Visible-Light photo-Arbuzov reaction of aryl bromides and trialkyl phosphites yielding aryl phosphonates

Article abstract of DOI:10.1021/acscatal.6b02591

Aryl phosphonates are functional groups frequently found in pharmaceutical and crop protection agents. For their synthesis via C?P bond formation typically transition-metal-catalyzed reactions are used. We report a visible-light photo-Arbuzov reaction as an efficient, mild, and metal-free alternative. Rhodamine 6G (Rh.6G) is used as the photocatalyst, generating aryl radicals under blue light. Coupling of the radicals with a wide range of trivalent phosphites gives aryl phosphonates in good to very good isolated yields. The mild reaction conditions allow the introduction of a phosphonate group into complex and sensitive pharmaceutically active molecules such as benzodiazepams and nicergoline by the activation of a carbon?halogen bond.

Full text of DOI:10.1021/acscatal.6b02591

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Article
Visible Light Photo-Arbuzov Reaction of Aryl Bromides
and Trialkyl Phosphites Yielding Aryl Phosphonates
Rizwan Shaikh, Simon Duesel, and Burkhard König
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02591 • Publication Date (Web): 10 Nov 2016
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Visible Light Photo‐Arbuzov Reaction of Aryl Bromides and Trialkyl
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Phosphites Yielding Aryl Phosphonates
Rizwan S. Shaikh, Simon Düsel, Burkhard König*
Faculty of Chemistry and Pharmacy, University of Regensburg, D‐93040 Regensburg, Germany.
ABSTRACT: Aryl phosphonates are functional groups frequently found in pharmaceutical and crop protection agents. For
their synthesis via C‐P bond formation typically transition metal catalyzed reactions are used. We report a visible light
photo‐Arbuzov reaction as an efficient, mild and metal‐free alternative. Rhodamine 6G (Rh.6G) is used as photocatalyst
generating aryl radicals under blue light. Coupling of the radicals with a wide range of trivalent phosphites gives aryl
phosphonates in good to very good isolated yields. The mild reaction conditions allow the introduction of a phosphonate
group into complex and sensitive pharmaceutically active molecules, such as benzodiazepams or nicergoline by the acti‐
vation of a carbon halogen bond.
KEYWORDS: Rhodamine 6G, Phosphonate, Photo‐Arbuzov, Visible light, C‐P bond formation.
eration of reactive aryl radicals, which serve as arylating
agents to suitable precursors. We have recently introduced
the photoredox catalyzed generation of transient aryl radi‐
cals from electron deficient arenes, such as diazonium salts14
or perfluorophenyl bromide15 by a visible light PET process.
Using a consecutive photoinduced electron transfer process
(conPET)16 the scope of starting materials for the generation
of aryl radicals was extended to less reactive aryl bromides or
aryl chlorides bearing very negative reduction potentials.17
The xanthene dye rhodamine 6G (Rh.6G)18 is a particular
useful conPET photocatalyst allowing the chromoselective C‐
X bond activation.19
INTRODUCTION
Aryl phosphonates and phosphine oxides are structural mo‐
tifs found in many pharmaceutically active molecules (Figure
1). In addition, they find applications in organic materials
and as synthetic intermediates.1 The phosphorus containing
moiety act as ligand coordinating to transition metals or
binding to biological receptors and thereby regulating physi‐
ological processes or material functions.2 The broad use of
organophosphorus compounds in different areas of science
triggered the development of many methods for their prepa‐
ration (Scheme 1). Classic synthesis of aryl phosphonates
involves transition metal catalyzed cross coupling of
R2P(O)H with a coupling partner via P‐H bond activation.3
Palladium‐catalysed C‐P coupling has been well established
using various precursors such as aryl halides,4 triflates,5 bo‐
ronic acids,6 aryl sulfonates,7 and triarylbismuth.8 Other
methods involve nickel and copper catalysed C‐P bond for‐
mation,9 but harsh reaction conditions or substrate scope
limitations call for the development of new routes for the
synthesis of aryl phosphonates. Recently, a combination of
transition metal and visible light catalysis was used for the
synthesis of aryl phosphonates via C‐X bond activation, but
expensive metal complexes and iodobenzene or diazonium
salts are required as starting materials.10
Previous work
O
OEt
O
P
X
a)
P
Cat. Ni, Pd, or Cu
H
H
OEt
OEt
+
+
OEt
X = I, Br, OTf
X
O
P
R
R
O
Cat. Pd
R
P
R
b)
X = OTs, OMs, B(OH)2
O
O
This work
O
HCl
N
N
O
H
Br
P
OR
Rh-6G
Ar
1
Ar
2
OR
P(OR)3, DMSO, DIPEA, 25 °C
455 nm Blue LED
Scheme 1. Rh.6G catalyzed mild C‐P bond formation via
C‐Br bond activation.3,4,5,6,7,8,9
Upon irradiation with blue light (455 nm) in the
presence of an electron donor under nitrogen atmosphere
Rh.6G yields the corresponding excited radical anion
Figure 1. Aryl Phosphonates in pharmaceutical agents.1,2
(Rh.6G˙ˉ*),20,21 which has
a reduction potential of
Visible light photoredox catalysis has emerged into a useful
ca. ‐2.4 V vs SCE19 that is sufficient to activate aryl hal‐
ides.16, 22 The initial aryl halide radical anion may cleave
into a halide anion and an aryl radical, which can further
react with different coupling partners.
tool for the C‐X bond activation by functionalization via
12
,
photoinduced electron transfer (PET) processes.11 13 A high‐
,
ly anticipated application of visible light catalysis is the gen‐
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RESULTS AND DISCUSSION
bromobenzonitrile in DMSO in the presence of Rh.6G
and N,N‐diisopropylethylamine (DIPEA), as sacrificial
electron donor, under nitrogen atmosphere to generate a
highly reactive p‐cyano phenyl radical. Trapping of the
radical with triethyl phosphite delivered the correspond‐
ing diethyl (4‐cyanophenyl) phosphonate in 71% (see
table 1, entry 3, 4, 5). Reactions were also performed in the
presence of air (entry 8), which reduced the product yield
significantly, and under oxygen (entry 9), giving only
traces of the product. Typical test reactions were run at
0.1 – 0.3 mmol scale, but larger scales are possible. A reac‐
tion done at 2.20 mmol scale gave 52% isolated product
and in a flow reactor 76 % product yield.
As part of our efforts towards the development of
mild methods for C‐X bond activation, we report here the
visible light photoredox catalyzed synthesis of aryl phos‐
phonates via photo‐Arbuzov reaction. The photo‐Arbuzov
reaction is a variation of the classical Michaelis‐Arbuzov
reaction,23 and was first demonstrated in 1966 using UV
light (quartz mercury vapor lamp) at low temperature and
mostly aryl iodides as substrates. The obtained product
yields were low to good in the range of 34‐90%.23 To im‐
prove the method we aimed to develop a more broadly
applicable radical phosphonylation reaction employing
visible light photocatalysis using Rh.6G and commercially
available bench stable aryl halides.
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The same reaction conditions with different tri‐
ethyl phosphite concentrations did not show much influ‐
ence on the isolated yield. The reaction in CH3CN gave
excellent conversion, but it was limited to the standard
substrate. In the absence of DIPEA only traces of the
desired product were observed. This might be rational‐
ized by a single electron transfer from the trapping rea‐
gent to the excited photocatalyst, but the reaction pro‐
ceeds very sluggish with minor amounts of product
formed.
Table 1. Optimization of the reaction conditions for
the photocatalyzed Photo‐Arbuzov reaction.
Entry
Solvent
Time
(hrs)
P(OEt)3
Yield %[a]
1
2
DMSO
DMSO
DMSO
DMSO
DMSO
CH3CN[b]
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
6
20 eq.
20 eq.
20 eq.
3 eq.
35
52
12
17
17
18
18
18
18
18
18
8
3
71
4
76
5
5 eq.
76
5
20 eq.
20 eq.
20 eq.
3 eq.
93
6[c]
7[d]
8[e]
9[f]
10[g]
11[h]
12[i]
13[j]
34
0
36
3 eq.
Traces
76
3 eq.
21
18
18
3 eq.
52
3 eq.
40
28
[a] Yield of product with 20 eq. of the trapping reagent. [b] Yield of the prod‐
uct with 3 eq. of the trapping reagent. [c] 15% of the Rh.6G used. For detailed
information on the reaction conditions, see the supporting information.
3 eq.
[a] Isolated yield. [b] Substrate specific. [c] Reactions performed in the
absence of DIPEA. [d] Reactions performed in the absence of light. [e]
Without degassing [f] Under O2 atmosphere [g] In PTFE flow reactor.
P(OEt)3 = triethyl phosphite. [h] Reaction performed with 2.2 mmol. [i]
Reaction with 5% Ru(bpy)3Cl2 x 6 H2O as photocatalyst [j] Reaction with
5% Ir(ppy)3 as photocatalyst. DIPEA = N,N’‐disopropylethylamine. LED =
light emitting diode. *10 mol % of photocatalyst was used unless mentioned
otherwise. Reactions were monitored using gas chromatography.
Scheme 2. Scope or aryl bromide for Photo‐Arbuzov
reaction.
Based on the optimized reaction conditions, we ex‐
plored the scope of visible light induced phosphonylation
of aryl bromides further.
We began our investigations towards a visible light
mediated photo‐Arbuzov reaction with irradiating 4‐
The reaction proceeds smoothly with a wide func‐
tional group tolerance. Mostly, 10‐15 mol% loading of the
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catalyst was required for a complete conversion of the
In addition, electron rich heterocycles such as 6d,
6e, 6f, and 6g gave excellent conversion. Moreover, iso‐
quinoline derived substrates such as 6h and 6i showed
exceptionally good conversion to the corresponding
phosphonates 7h and 7i giving 94 and 92% yield, respec‐
tively, in 33 h. (Scheme 3).
substrate. Precursors bearing nitrile, ester, methoxy, vinyl
functionality (1a‐1l) gave the phosphonylated product in
good yield. In general, electron deficient arenes (1a, 1b,
1g, 1h, 1i) showed higher conversion to the corresponding
phosphonylated product than electron‐rich arenes (1c, 1d,
1j). Contradictory to the observation, 4‐bromobenzo‐
trifluoride (1e) gave the corresponding phosphonate in a
yield of only 27%. One‐pot conversion of 1,4 dibromoben‐
zene (1k) to the corresponding di‐phosphonate was
achieved in moderate yield. Also, 4‐bromostyrene (1j)
gave the interesting building block 2j in good yield. Ap‐
parently, bromobenzonitriles (1a, 1b) are more reactive
substrates in the photocatalytic phosphonylation reac‐
tion, with comparatively shorter reaction time (17h) re‐
quired for a complete conversion.
Next, we examined aryl chloride and aryl triflates
as aryl radical precursors under the optimized conditions
(Scheme 4). A longer reaction time was required for the
conversion of p‐chlorobenzonitrile (10a) to the corre‐
sponding phosphonate, because of the more negative
reduction potential of the substrate.16,21 The reaction with
aryl chlorides and aryl triflates gave the corresponding
phosphonate 12ac in 61% and 12ad in 41% yield. A triflate
derivative of the natural product Carvacrol (11c) was con‐
verted to its diethyl phosphonate analogue 12cd. The di‐
methoxy‐protected uracil 10d gave the expected product
12d in 86% yield. This compound can be further convert‐
ed to the phosphoric acid analogue of uracil.24
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Different phosphites, such as P(OMe)3, P(OPh)3 and
P(OiPr)3, were coupled with aryl bromides under the
optimized reaction conditions (Scheme 2, entry 2g, 3a‐5a,
3h, 3i, 2l). Varying electronic properties and steric bulk
were tolerated: aryl‐dimethyl, diphenyl and di‐isopropyl
phosphonates were synthesised in 62 to 80% yield.
Subsequently, we explored the use of heteroaryl
bromides in this photo‐Arbuzov reaction protocol. To our
delight, a range of six membered and fused heterocyclic
bromides were suitable for the coupling with triethyl
phosphite as well as other trialkyl phosphites (Scheme 3)
in moderate to excellent yield. The ability of the catalytic
system to activate electron‐rich as well as electron‐
deficient heteroarenes demonstrates a remarkable scope.
Bromopyridine analogues 6a and 6b showed fast conver‐
sion to the corresponding phosphonates (14h) with 75‐
78% yield.
Amount of starting material in all cases 0.1 – 0.3 mmol, reaction time 24‐68 h.
[a] Yield of product with 20 eq. of trapping reagent. [b] 15% of the Rh.6G used.
[c] Aryl chloride as a substrate. [d] Aryl triflate as substrate. [e] Yield of prod‐
uct with 3 eq. of trapping reagent. For detailed information on the reaction
conditions, see the supporting information.
Scheme 4. Scope of aryl chloride and aryl triflates for
the visible light induced phosphonylation.
Encouraged by the results, we applied the devel‐
oped conditions to the late stage metal‐free phosphonyla‐
tion of pharmaceutically active molecules (Scheme 5). We
chose bromazepam, a benzodiazepine derivative and anti‐
anxiety agent and nicergoline, an ergot alkaloid used for
the treatment of metabolic vascular disorders, both con‐
taining a bromide group in the aryl ring.
The phosphonylation of bromazepam 13 gave the
Starting material in all the cases 0.1 – 0.3 mmol, reaction time 15‐33 h. [a] Yield
of product with 20 eq. of trapping reagent. [b] Yield of the product with 3 eq.
of the trapping reagent. [c] 15% of the Rh.6G used. For detailed information on
the reaction conditions, see the supporting information.
respective azepam phosphonate (14) in 37% isolated yield,
tolerating the amide and imine functional groups. Like‐
wise, the reaction of highly functionalized nicergoline 15
yielded the phosphonylated product in 66%.
Scheme 3. Scope of visible light induced phospho‐
nylation of heteroaryl bromides.
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fully excluded at the present stage of the mechanistic
investigations.[26]
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CONCLUSION
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We have demonstrated the metal‐free visible light driven
phosphonylation of aryl halides to their corresponding
phosphonates. The mild reaction conditions are compati‐
ble with many functional groups and the scope of trialkyl
phosphites and halogenated heteroarenes is broad provid‐
ing various phosphonates with consistent yields. Fur‐
thermore, the method allows a late stage functionaliza‐
tion of biological active aryl halides that can be converted
into the phosphoric acid analogues. We believe this new
photoredox C‐P bond formation via a visible light photo‐
Arbuzov reaction will find applications in pharmaceutical
and academic research.
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Scheme 5. Late stage phosphonylation of biologically
active compounds.
PROPOSED MECHANISM
A proposed mechanism for the visible light induced
Rh.6G catalysed photo‐Arbuzov reaction is outlined in
Scheme 6.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the
http://pubs.acs.org.”
General information, experimental procedure, characteriza‐
1
tion of the compounds, and copies of H NMR, 13C, NMR, 31P
NMR. Reaction setup
AUTHOR INFORMATION
Corresponding Author
E‐mail: Burkhard.Koenig@ur.de
*Fax: (+49)‐941‐943‐1717. Phone: (+49)‐941‐943‐4575.
FUNDING SOURCE
Scheme 6. Proposed mechanism for the visible light
photo‐Arbuzov reaction.
Deutsche Forschungsgemeinschaft (DFG, GRK 1626, Chemi‐
cal Photocatalysis).
Upon blue light photoexcitation Rh.6G photooxidizes
DIPEA giving the radical anion Rh.6G˙ˉ and the radical
cation of DIPEA˙+. Continuous irradiation of the radical
anion Rh.6G˙ˉ with blue light triggers a single electron
transfer from the excited Rh.6G˙ˉ* 17,19,20 to the aryl halide
producing the transient Ar‐Br˙ˉradical anion and regener‐
ating the neutral Rh.6G completing the catalytic cycle.
Further, Ar‐Br˙ˉ undergoes fragmentation to release an
aryl radical and a bromide anion; this process is in com‐
petition with a non‐productive back electron transfer.
The aryl radical reacts with trivalent phosphorous form‐
ing a C‐P bond and the unstable phosphoranyl radical.[25]
Release of an ethyl radical and rearrangement results in
the formation of the target aryl phosphonate. The reactive
ethyl radical may abstract a hydrogen atom from DIPEA˙+
or the solvent. In competition with the desired C‐P bond
forming reaction, the aryl radical can abstract a hydrogen
atom from DIPEA˙+ or the solvent giving the respective
reduced product. This process is confirmed by the gas
chromatography mass spectrometry (GS‐MS) data show‐
ing the formation of diisopropylamine and the dehalo‐
genated arenes as by‐products in the reaction mixture.
However, an alternative mechanism forming the aryl
phosphonate via oxidation of the radical to a phosphoni‐
um intermediate and elimination of ethene cannot be
ACKNOWLEDGMENT
We thank the Deutsche Forschungsgemeinschaft (GRK1626)
for financial support and Dr. R. Vasold and Ms. R. Hoheisel
for GC‐MS and CV measurements.
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