Organocatalytic Approach to Photochemical Lignin Fragmentation

Organocatalytic Approach to Photochemical Lignin Fragmentation

Letter

̈ ̈

Cheng Yang, Markus D. Karkas, Gabriel Magallanes, Kimberly Chan, and Corey R. J. Stephenson*

Cite This: https://dx.doi.org/10.1021/acs.orglett.0c03029

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sı Supporting Information

ABSTRACT: Herein, an organocatalytic method for photochemical

C−O bond cleavage of lignin systems is reported. The use of

photochemistry enabled fragmentation of the β-O-4 linkage, the

primary linkage in lignin, provides the fragmentation products in good

to high yields. The approach was merged with reported oxidation

conditions in a one-pot, two-step platform without any intermediary

puriﬁcation, suggesting its high ﬁdelity. The future utility of the

organocatalytic method was illustrated by applying the visible light-mediated protocol to continuous ﬂow processing.

tilization of biomass for the production of fuels and

chemicals is of the utmost priority in order to sustain a

−3

global economy that is intimately tied to its energy usage.

Conversion processes in the petrochemical industry are well-

established; however, eﬃcient biotechnologies and catalytic

processes for converting biomass to useful fuels and ﬁne

chemicals are lacking. Lignin, identiﬁed as an underutilized

renewable source for the production of value-added chemicals,

is largely produced as a waste product by the paper and

pulping industry. As the use of renewable sources for the

production of commodity chemicals and energy sources in an

attempt to replace nonrenewable fossil fuels as the source for

aromatic chemicals has become a focus, lignin has gained

increasing attention in recent years.

Lignin, a component of lignocellulosic biomass alongside

cellulose and hemicellulose, is a biopolymer composed of three

phenylpropanol subunits (coumaryl alcohol, coniferyl alcohol,

and sinapyl alcohol) that are oxidatively polymerized to aﬀord

various linkages that constitutes 30% of the organic carbon in

the biosphere and is the largest source of aromatic building

Figure 1. Photocatalytic depolymerization of lignin.

−8

blocks on the planet (Figure 1).

One of the two strategies

including moderate yields, as well as the use of high

for lignin valorization is hydrogenolysis. The other approach

involves the selective and mild deconstruction of the polymeric

network to its aromatic small molecule products while

13,14

temperatures and pressures.

Even during the advent of

newly developed catalytic approaches, technologies for lignin

depolymerization that aﬀord predictable monomeric units are

maintaining native functionality.

still underdeveloped.

The β-O-4 linkage is the most investigated linkage moiety

The Stephenson group has been interested in the

depolymerization of lignin employing redox-based methods.

Our previous studies on lignin fragmentation have exploited

the unique reactivity of the β-O-4 linkage by targeting the

for fragmentation due to its abundance in lignin (around 45−

0% of all lignin linkages). Various advancements have been

made for the cleavage of lignin model systems, as well as native

lignin polymers. Despite the variety of methods that have

been reported for lignin functionalization and depolymeriza-

tion, achieving high yields and selectivity under ambient

conditions still remains a challenge. An example is the

conversion of lignosulfonate to vanillin. The production of

vanillin from a renewable source is of high interest to food

Received: September 10, 2020

companies, as well as the ﬂavor and fragrance industries.

However, several drawbacks exist in current methods,

XXXX American Chemical Society

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

Figure 2. Organocatalytic photochemical C−O bond cleavage. Reaction conditions: 1 (0.25 mmol), N-phenylphenothiazine (PhPTH, 0.0125

mmol), formic acid (HCO H, 0.25 mmol), diisopropylethylamine ( Pr EtN, 0.5 mmol), acetone (5 mL), N protection. Reactions were irradiated

by violet LED strips (Method A) or Kessil 390 nm LED light (Method B). Isolated yields are reported. 1 mmol reaction was carried out in

continuous ﬂow. 0.25 mmol reaction was carried out in continuous ﬂow.

benzylic alcohol for oxidation. Capitalizing on this oxidation

additives (Table S1), full conversion of the oxidized lignin

substrate 1a was obtained after 96 h.

event, several two-step oxidation−fragmentation sequences

were developed (Figure 1). However, one major drawback of

our reported methods for biomass valorization is the necessity

of using iridium-based photocatalysts, which has limited the

practical application of photochemical depolymerization of

lignin in industry. In comparison with the production of

pharmaceuticals and other ﬁne chemicals, biomass valorization

is anticipated to operate on a much larger scale, wherein

catalyst cost becomes a critical factor in process development.

In this paper, an organocatalytic approach to photochemical

fragmentation of oxidized lignin substrates is reported.

To increase reaction eﬃciency, a higher-powered LED

source (Kessil, PR160−390 nm) was used, in which photon

ﬂux is determined to be 16 times larger than that of the violet

LED strip (SI). This provided a nearly 30-fold reduction in

reaction time, allowing the reaction to proceed in merely 3 h to

aﬀord full conversion. The observation is consistent with

reports that demonstrate the dependence of reaction eﬃciency

on photon ﬂux.

The organocatalytic photochemical fragmentation of a

variety of oxidized lignin model substrates was performed.

The fragmentation products were isolated in good yields,

ranging from 66% to 95% (Figure 2). In addition, the

organocatalytic photochemical C−O bond fragmentation

could also be carried out in continuous ﬂow and similar yields

were obtained in an even shorter reaction time of 28 min

(Figure 2). A scaled-up reaction of 1a (1 mmol) was

performed, and acceptable yields were achieved (Figure 2).

To explore further application of this method, compatibility

with oxidation conditions was studied. The organocatalytic

photochemical approach was successfully merged with Stahl’s

Initially, several reducing photocatalysts were evaluated.

Photocatalyst perylene di-imide (PDI), which has previously

been used for reductive dehalogenation reactions via a

presumed reductive quenching cycle, resulted in no reactivity

with the irradiation of blue LEDs (see the Supporting

Information (SI), Table S1 ). Subsequently, N-phenylpheno-

thiazine (PhPTH) was identiﬁed as a photocatalyst (E

(

•

PhPTH /PhPTH*) = −2.1 V vs SCE) and resulted in

fragmentation products in reasonable yield. The absorbance

spectrum of PhPTH was obtained (Figure S1) and showed a

maximum absorbance at the wavelength 315 nm, suggesting

the need for more violet-centered light sources. Thus, a violet

LED strip (λ_m_a_x= 400−410 nm) was utilized initially. Upon

optimization of the reaction conditions, paying particular

attention to solvent, concentration, and equivalence of

10a

disclosed aerobic and our electrochemical

oxidation

manifolds of lignin into a one-pot, two-step platform. After

chemical or electrochemical oxidation, the solvent was

evaporated. Without any additional workup or puriﬁcation,

the crude residue dissolved in acetone was directly subjected to

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

orcid.org/0000-0003-2097-1325

Letter

Figure 3. Organocatalytic one-pot, two-step fragmentation of lignin systems. All reactions were performed on a 0.25 mmol substrate scale in a one-

pot, two-step platform without any intermediary puriﬁcation. Method A is carried out according to ref 21 and condition A (irradiated by violet LED

strips). Method B is carried out according to ref 10a and condition B (irradiation by Kessil 390 nm LED light). Detailed procedures are described

in the Supporting Information . Isolated yields are reported.

the photochemical conditions. The one-pot, two-step

oxidation/reduction approach aﬀorded the fragmentation

products in good to high yields (Figure 3).

SE-100 44 Stockholm, Sweden; orcid.org/0000-0002-

6089-5454

Gabriel Magallanes − Department of Chemistry, University of

Michigan, Ann Arbor, Michigan 48109, United States;

Kimberly Chan − Department of Chemistry, University of

Michigan, Ann Arbor, Michigan 48109, United States

In conclusion, the disclosed methods will add to the growing

ﬁeld of lignin depolymerization and valorization as investment

in green technologies continues to rise. Using organic

photocatalysts, the fragmentation of oxidized lignin strategies

under mild conditions is viable. We anticipate the incorpo-

ration of these strategies and principles for lignin valorization

to be of great value to the lignin community in the venture to

convert renewable biomass feedstocks into useful chemicals.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.orglett.0c03029

Notes

The authors declare no competing ﬁnancial interest.

C.Y., M.D.K., and G.M. contributed equally.

ASSOCIATED CONTENT

sı Supporting Information

■

ACKNOWLEDGMENTS

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.orglett.0c03029.

■

C.R.J.S. acknowledges ﬁnancial support from the NSF (No.

CHE-1565782), the Camille and Henry Dreyfus Foundation,

and the University of Michigan. M.D.K. acknowledges ﬁnancial

support from the Swedish Research Council (No. 637-2013-

Experimental Procedures, characterization data for all

compounds along with copies of spectra (PDF)

314) and the Royal Swedish Academy of Agriculture and

AUTHOR INFORMATION

Forestry (Kungliga Skogs- och Lantbruksakademien) for a

postdoctoral fellowship. G.M. acknowledges the Rackham

Graduate School at the University of Michigan for a Rackham

Merit Fellowship.

■

Corresponding Author

Corey R. J. Stephenson − Department of Chemistry, University

of Michigan, Ann Arbor, Michigan 48109, United States;

orcid.org/0000-0002-2443-5514; Email: crjsteph@

umich.edu

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