Photochemical O-H Functionalization of Aryldiazoacetates with Phenols via Proton Transfer

Article abstract of DOI:10.1021/acs.orglett.0c02564

In this work, we report a thorough investigation of the reaction of phenols with aryldiazoacetates. Mechanistic studies using different spectroscopic methods and theoretical calculations suggest a hydrogen bond between phenol and aryldiazoacetates, which can be modulated by the phenol acidity. The pKA of phenol and therefore the hydrogen bond plays an important role in a subsequent photoinduced proton transfer reaction to give the formal O-H functionalization product of phenols.

Full text of DOI:10.1021/acs.orglett.0c02564

pubs.acs.org/OrgLett
Letter
Photochemical O−H Functionalization of Aryldiazoacetates with
Phenols via Proton Transfer
Claire Empel, Sripati Jana, Chao Pei, Thanh Vinh Nguyen,* and Rene M. Koenigs*
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ABSTRACT: In this work, we report a thorough investigation of the reaction of phenols with aryldiazoacetates. Mechanistic studies
using different spectroscopic methods and theoretical calculations suggest a hydrogen bond between phenol and aryldiazoacetates,
which can be modulated by the phenol acidity. The pKA of phenol and therefore the hydrogen bond plays an important role in a
subsequent photoinduced proton transfer reaction to give the formal O−H functionalization product of phenols.
in the presence of strong Brønsted acids as reaction partners
(Scheme 1b).13
rganic transformation via photoexcited states is currently
Oan important strategy to conduct highly efficient
synthesis by exploiting reaction pathways that are inaccessible
In our previous report on the reaction of fluorinated alcohols
with aryldiazoacetates,10 we observed a hydrogen bond of an
acidic alcohol proton and the ester carbonyl group of the
aryldiazoacetate (Scheme 1c). Moreover, the efficiency of the
photoinduced proton transfer reaction depends on the fluorine
content of the used alcohol, and thus the pKA of the alcohol
may play an important role in this reaction. On the basis of
these results, we planned to further investigate the influence of
the pKA to gain deeper insight into the mechanism of such a
transformation. In this Letter, we focus on the reaction of
phenols with methyl phenyldiazoacetate. Phenols are a
particularly suitable class of substrates because the pKA of
phenols can be easily modulated by the electronic properties of
the aromatic substituents, and thus the influence of pKA on the
reaction efficiency can be monitored.
In the first step, we studied the interaction of phenol 10a
and methyl phenyldiazoacetate 6a by 1H and 13C NMR
spectroscopy (Scheme 2). When investigating a solution of a
stoichiometric mixture in CDCl3 (c = 0.1 M for 6a and 10a),
we observed a significant chemical shift perturbation for both
the 10a O−H proton (0.30 ppm) and the 6a carbonyl carbon
(0.26 ppm), which agreed with our previously proposed
hydrogen-bond interaction (Scheme 1c). To find further
in the ground state.1,2 In this context, the classic Paterno−
̀
Bu
̈
chi reaction3 and, more lately, photoredox catalysis4 are
predominant examples, with broad applications in organic
synthesis, that rely on the photochemical activation of reaction
partners or catalysts. On the contrary, there are rare synthetic
applications of photoexcited proton transfer (PPT) reactions
to date,5−9 although photoacids6,7 or -bases8 have been known
for interesting usages such as conducting pH jumps or
triggering protein folding.5 Early examples of PPT reactions
reported recently6 often rely on the UV-light-mediated
photoexcitation of a Brønsted acidic catalyst. The excited
state of the catalyst exhibits a lower pKA value than the ground
state and can thus be used to conduct photocatalytic
transformations based on a proton transfer reaction. In a
similar fashion, the photoexcitation of fluorene-based imine 1
furnishes a representative photobase that is sufficiently basic to
deprotonate alcohols to give the ion pair [1·H]+EtO− (Scheme
1a).8,9
Recently, our group reported on PPT reactions to conduct
efficient O−H functionalization reactions of halogenated
alcohols with aryldiazoacetates 6 under photochemical
conditions (Scheme 1c).10,11 This reaction proceeds via
photoexcitation of a diazoacetate and enriches the reactivity
of diazoalkanes and differentiates from the conventional
reactivity in carbene transfer reactions or in the reaction with
strong acids.12 The latter traditionally proceeds via a metal-
catalyzed insertion reaction of a carbene intermediate into O−
H bonds or via a protonation substitution reaction mechanism
Received: July 31, 2020
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© XXXX American Chemical Society
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Letter
the hydrogen-bond interaction, the O−H bond in phenol 10a
and the CO bond in diazoacetate 6a each lengthen by 0.007
Å, which is consistent with the NMR chemical shift
perturbations.
Scheme 1. Photoinduced Proton Transfer Reactions and
Reactivity of Diazoalkanes
Having solid experimental and theoretical confirmations for
the hydrogen-bonding interaction, we subsequently inves-
tigated different para-substituted phenols 10b−i to study the
influence of the phenol pKA on the complexation (Table 1).16
Table 1. Data on Hydrogen-Bonding Complex Formation
ΔEint (ΔE
)
Δδ (O−H) Δδ (CO)
(ppm) (ppm)
16
soal
(kcal/mol)
b
b
no.
R
pKA
11a
11b
11c
11d
11e
11f
11g
11h
11i
H
4-CN
4-NO2
4-F
4-Cl
4-CF3
4-OMe
4-OCF3
4-CO2Me
2-NO2
9.98
7.59
7.15
9.95
9.38
−8.0 (−5.4)
−9.5 (−6.2)
−9.5 (−6.2)
−8.4 (−5.6)
−8.6 (−5.8)
−9.1 (−6.1)
−8.0 (−5.5)
−9.1 (−6.0)
−8.6 (−5.9)
−4.5 (−2.8)
0.30
0.63
0.71
0.40
0.40
0.43
0.25
0.48
0.32
0.00
0.26
0.49
0.73
0.33
0.39
0.51
0.21
0.42
0.32
0.00
10.21
11j
7.23
a
Interaction energies calculated at the MP2/6-311+G(d,p)//B3LYP/
b
6-31G(d) level with BSSE correction. Determined by H and 13C
NMR spectroscopy of a 0.1 M solution of a 1:1 mixture of phenol 10
and methyl phenyl diazoacetate 6a in CDCl3.
1
Scheme 2. Formation of Hydrogen Bond Complex 11a and
Selected Properties
For weakly acidic phenol derivatives, for example, para-
methoxy phenol 10g, we observed a smaller perturbation of the
chemical shift in the hydrogen-bonding complex 11g as
compared with the parent phenol 10a. When studying more
acidic phenols, for example, para-cyano phenol 10b, a larger
chemical shift perturbation was seen. This dependency of pKA
and the chemical shift perturbation was observed for all para-
substituted phenols (Table 1). Furthermore, we have
calculated the single point energy change for the formation
of 11 for different para-substituted phenols 10b−i with methyl
phenyldiazoacetate 6a upon complex formation. The inter-
action of acidic phenols of 10 with 6a via a hydrogen bond is
energetically more favored compared with less acidic phenols
(−9.58 kcal/mol for the complex with para-NO2−phenol 11c
vs −8.05 kcal/mol for the complex with para-MeO−phenol
11g), which represents a similar trend, as observed by NMR
studies.
We also studied the hydrogen-bonding complexes of meta-
and ortho-substituted phenols with methyl phenyldiazoacetate
6a. (For details, see Table S1.) The introduction of
substituents in the ortho position opens up the possibility
for intramolecular hydrogen bonds of Lewis-basic substituents
with the phenol proton. For the 1:1 mixtures of ortho-NO2−
phenol (11j, Table 1) and methyl phenyldiazoacetate 6a, no
perturbation of chemical shifts was observed, which can be
attributed to an intramolecular hydrogen bond of the phenolic
O−H proton. This is further supported by the calculated
interaction energies. IR studies for all hydrogen-bond
complexes did not reveal an obvious correlation between
evidence for this, we performed IR studies, in which we
examined a 1:1 mixture of both substrates. We observed a shift
of the carbonyl (CO) and the NN absorption bands of
22.7 and 5.4 cm−1, respectively, from the original IR spectrum
for 6a, which is also supportive of a hydrogen-bond interaction.
To give further insights, we subsequently performed
calculations at the MP2/6-311+G(d,p)//B3LYP/6-31G(d)
level of theory with basis set superposition error (BSSE)
correction.14,15 (For details, see the SI.) The interaction
between phenol 10a and methyl phenyldiazoacetate 6a was
found to be −8.0 kcal/mol in free energy change. A hydrogen
bond length of 1.85 Å was estimated, which indicates a strong
intermolecular hydrogen bond in this complex. As a result of
B
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differences in the wavenumber and the acidity of the phenol.15
(For details, see Table S1.)
Scheme 3. Scope of Different Diazoalkanes
To study the influence of the hydrogen-bond strength on the
reactivity of the O−H moiety, we subjected these phenols to
the blue-light-induced PPT reaction with a diazo compound.
First, we investigated the reaction of para-cyano phenol 10b
with methyl phenyldiazoacetate 6a, as they form a strong
hydrogen-bonding complex (11b, ΔEint = −9.51 kcal/mol,
Table 1). Gratifyingly, most common organic solvents were
compatible with this reaction, and the desired product was
obtained in moderate yields (Table 2, entries 1−6). The
Table 2. Optimization of Reaction Conditions
a
no.
solvent
ratio (6a/10b)
yield (%)
50
53
46
47
1
2
3
4
5
6
7
8
9
10
11
12
13
1,2-DCE
DCM
1:1
1:1
1:1
1:1
1:1
1:1
1:1
2:1
1:2
1:2
1:2
1:2
1:2
CHCl3
toluene
n-hexane
EtOAc
MeOH
DCM
17
52
b
no product
76
c
DCM
80
75
61
c
1,2-DCE
EtOAc
DCM
c
d
no reaction
traces
e
DCM
a
Reaction conditions: 6a and 10b were dissolved in 1.0 mL of the
indicated solvent and irradiated with blue LEDs overnight at ambient
1
temperature (27 °C). Yields were calculated by H NMR spectros-
b
copy using mesitylene as an internal standard. Product of O−H
c
functionalization of MeOH from crude NMR. Isolated yield.
d
e
Reaction in the dark. Irradiation with a 23W CFL lamp overnight
at ambient temperature (27 °C).
highest reaction yield was obtained using DCM as the solvent.
In n-hexane, a heterogeneous reaction mixture was observed,
which resulted in a reduced reaction efficiency (17% yield).
When using MeOH as the solvent, only O−H functionaliza-
tion of methanol instead of para-cyano phenol was observed by
crude NMR of the reaction mixture (Table 2, entry 7). Next,
we investigated the reaction stoichiometry, and 2 equiv of
phenol was identified as being optimal for the reaction (Table
2, entry 9). The necessity for an excess of phenol can be
attributed to the rapid decomposition of diazo compounds
under blue-light irradiation conditions to form highly reactive
carbene or (in our case) carbocationic intermediates. Thus the
use of an excess of the reaction partner is often needed.11 No
reaction took place when the reaction was performed in the
dark, which underlines the necessity of photoirradiation for the
O−H functionalization reaction (Table 2, entry 12).
With the optimized reaction conditions in hand, we
investigated the reaction of different aryldiazoacetates in the
reaction with para-cyano phenol 11b (Scheme 3). To our
delight, different substituted esters were well tolerated under
the present reaction conditions, and the corresponding O−H
functionalization reaction products were isolated in high yields
(12a−e). Moreover, different electron-withdrawing and
electron-donating substituents on the aromatic ring and
polycyclic or heterocyclic aromatic systems are compatible
with the PPT reaction. The low yield of 12q can be explained
by the lower basicity of the pyridyl-substituted diazo
compound. On the contrary, electron-rich heterocyclic
systems, such as the 2-thienyl-substituted diazoacetate gave
significantly improved yields (12r). (1-diazo-2,2,2-
trifluoroethyl)benzene gave the O−H functionalization re-
action product 13 in 43% yield. When using ethyl diazoacetate
14 (EDA) under irradiation with UV light, only the
decomposition of the diazo compound was observed.
We subsequently studied a range of phenols with different
electron-donating and electron-withdrawing substituents under
the optimized reaction conditions (Scheme 4). On the basis of
our studies on the formation of the hydrogen-bonding
complex, we expected higher yields for electron-withdrawing
substituents as the acidity of the phenol increased. Indeed, this
was the case, as we observed higher yields for electron-
withdrawing groups compared with electron-donating groups
(e.g., para-nitro phenol vs para-chloro phenol: 85 vs 38% yield;
see Scheme 4). Moreover, the yield decreased when changing
from para- to meta- or ortho-substituted phenols, which is in
good correlation to previously observed trends in the
perturbation studies (Table 1). When investigating methyl 2-
C
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Scheme 4. Scope of Different Phenols and N-Heterocyclic
Phenols
Figure 1. Scope of different phenols and N-heterocyclic phenols.
substrates, including different aryldiazoacetates, phenols, and
hydroxyl-substituted N-heterocycles (32 examples, 20−85%
yield). The analysis of the studies on the perturbation of
chemical shifts against product yields shows a linear
correlation, which underlines the importance of the hydro-
gen-bonding complex in photoinduced proton transfer
reactions of aryldiazoacetates.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02564.
Experimental procedures, characterizations, theoretical
calculations, and analytical data (PDF)
hydroxybenzoate (10y), no reaction occurred, as the presence
of a strong intramolecular hydrogen bond might have
interfered with the complexation. Similarly, 4-methoxy phenol
(10g) did not give the desired reaction product, which might
be related to the weak hydrogen-bond complex (cf. Table 1). It
is important to note that in the case of low-yielding O−H
functionalization, no byproducts from the undesired C−H
insertion into an C(sp2)−H bond were observed.
AUTHOR INFORMATION
Corresponding Authors
■
Rene M. Koenigs − Institute of Organic Chemistry, RWTH
Aachen University, D-52074 Aachen, Germany; School of
Chemistry, University of New South Waley, Sydney 2052,
Australia; orcid.org/0000-0003-0247-4384;
For a more general concept of this PPT reaction, we finally
studied hydroxy-substituted N-heterocycles under the opti-
mized reaction conditions (Scheme 4). When using 2-hydroxy
pyridine or 2-hydroxy quinoline, the desired products were
isolated in 76 and 70% yield, respectively. The intensely yellow
8-hydroxy quinoline 16 did not react under the blue-light
reaction conditions, which might be explained by intermo-
lecular hydrogen bonds of 16.17
To further rationalize the relationship between the chemical
shift perturbation and the efficiency of the PPT reaction, we
plotted the isolated yield against the chemical shift
perturbation of the O−H proton of phenols 10 (Figure 1).
For para-substituted phenols, a linear trend was observed;
when using meta-substituted phenols, a similar trend was
obtained, yet with a larger deviation. When investigating ortho-
substituted phenols, the opposite trend was obtained. (For
details, see Scheme S1.) These observations further emphasize
the influence of the intramolecular interactions of the O−H
proton.
Email: rene.koenigs@rwth-aachen.de
Thanh Vinh Nguyen − School of Chemistry, University of New
South Waley, Sydney 2052, Australia; orcid.org/0000-
0002-0757-9970; Email: t.v.nguyen@unsw.edu.au
Authors
Claire Empel − Institute of Organic Chemistry, RWTH Aachen
University, D-52074 Aachen, Germany; School of Chemistry,
University of New South Waley, Sydney 2052, Australia
Sripati Jana − Institute of Organic Chemistry, RWTH Aachen
University, D-52074 Aachen, Germany; orcid.org/0000-
0001-7155-8384
Chao Pei − Institute of Organic Chemistry, RWTH Aachen
University, D-52074 Aachen, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02564
Notes
The authors declare no competing financial interest.
In summary, we describe detailed studies of photoinduced
proton transfer reactions for O−H functionalization reactions
of phenols. The hydrogen-bonding interaction of aryldiazoa-
cetates and phenols was investigated by IR and NMR
spectroscopy and further supported by computational
calculations. This method is applicable to a broad range of
ACKNOWLEDGMENTS
■
R.M.K. thanks the German Science Foundation and Dean’s
Seed Fund of RWTH Aachen Foundation for financial support.
We acknowledge the DAAD-PPP and Universities Australia for
D
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E
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