Visible light induced redox neutral fragmentation of 1,2-diol derivatives

Article abstract of DOI:10.1039/c9cc06904f

A homogeneous, redox-neutral photo fragmentation of diol derivatives was developed. Under photo/hydrogen atom transfer (HAT) dual catalysis, diol derivatives such as lignin model compounds and diol monoesters undergo selective β C(sp³)-O bond cleavage to afford ketones, phenols and acids effectively.

Full text of DOI:10.1039/c9cc06904f

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Visible light induced redox neutral fragmentation
of 1,2-diol derivatives†
Cite this: Chem. Commun., 2019,
55, 13144
Kang Chen, ‡ Johanna Schwarz, ‡ Tobias A. Karl,
and Burkhard Konig
Anamitra Chatterjee
Received 4th September 2019,
Accepted 7th October 2019
*
¨
DOI: 10.1039/c9cc06904f
rsc.li/chemcomm
A homogeneous, redox-neutral photo fragmentation of diol deri-
We began the optimization of the reaction conditions with
vatives was developed. Under photo/hydrogen atom transfer (HAT) diol derivative 1a. After screening of metal photocatalysts, HAT
dual catalysis, diol derivatives such as lignin model compounds and catalysts, and solvents (see ESI,† for detailed information), we
diol monoesters undergo selective b C(sp3)–O bond cleavage to discovered that the combination of [Ir(ppy)2dtbpy]PF6 and
afford ketones, phenols and acids effectively.
methyl 2-mercaptoacetate as co-catalyst in DMA solution
facilitates the cleavage of 1a into ketone (2a) and phenol (3a)
Diol and polyol derivatives are the fundamental structural units in 91% and 81% yield, respectively (Table 1, entry 1). Sodium
of lignin, which is an important renewable non-fossil organic dibutylphosphate was the crucial base for this reaction, because
carbon source.1 The selective C(sp3)–O bond cleavage of diol or it shows good solubility in organic solvents and accelerates the
polyol derivatives could offer a sustainable approach to gain generation of thiyl radicals (S–H BDE = 87 kcal molÀ1)9 as HAT
valuable oxygen-containing feedstock chemicals from lignin agents.10 Moreover, this base is also a good hydrogen bond
biomass.2 Photochemical C–O bond cleavage processes may acceptor, which could effectively activate the a C–H bond of
proceed under mild conditions,3 opening perspectives of solar the hydroxyl group to promote the following H-atom abstraction
driven lignin valorization. The selective deoxygenation of lignin (a C–H bond BDE = 86 kcal molÀ1).11,12 Notably, the use of
b-O-4 model compounds, a type of aromatic backbone diol stoichiometric phosphate was necessary to achieve high yields
derivative, has been realized by using Ir photocatalysts, organic of products (Table 1, entries 1 and 2). Other insoluble inorganic
copolymers or semi-conductors under UV-Vis irradiation bases (K2CO3, Na2HPO4 etc.) or the organic base 2,4,6-collidine gave
(Scheme 1A).4–6 These step-wise deoxygenation reactions involved significantly lower yields (Table S2, ESI†). Control experiments
oxidation and reduction processes to achieve an overall redox-
neutral transformation. However, the use of stoichiometric
amounts of external oxidants and reductants were still necessary.
The hydroxyl groups in diols are already a reductive hydrogen
source. Thus, the catalytic hydrogen transfer7 from the hydroxyl
group to the b C(sp3)–O bond would be an ideal strategy for the
redox-neutral deoxygenation in one single step without external
reductants. Very recently, heterogeneous photocatalysts such as
ZnIn2S4 and CdS quantum dots were applied in the redox-neutral
degradation of lignin b-O-4 model compounds in absence of
stoichiometric external reductants (Scheme 1B).8 Herein, we report
the homogeneous, redox neutral photocatalytic fragmentation of
diols using two different photo/HAT dual catalytic systems
(Scheme 1C).
Faculty of Chemistry and Pharmacy, University of Regensburg, 93040 Regensburg,
Germany. E-mail: burkhard.koenig@ur.de
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cc06904f
Scheme 1 Photo fragmentation of diol derivatives.
‡ These authors contributed equally to this work.
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Table 1 Optimization of redox-neutral fragmentation of 1a
Table 2 Photo fragmentation of lignin b-O-4 model diol derivatives
Entrya Catalyst (x mol%)
Solvent Ketone
Phenol
Entrya Substrate
Ketone
Phenol
1
[Ir(ppy)2dtbpy]PF6 (1) DMA
[Ir(ppy)2dtbpy]PF6 (1) DMA
[Ir(ppy)2dtbpy]PF6 (1) DMA
91% (86%)b 81% (71%)b
2c
40%
16%
n.d.
30%
Trace
n.d.
3d
1
4
5
Eosin Y (5)
Perylene (5)
4CzIPN (5)
4CzIPN (5)
4CzIPN (10)
4CzIPN (10)
4CzIPN (8)
4CzIPN (4)
4CzIPN (4)
DMA
DMA
n.d.
n.d.
6
DMA
DMA
DMA
DMSO
DMSO
DMSO
DMSO
26%
59%
72%
73%
76%
82%
26%
48%
44%
54%
58%
57%
7d
8d,e
9d,e
10d,e
11d,e
12c,d,e
2
3
4
5
80% (79%)b 57% (63%)b
a
0.1 mmol scale reaction; yields determined by GC analysis with an
b
c
d
internal standard. Isolated yield. 50 mol% NaOP(O)(OBu)2. Under
air atmosphere. 40 mol% HSCH2CO2Me.
e
indicated that Ir catalyst, thiol catalyst, base, light irradiation
(Table S5, ESI†) and inert atmosphere (Table 1, entry 3) were all
essential for this transformation.
Although the Ir catalyst performs quite well in this reaction,
the high price and toxicity of iridium limits its application.
We therefore tested different redox active organic dyes13,14 to
explore the possibility of a transition-metal free catalytic system
for this transformation. Eosin Y and perylene did not catalyze
the C(sp3)–O bond cleavage (Table 1, entries 4 and 5) while with
5 mol% 4CzIPN as catalyst, the generation of aryl ketone and
phenol was observed, albeit in low yields (Table 1, entry 6).
To our surprise, the 4CzIPN catalyzed reaction performed even
better under air atmosphere (Table 1, entry 7). We could further
improve the reaction efficiency by tuning the loading of 4CzIPN
and thiol, as well as by using DMSO, instead of DMA (Table 1,
entries 8–11). Additionally, the yields of both products remained
almost the same even when the amount of NaOP(O)(OBu)2 was
decreased to 0.5 equiv. (Table 1, entry 12). Compared with Ir
catalysis, the 4CzIPN catalyst system exhibits similar reaction effi-
ciency under more robust conditions (Table 1, entries 1 and 12).
Thus, we chose 4CzIPN catalysis for substrate scope investigation.
To probe the generality of this methodology, various lignin
b-O-4 model compounds were investigated under the optimized
conditions (Table 2). The electronic properties of both aromatic
rings (Ar1 and Ar2) influence the reactivity of substrates (1b–d).
Generally, more electron-poor Ar1 (1d) and more electron-rich
Ar2 (1b) decreased the conversion of starting material so
that comparatively lower product yields were obtained. The
coniferyl-derived substrates (1e–g) convert smoothly, affording
6
7
8
9
a
Reactions were carried out in 0.1 mmol scale, isolated yields are
b
reported. 1 mol% of [Ir(ppy)2(dtbpy)]PF6, 20 mol% of HSCH2CO2Me
and 1.0 equiv. of NaOP(O)(OBu)2 in 2.0 mL DMA (0.2 mmol scale) under
c
N2 atmosphere. Determined by GC analysis.
good to excellent yields of the corresponding fragmentation steric hindrance of 1i inhibiting the benzylic C–H abstraction
products. However, the sinapyl-derived substrate 1h was not by thiyl radicals may be a reason for the observation.
compatible with the aerobic conditions and reacted less
Furthermore, we investigated the influence of different leaving
efficient. The branched b-O-4 model compound 1i did not react groups (Table 3). Similar to the diol mono-phenoxyethers, mono-
well with 4CzIPN or the Ir catalysis. Traces of deoxygenated acetates of diols bearing different substituents underwent selec-
ketone and not more than 20% phenol were detected. Increased tive b C(sp3)–O bond cleavage to generate aryl ketones and the
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Table 3 Photo fragmentation of diol derivatives with various backbones
and leaving groups
Entrya
1–4
Substrate
Products
Scheme 2 Control experiments with unprotected and full-protected diol
derivatives.
According to quenching experiments (see Fig. S3–S6 in the
ꢀ
ESI†), we assume the excited photocatalyst 4CzIPN (E(PC*/PC
À =
)
+1.43 V vs. SCE)15 is reductively quenched in the presence of
thiol (Eox = +0.85 V vs. SCE)10a and phosphate. Then the
generated thiyl radical abstracts the a H-atom of benzylic
alcohol 1 to give a benzylic ketyl radical A, which is further
oxidized to the corresponding ketone B. The generation of
ketone B could be proven by GC analysis of the crude reaction
mixture after four hours of irradiation (see Fig. S7 in the ESI†).
A SET reduction of ketone B by the reduced photocatalyst
results in ketyl radical anion C.16 The intermediate C is not
stable so that a following b C(sp3)–O bond cleavage takes place
to form the carbonyl radical D and the anion of leaving group
E,3e,4,8 which are further converted to the final fragmentation
products (Scheme 3).
In summary, we have developed a redox neutral fragmenta-
tion of diol derivatives via photo/HAT dual catalysis without the
use of stoichiometric external reductants. Both 4CzIPN and Ir
photocatalysts facilitate the fragmentation of diol derivatives
such as lignin b-O-4 model compounds and diol monoesters.
Mechanistic studies suggest that the substrates are trans-
formed to the corresponding ketones via HAT and oxidation.
Subsequently, the photo reduction of the ketone generates the
ketyl radical anion and induces the b C(sp3)–O bond cleavage to
afford the desired products. The photocatalytic C–O bond
cleavage strategy may find application in the valorization of
biomass molecules.
5
6
7b
a
Reactions were carried out in 0.1 mmol scale, isolated yields are
b
reported. 2 mol% of [Ir(ppy)2(dtbpy)]PF6, 40 mol% of HSCH2CO2Me
and 1.0 equiv. of NaOP(O)(OBu)2 in 1.0 mL DMA (0.1 mmol scale) under
N2 atmosphere for 72 h. Yield determined by GC analysis with an
internal standard.
volatile AcOH (1j–m). Halogen atoms such as F (1m) and Cl (1l)
are tolerated. Moreover, we could isolate both aryl ketone and
benzoic acid in good yields from the reaction of mono-benzoate
(1n). An intramolecular fragmentation occurred with dihydro-
benzofuran derivative 1o to form the ring-open product
2-hydroxyacetophenone 2h in moderate yield.
Finally, we also explored the conversion of aliphatic diol
derivatives with the photo/HAT dual catalysis strategy. Unfortu-
nately, the 4CzIPN system was not able to catalyze the
fragmentation of 1p under aerobic conditions. The Ir catalyst
under inert atmosphere gave the desired phenol product, but
higher Ir and thiol catalyst loadings and a longer reaction time
were required (Table 3, entry 7). Due to the higher BDE of the
aliphatic alcohol a C–H bond (BDE = 92 kcal molÀ1, vs. benzylic
alcohol a C–H BDE = 86 kcal molÀ1),11 cleavage of the a C–H
bond of the aliphatic alcohol is more difficult with the thiyl
radical.
This work was supported by the German Science Foundation
(DFG) (GRK 1626, Chemical Photocatalysis; KO 1537/18-1).
We also explored the reactivity of free diol and full-protected
diester. In case of diol 4, the ketone product 2a was detected
only in a very low yield (Scheme 2, eqn (1)). Furthermore, the
diol diacetate 5 remained untouched under the standard
conditions (Scheme 2, eqn (2)). These results suggest that both
unprotected hydroxyl group and the adjacent leaving group are
crucial for the desired b C(sp3)–O bond cleavage.
Scheme 3 Proposed mechanism for the photo fragmentation of diol
derivatives.
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vs. SCE) is more negative than that of 4CzIPNꢀÀ (E(PC/PC
À
) = À1.24 V vs.
ꢀ
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reduced to afford the fragmentation products, see ESI†.
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Chem. Commun., 2019, 55, 13144--13147 | 13147