Rhodium-Catalyzed Direct C-H Phosphorylation of (Hetero)arenes Suitable for Late-Stage Functionalization

Article abstract of DOI:10.1002/adsc.201600014

Efficient rhodium-catalyzed direct C-H phosphorylation of (hetero)arenes was developed. Various directing groups and a wide range of substrates, including heterocycles, can be utilized in this C-H phosphorylation process, allowing for the rapid installation of the phosphonate group into medicinally and biologically important privileged scaffolds. The efficient and straightforward method could serve as a new tool to streamline late-stage C-H functionalization for preparing aryl phosphonates, which are important structural motif in synthetic and medicinal chemistry.

Full text of DOI:10.1002/adsc.201600014

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DOI: 10.1002/adsc.201600014
Very Important Publication
Rhodium-Catalyzed Direct C–H Phosphorylation of (Hetero)-
arenes Suitable for Late-Stage Functionalization
Minsik Min,a,b Dahye Kang,a,b Sungwoo Jung,a,b and Sungwoo Honga,b,*
a
Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea
Fax: (+82)-42-350-2810; Phone: (+82)-42-350-2811; e-mail: hongorg@kaist.ac.kr
Center for Catalytic Hydrocarbon Functionalization, Institute for Basic Science (IBS), Daejeon, 305-701, Korea
b
Received: January 5, 2016; Revised: February 8, 2016; Published online: March 4, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600014.
Abstract: Efficient rhodium-catalyzed direct C–H serve as a new tool to streamline late-stage C–H
phosphorylation of (hetero)arenes was developed. functionalization for preparing aryl phosphonates,
Various directing groups and a wide range of sub- which are important structural motif in synthetic and
strates, including heterocycles, can be utilized in this medicinal chemistry.
C–H phosphorylation process, allowing for the rapid
installation of the phosphonate group into medicinal-
ly and biologically important privileged scaffolds. Keywords: catalyst; C–H activation; late-stage modi-
The efficient and straightforward method could fication; phosphorylation; rhodium
Introduction
Although these methods remarkably improved the
efficiency of direct C–H phosphorylation, the prece-
Arylphosphonate derivatives have been extensively dent examples were limited to substrates bearing non-
investigated in the field of medicinal chemistry,[1] ma- removable, pyridine-based directing groups. As
terial chemistry,[2] catalysis and ligand chemistry.[3] a result, synthetic applications for rapidly exploring
Since the development of the Hirao reaction,[4] Pd- structure-activity relationships (SAR) of a variety of
À
catalyzed carbon phosphorus bond formation has biologically active molecules are not feasible. In this
provided an efficient route to synthesize arylphospho- regard, the development of new catalytic systems is
nate derivatives.[5] Despite the significant progress in highly desirable for expanding the scope and utility of
the area of Pd-catalyzed C–H functionalization,[6] C–H phosphorylation. Herein, we describe the first
III
À
À
direct C P bond formation via directed C–H activa- example of Rh -catalyzed, C P bond formation of
tion poses challenges because of the strong coordina- (hetero)arenes (Scheme 1). Significantly, various di-
tion ability of the phosphorus reagent to transition recting groups (oxime, pyridine, anilide, amide, and
metals.[7] In 2013, the Yu group reported on pyridine imine) could be applied in this new C–H phosphoryla-
directed C–H phosphorylation using a Pd catalyst and tion reaction, which would be suitable for late-stage
by slowly adding a phosphorus reagent using a syringe functionalization of complex bioactive small mole-
pump.[8] The Murakami group successfully explored cules.
Pd-catalyzed C–H phosphorylation, employing a-hy-
droxylalkylphosphonate as the masked phosphonating
agent to prevent catalyst deactivation (Scheme 1).[9]
Results and Discussion
We commenced our study by investigating the reactiv-
ity of diethyl phosphonate with substrate 1a bearing
O-methyl oxime group which is easily removable or
transformable into other functional groups. The feasi-
bility of this process was tested with a Cp*RhIII cata-
lyst that has been widely used in many bond-forming
reactions.[10,11] When 1a was subjected to conditions
using [RhCp*Cl2]2 as a catalyst, only negligible reac-
Scheme 1. Direct C–H phosphorylation
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Rhodium-Catalyzed Direct C–H Phosphorylation
Table 1. Optimization of the reaction conditions[a]
Entry
Cat. (5 mol%)
Additive
Oxidant
Solvent
Yield (%)[b]
1[c]
2
3
4
5
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*(CH3CN)3](SbF6)2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
Pd(OAc)2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
–
AgNTf2
AgSbF6
AgPF6
AgNTf2
AgNTf2
NMMI
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
AgOAc
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
AgOAc
1,2-DCE
dioxane
DCM
trace
11
20
54
39
trace
44
41
70
87
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE/DCM (1:1)
tBuOH
6
7
8
9[d]
10[d]
11[e]
15
[a]
Reactions were performed by using 1a (0.1 mmol), 2a (1.4 equiv), catalyst, additive (0.4 equiv), oxidant (2.5 equiv) and
solvent (1.0 mL) at 1208C for 8 h.
Yields were determined by H NMR.
Diethyl phosphonate (1.4 equiv) was used instead of 2a.
Additive (40 mol%) was used.
[b]
[c]
[d]
[e]
1
Pd(OAc)2 (0.1 equiv), NMMI (0.4 equiv) and K2HPO4 (4.5 equiv) were added. NMMI=N-Methylmaleimide
tivity was observed (Table 1, entry 1). Inspired by (5 mol%), AgNTf2 (40 mol%), and Ag2CO3
Murakamiꢁs recent work, we turned our attention to (2.5 equiv) in 1,2-DCE/DCM at 1208C generated
a-hydroxylalkylphosphonate as
a phosphorylating product 3a in an 87% yield. A control experiment in
agent to prevent poisoning of Rh catalyst. To our de- the absence of [RhCp*Cl2]2 catalyst was carried out,
light, the use of a-hydroxylalkylphosphonate (2a) was and no desired product was obtained (see SI).
found to initiate the phosphorylation reaction and
With the optimized conditions in hand, we next in-
provided the desired product 3a, albeit only in 11% vestigated the scope of both oxime substrates and
yield (entry 2). After extensive screenings, 1,2-DCE phosphorylation reagents (Table 2). Various moieties
was found to be the optimal solvent, and the product of phosphonates, such as ethyl, isopropyl, butyl, isobu-
yield dramatically increased to 54% (entry 4). The re- tyl, benzyl, and 2-ethylhexyl groups, were surveyed
action only occurred at the sterically more accessible under the optimized conditions and showed similar
À
C H bond, generating a monophosphorylated prod- efficiency. The phosphorylation reaction, however,
uct. The catalyst [RhCp*(CH3CN)3](SbF6)2 gave was not operative with diaryl phosphine oxides.
a lower yield (entry 5). The oxidizing agent was also Oxime derivatives bearing either electron-rich (4c,
critical to the efficiency of the transformation, and no 4d, 4e, 4 f, 4k, 4l, and 4m) or electron-deficient
detectable desired product was obtained with the use groups (4g, 4h, 4i, and 4j) are compatible to the C–H
of AgOAc or Cu(OAc)2 (entry 6 and see the Support- phosphorylation reaction to provide the desired cou-
ing Information). Changing the silver additive to pling products in moderate to good yields. Consistent
AgSbF6 or AgPF6 slightly diminished the reaction ef- with the previous observation, only monophosphory-
ficiency (entries 7 and 8). PdII catalytic systems which lated products were observed. In addition, the reac-
are known to facilitate C–H phosphorylation reac- tion showed high functional group tolerance, includ-
tions of arylpyridines, were considerably less effective ing ethers (4d, 4e, 4k, 4m, and 4t), halides (4g, 4h,
À
in the C P bond formation of oxime substrate and 4i), acetate (4 f), and ester (4j). Notably, bromo
(entry 11). Other catalytic systems, including [Ru(p- and iodo groups were intact under the reaction condi-
cymene)Cl2]2 and [IrCp*Cl2]2, were not operative (see tions, enabling further functionalization at these posi-
SI). Further optimization studies revealed that the tions. When meta-substituted oximes were used, most
best result was obtained when the reaction was per- of the phosphorylation occurred selectively at the
formed in a 1:1 mixture of 1,2-DCE and DCM as the sterically more accessible position (3a–3 f, 4k, and
co-solvent (entry 10). Under the optimized reaction 4s). In contrast, for the reaction of 3,4-(methylene-
conditions, the C–H phosphorylation process of 1a dioxy)ketoxime (1l) with 2a, phosphorylation takes
with 2a (1.4 equiv) in the presence of [RhCp*Cl2]2 place site-selectively at a sterically more hindered po-
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Minsik Min et al.
Table 2. Substrate scope studies[a]
Table 3. Substrate scope of directing groups[a]
[a]
Reactions were carried out by using 1 (0.2 mmol), 2a
(1.4 equiv), [RhCp*Cl2]2 (5 mol%), AgNTf2 (40 mol%),
Ag2CO3 (2.5 equiv) and 1,2-DCE/DCM (1 mL : 1 mL) at
1208C for 8 h.
Reactions were carried out by using 1,2-DCE (2 mL) at
1208C for 2 h.
[a]
[b]
Reactions were carried out by using 5 (0.1 mmol), 2
(1.4 equiv), [RhCp*Cl2]2 (5.0 mol%), AgNTf2
(20 mol%), Ag2CO3 (2.5 equiv) and DCE (1 mL) at
1208C for 8 h.
[RhCp*Cl2]2 (2.5 mol%), and AgNTf2 (10 mol%) were
used.
THF (1 mL) was used.
[RhCp*Cl2]2 (8 mol%), AgNTf2 (40 mol%) and iPrOAc
(1 mL) were used.
iPrOAc (1 mL) was used.
PhCF3 (1 mL) was used.
[RhCp*Cl2]2 (8 mol%), and AgBF4 (40 mol%) were
used. Isolated yields are shown.
[b]
[c]
8 mol% of [RhCp*Cl2]2 was used. Isolated yields are
shown.
[c]
[d]
sition, affording 4l. In addition, the use of ortho-sub-
stituted ketoxime (1m) provided the phosphorylated
product 4m in a yield of 84%. The scope of this
transformation with respect to heteroarene substrates
was next evaluated, and the desired products were ob-
tained in good yields (4o, 4p, 4q, and 4r). Naphthyl
substrate was also suitable for this transformation,
generating the desired product in an 86% yield (4s).
[e]
[f]
[g]
Oxime substrate derived from aldehyde was also com- ene substrates, and the utility of the present method
patible to the reaction condition to afford the phos- was broadened by the phosphorylation of prominent
phorylated product 4t.
structural motifs, including indole, isoquinolone, thio-
For broad utility, the scope of other directing phene, and benzofuran. We further investigated addi-
groups was investigated (Table 3). Considering the tional directing groups and were pleased to observe
frequency of heterocyclic moieties within biologically that Rh-catalyzed phosphorylation of anilides (6g–k)
active molecules, we subsequently explored heteroar- and amides (6n–p) worked well in the optimized
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Rhodium-Catalyzed Direct C–H Phosphorylation
system to produce the desired products. In a similar
fashion, imines underwent phosphorylation reactions
without difficulty, generating the corresponding prod-
ucts (6l, and 6m).
The utility of the direct C–H phosphorylation reac-
tion as a tool for late-stage functionalization of com-
plex molecules is significant. Therefore, we next as-
À
sessed the potential that our method for C H bond
phosphorylation could be applied to bioactive com-
plex molecules containing several functional groups.
To our delight, the phosphorylation of diazepam 7a
(tranquilizer) and halazepam 7b (anti-anxiety drug)
site-selectively took place under the present catalytic
system to yield desired products (Scheme 2a, 2b). Ke-
Scheme 3. Preliminary mechanistic studies.
toxime 7c that was derived from zaltoprofen (anti-in- would be reversible. In addition, kinetic isotope effect
flammatory drug) was subjected to phosphorylation (KIE) experiments were performed under the opti-
conditions to afford the corresponding product 8c mized conditions (Scheme 3b). A low values of inter-
(Scheme 2c). In addition, the phosphorylation of 6-ar- molecular kinetic isotope effect were observed in
ylpurine 7d proceeded with excellent ortho-selectivity both parallel reactions (kH/kD =1.1) and competitive
À
to produce 8d (Scheme 2d).
reaction (kH/kD =1.2), implying that the C H bond
To elucidate the mechanism of the phosphorylation cleavage may not be involved in the rate-limiting
process, preliminary experiments were carried out by step.[12]
means of H/D exchange experiments (Scheme 3). A
A plausible mechanism for the Rh-catalyzed phos-
significant level of H/D exchange was observed by phorylation process is shown in Scheme 4. The acti-
the rhodium catalytic system in the presence of vated cationic RhIII species is generated in situ by the
CD3OD, indicating that the C–H activation step [RhCp*Cl2]2/AgNTf2 catalytic system. Subsequently,
Cp*RhIII catalyst reacts with the O-methyl oxime 1a
via C–H activation, providing the five-membered rho-
dacycle complex I with one accessible vacant site.[13]
Then, a-hydroxylalkylphosphonate (2a) would
occupy the vacant site of the metal center to generate
complex II. Next, b-phosphonate elimination,[14] with
the concomitant release of acetone, leads to the for-
mation of complex III, which undergoes reductive
elimination to afford the desired phosphorylation
product. The oxidation of RhI by AgI regenerates the
RhIII species to complete the catalytic cycle.
Scheme 2. Late-stage C–H bond phosphorylation.
Scheme 4. Proposed Rh-catalyzed phosphorylation pathway.
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Minsik Min et al.
1
was analyzed by integration of H NMR using internal stan-
dard. Product yield of 4b was 36% and that of [D4]-4b was
33%, leading to KIE value of 1.1.
Conclusions
In summary, we developed an efficient RhIII-catalyzed
C–H phosphorylation reaction, which broadens the
utility and its substrate scope. Various directing
groups were applicable in the process, providing an
efficient and straightforward route for preparing aryl
phosphonate derivatives that have high synthetic utili-
ty. The present strategy has been verified in late-stage
functionalization for the rapid derivatization of me-
dicinally important molecules. Further application to
the late-stage drug modification and bioevaluation
are currently ongoing.
b) Competitive experiment: O-methyl oxime (1b)
(0.05 mmol) and O-methyl oxime ([D5]-1a) (0.05 mmol)
were added to same screw cap test tube with diethyl (2-hy-
droxypropan-2-yl)phosphonate (1.4 equiv), [RhCp*Cl2]2
(5 mol%), AgNTf2 (40 mol%), Ag2CO3 (2.5 equiv) and 1,2-
DCE/DCM (0.5 mL/0.5 mL). The reaction mixture was
stirred at 1208C for 1 h. The reaction mixture was diluted
and filtered through Celite with DCM. The residue was con-
centrated, and evaporated to dryness under high vacuum.
The ratio of 4b/[D4]–4b was analyzed by integration of
1H NMR to give a KIE value of 1.2.
Experimental Section
Acknowledgements
General procedure for RhIII-catalyzed
phosphorylation
This research was supported financially by Institute for Basic
Science (IBS-R010-G1). M. Min is the recipient of a Global
Ph.D Fellowship (NRF-2011–0007511).
Caution in required because of the internal pressure at
1208C. The reactions were conducted in Ace pressure tubes
(Aldrich). O-methyl oxime (0.2 mmol), diethyl (2-hydroxy-
propan-2-yl)phosphonate
(1.4 equiv),
[RhCp*Cl2]2
References
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material, the reaction mixture was diluted and filtered
through Celite with DCM. The organic layer was dried over
MgSO4. After removal of solvent, the residue was purified
by flash chromatography on silica gel to give desired prod-
uct. Full characterization data and copies of relevant spectra
are provided in the Supporting Information.
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Caution in required because of the internal pressure at
1208C. The reactions were conducted in Ace pressure tubes
(Aldrich). O-methyl oxime (1d) (0.1 mmol), [RhCp*Cl2]2
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General procedure for kinetic isotope effect study
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O-methyl oxime ([D5]-1a) (0.1 mmol) were added to two
separate screw cap test tubes with diethyl (2-hydroxypro-
pan-2-yl)phosphonate (1.4 equiv), [RhCp*Cl2]2 (5 mol%),
AgNTf2 (40 mol%), Ag2CO3 (2.5 equiv) and 1,2-DCE/DCM
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