Angewandte
Chemie
produced as a mixture with other phenolic monomers through
bimetallic hydrogenolysis of organosolv lignin in water.[17a]
Finally, 8 was converted to the monolignol sinapyl alcohol
16, which is known to possess anti-inflammatory and anti-
nociceptive activity,[24] and which is a potential substrate for
a wide range of chemical transformations that could add
complexity and hence value.[25]
In summary, we report the depolymerization of birch
lignin leading to the isolation of a pure phenolic monomer 8
as the major product in a 5 wt% yield. To the best of our
knowledge, this is the first time that compounds such as 8 (and
7) have been isolated as the major products in the depolyme-
rization of lignin. Our one-pot method involves an initial
aerobic catalytic oxidation of the b-O-4 linkages in lignin
followed by zinc-mediated cleavage of the C-OAryl bond.
The method reported here was developed through the use of
simple model compounds and more complex b-O-4 model
polymers. This strategy has allowed us to observe, for the first
time, a clear difference in reactivity between simple and
polymeric models, which is of potential importance in the
development of future methods for lignin valorization. We
also demonstrate that 8, a compound not readily obtainable
from lignin by other methods, is a useful starting point for
subsequent synthetic sequences aimed at delivering valuable
products from this recalcitrant, but highly abundant renew-
able biopolymer.
Scheme 3. Depolymerization of lignin to phenolic monomers.
DME=1,2-dimethoxyethane.
depolymerization of lignin could also be conducted in one pot
with no impact on the yield of 8. A key advantage of our lignin
depolymerization method is the retention of functional
groups in the major phenolic product 8, as these groups are
frequently lost in other lignin depolymerization process-
es.[13,17] Future studies will focus on integrating methods to
recover and recycle both the organic solvents used and the
solubilized zinc salts in the aqueous waste stream. The
environmental and cost implications of using a halogenated
organic catalyst, such as DDQ, will also be assessed.
Optimizing these and other factors may enable this promising
initial reaction sequence to be developed into a large-scale
process.
To assess whether 8 is of potential relevance to the
valorization of lignin, several chemical transformations start-
ing from 8 were assessed (Scheme 4).[18] Products from these
reactions include potentially polymerizable monomer 10,[19]
from which propiophenone 9 and syringylpropane 11[20] could
be accessed, dihydrosinapyl alcohol 12 and b-amino acid
precursor 14a.[21–23] A recent report has shown that 12 can be
Received: September 23, 2014
Published online: November 5, 2014
Keywords: biomass · depolymerization · lignin · oxidation ·
.
renewable chemicals
375; b) M. FitzPatrick, P. Champagne, M. F. Cunningham, R. A.
[4] J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius, B. M. Weck-
[5] F. T. ꢀhman, H. Tomani, P. Axegard, (LignoBoost AB), US
8486224 B2, 2006.
[6] J. Ralph, L. L. Landucci in Lignin and Lignans: Advances in
Chemistry (Eds.: C. Heitner, D. Dimmel, J. Schmidt), CRC, Boca
Raton, 2010, pp. 137 – 243.
[7] a) A. Rahimi, A. Azarpira, H. Kim, J. Ralph, S. S. Stahl, J. Am.
Scheme 4. Synthetic conversion of phenolic monomer 8 to various
chemical compounds. Reaction conditions: a) PPh3, DDQ, NBu4I,
DBU, CH2Cl2, RT, 4 h, 78%; b) Zn dust, AcOH, RT, 12 h, 63% for 9;
c) NaBH3CN, BF3·OEt2, RT, 12 h, 89% for 11, 82% for 12; d) TBS-Cl,
DMAP, imidazole, DMF, RT, 1 h, 94%; e) (R)-tert-butyl sulfinamide,
Ti(OEt)4, PhMe, 1008C, 12 h, 60%; f) l-selectride, THF, ꢀ208C!RT,
4 h, 80%; g) TBAF, THF, 3ꢀ M.S., RT, 2 h, 63%; h) H2NNH2·H2O,
1008C, 12 h, 82%; i) I2, Et2O, 08C, 30 min, 51%. DBU=1,8-diaza-
bicyclo[5.4.0]undec-7-ene; DMAP=4-dimethylaminopyridine;
DMF=N,N-dimethylformamide; M.S.=molecular sieves; TBAF=
tetra-n-butylammonium fluoride; TBS=tert-butyldimethylsilyl.
[8] F. Tran, C. S. Lancefield, P. Kamer, T. Lebl, N. Westwood, Green
Chem. 2014 in Press, DOI: 10.1039/c4gc01012d.
[9] a) Z. Shen, L. Sheng, X. Zhang, W. Mo, B. Hu, N. Sun, X. Hu,
Cosner, P. J. Cabrera, K. M. Byrd, A. M. A. Thomas, P. Helquist,
[10] See Figure S4 in the Supporting Information.
Angew. Chem. Int. Ed. 2015, 54, 258 –262
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
261