Copper catalyzed C–O bond formation by direct C–H activation of THF with phenols: an approach to the synthesis of phenyl tetrahydrofuranyl ethers

Article abstract of DOI:10.1080/00397911.2020.1858109

A direct oxidative transformation of phenol derivatives to the corresponding tetrahydrofuranyl ethers via C–H bond activation, in the presence of copper catalyst and desired products were achieved in good yields using TBHP as an oxidant.

Full text of DOI:10.1080/00397911.2020.1858109

Synthetic Communications
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Copper catalyzed C–O bond formation by direct
C–H activation of THF with phenols: an approach
to the synthesis of phenyl tetrahydrofuranyl
ethers
Manoj Kumar Vala , Suryakumari Adurthi & Sudhakar Chithaluri
To cite this article: Manoj Kumar Vala , Suryakumari Adurthi & Sudhakar Chithaluri (2020):
Copper catalyzed C–O bond formation by direct C–H activation of THF with phenols: an
approach to the synthesis of phenyl tetrahydrofuranyl ethers, Synthetic Communications, DOI:
10.1080/00397911.2020.1858109
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https://doi.org/10.1080/00397911.2020.1858109
Copper catalyzed C–O bond formation by direct C–H
activation of THF with phenols: an approach to the
synthesis of phenyl tetrahydrofuranyl ethers
Manoj Kumar Valaa,b, Suryakumari Adurthia,b, and Sudhakar Chithaluria
b
aDepartment of Chemistry, GITAM Deemed to be University, Rudraram, India; Department of
Chemistry, TARA Govt Degree College, Sangareddy, India
ABSTRACT
ARTICLE HISTORY
Received 25 September 2020
A direct oxidative transformation of phenol derivatives to the corre-
sponding tetrahydrofuranyl ethers via C–H bond activation, in the
presence of copper catalyst and desired products were achieved in
good yields using TBHP as an oxidant.
KEYWORDS
C–H activation; Cu(OAc)2;
phenol substrates;
GRAPHICAL ABSTRACT
tetrahydrofuranyl ethers
THF, R = -CHO
THF, R = -H
Cu(OAc)2 (10 mol%l)
OH
O
O
O
O
O
Cu(OAc)2 (10 mol%l)
R
H
1,10 phen., (10 mol%)
1,10 phen., (10 mol%)
TBHP, 60°C, 1 h
TBHP, 60°C, 1 h
R1
R1
R1
4 examples
up to 80% yield
14 examples
up to 95% yield
Introduction
The transition-metal catalyzed synthetic protocols of carbon–heteroatom bond forma-
tion have been much more advanced in the trends of synthetic organic chemistry.[1]
Apart from the well-established classical cross-coupling strategies are versatile methods
for C–O bond formations that have found broad applications in organic synthesis, pro-
tecting groups and related disciplines.[2] In particular, transition-metal-catalyzed reac-
tions have attracted significant interest in the synthetic community.[3]
Copper has been widely studied in organic synthesis because of its promise as a high
efficiency catalyst, and Cu-catalyzed C–H activation reactions have been described.[2d,4]
Recently, the direct functionalization of O–H bonds in simple phenols and alcohols has
received significant attention in organic synthesis because of their potential possibility
for diverse transformation into a variety of useful derivitatives.[5] In the past five
decades, a multitude of different strategies has been developed for the protection of
important functional groups in organic compounds.[6] Tetrahydropyranylation[7] and
tetrahydrofuranylation[8,9] have become widely used methods in organic synthesis for
the protection of hydroxyl groups.
CONTACT Sudhakar Chithaluri
schithal@gitam.edu; sudha1iict@gmail.com
Department of Chemistry, GITAM
Deemed to be University, Rudraram, Hyderabad 502329, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
M. K. VALA ET AL.
Scheme 1. Synthesis of 2-phenoxytetrahydrofuran derivatives Reaction conditions: Phenol derivative
(1 mmol), Cu(OAc)2 (10 mol%), 1,10 phen. (10 mol%), TBHP in water (3 equiv.), THF 3 mL, 60 ꢀC, 1 h.
A very important method for protecting alcohol functional groups in synthesis
involves their conversion to acetals.[10] Conversion to tetrahydropyranyl (THP) ethers is
very common and these are generally prepared from an acid-catalyzed addition of the
alcohol to dihydropyran (DHP). Tetrahydropyranyl (THP) and tetrahydrofuranyl (THF)
ethers make ideal protecting groups because of their ease of preparation and stability
toward a broad range of reaction conditions, such as strongly basic media, hydrides,
and acylating and alkylating agents.
There have been several methods have been reported for the synthesis of THF ethers.[9]
In addition, the activation of tetrahydrofuran has recently been reported with a variety of
one-electron oxidants, including p-TsOH,[11] cerium (IV) reagents,[12] peroxodisulfates,[13]
CrCl2,[14] alkylperoxy-k3-iodane,[15] Manganese powder,[16] aluminum triflate,[17] allyl
chloride[18] and Mn3O4 (SMONP).[19] Unfortunately, there are issues associated with all
of these methods, including the use of expensive reagents, elevated temperatures, strongly
acidic conditions, toxic reagents, and low levels of functional group tolerance.
Results and discussion
In this context, we look for the efficient and environmentally benign direct coupling of
phenol derivatives with tetrahydrofurans in the presence of external ligand for the syn-
thesis of 2-phenoxytetrahydrofuran derivatives via O–H bond activation (Scheme 1).
During these investigations, desired product was observed, under oxidative cross cou-
pling of phenols with THF in the presence of copper catalyst, 1, 10-phenonthroline ligand
and TBHP as an external oxidant. This is a ligand-assisted copper-catalyzed tetrahydro-
furanylation of phenols and alcohols under mild conditions (Scheme 1). Interestingly, this
transformation represents a direct cross coupling of sp3 C–H bond and O–H bond.
In the initial experiments, simple phenol was chosen as a model substrate and treated
with excess amount of THF, using 10mol% of CuI as catalyst and 10mol% of 1, 10 phe-
nanthroline as a ligand, TBHP as the external oxidant (Table 1, entry 1). A small amount
of carbamate product was formed at 60ꢀC for 1 h. In these reaction conditions hydroxyl
group of phenol participated in the oxidative coupling with the THF and afforded the 2-
phenoxytetrahydrofuran, which was the desired product. Further purification of the prod-
1
uct and analysis by H, 13C NMR and ESI mass revealed that the coupled product is
indeed the 2-phenoxytetrahydrofuran. Based on this initial result, further optimization
was carried out under different reaction conditions (Table 1). Absence of any coupled
product in blank experiments clearly shows the importance of both catalyst and oxidant
for this reaction (Table 1, entry 7, 8). It has been observed that both Cu(I) and Cu(II)
salts are quite active for the coupling reaction (Table 1, entries 2–6), among the different

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SYNTHETIC COMMUNICATIONSV
3
copper salts, Cu(OAc)2 provided relatively higher yields of the product (Table 1, entry 6).
Then the role of different oxidants was evaluated, and no product formation was
observed with the use of H2O2, m-CPBA and NaOCl as oxidants (Table 1, entries 9–11).
Further, optimization experiments were carried out with tetrahydrofuran (THF) and
simple phenol as the reaction partners in presence of Cu(OAc)2 by using various
ligands (Table 2). Among them, bidentate ligands viz., 2,20-bipyridine (BPY) and
1,10-phenanthroline (phen.) have shown relatively higher activity. In these optimization
studies, the absence of the product in control experiments also clearly indicates that the
ligand is essential for the activation of phenol by the metal ion (Table 2, entry 3).
During these investigations various ligands were further tested (Table 2). Whereas, the
product yield was not increased when the reaction was carried out in presence of other
ligands DMAP, DMEDA, and TMEDA (Table 2, entries 4–6). As shown in the table,
Table 1. Catalyst optimization studies of reaction conditions.a
Entry
Catalyst
Oxidant
Yield[%]b
1
2
3
4
5
6
7
8
CuI
CuBr
CuBr2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
–
20
25
42
48
64
83
NR
NR
NR
NR
NR
CuCl2 2H2O
Cu(OAc)
Cu(OAc)2
–
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
9
10
11
NaOCl
m-CPBA
H2O2
aReaction conditions: Phenol (1 mmol), catalyst (10 mol%), 1,10- phenanthroline (10 mol%), THF (3 mL, 30 mmol), 3
equiv., TBHP(tert-Butyl hydroperoxide—70 wt % in H2O), 60 ꢀC, 1 h. bIsolated yields.
Table 2. Ligand optimization studies of 2-phenoxytetrahydrofuran formation.a
Entry
Ligand
Yield [%]b
1
2
3
4
5
6
7
8
9
Triethylamine
Ethylene diamine
–
DMAP
DMEDA
10
14
NR
25
30
33
42
48
83
TMEDA
2-aminomethyl pyridine
2,20-bipyridine
1,10-Phenanthroline
aReaction conditions: Phenol (1 equiv.), catalyst (10 mol%), THF (3 mL), TBHP in water (3 equiv.), ligand (10 mol%), 60ꢀ
C, 1 h. bIsolated yields.

4
M. K. VALA ET AL.
most of the ligands have shown some activity, still 1, 10-phenanthroline proved to be
the best one (Table 2, entry 9).
With these optimization studies, the general scope of the present oxidative cross cou-
pling protocol was carried out for different phenol substrates using 10 mol% of
Cu(OAc)2, 3 mL of tetrahydrofuran source, 3 equiv. of TBHP in water solution,
10 mol% of 1,10 phenanthroline at 60 ꢀC for 1 h and the results are summarized in
Table 3. Simple phenol and substrates having electron-donating groups at meta-position
provided the 2-phenoxytetrahydrofuran products in good yields (Table 3, 3a and 3b).
Similarly, substrates having electron withdrawing groups at para- position provided
the 2-phenoxytetrahydrofuran derivatives in high yields (Table 3, 3c–3g). In addition,
that there was a negligible meta-electron withdrawing group influence on the product
where moderate yield of the product was observed (Table 3, 3h and 3i). A similar
Table 3. Synthesis of tetrahydrofuranyl ether derivatives.a,b
aReaction conditions: Phenol substrate (1 mmol), catalyst (10 mol%), TBHP (3 equiv.), ligand (10 mol%), THF (solvent,
3 mL). bIsolated yields.

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5
reactivity pattern was also observed for other derivatives (Table 3, 3j). (Table 3, 3k–3n).
Further, to test the generality of this reaction with naphthalene, hetero aromatic phenols
such as 2-hydroxyisoindoline-1,3-dione and quinazolin-4-ol were used. These derivatives
provided the corresponding tetrahydrofuranyl ether products in moderate to good yields
Furthermore; the present coupling protocol was also extended to salicylaldehyde sub-
strates with tetrahydrofuran (THF) as an ether source, whose resulting products have
more importance like as protecting groups in synthetic organic chemistry. Moreover,
the recent reports on coupling chemistry involving tetrahydrofuran were mainly with
simple phenol derivatives and rarely reported with salicylaldehyde substrates. In the pre-
sent investigation, the reaction of salicylaldehyde substrate with THF under optimized
reaction conditions provided 70–80% yields of the 2-((tetrahydrofuran-2-yl)oxy)benzal-
dehyde product (Table 4, 5a–5d).
From the above results, ligand plays an important role in the activation of phenol
and salicylaldehyde substrates with a subsequent coupling of tetrahydrofuran. The reac-
tion between simple phenol and THF under 1:0; 1:1; 1:2 and 1:3 metal to ligand ratios
resulted in 0%, 83%, 60% and 35% yields of the tetrahydrofuranyl product respectively,
which clearly confirms the participation of ligand with metal ion in activation of phe-
nol. Similarly, no product formation was observed in the presence of radical scavenger
TEMPO, it has strongly supported that the present system undergoes via radical mech-
anism (Scheme 2).[19,20]
Table 4. Synthesis of 2-((tetrahydrofuran-2-yl) oxy) benzaldehyde derivatives.a,b
aReaction conditions: Salicylaldehyde substrate (1 mmol), catalyst (10 mol%), TBHP (3 equiv.), ligand (10 mol%), THF
(solvent, 3 mL). bIsolated yields.
Scheme 2. Plausible mechanism for 2-phenoxytetrahydrofuran formation

6
M. K. VALA ET AL.
Conclusion
The present copper-catalyzed oxidative C–O bond formation protocol in which the sim-
ple phenols and salicylaldehydes were directly coupled with simple tetrahydrofurans to
generate the unsymmetrical acetal scaffolds. It has been shown that both phenol- and
salicylaldehyde substrates were easily coupled under mild reaction conditions with THF,
resulting in tetrahydrofuranyl ethers, which are useful for protecting groups. Generally,
this alcohol functionality has affords protected by temporarily converting into simple
derivatives in which most often as ethers. This protocol provides a new route for the
protection of alcohols. The present method is useful in the synthesis of salicylaldehyde
tetrahydrofuranyl ethers, which are not easily available by other methods. Although in
some cases the yields are moderate, this new approach involving the oxidative cross-
coupling chemistry via C–H bond activation.
Experimental section
General procedure for the synthesis of 2-phenoxytetrahydrofuran
In a reaction vessel Cu(OAc)2 (0.1 mmol), 1,10-phenanthroline (0.1 mmol) was dissolved
with 3 mL of THF source. The reaction mixture stirred for five minutes and added the
phenol (1.0 mmol) substrates. To the above reaction mixture, 3.0 equiv., of TBHP (tert-
Butyl hydroperoxide ꢁ70 wt % in H2O) was added dropwise, with stirring over a period
of 5 min. Then the reaction temperature was increased to 60 ꢀC and stirred for one
hour. After cooling to room temperature, the reaction mixture was directly subjected to
purification with column chromatography on silica gel using 5–10% ethyl acetate and
hexane mixture to afford the required 2-phenoxy tetrahydrofuran derivatives.
2-Phenoxytetrahydrofuran(3a): yield (83%); m.p.: 18–20 ꢀC; 1H NMR (300 MHz,
CDCl3): d 7.34 (t, J ¼ 7.7 Hz, 2H, 2xCHAr), 7.12 (d, J ¼ 8.1 Hz, 2H, 2xCHAr) , 7.04 (m,
J ¼ 7.3 Hz, 1H, CHAr), 5.88 (t, J ¼ 4.34 Hz, 1H, O–CH–O), 4.15–3.97 (m, 2H, OCH2),
2.26–2.0 (m, 4H, –CH2–CH2–); 13C NMR (75 MHz, CDCl3): d 157.0 (O–CAr), 129.2
(CHAr), 121.4 (CHAr), 116.4 (CHAr), 102.1 (O–CH–O), 67.9 (O–CH2), 32.6 (CH2), 23.3
(CH2); IR (KBr): 28,351,529,138,012,500,000,000,000,000 cmꢁ1; HRMS (ESI) calcd for
C10H13O2 (M þ H)þ 165.09101, found 165.09060.
1
Full experimental detail, H and 13C NMR spectra. This material can be found via
the “Supplementary Content” section of this article’s webpage.
Acknowledgements
We thank Department of Chemistry, GITAM Deemed to be University, Hyderabad Campus,
India, for providing space and instrumental support.
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