Enantioselective addition of diphenylphosphine to 3-methyl-4-nitro-5- alkenylisoxazoles

Article abstract of DOI:10.1002/adsc.201300164

An enantioselective Michael addition reaction of diphenylphosphine to substituted alkenylisoxazoles has been developed. The reaction proceeds efficiently under mild conditions with high yields (up to 99%) and moderate to excellent enantioselectivities (up to 92%) thus providing a hitherto unavailable direct access to a library of chiral tertiary phosphine-functionalized isoxazoles. Copyright

Full text of DOI:10.1002/adsc.201300164

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DOI: 10.1002/adsc.201300164
Enantioselective Addition of Diphenylphosphine to 3-Methyl-4-
nitro-5-alkenylisoxazoles
Renta Jonathan Chew,a Yinhua Huang,a Yongxin Li,a Sumod A. Pullarkat,a
and Pak-Hing Leunga,*
a
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological
University, Singapore 637371
Fax : (+65)-6791-1961; phone: (+65)-6316-8905; e-mail: pakhing@ntu.edu.sg
Received: February 18, 2013; Revised: April 2, 2013; Published online: April 30, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300164.
Abstract: An enantioselective Michael addition reac- ble direct access to a library of chiral tertiary phos-
tion of diphenylphosphine to substituted alkenylisox- phine-functionalized isoxazoles.
azoles has been developed. The reaction proceeds ef-
ficiently under mild conditions with high yields (up
to 99%) and moderate to excellent enantioselectivi- Keywords: asymmetric catalysis; hydrophosphina-
ties (up to 92%) thus providing a hitherto unavaila- tion; isoxazoles; palladacycles; tertiary phosphines
Introduction
Isoxazoles and their derivatives have been reported
in recent years to exhibit antimycobacterial, COX-2
Chiral phosphines have been widely utilized as li- inhibitory, GABAA antagonist,[11] multidrug-resistance
gands for metal catalysis[1] as well as metal-free catal- transporter (MDR-1) inhibitor[12] as well as potential
ysis[2] by virtue of their ability to impart stereoselec- anti-cancer properties.[13] Significant interest has
tivity and also due to their unique steric and electron- therefore been focused on the generation of function-
ic properties. However, their conventional synthetic alized analogues of isoxazoles. However, their func-
routes usually need an enantiopure substrate, use of tionalization via nucleophilic addition to electron-de-
stoichiometric amounts of chiral auxiliary or a tedious ficient olefinic side chains has not been well ex-
and inefficient resolution process.[3] Asymmetric catal- plored.[14] Recently, the ability of styryloxazoles to
ysis is an ideal solution for the aforementioned chal- function as an excellent Michael acceptor in an asym-
lenges. Glueck and co-workers have pioneered the metric organocatalytic cyclopropanation reaction has
Pt(0)-[(R,R)-Me-Duphos]- and Pt(0)-(diphos)-cata- been reported.[15] In line with our interest in both ter-
lyzed addition of secondary phosphines to activated tiary phosphine synthesis as well as the study of cyto-
olefins.[4] Since then the catalytic hydrophosphinations toxic properties of various gold phosphine com-
of unsaturated aldehydes,[5] ketones,[6] imines,[6f] ni- plexes,[16] we herein disclose the efficient enantioselec-
triles,[4,7] esters[8] and nitroalkenes[9] have been report- tive synthesis of an array of enantioenriched, tertiary
ed. In many such protocols, phosphines being air sen- phosphine-functionalized isoxazoles in high to excel-
sitive are isolated as oxides, sulphides or borane ad- lent yields and appreciable enantioselectivities.
ducts. The removal of borane or further reduction to
yield free tertiary phosphines has low functional
group tolerance and is often associated with concomi- Results and Discussion
tant loss in optical purity.[10] Therefore, the direct syn-
thesis of tertiary phosphine ligands incorporating vari- Our preliminary investigations were conducted with
ous functionalities is critical. Access to such a method (E)-3-methyl-4-nitro-5-styrylisoxazole 1a and pallada-
not only allows for their direct use in organocatalysis cycle (S)-3[6c] as the catalyst (Table 1). In our hands,
but also in the generation of transition metal com- palladacycle (S)-3 and its analogues have been found
plexes and the in situ generation of the same in a cata- to be highly efficient in synthetic scenarios such as cy-
lytic reaction.
cloaddition,[16b,17] hydroamination,[18] heterocycle syn-
thesis[19] and in hydrophosphination reactions.[20] In
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Table 1. Optimization of conditions for the enantioselective hydrophosphination of (E)-3-methyl-4-nitro-5-alkenylisoxazole
1a with Ph2PH.[a]
Entry
Solvent
Temp [oC]
Cat. loading [mol%]
Base [equiv.]
Time [h]
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
THF
THF
THF
THF
THF
THF
acetone
toluene
CHCl3
DCM
DCM
DCM
À45
À45
À45
À45
À45
À45
À45
À45
À45
À45
À60
À80
5
3
1.5
1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
1
1
1
0.5
0.2
1
1
1
1
1
1
1
1
1
1
13
1
1
1
1
99
99
99
99
96
82
99
68
99
99
99
99
28
30
39
19
34
2
63
26
93
89
92
92
9
10
11
12
1
1
1
[a]
Conditions: 0.2 mmol of Ph2PH, 0.2 mmol of 1a, 5 mL degassed solvent.ꢁ
[b]
[c]
Yield is calculated from the 31P{1H} NMR spectrum of the crude product.
The ee is calculated from the 31P{1H} NMR integration of signals of diastereomers 5 and 6 arising from the treatment of
2a with chiral resolving agent (S)-4.
lyst present did not proceed even after 14 h. As previ-
ous studies have shown that triethylamine (Et3N) was
the ideal base, (due to its appropriate basicity and
ease of removal under reduced pressure)[6b,c,e,f] it was
employed in the current study. Nevertheless, we at-
tempted to reduce the amount of base used but un-
fortunately, both conversions and ees suffered
(Table 1, entries 5 and 6). The enantiomeric excess of
tertiary phosphine products was determined from the
the context of hydrophosphination, a significant chal- 31P{1H} NMR signals of diastereomers (S,R)-5a and
lenge posed by isoxazoles (especially the nitro-substi- (S,S)-5a obtained from the treatment of enantioen-
tuted variants) is that they possess several electron- riched 2a with a chiral resolving agent (S)-4.[21]
A
donating groups that can potentially coordinate to the single-crystal X-ray diffraction analysis of one such
catalyst. The hydrophosphination catalytic cycle in- derivative, 5b (Ar=4-Cl-C6H4) also revealed that the
volving palladacycles requires the simultaneous coor- absolute configuration of the newly formed chiral
dination of both isoxazole and diphenylphosphine on centre was R (Figure 1). A reaction conducted using
to the palladium in an appropriate orientation. The the (R)-3 as catalyst generated the S isomer in same
generation of a single isomer is therefore more chal- yields and similar enantiomeric excess.[22]
lenging since addition may occur when undesired
With the optimal conditions established, various
donors are bound to the Pd thus leading to loss of isoxazoles were screened and the results are present-
yields and enantioselectivity. The reaction when moni- ed in Table 2. The protocol can tolerate the presence
tored by 31P{1H} NMR was found to proceed smooth- of various substituted phenyls bearing electron-with-
ly under mild conditions. A systematic screening of drawing (cyano, ester), electron neutral (methyl) as
reaction conditions was carried out (Table 1). To our well as heterocycles (4-pyridyl, 2-furanyl, etc.) on the
delight, it was observed that in chloroform (CHCl3) C=C side chain of the isoxazole. In the context of the
the reaction afforded one of the highest conversions Michael addition of secondary phosphines to unsatu-
(99%) and enantiomeric excesses (ee) (93%) even rated systems it is generally observed that substrates
with a significantly low catalyst loading (1.5 mol%) at with electron-withdrawing substituents react more
a relatively mild temperature (À458C) (Table 1, rapidly when compared to those with electron-donat-
entry 9). It needs to be noted that a reaction set up ing moieties.[6a,c] However, in 1, the 4-nitro substituent
under exactly similar conditions but without the cata- functions as an excellent electron sink, thus greatly
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Enantioselective Addition of Diphenylphosphine to 3-Methyl-4-nitro-5-alkenylisoxazoles
clic rings afforded comparatively poorer ees and/or
lower reactivity (Table 2, entry 13).
It is indeed well known that phosphines possess
a high affinity towards transition metals. Therefore in
the current reaction scenario free diphenylphosphine
readily displaces the acetonitrile on the palladacycle
(Scheme 1). Due to the p-accepting effect of the aro-
matic naphthyl ring, the Ph2PH at the trans position
to it on Pd is labilized thus allowing 1 to coordinate
through the oxygen atom of the heterocyclic ring
yielding
a
monophosphine
intermediate
(Scheme 2).[6b] This marked oxophilicity of the posi-
tion trans to the C of the palladacycle has been clear-
ly established in an earlier report wherein a phos-
phine-sulphoxide ambidentate ligand prefers to form
À
a stable 6-membered P O chelate with the soft palla-
Figure 1. Molecular structure and absolute stereochemistry
of the derivative complex (S,R)-5b with 50% thermal ellip-
soids shown. Hydrogen atoms except those on the chiral
centres are omitted for clarity.[23]
À
dium rather than the expected 5-membered P S
ring.[24] Upon addition of a base, the secondary phos-
phine of the intermediate is now deprotonated to give
a highly reactive phosphido species that undergoes
a Michael addition to give the desired product. It
activating the conjugated olefin in the screened sub- needs to be noted in this context that the alternative
À
strates. Substituents at the meta and para positions mechanism of P H addition via oxidative addition
(Table 2, entries 2 and 3) of the phenyl ring afforded [usually involving Pd(0) and typically accompanied by
excellent selectivities while that at the ortho position the oxidation of the phosphines involved] was not ob-
gave much poorer selectivity (Table 2, entry 4). Simi- served in this instance. The superior performance we
larly, 3- and 4-substituted heterocyclic substrates observed for the phosphapalladacycle catalyst over
(Table 2, entries 14 and 15) too showed excellent re- the amine analogue in such scenarios is due to the en-
sults while substrates bearing 2-substituted heterocy- hanced ability of the phosphorus within the metalla-
Table 2. Substrate scope for the palladacycle (S)-3-catalyzed enantioselective hydrophosphination of (E)-3-methyl-4-nitro-5-
alkenylisoxazoles 1 with Ph2PH.[a]
Entry
Substrate 1
R
Time [h]
Product 2
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
Ph
1
1
1
1.5
1
1
1
1
1
1
1
1
25
1
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
>99
99
99
98
99
92
89
89
58
80
91
79
91
89
72
73
67
64
90
91
4-Cl-C6H4
3-Cl-C6H4
2-Cl-C6H4
4-F-C6H4
4-CF3-C6H4
4-Br-C6H4
4-MeO2C-C6H4
4-NC-C6H4
4-Me-C6H4
4-MeO-C6H4
2-furanyl
99
>99
>99
98
>99
99
98
93
99
99
9
10
11
12
13
14
15
2-pyrrolyl
3-pyridyl
4-pyridyl
1
[a]
Conditions: 0.2 mmol of Ph2PH, 0.2 mmol of 1, 5 mL degassed chloroform at À458C.
The ee is calculated from the 31P{1H} NMR integration of signals of diastereomers 5 and 6 arising from the treatment of 2
with chiral resolving agent (S)-4.
[b]
[c]
Yield is calculated from the 31P{1H} NMR spectrum of the crude product.
Adv. Synth. Catal. 2013, 355, 1403 – 1408
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Scheme 1. Proposed catalytic cycle.
Scheme 2. The possible intermediates in Scheme 1.
cycle to activate the coordinated diphenylphosphine addition step the eight-membered intermediate is
moiety prior to nucleophilic attack. This in turn is at- clearly unfavourable when compared to the six-mem-
tributed to the inherent ability of phosphorus to ac- bered intermediate formed for A. This we believe is
commodate extensive back donation from the palladi- a crucial factor in determining the product selectivity.
um centre. It should be highlighted that besides the
oxygen of the heterocycle, the oxygens on the nitro
substituent and to a lesser extent the nitrogen of the Conclusions
heterocycle can also potentially coordinate to the pal-
ladium yielding isomeric products (Scheme 2). From In summary, we have developed the first protocol for
a purely electronic point of view since the position carrying out the highly efficient enantioselective addi-
trans to the naphthalene carbon would prefer a more tion of diphenylphosphine to isoxazoles carrying
electron-rich moiety, the nitro oxygens are indeed fav- a wide range of side chain functionalities thus allow-
oured (B in Scheme 2). However, for the subsequent ing the direct transformation of a variety of alkenyl-
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Enantioselective Addition of Diphenylphosphine to 3-Methyl-4-nitro-5-alkenylisoxazoles
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ACHTUNGTRENNUNGisoxazoles to tertiary phosphines. This study also pro-
vides significant insights into factors affecting product
selectivity in hydrophosphination of substrates con-
taining multiple electron rich centres. We are current-
ly exploring the biological activity of the newly gener-
ated compounds as well as their potential application
in asymmetric catalysis.
Experimental Section
General Procedure for the Preparation of Chiral
Tertiary Monophosphines via Hydrophosphination of
Substituted Alkenylisoxazoles
To a solution of Ph2PH (37.2 mg, 0.2 mmol, 1 equiv.) in de-
gassed chloroform (CHCl3, 4 mL) was added (S)-3 (1.9 mg,
0.003 mmol, 1.5 mol%) and stirred at room temperature
until complete dissolution before cooling to À458C. Subse-
quently, substituted vinylisoxazole 1 (0.2 mmol, 1 equiv.) was
added, followed by dropwise addition of Et3N (20.2 mg,
0.2 mmol, 1 equiv.) in CHCl3 (1 mL) over a period of
20 min. The solution was stirred at À458C and the reaction
monitored by 31P{1H} NMR. Upon completion, the set-up
was allowed to warm to room temperature and the reaction
mixture filtered through a silica plug using a Pasteur pipette
fixed on a nitrogen-filled 2-necked Schenk flask to remove
(S)-3 and phosphine oxides (if any). Solvent was removed
from the eluent under reduced pressure to afford the chiral
tertiary phosphine 2 as the pure product. Enantiomeric
excess (ee) was determined from the integration of signals
of diastereomers 5 and 6 arising from the treatment of 2
with (S)-4 and/or (R)-4.
[9] J. J. Feng, M. Huang, Z. Q. Lin, W. L. Duan, Adv.
Acknowledgements
Synth. Catal. 2012, 354, 3122–3126.
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We thank Nanyang Technological University for supporting
this research (MOE-T2-2-065) and for a research scholarship
to R. J. Chew.
ˇ
Stankevic, K. M. Pietrusiewicz, Synthesis 2005, 1279–
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