19F Nuclear Magnetic Resonance Spectroscopy for the Elucidation of Carbonyl Groups in Lignins. 1. Model Compounds

Article abstract of DOI:10.1021/jf960154r

A new method for the detection of different classes of carbonyl groups in a series of carbonyl-containing lignin-like model compounds has been developed. The method is based on the selective fluoride-induced trifluoromethylation of carbonyl groups with (trifluoromethyl)trimethylsilane (TMS-CF₃) in the presence of tetramethylammonium fluoride (TMAF), followed by hydrolysis with aqueous HF or TMAF in the case of quinones. In this study a series of ketones, aldehydes, quinones, and dimeric-lignin model compounds were quantitatively trifluoromethylated followed by ¹⁹F NMR spectral analyses of the resulting fluorine-containing derivatives, allowing for a thorough understanding of their structure/¹⁹F chemical shift relationships. These studies have shown that the ¹⁹F-NMR chemical shifts of the trifluoromethyl groups vary significantly and consistently for various classes of carbonyl groups which may be present in complex lignocellulosic materials. These studies are to form the basis for the development of a novel and sensitive method that can be used to obtain quantitative information on the various carbonyl groups present in such materials.

Full text of DOI:10.1021/jf960154r

J. Agric. Food Chem. 1996, 44, 2167
−2175
2167
19F Nu clea r Ma gn etic Reson a n ce Sp ectr oscop y for th e Elu cid a tion of
Ca r bon yl Gr ou p s in Lign in s. 1. Mod el Com p ou n d s
Behzad C. Ahvazi and Dimitris S. Argyropoulos*
Department of Chemistry, McGill University, and Pulp and Paper Research Centre,
3420 University Street, Montreal, Quebec, Canada H3A 2A7
A new method for the detection of different classes of carbonyl groups in a series of carbonyl-
containing lignin-like model compounds has been developed. The method is based on the selective
fluoride-induced trifluoromethylation of carbonyl groups with (trifluoromethyl)trimethylsilane (TMS-
CF3) in the presence of tetramethylammonium fluoride (TMAF), followed by hydrolysis with aqueous
HF or TMAF in the case of quinones. In this study a series of ketones, aldehydes, quinones, and
dimeric-lignin model compounds were quantitatively trifluoromethylated followed by 19F NMR
spectral analyses of the resulting fluorine-containing derivatives, allowing for a thorough under-
standing of their structure/19F chemical shift relationships. These studies have shown that the
19F-NMR chemical shifts of the trifluoromethyl groups vary significantly and consistently for various
classes of carbonyl groups which may be present in complex lignocellulosic materials. These studies
are to form the basis for the development of a novel and sensitive method that can be used to obtain
quantitative information on the various carbonyl groups present in such materials.
Keyw or d s: Carbonyl groups; model compounds; Ruppert’s reagent; trifluoromethylation; nuclear
magnetic resonance (NMR); chemical shifts
INTRODUCTION
instance, technical lignins, and, in particular, alkali
lignins, contain appreciable amounts of R-carbonyl
groups in addition to benzaldehydes and o-quinones.
Investigations of carbonyl groups have continued to
expand aimed at the identification of these groups and
elucidation of their role in lignin reactivity. Recent
studies by Panchapakesan (1990) provided evidence and
new information on such groups in lignin.
These studies have shown that the presence of car-
bonyl groups in lignins, in particular those present as
o- and p-quinonoids, quinonemethides, and other ex-
tended conjugate enone systems, are not only contribu-
tors to the color observed in lignified plant tissues (Hon
and Glasser, 1979; Lebo et al., 1990) but also sensitizers
in the photoyellowing of lignocellulosics (Brunow and
Eriksson, 1971; Lin and Kringstad, 1970). Oxidation
of lignins with reagents such as molecular oxygen,
hydrogen peroxide, and chlorine may further increase
their carbonyl content (Chang and Allen, 1971). The
o- and p-quinone structures so generated in residual
lignins of pulps may further alter the color of the pulps
and promote their capacity to be photosensitized (Pan-
chapakesan, 1990; Gierer and Lin, 1972).
Despite our advances aimed at understanding and
elucidating the role of the carbonyl groups in lignins,
there are still no fundamental techniques available for
classifying these groups. The objective of this study is
to identify and determine all distinct classes of carbonyl
groups in lignin by implementing an innovative method.
The strategy followed toward the development of such
a method was as follows:
Various carbonyl groups present in lignins and sev-
eral model compounds were to be quantitatively deriva-
tized with a reagent which readily reacts with carbonyls
and which contains an NMR sensitive heteroatom.
Next, the structure/chemical shift relations must be
evaluated in order to allow for the precise qualification
of the different classes of carbonyls in complex mixtures.
Therefore, according to these criteria, Ruppert’s reagent
The carbonyl groups present in wood and pulps play
an important role in determining the reactivity of lignin
structures toward the yellowing of paper and the
bleaching of pulp (Heitner and Schmidt, 1991; Schmidt
and Heitner, 1991, 1993; Leary, 1994). The low contents
of these groups in wood, in pulp, and in paper have
made the elucidation of their role rather elusive. De-
tecting and determining these groups will pave the way
toward arriving at new conclusions in relation to a
variety of pressing issues as far as the yellowing of
paper and the bleaching of chemical and mechanical
pulps are concerned. Therefore, the necessity to obtain
essential information toward understanding the reactiv-
ity of lignin structures during yellowing and inhibition
initiated efforts at developing a novel method for the
fine structural elucidation and quantitation of the
various carbonyl groups present in lignin.
The presence of carbonyl groups in lignin was first
postulated by Klason (1922) as early as 1922. However,
Adler et al. (Adler, 1951; Adler and Ellmer, 1948; Adler
et al., 1948) and Pew (1951) were the first to obtain
reliable direct evidence for the presence of conjugated
carbonyl groups, in particular coniferyl aldehyde-type
structures in spruce lignin. Further investigations by
Geiger and Fuggerer (1979) resulted in the detection of
cinnamaldehyde-type structures of conjugated carbonyls
in milled wood lignin. Additional experimentation
suggested the presence of other types of conjugated and
possibly nonconjugated carbonyl structures in lignin.
These structures, including trace amounts of quinone
ketals (Adler and Marton, 1959; Larsson and Miksche,
1972), have been recognized to be a portion of the total
carbonyl content present in milled wood lignins. How-
ever, different lignin isolations may contain a variety
of other possible carbonyl-containing structures. For
* Author to whom correspondence should be ad-
dressed.
[(trifluoromethyl)trimethylsilane (TMS-CF
3)] was cho-
S0021-8561(96)00154-9 CCC: $12.00
© 1996 American Chemical Society

2168 J. Agric. Food Chem., Vol. 44, No. 8, 1996
Ahvazi and Argyropoulos
MHz. The model compound derivatives were dissolved in
deuterated chloroform (10-20 mg/1.0 mL of CDCl3) using
fluorotrichloromethane (CFCl3) as internal reference. The
measurements were carried out in a 5 mm tube at room
temperature. The acquisition time of 0.64 s was used followed
by the relaxation delay of 2 s. The number of scans acquired
was 8 per measurement. Pulse widths corresponding to a 45°
flip angle and a line broadening of 1 Hz were used during
acquisition and processing of the spectra.
(b) 1H NMR Spectroscopy. The NMR spectra were recorded
at a proton operating frequency of 200.1, 299.9, and 499.8 MHz
on Varian XL-200 FT-NMR, Varian XL-300 FT-NMR, and
Varian unity 500 spectrometers, respectively. The model
compounds containing carbonyl groups and their derivatives
were dissolved in deuterated chloroform (30 mg/1.0 mL CDCl3)
using TMS as internal reference. The measurements were
sen for the selective and quantitative fluoride-induced
trifluoromethylation of carbonyl groups. This was fol-
lowed by 19F NMR spectroscopy aimed at probing the
resulting fluorine-containing derivatives.
The introduction of trifluoromethyl groups into or-
ganic molecules and macromolecules has been the
subject of significant interest by both synthetic and
medicinal chemists (Liebman et al., 1988). These
compounds have been examined for their potential as
biologically active drugs and agrochemicals (Banks,
1979). Trifluoromethyl-substituted compounds often
exhibit significant effects in their physical and chemical
properties. In 1989 Prakash and co-workers (Prakash
et al., 1989) reported the efficient and selective fluoride-
induced trifluoromethylation of carbonyl compounds
with TMS-CF3 (Ruppert et al., 1984). The work of this
group has been focused at developing new synthetic
methods for introducing perfluoroalkyl groups into
different classes of carbonyl compounds by using ef-
ficient nucleophilic trifluoromethylating agents. Vari-
ous carbonyl containing-compounds when reacted with
TMS-CF3 in the presence of a catalytic amount of tetra-
n-butylammonium fluoride (TBAF) have been shown to
undergo the following reaction.
carried out in
a 5 mm tube at room temperature. No
relaxation delay was used. The acquisition times for Varian
XL-200 FT-NMR, Varian XL-300 FT-NMR, and Varian unity
500 spectrometers, were 3.752, 2.496, and 1.892 s, respectively.
The number of scans for the 1H NMR signals were 128.
(c) 13C NMR Spectroscopy. The model compounds containing
carbonyl groups and their derivatives were dissolved in
deuterated chloroform (30 mg/1.0 mL CDCl3), and quantitative
13C NMR spectra were obtained on both Varian XL-300 FT-
NMR and Varian unity 500 spectrometers operating at 75.4
and 125.7 MHz, respectively. The internal reference was TMS.
The measurements were carried out in a 5 mm tube at room
temperature with heteronuclear decoupling. No relaxation
delay was used. The acquisition times on Varian XL-300 FT-
NMR and Varian unity 500 spectrometers were 0.970 and
1.083 s, respectively. The number of scans ranged between
65 000 and 80 000.
(d) 31P NMR Spectroscopy. The 31P NMR spectra were
obtained by using inverse gated decoupling on a Varian XL-
300 FT-NMR spectrometer at 121.5 MHz. The internal
deuterium lock was provided by the deuterium atoms present
in the deuterated chloroform, used as the solvent. The
external standard was 85% H3PO4. All downfield shifts from
H3PO4 were considered positive. A sweep width of 10 000 Hz
was observed, and spectra were accumulated with a time delay
of 10 s between successive pulses. A pulse width correspond-
ing to a 45° flip angle and a line broadening of 2 Hz were used
for acquiring the processing of the spectra. For the 45° flip
angle a series of experiments showed that there was no further
increase in signal intensity after a 7 s pulse delay was applied.
All chemical shifts reported are relative to the dimeric model
compound with water, which has been observed to give a sharp
31P signal at 121.1 ppm (Argyropoulos et al., 1993; Argyropou-
los, 1994).
This reaction works equally well for different classes
of carbonyl groups, such as aldehydes, ketones, enones,
carboxylic acid, and even ketoesters (Prakash and
Pichika, 1991) without being affected by moisture.
In this work, a series of organic compounds containing
different classes of carbonyl groups were subjected to
the proposed chemical reaction and their carbonyl
groups were converted to their trifluoro analogues.
Finally, the resulting fluorine-containing derivatives
were examined by 19F NMR spectroscopy. As a result,
numerous signals, due to the different classes of carbo-
nyl groups, were detected and studied.
MATERIALS AND METHODS
I. Rea ction s. Acetylation of Lignin Model Compounds.
The carbonyl-containing compound (∼200 mg) was acetylated
with 2-4 mL of acetic anhydride/pyridine (1:1, (v/v)) at room
temperature overnight in a 100 mL flask. Ethanol (50 mL)
was added, and, after 30 min, the solvents were removed by
film evaporation. Repeated addition and removal (film evapo-
ration) of ethanol (5-10 times) resulted in the removal of acetic
acid and pyridine from the sample. The acetate was dried in
vacuo over KOH and P2O5 or purified as described below prior
to drying.
Gas Chromatography/ Mass Spectrometry. GC-MS analyses
were carried out on a Hewlett-Packard 5972 mass spectrom-
eter interfaced to a Hewlett-Packard 5890A gas chromato-
graph with a 30 m × 0.25 mm packed silica capillary column
DB-5. The injection port temperature was 280 °C and the oven
temperature was varied from 100 to 250 °C, with a gradient
of 5 °C/min. The CF3-containing lignin model compounds were
analyzed after silylation.
Trifluoromethylation Procedure. The carbonyl-containing
model compound (10 mmol) was dissolved in 3 mL of high-
purity tetrahydrofuran (THF). To this solution, an excess (15
mmol) of TMS-CF3 was added. The mixture was then treated
with a catalytic amount (20 mg) of tetramethylammonium
fluoride (TMAF) under constant stirring at 0 °C for 0.5 h. It
was then brought to room temperature, and the reaction was
allowed to continue to completion (TLC). The intermediate
siloxy adducts were hydrolyzed by the addition of (4 mL) of
aqueous hydrofluoric acid (10% HF). Finally, the hydrolysed
mixture was extracted with chloroform (3 × 15 mL) and dried
over anhydrous magnesium sulfate. The hydrolysis of the
quinones was accomplished by replacing HF with an equimolar
excess of TMAF. The isolated products were characterized by
19F and 1H NMR and GC-MS.
III. Ch a r a cter iza tion of Tr iflu or om eth yla ted Com -
p ou n d s. Compound 1. 1H NMR (CDCl3, TMS): δ 1.77 (s, 3H);
2.60 (s, 1H), 7.37-7.46 (m, 3H), 7.56-7.61 (m, 2H) ppm. 19F
NMR (CDCl3, CFCl3): δ -81.35 (s) ppm. MS m/ z 190 (M+),
151, 127, 121, 105, 91, 77, 51. Anal. Calcd for C9H9F3O: C,
56.84; H, 4.77; F, 29.97. Found: C, 56.92; H, 4.69; F, 29.92.
96% yield.
Compound 2.1H NMR (CDCl3, TMS): δ 1.74 (s, 3H), 2.35
(s, 1H), 5.67 (s, 1H), 6.829 (d, 2H, J ) 8.78 Hz), 7.432 (d, 2H,
J ) 8.58 Hz) ppm. 19F NMR (CDCl3, CFCl3): δ -81.76 (s) ppm.
MS m/ z 206 (M+), 188, 167, 149, 137, 119, 107, 91. Anal.
Calcd for C9H9F3O2: C, 52.43; H, 4.40; F, 27.65. Found: C,
52.48; H, 4.49; F, 27.67. 98% yield.
Compound 3. 1H NMR (CDCl3, TMS): δ 1.74 (s, 3H), 2.90
(s, 1H), 3.879 (d, 3H, J ) 6.64 Hz), 5.84 (s, 1H), 6.86-7.13 (m,
3H) ppm. 19F NMR (CDCl3, CFCl3): δ -81.60 (s) ppm. MS
m/ z 236 (M+), 197, 167, 151, 124. 110, 69, 51. Anal. Calcd
II. In str u m en ta tion . NMR Spectroscopy. (a) 19F NMR
Spectroscopy. All spectra were recorded on a Varian Unity
500 FT-NMR spectrometer at an operational frequency of 470.3

19F NMR Elucidation of Carbonyl Groups in Lignin
J. Agric. Food Chem., Vol. 44, No. 8, 1996 2169
for C10H11F3O3: C, 50.85; H, 4.69; F, 24.13. Found: C, 50.91;
H, 4.68; F, 24.19. 96% yield.
-80.67 (s), -77.03 (s) ppm. MS m/ z 337 (M+ - CF3), 321,
268, 229, 203, 175, 147, 127.
Compound 4. 1H NMR (CDCl3, TMS): δ 1.73 (s, 3H), 2.54
(s, 1H), 3.879 (d, 6H, J ) 10.26 Hz), 6.78 (s, 2H), 7.22 (s, 1H)
ppm. 19F NMR (CDCl3, CFCl3): δ -81.47 (s) ppm. MS m/ z
266 (M+), 227, 197, 181, 155, 123, 93, 69. Anal. Calcd for
C11H13F3O4: C, 49.63; H, 4.92; F, 21.41. Found: C, 49.69; H,
4.85; F, 21.35. 95% yield.
Compound 17. 1H NMR (CDCl3): δ 2.02-2.03 (d, 3H, J )
1.16 Hz), 2.30 (br, 2H), 5.98-5.99 (d, 1H, J ) 1.72 Hz), 6.24-
6.25 (t, 2H, J ) 2.16 Hz) ppm. 19F NMR (CDCl3, CFCl3): δ
-81.21 (s), -77.25 (s) ppm. MS m/ z 193 (M+ - CF3), 173,
145, 124, 69, 51. Anal. Calcd for C9H8F6O2: C, 41.24; H, 3.08;
F, 43.48. Found: C, 41.27; H, 3.15; F, 43.50. 95% yield.
Compound 18. 1H NMR (CDCl3): δ -0.137 to -0.125 (d,
18H, J ) 2.38 Hz), 1.36-1.39 (d, 9H, J ) 5.30 Hz), 7.45-7.51
(m, 3H), 7.80-7.95 (m, 4H) ppm. 19F NMR (CDCl3, CFCl3): δ
-78.56 (s), -78.55 (s) ppm. MS m/ z 479 (M+ - CF3), 441,
410, 372, 344, 301, 267, 233.
Compound 19. 1H NMR (CDCl3): δ 1.37-1.41 (d, 9H, J )
8.60 Hz), 3.19-3.30 (d, 2H, J ) 21.23 Hz), 7.52-7.60 (m, 3H),
7.89-8.03 (m, 4H) ppm. 19F NMR (CDCl3, CFCl3): δ -78.61
(s), -78.29 (s) ppm. MS m/ z 335 (M+ - CF3), 320, 266, 251,
209, 181, 152, 112. Anal. Calcd for C20H18F6O2: C, 59.41; H,
4.49; F, 28.19. Found: C, 59.38; H, 4.46; F, 28.22. 94% yield.
Compound 20. 1H NMR (CDCl3): δ 0.021 (s, 18H), 1.03 (s,
9H), 1.20 (s, 9H), 5.39 (s, 1H), 6.01 (s, 1H) ppm. 19F NMR
(CDCl3, CFCl3): δ -68.84 (s), -74.89 (s) ppm. MS m/ z 504
(M+ - CF3), 489, 435, 399, 379, 327, 285, 259.
Compound 5. 1H NMR (CDCl3, TMS): δ 1.73 (s, 3H), 2.72
(s, 1H), 3.841 (d, 6H, J ) 2.16 Hz), 6.825 (d, 1H, J ) 8.48 Hz),
7.03-7.11 (m, 2H) ppm. 19F NMR (CDCl3, CFCl3): δ -81.56
(s) ppm. MS m/ z 250 (M+), 211, 181, 139, 124, 107, 95, 77.
Anal. Calcd for C11H13F3O3: C, 52.80; H, 5.24; F, 22.78.
Found: C, 52.85; H, 5.28; F, 22.80. 95% yield.
Compound 6. 1H NMR (CDCl3, TMS): δ 2.93 (s, 1H), 7.34-
7.38 (m, 6H), 7.47-7.52 (m, 4H) ppm. 19F NMR (CDCl3,
CFCl3): δ -74.79 (s) ppm. MS m/ z 252 (M+), 233, 213, 183,
165, 127. 105, 77. Anal. Calcd for C14H11F3O: C, 66.67; H,
4.40; F, 22.60. Found: C, 66.71; H, 4.32; F, 22.66. 94% yield.
Compound 7. 1H NMR (CDCl3, TMS): δ 3.22 (s, 1H), 3.84
(s, 6H), 6.87-7.03 (m, 4H), 7.03-7.47 (m, 4H) ppm. 19F NMR
(CDCl3, CFCl3): δ -75.11 (s) ppm. MS m/ z 312 (M+), 273,
243, 212, 168, 135, 108, 77. Anal. Calcd for C16H15F3O3: C,
61.54; H, 4.84; F, 18.25. Found: C, 61.56; H, 4.88; F, 18.17.
96% yield.
Compound 8. 1H NMR (CDCl3, TMS): δ 2.60 (s, 1H), 4.989
(q, 1H, J ) 6.60 Hz), 7.38-7.48 (m, 5H) ppm. 19F NMR (CDCl3,
CFCl3): δ -78.848 (d, J F-H ) 6.1 Hz) ppm. MS m/ z 176 (M+),
159, 127, 107, 89, 79, 51. Anal. Calcd for C8H7F3O: C, 54.55;
H, 4.01; F, 32.36. Found: C, 54.58 H, 3.99; F, 32.40. 97%
yield.
Compound 9. 1H NMR (CDCl3, TMS): δ 2.59 (s, 1H), 3.89
(s, 3H), 4.925 (q, 1H, J ) 6.78 Hz), 5.72 (s, 1H), 6.947 (d, 3H,
J ) 13.93 Hz) ppm.19F NMR (CDCl3, CFCl3): δ -78.981 (d,
J F-H ) 6.1 Hz) ppm. MS m/ z 222 (M+), 205, 183, 153, 125,
93, 65, 51. Anal. Calcd for C9H9F3O3: C, 48.66; H, 4.08; F,
25.65. Found: C, 48.70; H, 4.11; F, 25.69. 95% yield.
Compound 10. 1H NMR (CDCl3, TMS): δ 2.18 (s, 1H), 3.85
(s, 6H), 4.924 (q, 1H, J ) 6.58 Hz), 6.68 (s, 2H) ppm. 19F NMR
(CDCl3, CFCl3): δ -78.890 (d, J F-H ) 6.1 Hz) ppm. MS m/ z
252 (M+), 205, 183, 167, 155, 140, 123, 95. Anal. Calcd for
C10H11F3O4: C, 47.63; H, 4.40; F, 22.60. Found: C, 47.62; H,
4.42; F, 22.58. 98% yield.
Compound 11. 1H NMR (CDCl3, TMS): δ 2.50 (s, 1H), 3.87
(s, 6H), 4.943 (q, 1H, J ) 6.64 Hz), 6.841 (t, 1H, J ) 4.88 Hz),
6.976 (d, 2H, J ) 3.42 Hz) ppm.19F NMR (CDCl3, CFCl3): δ
-78.916 (d, J F-H ) 6.1 Hz) ppm. MS m/ z 236 (M+), 219, 197,
167, 139, 124, 108, 96. Anal. Calcd for C10H11F3O3: C, 50.85;
H, 4.69; F, 24.13. Found: C, 50.92; H, 4.66; F, 24.11. 99%
yield.
Compound 12. 1H NMR (CDCl3, TMS): δ 2.97 (s, 1H),
4.58-4.66 (m, 1H), 6.20 (q, 1H, J ) 9.46 Hz), 6.842 (d, 1H, J
) 15.93 Hz), 7.31-7.44 (m, 5H) ppm. 19F NMR (CDCl3,
CFCl3): δ -79.458 (d, J F-H ) 6.1 Hz) ppm. MS m/ z 202 (M+),
184, 165, 133, 115, 91, 77, 55. Anal. Calcd for C10H9F3O: C,
59.41; H, 4.49; F, 28.19. Found: C, 59.45; H, 4.52; F, 28.17.
99% yield.
Compound 13. 1H NMR (CDCl3, TMS): δ 2.25 (s, 1H), 3.84
(s, 3H), 4.531 (t, 1H, J ) 13.19 Hz), 5.65 (s, 1H), 5.961 (q, 1H,
J ) 9.12 Hz), 6.689 (d, 1H, J ) 15.73 Hz), 6.82-6.88 (m, 3H)
ppm. 19F NMR (CDCl3, CFCl3): δ -79.607 (d, J F-H ) 6.1Hz)
ppm. MS m/ z 248 (M+), 219, 199, 179, 161, 147, 119, 91. Anal.
Calcd for C11H11F3O3: C, 53.23; H, 4.47; F, 22.96. Found: C,
53.28; H, 4.44; F, 22.97. 99% yield.
Compound 14. 1H NMR (CDCl3): δ 0.091 (s, 18H), 6.16 (s,
4H) ppm. 19F NMR(CDCl3, CFCl3): δ -80.51 (s) ppm. MS
m/ z 323 (M+ - CF3), 307, 285, 265, 254, 223, 219, 189.
Compound 15. 1H NMR (CDCl3): δ 2.42 (br, 4H), 6.19 (s,
4H), 6.27 (s, 4H) ppm. 19F NMR (CDCl3, CFCl3): δ -80.81
(s), -80.76 (s) ppm. MS m/ z 179 (M+ - CF3), 159, 143, 110,
83, 69, 53. Anal. Calcd for C8H6F6O2: C, 38.73; H, 2.44; F,
45.94. Found: C, 38.75; H, 2.49; F, 45.96. 96% yield.
Compound 16. 1H NMR (CDCl3): δ 0.08-0.09 (d, 18H, J )
2.5 Hz), 1.93-1.94 (d, 3H, J ) 1.26 Hz), 5.85-5.86 (d, 1H, J
) 1.18 Hz), 6.16 (s, 2H) ppm. 19F NMR (CDCl3, CFCl3): δ
Compound 21. 1H NMR (CDCl3): δ 1.05 (s, 9H), 1.25 (s,
9H), 2.45 (s, 1H), 3.05 (s, 1H), 5.19 (s, 1H), 6.06 (s, 1H) ppm.
19F NMR (CDCl3, CFCl3): δ -70.13 (s), -75.90 (s) ppm. MS
m/ z (silylated) 504 (M+ - CF3), 489, 399, 379, 327, 285, 259,
239. Anal. Calcd for C16H22F6O2: C, 53.33; H, 6.15; F, 31.63.
Found: C, 53.40; H, 6.10; F, 31.59. 89% yield.
Compound 22. 1H NMR (CDCl3): δ 0.042 (s, 18H), 7.22-
7.31 (m, 3H), 7.36-7.61 (m, 4H), 7.76-7.83 (m, 3H) ppm. 19F
NMR (CDCl3, CFCl3): δ -74.52 (s) ppm. MS m/ z 247 (M+
11H14F3OSi), 213, 181, 165, 135, 105, 77, 51.
Compound 23. 1H NMR (CDCl3): δ 4.75 (s, 2H), 7.25-7.34
-
C
(m, 2H), 7.41-7.55 (m, 6H), 7.67-7.72 (t, 2H, J ) 7.32 Hz)
ppm. 19F NMR (CDCl3, CFCl3): δ -73.55 (s) ppm. MS m/ z
175 (M+ - C8H6F3O), 152, 105, 77, 51. Anal. Calcd for
C
16H12F6O2: C, 54.87; H, 3.45; F, 32.54. Found: C, 54.91; H,
3.50; F, 32.58. 98% yield.
Compound 25. 1H NMR (CDCl3, TMS): δ 3.63 (s, 1H), 3.78
(s, 3H), 3.863 (d, 6H, J ) 5.40 Hz), 4.07-4.12 (m, 2H), 4.449-
(t, 1H, J ) 1.80 Hz), 5.57 (s, 1H), 6.692 (q, 1H, J ) 3.60 Hz),
6.77-6.83 (m, 2H), 6.865 (d, 1H, J ) 5.10 Hz), 6.95-6.99 (m,
1H), 7.193 (d, 1H, J ) 4.80 Hz), 7.294 (d, 1H, J ) 1.2 Hz) ppm.
13C NMR (CDCl3, TMS): δ 55.85, 56.04, 56.17, 61.36, 79.59,
82.64, 109.87, 110.48, 112.07, 118.25, 120.72, 121.66, 123.83,
124.62, 124.97 (q, J C-F ) 286.59 Hz), 126.11, 128.93, 146.18,
148.56, 148.89, 151.53 ppm. 19F NMR (CDCl3, CFCl3):
δ
-75.53 (s) ppm. MS m/ z (silylated) 402 (M+), 302, 278, 248,
235, 221, 181, 165. Anal. Calcd for C19H21F3O6: C, 56.72; H,
5.26; F, 14.16. Found: C, 56.74; H, 5.29; F, 14.22. 95% yield.
Compound 27. 1H NMR (CDCl3, TMS): δ 3.84 (s, 3H), 3.87
(s, 3H), 3.90 (s, 3H), 3.93 (d, 1H, J ) 3 Hz), 4.24 (d, 1H, J )
12 Hz), 4.640 (d, 1H, J ) 12 Hz), 5.865 (d, 1H, J ) 3 Hz),
6.84-7.12 (m, 6H) ppm. 13C NMR (CDCl3, TMS): δ 56.11,
56.19, 56.64, 79.85, 108.78, 110.06, 111.44, 113.40, 120.41,
121.80, 122.95, 124.46, 125.60 (q, J C-F ) 286.59 Hz), 126.74,
127.88, 147.37, 148.68, 149.16, 149.45 ppm. 19F NMR (CDCl3,
CFCl3): δ -77.63 (s) ppm. MS m/ z 372 (M+), 303, 248, 235,
217, 189, 180. Anal. Calcd for C18H19F3O5: C, 58.06; H, 5.14;
F, 15.31. Found: C, 58.21; H, 5.20; F, 15.29. 96% yield.
Compound 29. 1H NMR (CDCl3, TMS): δ 2.25 (s, 3H),
3.75 (s, 1H), 3.79 (s, 3H), 3.88 (s, 3H), 3.935 (d, 1H, J ) 9 Hz),
4.12 (d, 1H, J ) 5.99 Hz), 4.33 (s, 1H), 5.60 (s, 1H), 5.70 (s,
1H), 6.55-6.64 (m, 3H), 6.94 (d, 1H, J ) 5.40 Hz), 7.10 (d,
1H, J ) 4.80 Hz), 7.311 (d, 1H, J ) 1 Hz) ppm. 13C NMR
(CDCl3, TMS): δ 21.19, 55.88, 55.93, 61.31, 79.52, 82.64,
109.59, 112.95, 113.93, 118.43, 121.01, 122.19, 123.93, 125.07
(q, J C-F ) 286.59 Hz), 126.21, 128.71, 134.83, 143.81, 145.62,
146.35, 151.36 ppm.19F NMR (CDCl3, CFCl3): δ -75.48 (s)
ppm. MS m/ z (silylated) 618 (M+), 480, 451, 411. 365, 343,
323, 271. Anal. Calcd for C19H21F3O6: C, 56.72; H, 5.26; F,
14.16. Found: C, 56.78; H, 5.19; F, 14.18. 93% yield.

2170 J. Agric. Food Chem., Vol. 44, No. 8, 1996
Ahvazi and Argyropoulos
Ta ble 1. F lu or id e Ion In d u ced Tr iflu or om eth yla tion of Keton es w ith Tr iflu or om eth yltr im eth ylsila n e
overall 19F NMR
entry
precursor
product
reacn conditions
% yield (ppm)
GC-MS m/e
1
0 °C, 30 min 25 °C,
16 h hydrolysis, 8 h
96
98
96
-81.35 MS m/z 190 (M+), 151,
CF3
C
O
C
127, 121, 105, 91
CH3
CH3
O
OH
2
3
CF3
0 °C, 30 min 25 °C,
6 h hydrolysis, 2 h
-81.76 MS m/z 206 (M+), 188,
167, 149, 137, 119
HO
HO
C
CH3
CH3
HO
HO
C
CH3
CH3
OH
CF3
C
0 °C, 30 min 25 °C,
14 h hydrolysis, 5 h
-81.60 MS m/z 236 (M+) 197,
O
C
167, 151, 124, 110
OH
H3CO
H3CO
H3CO
H3CO
4
5
0 °C, 30 min 25 °C,
18 h hydrolysis, 2 h
95
95
-81.47 MS m/z 266 (M+), 227,
O
C
CF3
C
197, 181, 155, 123
HO
CH3
HO
CH3
OH
H3CO
H3CO
H3CO
H3CO
0 °C, 30 min 25 °C,
14 h hydrolysis, 5 h
-81.56 MS m/z 250 (M+) 211,
O
CF3
181, 139, 124, 107
C
CH3
C
CH3
OH
H3CO
H3CO
6
7
CF3
C
0 °C, 30 min 25 °C,
16 h hydrolysis, 5 h
94
96
-74.79 MS m/z 252 (M+), 233,
O
C
213, 183, 165, 127
OH
0 °C, 30 min 25 °C,
16 h hydrolysis, 5 h
OCH3
-75.11 MS m/z 312 (M+), 273,
CF3
C
O
C
243, 212, 135, 108
H3CO
H3CO
OCH3
OH
RESULTS AND DISCUSSION
may be coupled not only with each other but also with
protons. Absorption by fluorine does not appear in the
proton NMR spectrum since it appears far off the proton
observation scale, but fluorine splitting of proton signals
can be seen. This was evident for all the trifluoromethy-
lated aldehyde derivatives examined during this work
(Tables 1 and 2).
Sa lien t F ea tu r es of th e Rea ction . The trifluoro-
methylation of most of the examined organic compounds
was first carried out in the presence of catalytic amounts
of tetra-n-butylammonium fluoride (TBAF) acting as an
initiator for the reaction. Although this salt was effec-
tive, other catalysts were investigated since the reaction
yields were consistently low. Potassium tert-butoxide
and tetra-n-butylammonium iodide (TBAI) have been
used with success as initiators for the trifluoromethy-
lation of acetophenone (Prakash et al., 1989). In
comparison to TBAF, both were found to be equally
effective. In fact the final yields of the reaction in-
creased to about 88% when potassium tert-butoxide was
employed. However, there are some drawbacks associ-
ated with the use of such initiators, restricting their use
in trifluoromethylating lignins.
In general the treatment of carbonyl-containing com-
pounds with Ruppert’s reagent in the presence of TBAF
only resulted in qualitatively converting these com-
pounds to their siloxy intermediates. The hydrolysis of
trimethylsiloxytrifluoromethylated intermediates was
found to be inefficient when dilute HCl was used,
particularly for the case of benzophenone (5). Such
problems were also observed by another group in their
attempt to trifluoromethylate hindered steroidal ketones
(Wang and Ruan, 1994). Both of these issues were
found to be responsible for the low overall yields of the
reaction. Such yields could not qualify this reaction as
an analytical tool. Wang and Ruan succeeded in
improving the yields of this reaction to quantitative
levels, by using the more effective tetramethylammo-
nium fluoride (TMAF) as an initiator.
Initially a series of model compounds containing
carbonyl groups, including various quinones, were tri-
fluoromethylated, and their 19F NMR spectra were
examined . This was done in order to understand the
relationships between the chemical environment of CF3-
containing derivatives and their 19F NMR chemical
shifts. Detailed work with carbonyl-containing aryl-
glycerol â-O-4 ether model compounds followed, aimed
at understanding the trifluoromethylation reaction so
that it can eventually be applied to lignins, solid wood,
and paper samples.
Ar om a t ic Com p ou n d s Con t a in in g Ca r b on yl
Gr ou p s. Various organic compounds containing car-
bonyl groups in different chemical environments were
subjected to the described trifluoromethylation reac-
tion. This eventually allowed for the correlation of
trifluoromethylated structures to their 19F resonance
for carbonyl environments that may be present in
lignins. These studies indicated that the 19F NMR
chemical shifts vary significantly for several classes of
carbonyl groups. For example, the 19F NMR signal of
various trifluoromethylated ketones, such as acetophe-
none (1) was observed at -81.35 ppm, distinguishing
it from that of p-hydroxyacetophenone (2) recorded at
-81.76 ppm upfield from CFCl3. Such variations of 19
chemical shifts were also apparent for different alde-
hydes.
F
The 19F NMR chemical shift of trifluoromethylated
benzaldehyde (8) was recorded (doublet) at -78.848
ppm, while the 19F NMR signals moved progressively
downfield with the addition of different substituents on
the benzene ring. This demonstrates that the chemical
shift of 19F is quite sensitive to the environment (Gerig,
1978), and therefore 19F spectroscopy would be a useful
tool for measuring and probing different classes of
compounds containing fluorine and fluorine derivatives.
It is well-known that the fluorine (19F) nucleus has
similar magnetic properties to those of the proton and

19F NMR Elucidation of Carbonyl Groups in Lignin
J. Agric. Food Chem., Vol. 44, No. 8, 1996 2171
Ta ble 2. F lu or id e Ion In d u ced Tr iflu or om eth yla tion of Ald eh yd es w ith Tr iflu or om eth yltr im eth ylsila n e
overall
% yield
19F NMR
(ppm)
entry
precursor
product
reacn conditions
GC-MS m/e
8
CF3
0 °C, 30 min 25 °C,
20 h hydrolysis, 5 h
97
-78.848
(d, J F-H ) 6.1 Hz)
MS m/z 176 (M+),
159, 127, 107, 89, 79
O
C
H
C
H
OH
9
H3CO
HO
0 °C, 30 min 25 °C,
20 h hydrolysis, 5 h
95
-78.981
(d, J F-H ) 6.1 Hz)
MS m/z 222 (M+),
205, 183, 153, 125, 93
H3CO
HO
O
C
CF3
C
H
H
H
OH
10
H3CO
HO
H3CO
HO
0 °C, 30 min 25 °C,
24 h hydrolysis, 5 h
98
99
-78.890
(d, J F-H ) 6.1 Hz)
MS m/z 252 (M+),
205, 183, 167, 155, 140
O
C
CF3
C
H
OH
H3CO
H3CO
H3CO
H3CO
11
0 °C, 30 min 25 °C,
24 h hydrolysis, 5 h
-78.916
(d, J F-H ) 6.1 Hz)
MS m/z 236 (M+),
219, 197, 167, 139, 124
O
CF3
C
H
C
H
OH
H3CO
H3CO
12
13
0 °C, 30 min 25 °C,
16 h hydrolysis, 5 h
99
99
-79.458
(d, J F-H ) 6.1 Hz)
MS m/z 202 (M+),
184, 165, 133, 115, 91
CF3
C
O
C
C
H
C
C
C
H
OH
CF3 0 °C, 30 min 25 °C,
-79.607
(d, J F-H ) 6.1 Hz)
MS m/z 248 (M+),
219, 199, 179, 161, 147
O
C
18 h hydrolysis, 5 h
HO
C
C
C
H
HO
C
C
H
OH
H3CO
H3CO
In our work we addressed the issue of efficiently
cleaving the silyl ethers by using 40% aqueous HF
solution in acetonitrile (CH3CN) (Greene and Wuts,
1991). After careful isolation of the products, their yield
was calculated and compared to those with TBAF. The
combination of TMAF and aqueous HF was therefore
used in all subsequent efforts of trifluoromethylating
the various sets of compounds discussed in this report
(quinones were hydrolyzed with TMAF).
Qu in on es. A number of studies have shown that
quinones are present, in small amounts, in wood and
high-yield mechanical pulps (Lebo et al., 1990; Argy-
ropoulos et al., 1995; Argyropoulos and Heitner, 1994).
Despite their low abundance, they have been held
responsible as one of the significant factors for the
yellowing of paper (Lin and Kringstad, 1971; Kringstad,
1973; Forsska˚hal and J anson, 1991; Castellan et al.,
1993; Gellersted and Pattersson, 1977; Lebo et al.,
1990).
their corresponding alcohols, resulting in the develop-
ment of an improved method for cleaving silyl ethers.
After hydrolysis, the isolated compound (15) was sub-
jected to 19F and 1H NMR investigations in order to
assign the signals for the corresponding fluorine and
alcohol groups of the derivative. The 19F NMR spectrum
showed two signals with equal intensity and 0.05 ppm
1
difference in their chemical shifts. The H NMR spec-
trum confirmed the complete hydrolysis of the silyl
ethers since a new signal due to the hydroxyl group
appeared at 2.42 ppm. In addition, two other signals
corresponding to the four unsaturated protons of each
stereoisomer were observed.
These data, in conjunction with its GC-MS spectrum,
confirmed the purity of the products. As such, the
formation of two stereoisomers (syn and anti) as a result
of trifluoromethylation and hydrolysis became evident.
The absence of these geometric isomers prior to hy-
drolysis can only be explained by the presence of the
bulky trimethylsiloxy groups which considerably hin-
ders the formation of the “syn” isomer, therefore giving
rise to only one signal in the 19F NMR spectrum at
-80.51ppm.
The isolated trifluoromethyltrimethylsiloxy interme-
diate of methyl-p-benzoquinone (16), an unsymmetric
derivative of p-benzoquinone, displayed two well-
resolved signals at -77.03 and -80.67 ppm. These
signals were assigned to both CF3 substituted on the
former carbonyl carbons adjacent and nonadjacent to
the methyl group, respectively.
When 2-tert-butyl anthraquinone was subjected to
trifluoromethylation, the 1H NMR spectrum of the
isolated intermediate (18) showed the presence of
trimethylsiloxy groups, and its 19F NMR spectrum
displayed two overlapping signals separated by 0.007
ppm. Such evidence supported the observation of two
stereoisomers of derivatized p-benzoquinones. This was
concluded since the tert-butyl group, though far removed
from the carbonyl groups is, less sterically hindering
than the neighboring methyl group in methyl-p-benzo-
quinone (Figure 1).
The trifluoromethylation of quinones by adding only
to one of the carbonyl carbon atoms has been studied
by Stahly and Bell (1989). In an effort to evaluate the
reactivity of quinones (on both carbonyl carbon atoms)
with Ruppert’s reagent, a number of different com-
pounds were selected and trifluoromethylated. The
isolated products at each step of the reaction were
1
thoroughly examined by 19F and H NMR and GC-MS.
Initially p-benzoquinone was trifluoromethylated.
The CF3-containing trimethylsiloxy intermediate (14)
1
was isolated and later examined by H and 19F NMR
spectroscopies. The resulting spectra confirmed the
trifluoromethylation of both carbonyl groups.
The subsequent hydrolysis of the CF3-containing
siloxy intermediates with 40% HF solution was limited
since such concentrations would result in the cleavage
of several arylglycerol â-O-4 ether bonds when applied
to lignins. However, lower concentrations of HF (i.e.
10%) failed to cleave silyl ether bonds completely due
to the low solubility of quinones in acetonitrile. This
problem was resolved by the addition of TMAF in THF
at room temperature. Such a combination was able to
cleave the trimethylsiloxy intermediates effectively to

2172 J. Agric. Food Chem., Vol. 44, No. 8, 1996
Ahvazi and Argyropoulos
F igu r e 1. 19F NMR spectra of trifluoromethylated quinones before (left) and after (right) hydrolysis.
1
The H NMR spectrum of 19 showed a broad signal
due to the hydroxyl groups at 3.19 ppm, while its
trifluoromethylated 19F NMR spectrum showed two
distinct signals at -78.29 and -78.61 ppm due to the
desilylation of CF3 intermediates during hydrolysis with
TMAF. In addition to p-quinones, o-quinones are
another important class of dicarbonyl-containing com-
pounds that may be found in lignin. o-Quinones may
be formed during demethylation and/or deprotonation
of several syringyl- and guaiacyl-lignin structures as a
result of various chemical reactions.
fluoromethylated (20) and hydrolyzed (21). According
to the obtained data, o-quinones were found to undergo
this process quite efficiently, as shown in Table 3.
Furthermore, the same procedure was carried out for
benzil, and the chemistries of the trifluoromethylated
intermediates (22) and alcohols (23) were studied. The
19F NMR spectra of the both 22 and 23 depicted only
one signal due to the symmetric structure of benzil.
Lign in Mod el Com p ou n d s. Our efforts toward
examining the potential of Ruppert’s reagent for the
elucidation of carbonyl groups in lignins were then
focused on dimeric model compounds containing carbo-
nyl groups, resembling those present in lignins. Dimer-
To evaluate the reactivity of o-quinones with Ruppert’s
reagent, 3′,5′-di-tert-butyl-1,2-benzoquinone was tri-

19F NMR Elucidation of Carbonyl Groups in Lignin
J. Agric. Food Chem., Vol. 44, No. 8, 1996 2173
Ta ble 3. F lu or id e Ion In d u ced Tr iflu or om eth yla tion of Qu in on es w ith Tr iflu or om eth yltr im eth ylsila n e
overall
% yield
19F NMR
(ppm)
entry
precursor
product
reacn conditions
GC-MS m/e
14, 15
0 °C, 30 min 25 °C,
6 h hydrolysis, 5 h
96
95
94
14
15
{-80.51 MS m/z 323 (M+ - CF3),
307, 285, 265,
F3C
F3C
OR
O
254, 223
-80.75 MS m/z 179 (M+ - CF3),
{
-81.80
159, 143, 110, 83, 69
OR
O
O
14, R = SiMe3; 15, R = H
16, 17
0 °C, 30 min 25 °C,
7 h hydrolysis, 5 h
-77.03 MS m/z 337 (M+ - CF3),
F3C
OR
16
17
{
{
-80.67
321, 268, 229,
203, 175
CH3
CH3
-77.25 MS m/z 193 (M+ - CF3),
-81.21
173, 145, 124, 69, 51
F3C
OR
O
16, R = SiMe3; 17, R = H
18, 19
0 °C, 30 min 25 °C,
8 h hydrolysis, 6 h
-78.55 MS m/z 479 (M+ - CF3),
O
F3C
OR
18
19
{
{
-78.56
410, 372, 344,
301, 267
tert-butyl
tert-butyl
-78.29 MS m/z 335 (M+ - CF3),
-78.61
320, 266, 251,
209, 181
F3C
OR
O
18, R = SiMe3; 19, R = H
F3C
20, 21
22, 23
0 °C, 30 min 25 °C,
4 h hydrolysis, 5 h
89
98
-68.84 MS m/z 504 (M+ - CF3),
O
OR
OR
20
21
{
{
-78.89
489, 435, 399,
379, 327
O
CF3
-69.59 MS m/z 504 (silylated),
-75.87
(M+ - CF3), 489,
399, 379, 327, 285
tert-butyl
tert-butyl
tert-butyl
tert-butyl
20, R = SiMe3; 21, R = H
F3C OR
0 °C, 30 min 25 °C,
5 h hydrolysis, 4 h
22
23
{-74.52 MS m/z 247
O
C
O
C
CF3
RO
(M+ - C11H14F3OSi),
213, 181, 165,
135, 105
C
C
{-73.55 MS m/z 175
22, R = SiMe3; 23, R = H
(M+ - C8H6F3O),
152, 105, 77, 51
ic model compounds have often been used in NMR
studies aimed at the structural elucidation of lignins
(Adler et al., 1987; Lundquist, 1980; Lundquist and
Olsson, 1977; Brunow and Lundquist, 1980; Lundquist,
aqueous HF in acetonitrile as the hydrolysis medium.
These modifications not only increased the reaction
yields by more than 2-fold, they also gave clean trifluo-
romethylated products. The 19F NMR spectrum of 25
showed a single signal at -75.53 ppm due to substituted
CF3, and the 31P NMR spectra featured a signal at 127.4
ppm due to the formation of tertiary R-OH as a result
of the hydrolysis. The 13C NMR contained a new
quartet due to the CF3 group, while the complete
elimination of the carbonyl signal signified the trifluo-
romethylation of the carbonyls. This was verified by
examining the 1H NMR spectrum of the compound,
where a new signal at 5.57 ppm signified the formation
of R-OH.
To further confirm our findings, 3-methoxy-4-hydroxy-
R-(2-methoxy-4-methyl)-γ-hydroxypropiophenone (28)
was trifluromethylated under the same conditions. The
resulting CF3-containing compound (29) was then ex-
amined by GC-MS and 19F, 13C, and 1H NMR spec-
troscopies. Consequently, it was demonstrated that
trifluoromethylation of the dimeric lignin model com-
pounds can be quantitatively achieved with the more
effective TMAF as an initiator.
The next issue was whether or not the trifluoro-
methylation procedure could result in the scission of
arylglycerol â-O-4 ether bonds. This is a very important
issue as far as the primary objectives of this work are
concerned, since more than 40% of the structural units
in lignins are known to be linked together by â-ether
bonds (Lai and Sarkanen, 1972; Erickson et al., 1973;
Matsumoto et al., 1980).
Such an event may take place due to the presence of
trimethylsilyl fluoride, formed as a side product in
conjunction with trifluoromethyltrimethylsilane dur-
ing the reaction. In a series of publications that address
the selective cleavage of ether bonds in lignin, there is
1
1981). 31P, H, and 13C NMR spectroscopies and GC/
MS analyses were used in a manner similar to that
described so far in each step of the trifluoromethylation
reaction, and the obtained spectra were compared to
those of the starting materials aimed at monitoring all
changes that took place during the course of the reac-
tion.
Initial attempts to trifluoromethylate 3,4-dimethoxy-
R-(2-methoxyphenoxy)-γ-hydroxypropiophenone (24)
posed some difficulties since low yields were obtained
when stoichiometric amounts of TMS-CF3 in the pres-
ence of TBAF were employed; the presence of the
primary γ-OH was suspected as interfering. In an effort
to understand the role of the γ-OH in this reaction, two
avenues were followed. The compound was acetylated
prior to trifluoromethylation, thus protecting the pri-
mary γ-hydroxyl groups. Following acetylation, the
model compound was trifluoromethylated and hydro-
lyzed with 1 N HCl. The yield of the purified products
was calculated to be 46%. Although this procedure
improved the final yield to a small extent, the reaction
yield was still too low for the quantification of the
carbonyls in lignins. Trifluoromethylation of 3,4-
dimethoxy-R-(2-methoxyphenoxy)acetophenone (26) was
the other approach. The overall trifluoromethylation
of the R-carbonyl of 26 took place with somewhat more
facility than that of 24. Although the spectral analysis
of product 27 showed only one 19F NMR signal at -77.63
ppm due to the trifluoromethyl group, the yield of the
product never exceeded 46%.
Finally the yields of these reactions were considerably
improved by using TMAF as the initiator and 20%

2174 J. Agric. Food Chem., Vol. 44, No. 8, 1996
Ahvazi and Argyropoulos
Ta ble 4. F lu oid e Ion In d u ced Tr iflu or om eth yla tion of th e Ca r bon yl Gr ou p for Lign in Mod el Com p ou n d s w ith
Tr iflu or om eth yltr im eth ylsila n e
overall
% yield
19F NMR
(ppm)
entry
precursor
product
reacn conditions
GC-MS m/e
24
H
C
C
C
0 °C, 30 min 25 °C,
24 h hydrolysis, 24 h
95
-75.53
Ms m/z (silylated) 402, (M+),
302, 278, 248, 235, 221
H
C
C
C
H
H
OH
O
H
H
OH
O
O
HO
CF3
OCH3
OCH3
OCH3
OCH3
OCH3
H
OCH3
25
26
H
0 °C, 30 min 25 °C,
24 h hydrolysis, 24 h
96
-77.63
Ms m/z 372, (M+), 303, 248,
235, 217, 189
H
C
C
O
H
C
C
O
HO
CF3
O
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
H
27
28
H
0 °C, 30 min 25 °C,
24 h hydrolysis, 24 h
93
-75.48
Ms m/z (silylated) 618, (M+),
480, 451, 411, 365, 343
H
H
C
C
C
OH
H
H
C
C
C
OH
O
CH3
O
O
CH3
HO
CF3
OCH3
OCH3
OCH3
OCH3
OH
OH
29
substantial evidence that trimethylsilyl iodide is capable
of cleaving â-O-4 ether bonds. In fact several studies
have shown that this is an effective method of cleaving
the ether bonds between lignin side chains and aromatic
nuclei (Matsumoto et al., 1982; Makino et al., 1989,
1990; Fujino et al., 1992; Ho and Olah, 1976).
was found to be an efficient nucleophilic trifluoro-
methylating agent for derivatizing a variety of carbonyl-
containing compounds. Although their 19F NMR signals
were well-resolved, allowing differentiation among the
various classes of carbonyl groups, the chemical shifts
of trifluoromethylated quinones occupied a wide range,
overlapping with those of ketones.
Although the reaction was carried out in the absence
of trimethylsilyl iodide (TMSiI), the formation of tri-
methylsilyl fluoride (TMSiF) during the course of the
reaction was a possibility to consider since it could
behave similarly to TMSiI. For this reason, gas chro-
matography was employed to evaluate the integrity of
alkyl-aryl ether bonds of the lignin model compounds
(24 and 26) before and after the reaction. It was
confirmed that, under the conditions employed in this
work, the formation of TMSiF did not cleave the â-ether
bonds of the models used. Tetrabutylammonium iodide
cannot be used as an initiator for the reaction, since it
would result in the formation of TMSiI, which will in
return cleave the alkyl-aryl ether bonds in dimeric
model compounds and lignin. Potassium tert-butoxide
was found to be a very effective initiator of trifluoro-
methylation for a series of monomeric model compounds.
Its strongly basic character, however, poses the danger
of structural alterations to the delicate and complex
nature of lignin. The trifluoromethylation reaction in
the presence of TBAF resulted in low yields, precluding
this procedure from becoming a tool for the quantitative
analysis of carbonyl groups in lignins.
LITERATURE CITED
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Received for review March 4, 1996. Accepted May 10, 1996.X
This work was supported by the Canadian Pulp and Paper
Industry, a National Network Centres of Excellence grant, and
a research grant provided by the Government of Canada and
the Natural Science and Engineering Research Council
(NSERC).
J F960154R
X Abstract published in Advance ACS Abstracts, J uly
15, 1996.