 7.10 ꢁ 7.05 (m, 4H), 5.70 (s, 1H); 13C NMR
(100 MHz, CDCl3) d 163.67, 161.21, 135.26, 135.23, 129.09 (2 C), 129.01 (2 C), 115.82
(2 C), 115.61 (2 C), 67.12; HRMS (ESI) m/z (M þ H)þ calcd for C13H10F2N3
¼
246.0843, found 246.0846.
Prop-2-yne-1,1,3-triyltribenzene (5b): Colorless oil (82%); 1H NMR (400 MHz,
CDCl3) d 7.15 ꢁ 7.46 (m, 6H), 7.35 ꢁ 7.32 (m, 7H), 7.28 ꢁ 7.26 (m, 2H), 5.24 (s, 1H);
13C NMR (100 MHz, CDCl3) d 141.77 (2 C), 131.73 (2 C), 128.66 (4 C), 128.23 (2 C),
127.99 (1 C), 127.92 (4 C), 126.94 (2 C), 123.50 (1 C), 90.20 (1 C), 84.88 (1 C), 43.77
(1 C); HRMS (ESI) m/z (M þ H)þ calcd for C21H17 ¼ 269.1330, found 269.1332.
Disclosure statement
There are no conflicts to declare.
Funding
This research was supported by Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education [2018R1D1A1B07047572].
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