Pd(II)-catalyzed enantioselective synthesis of P-stereogenic phosphinamides via desymmetric C-H arylation

Article abstract of DOI:10.1021/ja512029x

We present the enantioselective synthesis of P-stereogenic phosphinamides through Pd-catalyzed desymmetric ortho C-H arylation of diarylphosphinamides with boronic esters. The method represents the first example of the synthesis of P-stereogenic phosphorus compounds via the desymmetric C-H functionalization strategy. The reaction proceeded efficiently with a wide array of reaction partners to afford the P-stereogenic phosphinamides in up to 74% yield and 98% ee. The efficiency was further demonstrated by gram scale syntheses. Moreover, the flexible conversion of the P-stereogenic phosphinamides into various types of P-stereogenic phosphorus derivatives was also elaborated. Thus, the protocol provides a novel tool for the efficient and versatile synthesis of P-stereogenic compounds.

Full text of DOI:10.1021/ja512029x

Communication
pubs.acs.org/JACS
Pd(II)-Catalyzed Enantioselective Synthesis of P‑Stereogenic
Phosphinamides via Desymmetric C−H Arylation
Zhi-Jun Du,†,‡,∥ Jing Guan,†,∥ Guo-Jie Wu,† Peng Xu,†,§ Lian-Xun Gao,† and Fu-She Han*,†,§
†Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China
‡University of the Chinese Academy of Sciences, Beijing 100864, China
§State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
S
* Supporting Information
prominent results, one may certainly consider the catalytic
ABSTRACT: We present the enantioselective synthesis
of P-stereogenic phosphinamides through Pd-catalyzed
desymmetric ortho C−H arylation of diarylphosphina-
mides with boronic esters. The method represents the first
example of the synthesis of P-stereogenic phosphorus
compounds via the desymmetric C−H functionalization
strategy. The reaction proceeded efficiently with a wide
array of reaction partners to afford the P-stereogenic
phosphinamides in up to 74% yield and 98% ee. The
efficiency was further demonstrated by gram scale
syntheses. Moreover, the flexible conversion of the P-
stereogenic phosphinamides into various types of P-
stereogenic phosphorus derivatives was also elaborated.
Thus, the protocol provides a novel tool for the efficient
and versatile synthesis of P-stereogenic compounds.
properties of the corresponding P-stereogenic derivatives such as
phosphoramides and phosphinamides, but to the best of our
knowledge, such derivatives have never been investigated
because of the paucity of suitable synthetic methods, although
amino-group-directed ortho lithiation is a potential option.6b,g
Thus, the exploration of a conceptually new protocol for the
synthesis of P-stereogenic phosphoramides or phosphinamides is
of prime importance in view of their wide potential applications.
On the basis of our recent work on the BINOL-based chiral-
phosphoramide-catalyzed asymmetric reaction of indol-2-yl
carbinols12 and TM-catalyzed C−H functionalization,13 we
became interested in developing a method for the synthesis of
P-stereogenic phosphoramides or phosphinamides and inves-
tigating their applications in asymmetric synthesis. As the first
step directed toward these goals, we report the successful
enantioselective synthesis of P-stereogenic phosphinamides
through a desymmetric C−H arylation strategy.
Our initial idea originated from a very recent study of Pd-
catalyzed racemic C−H arylation,13b wherein we found that
phosphinamide 1 and Pd(OAc)2 can form the stable palladacycle
2 in DMF (Scheme 1a). The reaction of 2 with boronic acid 3
proceeds smoothly to deliver o-arylphosphinamide 4 in excellent
yield (Scheme 1b). Crystallization of 2 from an acetone/H2O
solvent system revealed that the DMF in 2 was displaced by two
molecules of H2O (Scheme 1c). The coordination of a DMF
n recent decades, comprehensive studies have revealed the
importance of chiral phosphorus compounds in asymmetric
I
synthesis both as ligands in metal-catalyzed reactions1 and as
organocatalysts.2 However, the majority of these studies focused
on axial, planar, spiro, or carbon-centered chiral phosphorus
derivatives.1,2 Although P-stereogenic compounds have shown
prominent chiral induction stemming from chirality proximate to
the catalytic center,3 their wide application has been severely
restricted because of the lack of efficient synthetic methods.
Conventional approaches for their preparation include reso-
lution,3a,4 chiral-auxiliary-based approaches,4,5 and enantioselec-
tive lithiation−trapping of phosphine−boranes or related
analogues.6 Recently, transition metal (TM)-catalyzed asym-
metric cross-couplings of aryl halides with secondary phos-
phines,7 hydrophosphination of electron-deficient olefins,8
alkylation of secondary phosphines with alkyl halides,9 addition
reactions of secondary phosphines and benzoquinone,10 and
ring-closing metathesis of olefins11 have also been reported.
Despite these important advances, the scope and efficiency of
most of the approaches await further improvements. More to the
point, some types of potentially useful P-stereogenic compounds
are still inaccessible.
Scheme 1. Palladacycle 2 and Control Experiment13b
Intense interest in chiral BINOL- and spirocycle-based
phosphoramide Brønsted acid-catalyzed asymmetric reactions
has recently been observed.2 Such catalysts have the advantage of
catalyzing a broad range of transformations through various
activation modes such as Brønsted acidity, hydrogen bonding,
ion pairing, and metal phosphates.2e Stimulated by these
Received: November 24, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ja512029x
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Journal of the American Chemical Society
Communication
a
molecule and the ease of ligand exchange of 2 attracted our
attention because, conceptually, a P-stereogenic phosphinamide
should be generated if an appropriate DMF-like chiral ligand
could be used in place of H2O to exchange with the DMF in the
reaction system.
Table 1. Effect of the Palladium Catalyst and Base
Thus, a variety of amino acid derivatives were selected to
evaluate the reaction conditions. Our first consideration in using
amino acids as chiral ligands is that such compounds are
structurally similar to DMF, and their efficiency in the
asymmetric synthesis of C-centered chiral compounds has
been demonstrated by Yu.14 As expected, by screening an array
of amino acid derivatives and carefully tuning bases, oxidants, and
additives on the basis of the procedure for the racemic
arylation,13b we found that the arylation of phosphinamide 5
with boronic acid 3 in the presence of Boc-L-phenylalanine (Boc-
L-Phe-OH, L1) could afford the desired P-stereogenic
phosphinamide 6 in 65% yield with 75% ee with Pd(OAc)2 as
the catalyst, Ag2CO3 as the oxidant, benzoquinone (BQ) as an
additive, and CsF as the base (Scheme 2). Although further
b
c
entry
1
catalyst
base
yield (%)
ee (%)
Pd(OAc)2
Pd(OAc)2
PdCl2
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Li2CO3
K2CO3
Cs2CO3
NaHCO3
CsF
59
18
51
25
62
93
80
92
86
94
−
d
2
3
4
5
6
7
8
9
Pd(O2CCF3)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
e
n.r.
n.r.
60
−
96
trace
−
a
Reaction conditions: 7a (0.2 mmol), 8a (0.4 mmol), Pd catalyst (5
mol %), L1 (20 mol %), BQ (0.5 equiv), base (3.0 equiv), Ag2CO3 (1.5
equiv), and H2O (40.0 equiv) in 2 mL of anhydrous DMF at 40 °C
b
c
Scheme 2. Initial Result of Enantioselective C−H Arylation
under air, unless otherwise noted. Isolated yields. Determined by
d
e
chiral HPLC analysis on an AD-H column. No H2O. No reaction.
ab
,
Table 2. Evaluation of Chiral Ligands
exhaustive optimization of the reaction parameters to improve
the enantioselectivity proved to be futile, the preliminary result
was encouraging in terms of yield and enantioselectivity.
At this juncture, our attention was shifted to the effect of the
directing group of the phosphinamide substrate and the nature of
the boron compound. An orthogonal evaluation of various
phosphinamides and boron compounds showed phosphinamide
7a bearing a 2,3,5,6-tetrafluoro-4-cyanophenylamino (ArF)
directing group and boronic acid pinacol ester 8a to be a
promising combination of reaction partners, affording the
desired product 9a in 59% yield with 93% ee in the presence
of L1 (Table 1, entry 1). Interestingly, the addition of ca. 40 equiv
of H2O in anhydrous DMF was also crucial to improve both the
yield and the chiral induction (entry 1 vs 2), although the critical
role of H2O deserves a detailed clarification. We then screened an
array of Pd catalysts and bases (Table 1). Among a variety of
conditions examined, Pd(OAc)2 and Li2CO3 or NaHCO3 were
found to be the optimal catalyst and base, respectively, giving 9a
in over 60% yield with up to 96% ee (entries 5 and 8). The results,
together those of Yu on amide-group-directed C−H activation
for the synthesis of C-stereogenic molecules,14d indicate that the
electron-deficient polyfluorophenyl group may act as a powerful
substituent in enantioselective C−H activation.
a
Reaction conditions: 7a (0.2 mmol), 8a (0.4 mmol), Pd(OAc)2 (5
mol %), ligand (20 mol %), BQ (0.5 equiv), Li2CO3 (3.0 equiv),
Ag2CO3 (1.5 equiv), and H2O (40.0 equiv) in 2 mL of anhydrous
b
DMF at 40 °C under air. Shown are isolated yields and ee values
determined by chiral HPLC analysis on a CD-H column. No reaction.
c
Next, we systematically inspected the effect of the amino acid
derivative on this reaction using Pd(OAc)2 as the catalyst and
Li2CO3 as the base (Table 2). A general observation is that for an
array of Boc-protected amino acids L1−L8, the yield and
enantioselectivity are less affected by the steric and electronic
nature of the R group. In most cases 9a could be obtained in ca.
60% yield with a high enantioselectivity of over 90% ee, with Boc-
Tyr(tBu)-OH (L3) giving the best results in terms of yield (64%)
and enantioselectivity (94% ee). These results provide an
important advantage for flexibly choosing the chiral ligand within
a broad window. Nevertheless, we still found that precise tuning
of the steric nature of the R group is important as shown by a
comparison of Boc-Ala-OH (L6), Boc-Val-OH (L7), and Boc-
tert-Leu-OH (L8). Both the smallest (L6) and the largest (L8)
offered results inferior to that obtained with the middle-sized L7.
The ee value for the large L8 decreased even to 83%.
In contrast, the nature of the N-protecting group has a
dramatic effect on the reaction (Table 2). When the N-protecting
i
group was changed from Boc in L1 to PrOCO (L9), EtOCO
(L10), TcBoc (L11), and ultimately Fmoc (L12), both the yield
and enantioselectivity gradually decreased, although good
enantioselectivity was still maintained (79−88%). Surprisingly,
however, when the protecting group was replaced by an acyl
B
DOI: 10.1021/ja512029x
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Journal of the American Chemical Society
Communication
group (L13), only poor enantioselectivity (21% ee) was observed,
and the reaction was entirely suppressed when the unprotected
amino acid was used (L14). These results suggest that the
presence of a carbamate moiety in the ligand is of crucial
importance to induce good enantioselectivity. Finally, the
carboxylic acid group also plays a vital role in the reaction
since the enantioinduction was completely lost when the
carboxylic acid functionality was blocked by esterification (L15)
or conversion into a hydroxyl group (L16).
groups (9c−f) proceeded smoothly with phosphinamide 7a to
give the P-stereogenic phosphinamides in good yields (60−68%)
with high to excellent enantioselectivities (87−96% ee). A variety
of functional groups such as ether (9b), fluoro (9c), ester (9d),
trifluoromethyl (9e), and bromo (9f) were well-tolerated, and a
ring-fused nucleophile was also compatible, giving 9g in 64%
yield with 96% ee.
The reaction also works well for an array of diaryl-
phosphinamides. The ortho-substituted substrate gave the
product 9h in 50% yield with 92% ee. The somewhat decreased
yield of 9h compared with the meta-substituted analogues 9a−g
is presumably due to steric hindrance by the o-Me group. The
unsubstituted substrate also afforded the products in moderately
high yields with high enantioselectivities (9i and 9j). For such
substrates, small amounts of diarylated byproducts (<10%) were
isolated. A phosphinamide modified with a strong electron-
donating OMe group reacted very smoothly with a wide range of
boronic esters to produce the corresponding P-stereogenic
products in good yields of up to 74% as well as excellent
enantioselectivities of up to 98% ee (9k−r). The conditions were
also compatible with a wide range of functional groups. More to
the point, the reaction could be performed on a large scale with
consistent efficiency, as exemplified by the synthesis of 9k on a
gram scale (70−72% yield, 97−98% ee) and 9q on a 0.5 g scale
(72% yield, 95% ee). Finally, the arylation of substrates bearing
an electron-withdrawing CF3 group was sluggish at 40 °C, but
satisfactory results were obtained at 70 °C (9s and 9t). Of note,
only monoarylated products were produced with the ortho- or
meta-substituted substrates under the optimized conditions. The
absolute configuration of 9k was S as determined by X-ray
crystallography (Figure 1), and those of the other products were
assigned by analogy.
Having defined the optimal ligand L3, we explored the loading
of Pd(OAc)2 catalyst, the ratio of catalyst to L3, and the effect of
the atmosphere with the aim of further improving the reaction
efficiency. While significant improvement was not observed, the
use of a combination of 10 mol % catalyst and 20 mol % L3 did
slightly increase the efficiency, affording 9a in 68% yield with 96%
ee under an air atmosphere. Identical yields and enantioselectiv-
ities were obtained when the reaction was performed under
nitrogen and oxygen. Altogether, it was found that the optimized
conditions for the asymmetric synthesis of P-stereogenic
phosphinamides via desymmetric C−H arylation are 10 mol %
Pd(OAc)2, 20 mol % L3, 0.5 equiv of BQ, 1.5 equiv of Ag2CO3,
3.0 equiv of Li2CO3, and 40.0 equiv of H2O in anhydrous DMF at
40 °C under an air atmosphere. These conditions could be
reliably employed for large-scale synthesis with consistent yields
and enantioselectivities (vide infra).
Having established robust conditions, we investigated the
substrate scope of the protocol (Table 3). The reaction of a
variety of boronic esters decorated by either weak (9a) or strong
electron-donating groups (9b) or various electron-withdrawing
a b
,
Table 3. Evaluation of the Substrate Scope
Figure 1. Absolute configuration of 9k.
Finally, to further demonstrate the utility of this methodology,
the conversion of the P-stereogenic phosphinamides into other
types of potentially useful P-chiral phosphorus derivatives was
elaborated using 9k (Scheme 3). Treatment of 9k (97% ee) with
Lawesson’s reagent afforded thiophosphinamide 10 in high yield
and enantiomeric purity (95% ee).15 On the other hand, while
methanolysis of 9k with HCl proved to be sluggish,6g TfOH was
effective and afforded methyl phosphinate 11 in 62% yield. Most
significantly, only a negligible decrease in enantiomeric excess
(from 98% to 97% ee) was observed under the unoptimized
conditions. Subsequently, the conversion of 11 into P-stereo-
genic phosphine oxide 12 proceeded uneventfully under the
effect of MeLi (71% yield, 94.3% ee).5b Finally, reduction of 12
with LiAlH4 according to a known procedure16 gave P-
stereogenic phosphine 13 in high yield (70%) with slightly
reduced chirality (92.3% ee).
a
In summary, we have developed an unprecedented protocol
that realizes the asymmetric synthesis of conventionally
inaccessible P-stereogenic phosphinamides via Pd-catalyzed
desymmetric C−H arylation. The importance of this protocol
was further exemplified by the conversion of a chiral
Reaction conditions: 7 (0.2 mmol), 8 (0.4 mmol), Pd(OAc)2 (10
mol %), L3 (20 mol %), BQ (0.5 equiv), Li2CO3 (3.0 equiv), Ag2CO3
(1.5 equiv), and H2O (40.0 equiv) in 2 mL of anhydrous DMF at 40
b
°C under air. Shown are isolated yields and ee values determined by
chiral HPLC analysis. The reaction was carried out at 70 °C.
c
C
DOI: 10.1021/ja512029x
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Journal of the American Chemical Society
Communication
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Scheme 3. Derivatization of Phosphinamides
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ASSOCIATED CONTENT
* Supporting Information
Procedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*fshan@ciac.ac.cn
■
(10) Huang, Y.; Li, Y.; Leung, P.-H.; Hayashi, T. J. Am. Chem. Soc.
Author Contributions
2014, 136, 4865.
∥Z.-J.D. and J.G. contributed equally.
(11) Harvey, J. S.; Malcolmson, S. J.; Dunne, K. S.; Meek, S. J.;
Thompson, A. L.; Schrock, R. R.; Hoveyda, A. H.; Gouverneur, V.
Angew. Chem., Int. Ed. 2009, 48, 762.
Notes
The authors declare no competing financial interest.
(12) Qi, S.; Liu, C.-Y.; Ding, J.-Y.; Han, F.-S. Chem. Commun. 2014, 50,
8605.
ACKNOWLEDGMENTS
■
(13) (a) Xu, H.; Liu, P.-T.; Li, Y.-H.; Han, F.-S. Org. Lett. 2013, 15,
3354. (b) Guan, J.; Wu, G.-J.; Han, F.-S. Chem.Eur. J. 2014, 20, 3301.
(c) Wu, G.-J.; Guan, J.; Han, F.-S.; Zhao, Y.-L. ChemCatChem 2014, 6,
1589. (d) Du, Z.-J.; Gao, L.-X.; Lin, Y.-J.; Han, F.-S. ChemCatChem
2014, 6, 123.
We thank the NNSFC (21272225) and the State Key Laboratory
of Fine Chemicals (KF 1201) for financial support, Prof. Bao-
Min Wang at DUT for helpful discussions, and Wu-Ping Liao at
CIAC for X-ray crystallographic analysis.
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Chem. 1979, 182, 537. (b) Shi, B.-F.; Maugel, N.; Zhang, Y.-H.; Yu, J.-Q.
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