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Can. J. Chem. Vol. 79, 2001
′′
more cleavage of the tetrazole ether bond in 1′ under light
exposure; whether the increased yellowing of 1′ is due to a
colour-forming reaction from the cleaved tetrazole group,
the cleaved phenol, or both, is unclear.
propylbenzene (1 ) were prepared from the hydrogenation of
eugenol and 4-allyl-1,2-dimethoxybenzene over Pd/C, re-
spectively, (23). The residual light yellow colors in 1, 1 ,
and 4-hydroxy-3-methoxyacetophenone (2) (Aldrich) were
removed by filtration of their solutions in CH2Cl2 through
Celite–activated charcoal–Florisil–silica (1:5:5:1).
′′
The above photostability data show that effective photo-
stabilization of lignin phenols by conversion to tetrazole
ethers is favored by the presence of an α-carbonyl at the
C(1) position of the lignin aromatic ring. In softwood
lignins, the number of free phenols is probably <20 per 100
phenylpropane units (19). Among such phenols, only one
has been estimated to have a C(1) α-carbonyl group, the ma-
jority having an α-hydroxyl group (19). As the α-hydroxyl
group and alkyl group (such as the Pr substituent in 1) have
similar electronic properties, e.g., the Hammett σpara values
for -CH(OH)CH3 and n-Pr are –0.07 and –0.15, respectively,
(20), conversion of the hydroxyl group to an α-carbonyl
group may be necessary to obtain significant lignin photo-
stabilization by conversion of the lignin phenols to tetrazole
ethers. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
can oxidize the α-hydroxyl to the α-carbonyl group in lignin
model compounds (21) and lignin (22), but a reagent more
compatible with wood pulp reactions is needed, and cata-
lytic, oxidative-dehydrogenations are being considered by
us. The success of 3 in photostabilizing the acetophenone
derivative (2), via the formation of 2′, has also prompted us
to introduce a sulfonate group in to the phenyl ring of 3 to
give a water-soluble derivative. Future papers will describe
our efforts in these areas of research.
Synthesis of 2-methoxy-4-propylphenyl-1′-phenyl-5′-
tetrazole ether (1′)
To 1 (0.399 g, 2.4 mmol) in distilled water (5 mL) and ac-
etone (5 mL) in a 25 mL flask equipped with a condenser
and a magnetic stirring bar was added K2CO3 (0.663 g,
4.8 mmol) and 5-chloro-1-phenyl-1H-tetrazole (3) (0.433 g,
2.4 mmol). The mixture was stirred at 60°C overnight (ca.
18 h), cooled to room temperature (~20°C), acidified with
1 N HCl solution, and then extracted with Et2O (4 × 30 mL).
The combined ether extract was dried over anhydrous
MgSO4 for 20 min before being filtered. Removal of the fil-
trate solvent under reduced pressure gave 0.620 g of crude 1′
as a light-yellow solid (83% crude yield). Purification by
column chromatography on silica gel using hexane:ethyl ac-
etate (9:1 and then 1:1) as eluent, followed by recrystal-
lization from MeOH, gave 0.463 g of 1′ as white crystals
(62% yield). Crystals suitable for X-ray analysis were ob-
tained at –20°C from a diluted MeOH solution of 1′. EI-MS
m/z (%): 310 (M+, 42), 240 (58), 165 (85), 117 (100). H
1
NMR δ: 0.95 (t, 3H, CH2CH3), 1.63 (m, 2H, CH2CH3), 2.60
(t, 2H, CH2CH2), 3.75 (s, 3H, OCH3), 6.80–7.90 (m, 8H, ar-
omatic). Anal. calcd. for C17H18N4O2: C 65.79, H 5.85,
N 18.05; found: C 66.08, H 5.85, N 18.19.
Conclusions
The protection of the phenolic hydroxyl groups in lignin
model compounds, 2-methoxy-4-propylphenol (1) and 4-
hydroxy-3-methoxyacetophenone (2), as the tetrazole ethers
can be achieved in aqueous alkaline media using 5-chloro-1-
phenyl-1H-tetrazole (3) as the alkylating agent; 3 is nontoxic
and more stable toward alkali-induced hydrolysis than other
alkylating agents such as α-butylene oxide. The tetrazole
ether 2′ from 2 is much more photostable than the precursor
phenol on exposure to strong fluorescent light, while the
tetrazole ether 1′ from 1 is only slightly more stable than the
precursor phenol. Photostablization of lignin phenols by
conversion to tetrazole ethers seems to require conversion of
the α-hydroxyl to α-carbonyl groups, which is unlikely to be
commercially viable.
Synthesis of 3-methoxy-4-(1′-phenyl- 5′-tetrazol)-
oxyacetophenone (2′)
Identical to the preparation of 1′, 2 (0.407 g, 2.4 mmol)
was reacted with 3 (0.433 g, 2.4 mmol) and K2CO3 (0.663 g,
4.8 mmol) in H2O–acetone (1:1, 10 mL) to give 0.290 g of 2′
as a light-yellow crystalline solid (39% yield). Filtration of a
CH2Cl2 solution of this solid through Celite–activated char-
coal–Florisil–silica (1:5:5:1) gave 2′ as colorless crystals. EI-
MS m/z (%): 310 (M+, 12), 240 (13), 198 (10), 165 (34), 117
(100). 1H NMR δ: 2.62 (s, 3H, OCH3), 3.86 (s, 3H,
C(O)CH3), 7.40–7.90 (m, 8H, aromatic). Anal. calcd. for
C16H14N4O3: C 61.93, H 4.55, N 18.06; found: C 61.94,
H 4.56, N 17.80.
X-ray crystallographic analyses of 1′ and 2′
A needle crystal of 1′ (~ 0.45 × 0.25 × 0.20 mm) and a
plate crystal of 2′ (~ 0.40 × 0.10 × 0.50 mm) were each
mounted in a glass fibre. Measurements were made on a
Rigaku ADSC CCD area detector with graphite mono-
chromated Mo Kα (for 1′) or Cu Kα (for 2′) radiation. For 1′,
cell constants and an orientation matrix for data collection,
derived from 6295 reflections, corresponded to a monoclinic
cell, space group P21/c (No. 14), FW = 310.35 with dimen-
sions: a = 9.8679(6), b = 16.708(1), c = 10.2841(6) Å, β =
109.732(5)°, V = 1596.0(2) Å3, Z = 4, ρcalc = 1.37 g cm–3;
absent reflection: h0l: (l ≠ 2n), 0k0: (k ≠ 2n). The data were
collected at –100 ± 1°C to a max 2θ value of 55.8° in 0.50°
oscillations with 15.0 s exposures. A sweep of data was
done using Φ oscillations from 0.0 to 190.0° at χ = 0°, and a
second sweep was performed using ω oscillations between
Experimental
General procedures
All chemicals were purchased from Aldrich unless other-
wise specified. Reactions were carried out under Ar and
monitored by GC and (or) TLC. GC was performed on a
Hewlet-Packard model 5890 using a 25 m × 0.2 mm
Carbowax column (80°C for 2 min, then an increase to
220°C at a rate of 10°C per min, and a final time of 8 min at
220°C). TLC was performed on Al-backed, precoated silica
1
gel plates (E. Merck, type 5554). H NMR spectra were re-
corded in CDCl3 using SiMe4 as the internal standard on a
Bruker AC-200 spectrometer. Mass spectra were determined
on a Kratos-AEI model MS 50 spectrometer operating at
70 eV. 2-Methoxy-4-propylphenol (1) and 1,2-dimethoxy-4-
© 2001 NRC Canada