Direct C?H Phosphonylation of Electron-Rich Arenes and Heteroarenes by Visible-Light Photoredox Catalysis

Article abstract of DOI:10.1002/chem.201701283

The direct transformation of ubiquitous, but chemically inert C?H bonds into diverse functional groups is an important strategy in organic synthesis that improves the atom economy and faclitates the preparation and modification of complex molecules. In contrast to the wide applications of aryl phosphonates, their synthesis via direct C?H bond phosphonylation is a less explored area. We report here a general, mild, and broadly applicable visible-light photoredox C?H bond phosphonylation method for electron-rich arenes and heteroarenes. The photoredox catalytic protocol utilizes electron-rich arenes and biologically important heteroarenes as substrates, [Ru(bpz)₃][PF₆]₂ as photocatalyst, ammonium persulfate as oxidant, and trialkyl phosphites as the phosphorus source to provide a wide range of aryl phosphonates at ambient temperature under very mild reaction conditions.

Full text of DOI:10.1002/chem.201701283

A Journal of
Accepted Article
Title: Direct C-H Phosphonylation of Electron Rich Arenes and
Heteroarenes by Visible-Light Photoredox Catalysis
Authors: Rizwan Shaikh, Indrajit Ghosh, and Burkhard König
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Direct C–H Phosphonylation of Electron Rich Arenes and
Heteroarenes by Visible–Light Photoredox Catalysis
Rizwan S. Shaikh, Indrajit Ghosh, and Burkhard König*
Dedication ((optional))
]
Abstract: The direct transformation of ubiquitous, but chemically
inert C–H bonds into diverse functional groups is an important
strategy in organic synthesis that improves the atom economy and
allows preparing and modifying complex molecules more easily. . In
contrast to the wide applications of aryl phosphonates, their
synthesis via direct C–H bond phosphonylation is a less explored
area. We report here a general, mild, and broadly applicable visible
light photoredox C–H bond phosphonylation method for electron rich
arenes and heteroarenes. The photoredox catalytic protocol utilizes
electron rich arenes and biologically important heteroarenes as
substrates, [Ru(bpz)3][PF6]2 as photocatalyst, ammonium persulfate
as oxidant, and trialkyl phosphites as the phosphorus source to
provide a wide range of aryl phosphonates at ambient temperature
under very mild reaction conditions.
molecules.[13
Here, we report our attempts towards visible-light mediated
formal C–H bond phosphonylation of functionalized arenes and
heteroarenes. We propose the photoredox catalytic generation
of arene or heteroarene radical cations that serve as the key
reactive intermediates in C–H phosphonylation reactions. The
mild reaction conditions (room temperature and visible light)
utilizes readily available trialkyl phosphites as nucleophilic
phosphonylating reagent, ammonium persulfate as sacrificial
oxidant, and exhibits a wide substrate scope with excellent C–H
phosphonylation yields.
Aryl phosphonates and their derivatives are an important
]
class of organic compounds, because of their use in materials,[1
in medicinal chemistry,[2 and as ligands in catalysis.[3 Palladium
catalyzed cross coupling of H–phosphonates with aryl bromides,
triflates, boronic acid, tosylates, and aryl sulfonates is a typical
]
]
,
]
method for the synthesis of aryl phosphonates.[4
5
Among
others, copper and nickel catalysts were also used for the
]
synthesis of aryl phosphonates.[6 Recently, our group and others
have reported photoredox catalytic methods for the construction
of
C–P
bonds
using
diazonium
salts,
activated
tetrahydroisoquinolines, electron deficient aryl halides (such as –
Br, –Cl), or aryl triflates (–OTf) in the presence of electron
,
]
withdrawing groups.[7 8
The direct catalytic transformation of sp2-carbon-hydrogen
(C–H) bonds into carbon–phosphorus (C–P) is economical and
has practical advantages as it does not require prefunctionalized
]
Scheme 1. C–H Phosphonylation of arenes.
starting materials.[8c,
9
Recent advances in direct arene C–H
bond phosphonylation methods using transition metals or
photoredox catalytic methods are either restricted to specific
directing groups, limited substrate scopes, or require relatively
We began our investigations on the visible light mediated
C–H bond phosphonylation of arenes and heteroarenes by
irradiating a mixture of 1,2,4-trimethoxybenzene (test substrate,
see Table 1) and triethyl phosphite (P(OEt)3) in CH3CN in the
presence of [Ru(bpz)3][PF6]2 as photocatalyst and (NH4)2S2O8 as
the terminal oxidant. To our delight the phosphonylated product
diethyl (2,4,5-trimethoxyphenyl)phosphonate 2a was obtained in
63% yield (entry 1 in Table 1). Increasing the amounts of
(NH4)2S2O8 and triethyl phosphite (2.2 and 5.0 equiv.
respectively) provided the phosphonylated product in almost
quantitative (99%) yield. (NH4)2S2O8 was more effective as
sacrificial oxidant with respect to atmospheric oxygen under the
photoredox catalytic reaction condition (c.f. entry 3 and 5 in
Table 1). A series of control experiments (i.e., omitting each
individual component of the catalytic cycle) confirmed that the
catalytic components, i.e., [Ru(bpz)3][PF6]2, sacrificial electron
acceptor (NH4)2S2O8, and visible light irradiation are essential
(see entries 6–8 in Table 1) for the photoredox catalytic C–H
phosphonylation reactions to take place. When [Ru(bpy)3]Cl2
was used as a photocatalyst the phosphonylated product was
]
harsh reaction conditions.[10 Radical dehydrogenative cross
coupling between arenes and H-phosphonates were also
]
realized via the phosphoranyl radical using metal complexes[11
and electrochemical methods.[12 However, a narrow substrate
]
scope and relatively harsh reaction conditions limit their use for
synthesis.
Visible light mediated photoredox catalysis has emerged
into a powerful tool for the radical based C–X bond activation or
C–H bond functionalization via single electron transfer
processes (SET), and has played a vital role in the recent
development of new methodologies for the synthesis of complex
[a]
Dr. R. S. Shaikh, Dr. I. Ghosh, Prof. Dr. B. König
Universität Regensburg, Fakultät für Chemie und Pharmazie
93040 Regensburg (Germany)
E-mail: burkhard.koenig@ur.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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obtained in 98% isolated product yield. However, we choose
phosphonylated product 2k was obtained in 83% isolated yield
leaving the two alkyne moieties as building blocks for further
functionalization/s. It is worth mentioning here that when 1-
bromo-2,4-dimethoxybenzene was used as a starting material,
an ipso-substituted product 2c was obtained along with the
expected bromo-substituted phosphonylated product 2n (2c:2n
1:1, overall yield 72%). Other commercially available alkyl
phosphites, such as P(OMe)3 or sterically hindered P(OiPr)3
were equally effective as phosphonylating reagents providing
the corresponding phosphonates in excellent yields under the
photoredox catalytic protocol.
[Ru(bpz)3][PF6]2 as
a
photocatalyst for further synthetic
applications owing to its superior oxidation potential[13a] and
relatively faster reaction kinetics.[14
]
Table 1. Direct C–H phosphonylation of (hetero)arenes: Control reactions and
the optimization of the reaction condition.
Entry
Photocatalyst
[Ru(bpz)3][PF6]2
[Ru(bpz)3][PF6]2
[Ru(bpz)3][PF6]2
[Ru(bpy)3]Cl2
[Ru(bpz)3][PF6]2
None
Oxidant
1.7 eq.
1.7 eq.
2.2 eq.
2.2 eq.
Air.
P(OEt)3
3 eq.
5 eq.
5 eq.
5 eq.
5 eq.
5 eq.
5 eq.
5 eq.
5 eq.
Time (h)
20
Yield %[a]
63 %
1
2
20
80 %
3
20
99 %
4[b],[c]
5
20
98 %
20
15 %
6
2.2 eq.
None
20
No reaction
No reaction
No reaction
82 %
7
[Ru(bpz)3][PF6]2
[Ru(bpz)3][PF6]2
20
8[d]
2.2 eq.
2.2 eq.
20
9[e]
[Ru(bpz)3][PF6]2
32
[a]
[b]
[c]
Isolated yield. Substrate specific (see ref. 14). 5.0 mol % catalyst was
[d]
[e]
required.
Reactions performed in the absence of light.
Reaction
performed on 1.8 mmol scale. Abbreviations: P(OEt)3 = triethyl phosphite. LED
= light emitting diode.
With the optimized reaction conditions in hand, we then
explored the scope of the photoredox C–H phosphonylation
reactions using different arenes and heteroarenes as substrates.
Triethyl phosphite, P(OEt3), was used as a representative
coupling partner. Simple arenes such as trimethoxy or
dimethoxy benzenes (including 1,3–dimethoxy, 1,4–dimethoxy,
and 1,2–dimethoxy benzene) provided the phosphonylated
products in very good to excellent yields (yields up to 99%; see
Scheme 2). Anisole or benzene, however, could not be
phosphonylated using this photoredox catalytic protocol owing to
their extremely high oxidation potentials. Interestingly, Boc-
protected anilines were readily phosphonylated providing the
corresponding phosphonates in good yields (2g, 65% and 2h,
63%). NH-Boc protected dopamine was phosphonylated with
ease to access the product 2i in excellent yield. Arenes bearing
electron withdrawing groups, such as anilide, showed excellent
reactivity under the optimized photoredox C–P coupling protocol
providing the product 2j in 86% isolated yield. Polyaromatic
hydrocarbons (PAH’s) such as 2-methoxy naphthalene or
pyrene were phosphonylated in good isolated yields (90–98%).
Note that pyrene produced separable mono-phosphonylated and
di-phosphonylated products 2l and 2m in overall 96% yield
(2l:2m 1:1). Importantly, alkynes, present in the substrates as a
functional group, are not oxidized under this photoredox catalytic
protocol. For example, when 1,3-bis(prop-2-yn-1-yloxy)benzene,
bearing two alkyne groups, was used as a substrate the
Scheme 2. Scope of arenes and polyromantic hydrocarbons for the C–H
phosphonylation reaction: Amount of starting material 0.1 – 0.3 mmol. For
detailed information on the reaction conditions, see the supporting information.
Next, we explored the visible light mediated C–H
phosphonylation of heteroarenes (see Scheme 3). To our
delight, a range of electron rich heteroarenes were cleanly
converted into their corresponding phosphonylated products
using this photoredox C–H phosphonylation protocol.
Five membered heterocycles such as 3,4–dimethoxy
thiophene showed very good reactivity and provided the
phosphonylated product 6a in 75% yield. Pyridine derivatives, a
common scaffold of medicinally important compounds, such as
2,6–dimethoxypyridine were converted into their corresponding
phosphonylated products in excellent yields (up to 93% yield).
Among other nitrogen containing heterocycles, indole or
substituted indoles, which are an important structural motif in
]
pharmaceutical agents[15 react well as substrates. Unprotected
indole and 7-methyl indole easily underwent C–P coupling and
delivered the phosphonylated indoles in good yields (6c and 6d,
80% and 66% respectively). The electron rich derivative, 5-
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methoxy indole gave easily separable phosphonylated products
6e and 6ee (positional isomers) in overall 70% yield (6e:6ee
29:41). Interestingly, 3-methylindole produced the mono-
phosphonylated indole 6g (65%) and the di-phosphonylated
indole 6gg (29%) in overall 94% yield.
suggests that the photoredox cycle proceeds via a photoinduced
electron transfer from the (hetero)arene to the excited ruthenium
complex and not via a Ru(III) intermediate.
Finally, we applied the new photoredox catalytic protocol to
the late stage direct C–H phosphonylation of biomolecules,
including functionalized synthetic drug molecules. The optimized
reaction conditions were used for the phosphonylation of
tryptamine functionalized Gemfibrozil, a lipid lowering drug. The
phosphonylated product 6f was obtained in good yield (69%)
without affecting the amide and the ether moieties. Similarly, N-
Boc tryptamine and protected L-tryptophan were readily
converted to their corresponding phosphonylated products with
high regioselectivity and good yields (6h in 55% and 6i in 61%).
Fig. 1. (A) Changes in the luminescence spectra (in this case intensity) of
[Ru(bpz)3][PF6]2 upon successive addition of 1,2,4-trimethoxybenzene in
acetonitrile. In the inset, the changes in the absorption spectra of
[Ru(bpz)3][PF6]2 upon successive addition of 1,2,4-trimethoxybenzene are
shown. Note that the absorption spectra of [Ru(bpz)3][PF6]2 are almost
unaltered (other than the absorption arising below 360 nm due to the
absorption of 1,2,4-trimethoxybenzene) upon addition of 1,2,4-trimethoxy-
benzene demonstrating no interaction of 1,2,4-trimethoxybenzene with
[Ru(bpz)3][PF6]2 in the ground state. Figure (B) shows the Stern–Volmer
quenching plots of [Ru(bpz)3][PF6]2 in the presence of different additives
(1,2,4-trimethoxybenzene, 1,2-dimethoxybenzene, P(OEt)3, and anisole) in
acetonitrile. For the related changes in the luminescence spectra of
[Ru(bpz)3][PF6]2 in the presence of P(OEt)3, 1,2-dimethoxybenzene, and
anisole see the supporting information (Fig. S1, S2, and S3).
Scheme 3. Scope of heteroarenes for the C–H phosphonylation reaction:
Starting material in all cases 0.1 – 0.3 mmol; isolated product yields are given.
For detailed information on the reaction conditions, see the supporting
information.
Spectroscopic investigations gave valuable insight into the
mechanism of the photoredox catalytic cycle. The luminescence
of [Ru(bpz)3][PF6]2 (in this case intensity) is not quenched upon
addition of P(OEt)3. This suggests that the oxidation of the
investigated substituted arene or heteroarenes is the key step
initiating the C–H phosphonylation reaction. Indeed, the
luminescence of [Ru(bpz)3][PF6]2 is quenched in the presence of
1,2,4-trimethoxybenzene and 1,2-dimethoxy benzene having
Experimental results, spectroscopic investigations along
with the literature reports support the proposed photoredox
catalytic mechanism depicted in Scheme 4. Upon
photoexcitation, [RuII(bpz)3][PF6]2 (1) produces an excited state
]
[*RuII(bpz)3][PF6]2 (2),[17 which accepts an electron from the
substituted (hetero)arene substrates (7) converting them into the
corresponding (hetero)aryl radical cation (8). Ammonium
persulfate, (NH4)2S2O8, present in the reaction mixture could
take an electron from the reduced Ru1+ species to complete the
catalytic cycle yielding a sulfate dianion (5) and the sulfate
]
oxidation potentials of +1.12 V and +1.45 V vs SCE,[16
respectively. The values are lower or very similar to the
estimated excited state oxidation potential of [Ru(bpz)3][PF6]2
(+1.45 V vs SCE in acetonitrile).[13a] Anisole, however, does not
quench the luminescence of [Ru(bpz)3][PF6]2 and did not give
any phosphonylation product under the photoredox catalytic
conditions. Notably, anisole has an oxidation potential of 1.76 V
vs. SCE, which is higher than the estimated excited state
oxidation potential of [Ru(bpz)3][PF6]2, but lower than the ground
state oxidation potential of Ru3+ (+1.86 V vs. SCE in ACN). This
]
radical anion (6)[18 . The aryl radical cation (8) reacts with
nucleophilic triethyl phosphite to form the unstable intermediate
9. Hydrogen atom abstraction by SO4•– leads to the formation of
a transient aryl phosphonium intermediate (10), which upon
nucleophilic reaction in the presence of 5 and rearrangement
gives the desired product 11.[12b] The formation of ethyl and
diethyl sulfate as byproducts was confirmed by the gas
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yellowish–brown precipitate was observed during the progress
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]
mixture (acetonitrile as solvent) and probably in equilibrium with
•–
[Ru(bpz)3][PF6]2, excitation followed by an extrusion of SO4
could also oxidize the investigated arenes leading to their radical
cations. Such catalytic mechanism that could work alongside
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described in Scheme 4 is shown in the supporting information,
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In conclusion, we have developed
a
visible light
photoredox catalytic method for direct C–H bond
phosphonylation of arenes and heteroarenes utilizing an
oxidative quenching cycle of [Ru(bpz)3][PF6]2 under very mild
reaction conditions at ambient temperature. The method has a
broad scope with respect to both electron rich arenes and
heteroarenes and trialkyl phosphites producing a variety of aryl
phosphonates with excellent yields. Remarkably, this method
allows a C–H bond phosphonylation of tryptophan based amino
acid derivatives and tryptamine derived bioactive compounds.
We believe that the new photoredox catalytic protocol for direct
C–H bond phosphonylation will find applications in the synthesis
of pharmaceuticals and for the late state functionalization of
bioactive molecules, including drugs.
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Keywords: Photoredox catalysis • Phosphonylation • C–H
activation • Triethyl phosphite • Late-stage functionalization
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (GRK1626) for
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10.1002/chem.201701283
Chemistry - A European Journal
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