Iridium(III)-Catalyzed C H Amidation of Arylphosphoryls Leading to a P-Stereogenic Center

Article abstract of DOI:10.1002/chem.201404151

Direct C H amidation of arylphosphoryl compounds has been developed by using an Ir^IIIcatalyst system under mild conditions. A wide range of substrates could be employed with high functional-group tolerance. This procedure was successfully applied for the first time to the asymmetric reaction giving rise to a P-chirogenic center with a high diastereomeric ratio of up to 19:1 (90 % de).

Full text of DOI:10.1002/chem.201404151

DOI: 10.1002/chem.201404151
Communication
&
Asymmetric Synthesis
Iridium(III)-Catalyzed CÀH Amidation of Arylphosphoryls Leading
to a P-Stereogenic Center
Donghyeon Gwon,[a, b] Donggun Lee,[a, b] Jiyu Kim,[a, b] Sehoon Park,*[a, b] and
Sukbok Chang*[a, b]
Dedicated to Professor C.-M. Yu on the occasion of his 60th birthday
notable advance in the preparation of chiral phosphorus com-
pounds, the methods presently available often suffer from lim-
Abstract: Direct CÀH amidation of arylphosphoryl com-
pounds has been developed by using an IrIII catalyst
ited scope, multiple steps, requirement of optical resolution, or
harsh reaction conditions. In this regard, the development of
a more efficient and stereoselective procedure leading to chiral
phosphorus compounds is highly desirable.
system under mild conditions. A wide range of substrates
could be employed with high functional-group tolerance.
This procedure was successfully applied for the first time
to the asymmetric reaction giving rise to a P-chirogenic
center with a high diastereomeric ratio of up to 19:1
(90% de).
To address this issue, an asymmetric induction of the P-chiral
center was envisioned to be achieved through diastereoselec-
tive CÀH activation (Scheme 1),[8,9] and its results are described
herein. We have developed an efficient iridium catalyst system
Organophosphorus compounds containing PÀC and
PÀX bonds (X=O, N, S, etc.) have long belonged to
an important class of molecules because of their
wide applications in medicinal,[1] materials,[2] and syn-
thetic chemistry.[3] This versatility can probably be at-
tributed to their intrinsic nature of having multiple
oxidation states of the phosphorus atom and ubiqui-
ty in biological systems.[4] Undoubtedly, P-chiral phos-
Scheme 1. Our approach in the asymmetric CÀH amidation leading to a P-chiral center.
phorus compounds draw special attention since they
are a key motif consisting of important molecules in
pharmaceuticals, agrochemicals, materials, ligands,
to differentiate two diastereotopic aryl groups of diarylphos-
and organocatalysts.[5] In particular, the use of P-stereogenic
compounds as chiral ligands or catalysts has stimulated the re-
markable advance of asymmetric chemistry.[6] In this aspect,
a number of conventional routes has been devised for the syn-
thesis of P-chiral compounds. For instance, methods using
chiral auxiliary-based diastereomeric resolution, chiral base-
mediated asymmetric deprotonation followed by addition to
electrophiles, or enzymatic desymmetrization of prochiral
phosphorus compounds have been investigated.[7] Enantiose-
lective alkylation and alkoxylation of racemic phosphines using
chiral Ru or Pd complexes are also known.[7c–e] Despite of the
phoryl bearing a chiral auxiliary whereby the corresponding P-
chiral products could be attained with up to 90% diastereo-
meric excess under mild conditions (ratio of two diastereo-
mers, 19:1).
We first tried to optimize direct CÀH amidation conditions
initially using triphenylphosphine oxide (1) as a model sub-
strate in
a reaction with p-toluenesulfonyl azide (TsN3,
1.1 equiv). After extensive studies (see the Supporting Informa-
tion for details),[10] we found that the addition of pivalic acid
(PivOH, 12 mol%) was essential for promoting the amidation
with the combined use of [IrCl2(Cp*)]2 (2 mol%; Cp*=1,2,3,4,5-
pentamethylcyclopentadiene) and AgNTf2 (8.5 mol%) leading
to the desired amidated product 2 in 84% yield [Eq. (1)].[11] An
X-ray diffraction analysis of 2 revealed that an intramolecular
H-bonding exists between P=O and NHTs moieties. In addition,
a bis-amidated compound 3 was also formed (5%), and its
solid structure was also characterized to confirm that the
second amidation occurred at a different phenyl ring.
[a] D. Gwon, D. Lee, J. Kim, Dr. S. Park, Prof. Dr. S. Chang
Department of Chemistry
Korea Advanced Institute of Science and Technology (KAIST)
Daejeon 305-701 (Republic of Korea)
E-mail: sehoonp@kaist.ac.kr
sbchang@kaist.ac.kr
[b] D. Gwon, D. Lee, J. Kim, Dr. S. Park, Prof. Dr. S. Chang
Center for Catalytic Hydrocarbon Functionalizations
Institute of Basic Science (IBS)
With the optimal amidation conditions in hand, we were
next encouraged to undertake an asymmetric amidation study
(Table 1). Initially, we hypothesized that diarylphosphoryl com-
pounds bearing chiral auxiliaries would be a plausible template
Daejeon 305-701 (Republic of Korea)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404151.
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chiral, occurred exclusively at
the binaphthyl moiety instead of
two phenyl rings, thereby 6 was
not P-chiral.
We then proceeded to explore
a stereoselective CÀH amidation
by using diarylphosphorus com-
pounds possessing chiral auxilia-
ries. When diphenylphosphine
oxide bearing a chiral alkoxy
group, for example, (À)-menthol,
was amidated under the stan-
dard conditions, a desired prod-
uct (7) was obtained with a dia-
stereomeric ratio of 62:38
(24% de). Although this selectivi-
ty was not satisfactory, it encour-
aged us to further search for
more appropriate substrates that
allow for higher diastereoselec-
tivity. Indeed, it was shown that
the change of an alcohol auxil-
iary led to improved stereoselec-
tivity: 8a was smoothly amidat-
ed to give 8 with 41% de at
858C and 48% de at 558C. The
higher de in 8 than that in 7 im-
plies that a phenyl moiety on
the chiral auxiliary could affect
the diastereoselection step pre-
sumably by p–p interactions
with aryl rings on the P atom
(vide infra).[12] The diaryl part of
substrates was next modified by
electronically different substitu-
ents. When a p-methoxy group
was substituted, diastereoselec-
tivity of the obtained product (9)
was 44% de under the standard
conditions (858C), and increased
to 62% de at 508C (1 mol% of Ir
catalyst). A para-chloro-substitut-
ed substrate (10a) was amidated
slowly with 49% de at 858C
Table 1. Substrate scope of asymmetric CÀH amidations.[a]
[a] Reaction conditions: 4a–12a (0.20 mmol), azide (0.22 mmol), [IrCl2(Cp*)]2 (0.004 mmol), AgNTf2
(0.017 mmol), PivOH (0.024 mmol) in 1,2-DCE (0.5 mL) at 858C for 4 h (isolated as a diastereomeric mixture).
The de was determined based on the 31P{1H} NMR spectra of a crude reaction mixture. [b] For 12 h. [c] At 558C
for 48 h. [d] At 508C for 48 h with 1 mol% Ir catalyst. [e] At 558C for 72 h.
to lead to the diastereoselective formation of a key five-mem-
bered iridacycle by the IrIII-mediated asymmetric CÀH bond ac-
tivation on two diastereotopic aryl rings to give rise to P-chiral-
ity (Scheme 1). As a reference, when a nonchiral substrate (4a)
was reacted with a chiral azide, for example, (1S)-(+)-camphor-
sulfonyl azide, amidation took place with a low diastereomeric
ratio (52:48, 4). Similarly, a low level of diastereomeric excess
(de) was observed in a reaction of diphenylphosphinic amide
(5a) to afford 5 (6% de) in a reaction with the chiral azide. This
result ascertains our working hypothesis that an asymmetric in-
duction arises from a CÀH bond cleavage step rather than in
an azide insertion stage. Interestingly, a reaction of diphenyl(bi-
naphthyl)phosphine oxide (6a), wherein binaphthyl is axially
(58% de at 558C). However, the introduction of substituents at
the ortho-position on the aryl rings resulted in lower diastereo-
selectivity as seen by 11 and 12.
The observation that diastereoselectivity changed to some
extent depending on the substituent type in substrates led us
to study this pattern more quantitatively. For this purpose,
amidation of four substrates (10a, 8a, 9a, and 12a) was per-
formed at incremental temperatures between 25 and 858C
over 4 to 96 h (Table 2). Four sets of plots of ln(d.r.) versus 1/T
were obtained with excellently linear regressions (R2 >0.99),
which implied that the diastereoselective amidation takes
place through a common mechanistic pathway at each tem-
perature range (see the Supporting Information for details).[7c]
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dard conditions (2 mol% of Ir
catalyst and 12 mol% of PivOH
at 608C for 12 h), an enantiomer-
ic product 13-enan was obtained
with similar de when compared
to 13. Diastereomeric ratio was
determined by 31P NMR spectros-
copy and chiral-HPLC analysis of
the crude mixture in this study.
Diastereoselectivity was slight-
ly changed by substituents at
the para-position: 14 (82% de;
Table 2. Correlation between reaction temperature and diastereoselectivity in the amidation of 8a–10a and
12a with TsN3.[a]
Entry
Product
Slope
DDH° [kcalmolÀ1
]
y-intercept
DDS° [e.u.]
DDG°[b] [kcalmolÀ1
]
1
2
3
4
10 (R=4-Cl)
8 (R=H)
9 (R=4-MeO)
12 (R=2-MeO)
À964.4
À781.6
À671.2
À892.1
1.92
1.55
1.33
1.77
1.61
1.31
0.94
2.11
3.2
2.6
1.9
4.2
0.97
0.78
0.76
0.52
[a] 8a–10a and 12a (0.20 mmol), TsN3 (0.22 mmol), [IrCl2(Cp*)]2 (0.004 mmol), AgNTf2 (0.017 mmol), PivOH
(0.024 mmol) in 1,2-DCE (0.5 mL) for 4 to 96 h at 25 to 858C. The d.r. was referred to as a diastereomeric ratio
determined based on 31P{1H} NMR spectra of a crude reaction mixture. [b] Values at 298 K.
p-methyl
substituent),
15
(79% de; p-methoxy), and 16
By applying the obtained correlation between temperature
and d.r. to a differential Eyring equation, ln(d.r.)=-DDH°/RT+
ÀDDS°/R, we were able to calculate -DDH° and -DDS° for
each set of amidations, ranging
(77% de; p-chloro group). Interestingly, it was also influenced
to some extent by the azides employed. For example, while
the amidation of 17a with (p-methoxybenzene)sulfonyl azide
from 1.92 to 1.33 kcalmolÀ1 and
Table 3. Asymmetric CÀH amidation of phosphinic amides.[a]
from 4.2 to 1.9 e.u., respectively.
Notably, it was found that the
differential activation parameters
(ÀDDS°, ÀDDH°, and ÀDDG°)
increased as electron-withdraw-
ing groups were substituted at
the para-position, whereas the
values of a substrate containing
an ortho-methoxy group (12a)
were rather out of the above
trend.
Given the preliminary observa-
tion that phosphinic amides (e.g.
5a) exhibited excellent reactivity
in the current CÀH amidation
and that an aryl pendant on the
chiral auxiliary led to higher dia-
stereoselectivity
(Table 1,
7
versus 8), we designed a new
type of diarylphoryl bearing
a C2-symmetric chiral pyrrolidine
moiety[13] to explore the possibil-
ity of inducing P-chirality more
effectively (Table 3). A substrate
13a was smoothly amidated at
608C to give 13 in 84% yield,
more significantly, with a diaste-
reomeric ratio of 90:10 (80% de).
It was increased to 84% de
when the reaction was carried
out at 258C (24 h). A full conver-
sion was attained in 48 h even
with 0.5 mol% of catalyst at
608C, showing
a
turnover
number of approximately 200.
As expected, when a substrate
13a-enan, an enantiomer of
13a, was subjected to the stan-
[a] 13a–16a (0.20 mmol), azide (0.22 mmol), [IrCl2(Cp*)]2 (0.004 mmol), AgNTf2 (0.017 mmol), PivOH
(0.024 mmol) in 1,2-DCE (0.5 mL) at 608C for 12 h or at 258C for 24 h (isolated yields). The de was determined
based on 31P{1H} NMR spectra of a crude reaction mixture.
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took place with 80% de (17), which was identical to product
13 obtained with (p-methylbenzene)sulfonyl azide, 90% de
(18) was observed when benzylsulfonyl azide was reacted.
Slightly lower selectivity was ob-
to the observed stereochemical outcome is driven presumably
by the difference of the secondary interactions as shown in
Scheme 2.[15]
served with butanesulfonyl azide
(88% de of product 19) or (p-
chlorobenzene)sulfonyl
azide
(84% de of product 20), in all ex-
cellent chemical yields. Di-
arylphosphinic amides substitut-
ed with the p-methyl- or p-me-
thoxy group were amidated with
benzylsulfonyl azide to afford 21
(89% de at 608C and 90% de at
258C) and 22 (86% de at 608C
and 89% de at 258C), respective-
ly. Again, it needs to be empha-
sized that high chemical yields
(>90%) were obtained even
from room temperature reac-
tions. A similar level of de was
observed in
a reaction with
Scheme 2. A plausible pathway leading to the amidated product 13 with a (R)-configuration at the P-center.
a chiral azide to give 23 in high
yield. It needs to be emphasized
that optically pure P-chirogenic products were obtained in
high yields by a simple purification process from the optically
enriched crude reaction mixture.
Finally, we explored the substrate scope of arylphosphoryl
compounds, ArnR3ÀnP=O (R=alkyl, aryl),[16] which can be an al-
ternative template bearing a alkyl or aryl chiral auxiliary even-
tually for the perspective asymmetric CÀH functionalization
(Table 4). A series of triarylphosphine oxides was examined to
The absolute configuration at the newly generated P-chiro-
genic center of a major diastereomer of 13 was determined to
be (R) by X-ray crystallographic
analysis. Although additional
studies are required to reason
Table 4. Substrate scope of nonchiral CÀH amidation.[a]
the origin of the observed dia-
stereochemical outcome at the
P-chiral center, it is presently as-
sumed that two aryl groups in
diarylphosphinic amide sub-
strates become inequivalent
during the course of forming
the corresponding iridacycles.
Indeed, the DFT-optimized struc-
ture of 13a indicates that the P-
center is pseudo-stereogenic be-
cause of the different degree of
secondary interactions of each
phenyl ring on the P atom with
phenyl groups of a chiral auxil-
iary. The solid structure of ami-
dated product 13 revealed that
the P-chiral center has a (R)-con-
figuration and that an unamidat-
ed phenyl ring displays stronger
p–p interactions with a phenyl
group in auxiliary.[12,14] Based on
these structural features, we ten-
tatively propose that a path of
the CÀH bond activation leading
[a] 24a–37a (0.20 mmol), azide (0.22 mmol), [IrCl2(Cp*)]2 (0.004 mmol), AgNTf2 (0.017 mmol), PivOH
(0.024 mmol) in 1,2-DCE (0.5 mL) at 858C for 4 h (isolated yields).
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reveal that the direct CÀH amidation took place smoothly irre-
spective of the electronic properties of the substituents (2, 24–
27).[17] It was shown that the position of substituents at the
aryl rings influenced the reaction efficiency to some extent
(28–30). The amidation was highly regioselective, occurring at
the sterically less demanding CÀH bonds (31). A reaction of
tris(1-naphthyl)phosphine oxide took place selectively at the 2-
position, and its structure (32) was confirmed by X-ray crystal-
lographic analysis. A new type of substrate of diphenyl(mo-
noalkyl)phosphine oxides was also viable for the present CÀH
amidation that takes place at the aryl group. The correspond-
ing products were obtained in high yields irrespective of the
steric bulkiness of the alkyl groups. The structure of 37 was
confirmed by X-ray diffraction analysis.
In summary, for the first time we have developed an efficient
IrIII-catalyzed asymmetric CÀH amidation of arylphosphoryls
bearing chiral auxiliaries. Diastereomeric excesses of up to
90% were attained (ratio of two diastereomers, 19:1) with
a C2-symmetric chiral pyrrolidine auxiliary, and optically pure P-
chirogenic products could readily be obtained in high yields
by a simple purification process. While experimental observa-
tions led us to propose the formation of an iridacycle as a dia-
stereo-determining step, a pseudo-diastereomeric environment
of substrates derived from a different level of secondary inter-
actions was assumed to be the basis for the observed stereo-
chemical outcomes.
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Experimental Section
Representative procedure
In a 1 mL reaction vial, triphenylphosphine oxide (1) (55.7 mg,
0.20 mmol), [IrCl2(Cp*)]2 (3.2 mg, 0.004 mmol), and AgNTf2 (6.6 mg,
0.017 mmol) were dissolved in 1,2-dichloroethane (1,2-DCE, 0.5 mL)
and briefly shaken, to which tosyl azide (TsN3) (43.4 mg, 0.22 mmol)
and pivalic acid (PivOH, 48 mL from 0.5m stock solution in 1,2-DCE,
0.024 mmol) were subsequently added. The reaction mixture was
stirred at 858C for 4 h, and then cooled down to room tempera-
ture and filtered through a pad of Celite with ethyl acetate
(20 mL). The filtrate was concentrated in vacuo and purified by
column chromatography on silica gel (acetone/n-hexane=1:3 and
EtOAc/CH2Cl2/n-hexane=1:6:4) to give 2 (75.2 mg, 84%).
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Y.-M. Liang, Organometallics 2014, 33, 1905.
Keywords: asymmetric induction · CÀH functionalization ·
chirality · iridium catalysis · phosphine oxides
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[17] Deoxygenation of 2 was carried out efficiently by using HSiCl3 to afford
the corresponding amidated phosphine in 89% yield, and detailed ex-
perimental procedures are presented in the Supporting Information.
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Received: June 27, 2014
Published online on August 21, 2014
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