Ir(III)-catalyzed direct C-H functionalization of arylphosphine oxides: A strategy for MOP-type ligands synthesis

Article abstract of DOI:10.1021/acs.orglett.7b02710

Diazo compounds as coupling partners are efficiently applied to Ir(III)-catalyzed direct C-H functionalization of arylphos-phine oxides. Involving C-H activation, carbene insertion, and tautomerism, this reaction proceeds under mild conditions, thus proving an approach to the synthesis of MOP-type ligand precursor in a single step. The utility of this transformation has been further demonstrated in ligand synthesis as well as in the construction of phosphole framework.

Full text of DOI:10.1021/acs.orglett.7b02710

Letter
pubs.acs.org/OrgLett
Ir(III)-Catalyzed Direct C−H Functionalization of Arylphosphine
Oxides: A Strategy for MOP-Type Ligands Synthesis
Zhong Liu,† Ji-Qiang Wu,† and Shang-Dong Yang*,†,‡
†State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
‡State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, PR China
S
* Supporting Information
ABSTRACT: Diazo compounds as coupling partners are efficiently
applied to Ir(III)-catalyzed direct C−H functionalization of arylphos-
phine oxides. Involving C−H activation, carbene insertion, and
tautomerism, this reaction proceeds under mild conditions, thus proving
an approach to the synthesis of MOP-type ligand precursor in a single
step. The utility of this transformation has been further demonstrated in
ligand synthesis as well as in the construction of phosphole framework.
n the past decades, phosphorus ligands bearing a biaryl
substituted 2′-phosphorylbiphenyl-2-ol compounds.8 Despite
such important advances, using simple, cheap triphenylphos-
phine oxide (Ph3PO) to synthesize 2′-phosphorylbiphenyl-2-
ol compounds via the C−H activation appears undoubtedly as a
concise and highly efficient method, but which has yet to
succeed due to uncontrolled regioselectivity and a lack of an
appropriate pattern of activation.
I
scaffold have played a pivotal role in various transition-
metal-catalyzed reactions.1 2′-Phosphorylbiphenyl-2-ol com-
pounds as the P,O-ligand precursor have not only been used
to synthesize different MOP-type ligands,2 but are also easier to
modify and convert into a variety of other ligands or
organocatalysts.3 At present, reported methods (A, Scheme
1) for the synthesis of 2′-phosphoryl biphenyl-2-ol involve:
Transition-metal-catalyzed C−H functionalization involving
a metal carbene migratory insertion process has attracted
significant attention in recent years.9 In this context, diazo
compounds have been successfully employed as versatile and
efficient coupling partners in C−H activation/annulation
reactions.10 Yu and co-workers disclosed the first Rh(III)-
catalyzed intermolecular insertion of C(aryl)−H bonds into
diazo compounds.11 Subsequently, several research groups
made significant progress in the construction of heterocycles by
such approaches.12 Very recently, Kanai, Ackermann, Ellman,
and Glorius demonstrated that the Co(III) catalyst could also
catalyze some reactions similar to Rh(III) catalysts.13 At the
same time, Pd- and Au-catalyzed C−H bond activation with
metal carbene have also been reported by the Martin14 and
Zhang groups,15 respectively. Although Ir(III) has been
extensively used for C−H activations,16 a handful of reports
were available for C−H functionalization by means of
carbenoids and Ir(III)-based catalytic system. Recently, the
Patel group has reported two examples of Ir(III)-catalyzed C−
H annulation of N-hydroxyoximes and benzamides with α-
diazocarbonyl compounds, respectively.17 Wang disclosed a
Ir(III)-catalyzed C−H arylation with quinone diazides to
furnish arylated phenols.18 In addition, Wang has also
demonstrated an Ir(III)-catalyzed coupling of aromatic C−H
bonds with diazomalonates by carbene migratory insertions.19
On the other hand, despite the remarkable advances achieved
Scheme 1. Strategies for the Synthesis of Biarylphosphine
Oxides
phosphinylation of binaphthyl skeletons,4 Diels−Alder cyclo-
addition reaction of tetracyclone and functional aromatic
acetylenes,5 Pd-catalyzed Suzuki−Miyaura coupling between
an aryl metal and an aryl halide or pseudohalides6 and ring
opening of dibenzooxaphosphinine chloride with Grignard
reagent.7 We have also achieved a R2(O)P-directed Pd(II)-
catalyzed C−H direct hydroxylation to synthesize various
Received: August 31, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02710
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ab c
, ,
Table 1. Optimization of Reaction Conditions
entry
cat. (mol %)
Ag salts (mol %)
acids (mol %)
solvent
yield (%)
3a:3a′
1
2
3
4
5
6
7
8
9
[Cp*RhCl2]2 (3)
[Ru(p-cymene)Cl2]2 (3)
[Cp*Co(CO)l2] (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
[Cp*lrCl2]2 (3)
AgSbF6(12)
AgSbF6(12)
AgSbF6(12)
AgSbF6(12)
AgSbF6(12)
AgSbF6(12)
AgBF4(12)
AgNTf2 (12)
AgOTf (12)
AgSbF6(12)
AgSbF6(12)
AgSbF6(12)
AgSbF6(12)
AgSbF6(12)
AgSbF6(12)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
PivOH (20)
AdCO2H (20)
TMBzOH (20)
TMBzOH (15)
DCE
DCE
DCE
DCE
THF
toulene
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
0
−
−
−
−
−
−
−
−
0
0
61
31
40
53
62
59
86
88
88
85
88
88
−
d
10
6:1
14:1
20:1
15:1
>30:1
>30:1
d
11
d
12
d
13
de
,
14
15
df
,
a
b
The reaction was carried out on 0.2 mmol scale in 2.0 mL of DCE at 60 °C under Ar atmosphere for 12 h. Monoarylation:diarylation ratio was
c
d
e
f
determined by NMR. Isolated yields. Addition of 50 mg of 4 Å MS. TMBzOH = 2,4,6-trimethylbenoic acid. Substrate 1a was recovered up to
54% and conversion ratio is 95%.
in C−H activations assisted by phosphorus-containing directing
groups,20 the combination of P(O)R1R2-directed C−H
functionalization and intermolecular coupling with diazo
compounds has remained unexplored up to now. Herein, we
wish to report the example of Ir(III)-catalyzed direct C−H
aromatization of triphenylphosphine oxide (Ph3PO) and its
derivatives with diazo compounds used to synthesize 2′-
phosphorylbiphenyl-2-ol skeletons with high step- and atom-
economy (B, Scheme 1). This reaction involving C−H
activation, carbene insertion, and tautomerism proceeds under
mild conditions. Further transformations into building blocks of
different ligands and organocatalysts demonstrate the versatility
of the hydroxylation products.
Recently, the Chang group demonstrated a Ir(III)-catalyzed
direct C(sp2)−H amidation of arylphosphoryl compounds
using organic azides as the amino source.21 We were motivated
by this pioneering research as well as our own interest in
developing the R2(O)P-directed C−H functionalizations and
the potential uses of phosphrous ligand.22 Our initial
experiment was conducted with triphenylphosphine oxide
(1a) and 2,4-dichloro-6-diazocyclohe-xa-2,4-dienone (2a) in
the presence of various cyclopentadienyl metals (3 mol %) and
AgSbF6 (12 mol %) at 60 °C in DCE under the Ar atmosphere
for 12 h (Table 1, entries 1−4). To our delight, the desired
product (3a) was obtained in a 61% yield with [Cp*IrCl2]2 as
the catalyst. Different solvents screening showed that DCE was
the best choice (entries 5−6). Subsequently, we examined
various Ag salts; although the AgNTf2 gave the higher yield, we
still selected the common AgSbF6 as the additive (entries 7−9).
When we added 50 mg 4 Å MS to the reaction system, the yield
significantly improved to 86%, whereas the monoarylation (3a)
and diarylation (3a′) products were observed with a ratio of 6:1
based on the NMR analysis of the crude mixture (entry 10).
After adjusting the ratio of 1a and 2a to 2:1, we achieved the
product 3a in an 88% yield with excellent selectivity (mono:di
= 20:1) (entries 11−12). Finally, we also screened different
acids and found that the 2,4,6-trimethylbenoic acid
(TMBzOH) was much better than pivalic acid, the ratio of
3a and 3a′ improved to 30:1 (entries 13−14). Lowering the
amount of TMBzOH to 15 mol % made no difference in this
reaction (entry 15). Increasing or decreasing the amount of
AgSbF6 was not unfavorable. Control experiments indicated
that every component was indispensable to this reaction.
With optimized reaction conditions in hand, we first
investigated the scope of various arylphonphine derivatives
with 2,4-dichloro-6-diazocyclohexa-2,4-dienone (2a) (Scheme
2). Triphenylphosphine oxide bearing an electron-donating
group (1b) gave the desired product in good yield. The
introduction of the electron-withdrawing group to the para
position of the benzene ring (1c) led to a dramatically
decreased conversion, however. No corresponding product was
observed with a para-substituted trifluoromethyl group 1d,
which suggested that electron-withdrawing properties slowed
down the C−H activation in this catalytic cycle. The sterically
hindered substrate 1e afforded 3e in 62% yield. When
methyldiphenylphosphine oxide (1f) and cyclohexydiphenyl-
phosphine oxide were used as substrates, the corresponding
products of 3f and 3g were obtained in 73% (dr = 2.6:1) and
71% (dr = 3.7:1), respectively. Additionally, tris(2-thienyl)-
phosphine oxide (1h) and dimethyl(phenyl) phosphine oxide
(1i) were also worked well and led to the corresponding
products in 65% and 42% yields. In contrast to our previous
work, where biphenyl-2-yl-phosphine oxides were selectively
C−H functionalized in position 2′ of the biphenyl via a 7-
membered palladacycles,8,22 the ortho C−H functionalization
product 3j was achieved in 80% yield (the structure of 3j was
further determined by X-ray crystallography in Supporting
Information), which may illustrate that this reaction underwent
a five-membered cyclometalated transition state.
Next, we examined the scope of the diazo compounds. To
our delight, a wide range of substrates underwent the reaction
efficiently with the slow addition of diazo reactants in the
B
DOI: 10.1021/acs.orglett.7b02710
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 2. Scope of Arylphonphine Derivatives
Scheme 4. Gram-Scale Experiment and Derivatizations
treated 4e with Tf2O to obtain triflate 6 in 71% yield, which
could be transformed into 7 via a Pd-catalyed carbonylation
reaction, which was further applied to synthesize tropos
phosphine-oxazoline ligands.24 On the other hand, 7 could be
reduced with HSiCl3 to afford the intermediate 8 in 83% yield,
then followed through palladium-catalyzed phosphinylation to
give benzo-fused derivative 9 in 55% yield (b, Scheme 4), which
is utilized great importance in organic materials.25
To gain insight into the mechanism of the reaction, we
carried out the intermolecular competitive coupling of an
equimolar mixture of 1a and 1a-d5 with diazo compound 2a
under standard reaction conditions, and the value of the kinetic
isotope effect (KIE) was reached 6.4 (details see Supporting
Information), which indicated that C−H cleavage was likely
involved in a rate-limiting step. On the basis of the above
experiments and previous reports,11,12,18,21 we proposed a
plausible catalytic cycle in Scheme 5. First, Ir(III) coordinated
a
Reaction condition A: 1a (0.4 mmol), 2a (0.2 mmol), [Cp*IrCl2]2 (3
mol %), AgSbF6 (12 mol %), TMBzOH = 2,4,6-trimethyl benoic acid
(15 mmol %), 4 Å Ms (50 mg), 60 °C under Ar atmosphere for 12 h.
b
c
Isolated yields. For 24 h.
absence of 4 Å MS (Scheme 3, Condition B). The expected
products were obtained in moderate yields when different
a b
,
Scheme 3. Scope of Diazo Compounds
Scheme 5. Possible Reaction Mechanism
a
Reaction condition B: 1a (0.4 mmol), [Cp*IrCl2]2 (3 mol %),
AgSbF6 (12 mol %), and TMBzOH (15 mol %), were carried out in
0.5 mL DCE under Ar atmosphere and stirred for 15 min, 2a (0.2
mmol) was dissolved in 1.5 mL DCE and added via peristaltic pump
b
c
for 6 h. Isolated yields. 2a was carried out in 2 equiv (0.4 mmol).
with substrate 1 via PO directing group and subsequently
occurred electrophilic C−H bond cleavage at the ortho position
to form a five-membered Ir(III)-cycle intermediate A. Then,
diazo compounds 2 attacked A to generate an Ir-carbene
intermediate B by extrusion of N2, which underwent a
migratory insertion to give a six-membered iridacycle C.
Finally, protonation of C would release an Ir-catalyst and afford
the unstable intermediate D, which was transferred into the
desired product 3 by a tautomerism.
In summary, we have developed an intermolecular coupling
via Ir(III)-catalyzed direct C−H aromatization of triphenyl-
phosphine oxide (Ph3PO) and its derivatives with diazo
compounds under mild conditions. This method provides a
new synthetic approach to biarylphosphine and dibenzophosp-
hole oxides bearing a hydroxyl group in a single reaction.
halogen groups were introduced to the para- or meta-position
of ortho-quinone diazides (4b−4d). Meanwhile, a series of
substituted 1-diazonaphtho-quinones were converted smoothly
to desired products in generally good-to-excellent yields (4e−
4i). It is noteworthy that 2-diazo derivatives bearing chlorine,
acetoxy groups were also tolerated, albeit obtained in moderate
yields (4j−4k).
In order to highlight the availability of this method, we
carried out a gram-scale experiment with decreasing catalyst
loading to 1 mol % (a, Scheme 4), and the desired product 4e
was isolated in good yield (2.07g, 6 mol scales). We are all well-
aware of the importance of MOP-type ligands in catalytic
reactions.2,23 Therefore, we reduced the 4e with HSiCl3 to give
the new structural MOP-type ligand 5 in 83% yield. We also
C
DOI: 10.1021/acs.orglett.7b02710
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Chem. 2015, 80, 223. (f) Son, J.-Y.; Kim, S.; Jeon, W. H.; Lee, P. H.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02710.
■
S
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Experimental details and characterization data for all new
compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yangshd@lzu.edu.cn.
ORCID
2016, 54, 4508.
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(14) Gutier
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Shang-Dong Yang: 0000-0002-4486-800X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the NSFC (Nos. 21472076 and 21532001)
and PCSIRT (IRT_15R28 and lzujbky-2016-ct02) financial
support.
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D
DOI: 10.1021/acs.orglett.7b02710
Org. Lett. XXXX, XXX, XXX−XXX