Visible-light-induced C-C bond cleavage of lignin model compounds with cyanobenziodoxolone

Article abstract of DOI:10.1016/j.tetlet.2020.152420

The catalytic degradation of lignin to value-added chemicals has received considerable attention over the past decade. Photocatalysis provides promising approaches to enable previously inaccessible transformations. However, examples of the visible-light promoted degradation of lignin are still limited. In this work, the visible-light-induced selective C-C bond cleavage of β-O-4 lignin model compounds has been disclosed via β-scission of in situ generated alkoxy radical intermediates. With cyanobenziodoxolone as the oxidant, a variety of substrates could be transformed into aldehydes in moderate to good yields. In addition, unexpected acetal esters which could conveniently furnish formaldehyde and phenols by alcoholysis were observed.

Full text of DOI:10.1016/j.tetlet.2020.152420

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Visible-light-induced C-C bond cleavage of lignin model compounds
with cyanobenziodoxolone
⇑
⇑
Ming Zheng, Yan Huang, Le-Wu Zhan, Jing Hou , Bin-Dong Li
School of Chemical Engineering Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic degradation of lignin to value-added chemicals has received considerable attention over the
past decade. Photocatalysis provides promising approaches to enable previously inaccessible transforma-
tions. However, examples of the visible-light promoted degradation of lignin are still limited. In this
work, the visible-light-induced selective C-C bond cleavage of b-O-4 lignin model compounds has been
disclosed via b-scission of in situ generated alkoxy radical intermediates. With cyanobenziodoxolone as
the oxidant, a variety of substrates could be transformed into aldehydes in moderate to good yields. In
addition, unexpected acetal esters which could conveniently furnish formaldehyde and phenols by alco-
holysis were observed.
Received 16 June 2020
Revised 21 August 2020
Accepted 27 August 2020
Available online xxxx
Keywords:
Visible-light
C-C bond cleavage
Lignin model compounds
Cyanobenziodoxolone
Photocatalysts
Ó 2020 Elsevier Ltd. All rights reserved.
Introduction
and Ru [11], were also reported for the depolymerization of
b-O-4 model compounds. Notably, the Stahl group [12] and the
Lignin, a biopolymer of phenylpropanols formed through differ-
ent linkages, represents 20–35% by weight in lignocellulosic bio-
mass and is considered as a potential renewable resource of fuel
and platform chemicals [1]. While tremendous efforts have been
made with regard to the controlled degradation of lignin to
value-added chemicals, it remains a challenge, due to the rigid,
irregular and cross-linked structure [1,2]. The b-O-4 linkage, which
constitutes 43–62% of the polymer connectivity [3], is the main
target of the valorization studies of lignin. Although the cleavage
of C-O bonds of b-O-4 motifs could be achieved efficiently [4],
the selective cleavage of C-C bonds is challenging, due to the bond
energies of C-C bonds (69–123 kcal/mol) generally being higher
than C-O linkages (52–80 kcal/mol) [5,6]. In 2011, Hanson and
co-workers reported the CuCl/TEMPO catalysed aerobic oxidative
Zhang group [13] reported C-C bond cleavage under metal-free
conditions through two-step thermal depolymerization. Despite
these contributions, more catalytic activation modes are still
needed.
Photocatalysis has witnessed dramatic developments over the
past decade [14]. However, examples of the visible-light promoted
degradation of lignin are still limited [2c,15]. In 2015, Soo and
co-workers developed the visible-light driven C -Cb bond cleavage
a
of b-O-4 lignin model compounds employing a vanadium catalyst
to give aldehydes and formates (Scheme 1a) [16]. The Wang group
disclosed heterogenous catalysis systems such as a mpg semicon-
ductor for C-C bond cleavage (Scheme 1a) [17].
Alkoxy radicals are highly reactive intermediates, enabling
hydrogen atom transfer (HAT) or C-C bond b-scission [18]. The
Knowles group [19] reported the C-C bond cleavage of alcohols
through in situ generated alkoxy radicals via proton-coupled elec-
tron transfer (PCET) (Scheme 1b). The Chen group [20] also
reported the light-promoted generation of alkoxy radicals using a
cyclic iodine(III) reagent (Scheme 1c). Recently, Zhang and
co-workers reported the depolymerization of lignin model
compounds using CeCl3 as the catalyst via a ligand-to-metal
charge-transfer (LMCT) strategy [21]. Inspired by these works, we
envisioned that the combination of photocatalysts and cyclic
iodine(III) reagents might trigger the C-C bond cleavage of lignin
model compounds through b-scission of alkoxy radicals. Addition-
cleavage of the C -Cb bond of b-O-4 model compounds at 100 °C
a
[7]. Other transition metal catalysts, such as V [8], Fe [9], Co [10],
⇑
Corresponding authors.
E-mail addresses: chmhouj@njust.edu.cn (J. Hou), libindong@njust.edu.cn
(B.-D. Li).
https://doi.org/10.1016/j.tetlet.2020.152420
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: M. Zheng, Y. Huang, L. W. Zhan et al., Visible-light-induced C-C bond cleavage of lignin model compounds with cyanobenzio-
doxolone, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152420

2
M. Zheng et al. / Tetrahedron Letters xxx (xxxx) xxx
Scheme 1. Visible-light driven C-C bond cleavage.
ally, the cyclic iodine(III) reagents could react as SOMOphiles to
trap carbon radicals derived from the lignin model compounds
[22]. Herein, we provide support for this hypothesis. A visible-
light-induced selective C-C bond cleavage of b-O-4 motifs using
the cyclic iodine(III) reagent cyanobenziodoxolone (CBX) as the
oxidant is established. Interestingly, unexpected acetal esters
which could furnish formaldehyde and phenols by alcoholysis
were also observed (Scheme 1d).
Table 1
Screening for the optimal reaction conditions.a.
Results and discussion
We initiated our study by employing 1-(3,4-dimethoxyphenyl)-
2-(4-methoxyphenoxy)ethan-1-ol 1a as the model substrate under
blue LED irradiation. After extensive investigation (Table 1), we
found that the combination of the photoredox catalyst Ir[dF(CF3)
ppy]2(dtbbpy)PF6 (3 mol%) with Cs2CO3 (1.5 eq.) in the presence
of CBX (1.5 eq.) provided the optimal result, affording the C -Cb
a
bond cleavage product 2a in 50% (Entry 1) [23]. Notably, acetal
ester 3a was observed in 20% yield instead of the cyanation prod-
ucts [24]. No product was detected using other cyclic iodine(III)
reagents such as BI-OH and BI-OAc, suggesting that alkoxy radicals
could not be generated with these reagents (Entries 2 and 3). Addi-
tionally, using BI-OAc (1 eq.) as an additive resulted in no improve-
ment (Entry 4). Only a trace amount of the aldehyde was obtained
employing [Ir(dtbbpy)(ppy)2]PF6 or Ru(bpy)3(PF6)2 as the catalyst
(Entries 5 and 6). When the reaction was carried out with K2CO3
or without base, lower yields were obtained (Entries 7 and 8).
a
Reagents and conditions: 1a (0.15 mmol), Ir[(dF(CF3)ppy)2](dtbbpy)PF6 (3 mol%),
CBX (0.22 mmol), Cs2CO3 (0.22 mmol), DCE (2 mL), blue LED, rt, Ar balloon, 16 h.
b
Yield determined by GC analysis using n-hexadecane as an internal standard.
Isolated yield.
c
Please cite this article as: M. Zheng, Y. Huang, L. W. Zhan et al., Visible-light-induced C-C bond cleavage of lignin model compounds with cyanobenzio-
doxolone, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152420

M. Zheng et al. / Tetrahedron Letters xxx (xxxx) xxx
3
Table 2
Switching the reaction solvent from DCE to CH3CN resulted in a
decreased yield (Entry 9). Conducting the reaction under air condi-
tions, resulted in the formation of aldehyde 2a in only 10% yield
and 3a was not detected, suggesting that oxygen inhibited the
transformation (Entry 10). Control experiments indicated that the
photocatalyst and light irradiation are both critical for the reaction
(Entries 11 and 12).
Scope of the b-O-4 lignin model compounds.a.
With the optimized reaction conditions in hand, a variety of b-O-
4 lignin model compounds were subjected to the catalytic system.
As shown in Table 2, electron-rich benzylic alcohols with methoxy
groups (1a, 1b, 1d, 1e, 1f, 1g, 1i, 1o) proceeded well to afford the
corresponding aldehydes and acetal esters in moderate to good
yields. Nevertheless, the formation of acetal esters was significantly
inhibited by steric effects (1c, 1h, 1m), furnishing trace amounts of
products. Model compounds with unsubstituted benzylic alcohol
groups (1j-1n) were also investigated providing the desired prod-
ucts in moderate yields, indicating that electron-rich arenes were
essential for the reaction efficiency. Subsequently, the alcoholysis
of acetal ester 3a was assessed (Scheme 2). The transformation of
3a proceeded smoothly to provide phenol 5a, formaldehyde and
methyl benzoate 4a using K2CO3 as the base in methanol [13].
Hence, the selective degradation of the b-O-4 lignin model com-
pounds could be realized through the combination of visible-light
promoted C-C bond cleavage and alcoholysis of the acetal esters.
To gain insight into the reaction mechanism, a series of control
experiments were conducted. The addition of TEMPO to the reaction
of 1a gave a reduced yield of 2a (7%) and 6a was detected by GC–MS
(Scheme 3), suggesting a free radical mechanism. Luminescence
quenching experiments were performed where CBX and the b-O-4
lignin model compound 1d both quenched the excited state of Ir
[dF(CF3)ppy]2(dtbbpy)PF6 (Scheme 4). Therefore, both reductive
and oxidative quenching may occur; albeit the rate of CBX
(0.70 mMÀ1) is faster than 1d (0.36 mMÀ1). Subsequently, cyclic
a
voltammetry studies of CBX and 1d were carried out (Figs. S3 and
red
S4), which demonstrated that CBX (E = -0.42 V vs. SCE in DCE)
1/2
could be readily reduced by the excited state of Ir[dF(CF3)ppy]2(-
IV/*III
ox
dtbbpy)PF6 (E
= -0.89 V vs. SCE) and 1d (E = 0.73 V vs. SCE in
1/2
1/2
IV/III
DCE) could be oxidized by the [Ir](IV) intermediate (E
vs. SCE).
= 1.69 V
1/2
A plausible mechanism was proposed in the light of these
experimental data (Scheme 5). First, oxidative quenching of the
exited state of the Ir(III) catalyst by CBX gives Ir(IV) (C) and radical
intermediate D [25]. Then, substrate 1a is oxidized by Ir(IV) (C)
providing alkoxy radical E which undergoes b-scission to generate
2a and a carbon radical F [26]. Finally, radical coupling of D and F
affords acetal ester 3a [27]. However, we were unable to exclude
the possibility that acid anion (G) reacts with an oxonium ion (H)
to generate 3a.
Conclusion
We have described a mild, visible-light-induced selective C-C
bond cleavage of b-O-4 lignin model compounds to give aldehydes
and acetal esters which could be conveniently transformed into
simple phenols and formaldehyde. Detailed studies of the mecha-
nism and further usage of the catalytic system are in progress.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Reagents and conditions: 1 (0.15 mmol), Ir[(dF(CF3)ppy)2](dtbbpy)PF6 (3 mol%),
CBX (0.22 mmol), Cs2CO3 (0.22 mmol), DCE (2 mL), blue LED, rt, Ar balloon, 16 h.
b
Aldehyde yields were determined by GC analysis using n-hexadecane as an
internal standard. Isolated yields are provided for acetal esters.
Please cite this article as: M. Zheng, Y. Huang, L. W. Zhan et al., Visible-light-induced C-C bond cleavage of lignin model compounds with cyanobenzio-
doxolone, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152420

4
M. Zheng et al. / Tetrahedron Letters xxx (xxxx) xxx
Scheme 2. Alcoholysis of acetal ester 3a.
Scheme 3. Addition of TEMPO to the reaction of 1a.
Scheme 4. Luminescence quenching study.
Please cite this article as: M. Zheng, Y. Huang, L. W. Zhan et al., Visible-light-induced C-C bond cleavage of lignin model compounds with cyanobenzio-
doxolone, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152420

M. Zheng et al. / Tetrahedron Letters xxx (xxxx) xxx
5
Scheme 5. Proposed mechanism of C-C bond cleavage.
(b) S. Kim, S.C. Chmely, M.R. Nimlos, Y.J. Bomble, T.D. Foust, R.S. Paton, G.T.
Beckham, J. Phys. Chem. Lett. 2 (2011) 2846–2852.
Acknowledgments
[6] S. Guadix-Montero, M. Sankar, Top. Catal. 61 (2018) 183–198.
[7] B. Sedai, C. Díaz-Urrutia, R.T. Baker, R. Wu, L.A.P. Silks, S.K. Hanson, ACS Catal. 1
(2011) 794–804.
[8] J. Mottweiler, M. Puche, C. Rauber, T. Schmidt, P. Concepcion, A. Corma, C.
Bolm, ChemSusChem 8 (2015) 2106–2113.
[9] J. Mottweiler, T. Rinesch, C. Besson, J. Buendia, C. Bolm, Green Chem. 17 (2015)
5001–5008.
[10] B. Biannic, J.J. Bozell, Org Lett 15 (2013) 2730–2733.
[11] (a) J. Nichols, L. Bishop, R. Bergman, J. Ellman, J. Am. Chem. Soc. 132 (2010)
12554–12555;
(b) T.H. Vom Stein, T.D. Hartog, J. Buendia, S. Stoychev, J. Mottweiler, C. Bolm,
J. Klankermayer, W.A. Leitner, Chem. Int. Ed. 54 (2015) 5859–5863.
[12] A. Rahimi, A. Ulbrich, J.J. Coon, S.S. Stahl, Nature 515 (2014) 249–252.
[13] Y. Wang, Q. Wang, J. He, Y. Zhang, Green Chem. 19 (2017) 3135–3141.
[14] (a) L. Marzo, S.K. Pagire, O. Reiser, B. König, Angew. Chem. Int. Ed. 57 (2018)
10034–10072;
The work was supported by the National Natural Science Foun-
dation of China (No. 21875109 and No. 21901118), the Natural
Science Foundation of Jiangsu Province (No. BK20190430) and
the Fundamental Research Funds for the Central Universities (No.
30919011217)
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152420.
(b) M. Silvi, P. Melchiorre, Nature 554 (2018) 41–49, For selected recent
reviews, see:.
References
[15] For selected reviews, see: (a) Zhang, J. ChemSusChem, 2018, 11, 3071-3080;
(b) Liu, X.; Duan, X.; Wei, W.; Wang, S.; Ni, B. Green Chem. 2019, 21, 4266-
4289. For selected visible-light promoted C-O bond cleavage of lignin, see: (c)
Nguyen, J. D.; Matsuura, B. S.; Stephenson, C. R. J. Am. Chem. Soc. 2014, 136,
1218-1221; (d) Luo, J.; Zhang, X.; Zhang, J. ACS Catal. 2015, 5, 2250-2254; (e)
Luo, N.; Wang, M.; Li, H.; Zhang, J.; Liu, H.; Wang, F. ACS Catal. 2016, 6, 7716-
7721; (f) Karkas, M.; Bosque, I.; Matsuura, B. S.; Stephenson, C. R. Org. Lett.
2016, 18, 5166-5169; g) Cao, Y.; Wang, N.; He, X.; Li, H.; He, L. ACS Sustainable
Chem. Eng. 2018, 6, 15032-15039; (h) Chen, K.; Schwarz, J.; Karl, T. A.;
Chatterjee, A.; Konig, B. Chem. Commun. 2019, 55, 13144-13147.
[16] S. Gazi, W.K.H. Ng, R. Ganguly, A.M.P. Moeljadi, H. Hirao, H.S. Soo, Chem. Sci. 6
(2015) 7130–7142.
[1] (a) W. Boerjan, J. Ralph, M. Baucher, Annu. Rev. Plant Biol. 54 (2003) 519–546;
(b) A.J. Ragauskas, G.T. Beckham, M.J. Biddy, R. Chandra, F. Chen, M.F. Davis, B.
H. Davison, R.A. Dixon, P. Gilna, M. Keller, P. Langan, A.K. Naskar, J.N. Saddler, T.
J. Tschaplinski, G.A. Tuskan, C.E. Wyman, Science 344 (2014) 1246843;
(c) C. Li, X. Zhao, A. Wang, G.W. Huber, T. Zhang, Chem. Rev. 115 (2015)
11559–11624.
[2] (a) R. Rinaldi, R. Jastrzebski, M.T. Clough, J. Ralph, M. Kennema, P.C.A.
Bruijnincx, B.M. Weckhuysen, Angew. Chem. Int. Ed. 55 (2016) 8164–8215;
(b) M.V. Galkin, J.S.M. Samec, ChemSusChem 9 (2016) 1544–1558;
(c) A. Das, B. König, Green Chem. 20 (2018) 4844–4852;
(d) S. Gazi, Appl. Catal. B 257 (2019) 117936.
[3] J. Ralph, K. Lundquist, G. Brunow, F. Lu, H. Kim, P.F. Schatz, J.M. Marita, R.D.
Hatfield, S.A. Ralph, J.H. Christensen, W. Boerjan, Phytochem. Rev. 3 (2004) 29–
60.
[4] For selected examples: (a) Sergeev, A. G.; Webb, J. D.; Hartwig, J. F. J. Am. Chem.
Soc. 2012, 134, 20226-20229; (b) Chan, J. M. W.; Bauer, S.; Sorek, H.;
Sreekumar, S.; Wang, K.; Toste, F. D. ACS Catal. 2013, 3, 1369-1377; (c)
Lancefield, C. S.; Ojo, O. S.; Tran, F.; Westwood, N. J. Angew. Chem. Int. Ed.
2015, 54, 258-262; (d) Zhu, R.; Wang, B.; Cui, M.; Deng, J.; Li, X.; Ma, Y.; Fu, Y.
Green Chem. 2016, 18, 2029-2036.
[17] (a) T.T. Hou, N.C. Luo, H.J. Li, M. Heggen, J.M. Lu, Y.H. Wang, F. Wang, ACS Catal.
7 (2017) 3850–3859;
(b) H. Liu, H. Li, J. Lu, S. Zeng, M. Wang, N. Luo, S. Xu, F. Wang, ACS Catal. 8
(2018) 4761–4771.
[18] Suarez, E.; Rodriguez, M. S. In Radicals in Organic Synthesis; P. Renaud, M. P.
Sibi, Eds.;Wiley-VCH: Weinheim, 2001; Vol. 2, pp 440À454.
[19] (a) H.G. Yayla, H. Wang, K.T. Tarantino, H.S. Orbe, R.R. Knowles, J. Am. Chem.
Soc. 138 (2016) 10794–10797;
(b) E. Ota, H. Wang, N.L. Frye, R.R. Knowles, J. Am. Chem. Soc. 141 (2019)
1457–1462;
[5] (a) R. Parthasarathi, R.A. Romero, A. Redondo, S. Gnanakaran, J. Phys. Chem.
Lett. 2 (2011) 2660–2666;
Please cite this article as: M. Zheng, Y. Huang, L. W. Zhan et al., Visible-light-induced C-C bond cleavage of lignin model compounds with cyanobenzio-
doxolone, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152420

6
M. Zheng et al. / Tetrahedron Letters xxx (xxxx) xxx
(c) S.T. Nguyen, P.R.D. Murray, R.R. Knowles, Example from other groups, ACS
Catal. 10 (2020) 800–805;
(b) S.P. Morcillo, E.M. Dauncey, J.H. Kim, J.J. Douglas, N.S. Sheikh, D. Leonori,
Angew. Chem. Int. Ed. 57 (2018) 12945–12949.
(d) Y. Wang, Y. Liu, J. He, Y. Zhang, Science Bulletin 64 (2019) 1658–1666.
[25] F.L. Vaillant, M.D. Wodrich, J. Waser, Chem. Sci. 8 (2017) 1790–1800.
[26] The benziodoxole/alcohol complex generated from 1a and CBX may be
reduced to Ir(IV) to provide alkoxyl radical E (Fig. S5 in the ESI).
[27] (a) S.K. Rout, S. Guin, W. Ali, A. Gogoi, B.K. Patel, Org. Lett. 16 (2014) 3086–
3089;
[20] K. Jia, F. Zhang, H. Huang, Y. Chen, J. Am. Chem. Soc. 138 (2016) 1514–1517.
[21] Y. Wang, J. He, Y. Zhang, CCS Chem. 2 (2020) 107–117.
[22] X. Wang, A. Studer, Acc. Chem. Res. 50 (2017) 1712–1724.
[23] The ketone derived from the direct oxidation of 1a was detected.
[24] (a) M. Sun, Y. Wang, B. Xu, X. Ma, S. Zhang, Org. Biomol. Chem. 16 (2018)
1971–1975;
(b) L. Chen, E. Shi, Z. Liu, S. Chen, W. Wei, H. Li, K. Xu, X. Wan, Chem. Eur. J. 17
(2011) 4085–4089.
Please cite this article as: M. Zheng, Y. Huang, L. W. Zhan et al., Visible-light-induced C-C bond cleavage of lignin model compounds with cyanobenzio-
doxolone, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152420