Xu et al.
FULL PAPER
techniques.
dec-7-ene (DBU) were purchased from J&K. Anhy-
drous copper iodide, pyridine-2-carboxylic acid (pico-
linic acid), and anhydrous K3PO4 were also from J&K.
Di-2-methoxyphenyl ether, di-4-tert-butylphenyl
ether, and di-4-ethylphenylether were synthesized by
ourselves based on the reported procedures.[31] Typically,
a 100 mL round bottom flask was loaded with copper(I)
iodide (1 mmol), pyridine-2-carboxylic acid (2 mmol),
aryl iodide (10 mmol), phenol (12 mmol), potassium
phosphate (20 mmol), DMSO (30 mL) and a magnetic
stir bar. The reaction flask was sealed with a septum in
an argon atmosphere. After being stirred at 100 ℃ for
24 h, the reaction mixture was cooled down to room
temperature and diluted with a saturated aqueous solu-
tion of ammonium chloride. The product was extracted
with methylene chloride (2×100 mL), which was suc-
cessively washed with a 5% aqueous solution of potas-
sium hydroxide (100 mL), brine (100 mL) and dried
over anhydrous sodium sulfate. The crude product was
then purified by column chromatography.
Characterization
1H NMR and 13C NMR spectra were recorded on a
Bruker AV400 (400 MHz, 100 MHz) NMR 80 spec-
trometer in CDCl3 with tetramethylsilane as an internal
standard. The liquid products were analyzed by GC on
an Agilent 7890B Gas Chromatography equipped with
an HP-INNOWAX 19091N-133 (30 m×0.25 mm ID×
0.25 µm) capillary column (Agilent) and an FID detec-
tor. The following GC temperature program was used:
50 ℃ hold for 2 min, ramp 20 ℃/min to a final tem-
perature of 250 ℃, and hold for 8 min. The injector
temperature was held at 250 ℃. Nitrogen was used as a
carrier gas. GC-MS analyses were performed on a Shi-
madzu QP-2010 gas chromatography equipped with a
DB-5 30 m×0.25 mm×0.25 µm capillary column (Ag-
ilent). The GC was directly interfaced to an Agilent
5973 mass selective detector (EI, 70 eV). The following
GC temperature program was used: 50 ℃ hold for 2
min, ramp 20 ℃/min to a final temperature of 250 ℃,
and hold for 20 min. Nitrogen was used as a carrier gas.
The injector temperature was held constant at 250 ℃.
Di-2-methoxyphenyl ether NMR data are as fol-
1
lows. H NMR (400 MHz, CDCl3) δ: 7.03-6.83 (m,
8H), 3.85 (s, 6H); 13C {1H} NMR (100 MHz, CDCl3) δ:
155.4 (C), 151.6 (C), 119.6 (CH), 114.7 (CH), 55.7
(CH3).
Results and Discussion
Di-4-tert-butylphenyl ether NMR data are as fol-
lows. 1H NMR (400 MHz, CDCl3) δ: 7.32 (dd, J=8 Hz,
4H), 6.93 (dd, J=8 Hz, 4H), 1.31 (s, 18H); 13C {1H}
NMR (100 MHz, CDCl3) δ: 155.6 (C), 146.2 (C), 126.9
(CH), 118.7 (CH), 34.7 (C), 31.9 (CH3).
Di-4-ethylphenyl ether NMR data are as follows.
1H NMR (400 MHz, CDCl3) δ: 7.13 (d, J=8 Hz, 4H),
6.91 (d, J=8 Hz, 4H), 2.61 (q, J=8 Hz, 2H), 1.22 (t,
J=8 Hz, 3H); 13C {1H} NMR (100 MHz, CDCl3) δ:
155.6 (C), 138.9 (C), 129.1 (CH), 118.7 (CH), 28.3
(CH2), 15.8 (CH3).
Dibenzofuran is a model compound for the 4-O-5
linkage in lignin. The reduction of dibenzofuran was
chosen as a model reaction to optimize the reaction
conditions, and the results are listed in Table 1. Diben-
zofuran could not be reduced by NaH in the absence of
any base (Table 1, Entry 1). Excitingly, it was com-
pletely reduced to o-phenyl phenol by NaH in the pres-
ence of KOtBu at 140 ℃ (Table 1, Entry 2). Compared
to the reported results (Table 1, Entries 3 and 4), the
combination of NaH with KOtBu showed much higher
efficiency for this reaction than that of LiAlH4 with
NaOtBu in the presence of [Fe(acac)3] or that of Et3SiH/
KOtBu as the reaction system. More importantly, no
byproduct was detected, indicating obvious advantages
over the combination of triethylsilane with bases.[28] The
hydrogenation of aryl ring in dibenzofuran did not occur,
implying that the reductive power of NaH combined
with KOtBu was just appropriate for reductive cleavage
of C-O bond in ethers. Other hydrogen donors includ-
ing NaBH4, KBH4, diisobutyl aluminium hydride
(DIBAL) and Et3SiH were examined for this reaction in
the presence of KOtBu. It was indicated that NaBH4,
KBH4 and DIBAL were ineffective for this reaction
(Entries 5-7). Et3SiH afforded a product yield of 82%
under the same other conditions (Entry 8), however,
accompanied with several byproducts, consistent with
that reported previously.[28] The bases including NaOtBu,
LiOtBu, NaOEt, KOH and DBU instead of KOtBu were
examined in the reduction of dibenzofuran by NaH, and
no reaction occurred (Table 1, Entries 9-13). These
results indicated that the combination of NaH and
KOtBu was exclusively effective for the cleavage of aryl
General procedure for reduction of aryl ethers
The reactions were conducted in a 35 mL vial with
Teflon-lined screw cap (supplied by Synthware Com-
pany) and Teflon-coated magnetic stir bar. In a glove
box, the vial was loaded with corresponding substrate (1
mmol, 1 equiv.), base (2.5 equiv.) and a magnetic stir-
ring bar, followed by syringe addition of 5 mL of decane
and hydride source (2.5 equiv.). Then the vial was
sealed and heated at a desired temperature (e.g., 140 ℃)
for a desired time. After being cooled to room tempera-
ture, the reaction mixture was carefully quenched with 5
mL of 2 mol/L aqueous HCl at 0 ℃, accompanied with
addition of n-dodecane (internal standard for GC analy-
sis) to the reaction solution, and the liquid was separated.
Ethyl acetate (2×3 mL) was used to extract the aqueous
phase. The combined organic solution was subjected to
GC/FID for qualitative and quantitative identification.
The crude product was purified by column chromatog-
raphy on silica gel (the eluent was petro to petro/ethyl
acetate with a molar ratio of 15∶1 or 10∶1). The
purified products were characterized by NMR
2
© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2017, XX, 1—5