Investigating palladium pincer complexes in catalytic asymmetric hydrophosphination and hydroarsination

Article abstract of DOI:10.1039/c9dt00221a

Given the periodic relationship of phosphines and arsines, is remodeling the catalytic asymmetric hydrophosphination reaction an efficient manner to develop the corresponding hydroarsination reaction? Herein, a chiral PCP-Pd(ii) pincer complex adept at ge

Full text of DOI:10.1039/c9dt00221a

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Investigating palladium pincer complexes in
catalytic asymmetric hydrophosphination and
Cite this: DOI: 10.1039/c9dt00221a
hydroarsination†
Wee Shan Tay, Xiang-Yuan Yang,
Pak-Hing Leung
Yongxin Li, Sumod A. Pullarkat
and
*
Given the periodic relationship of phosphines and arsines, is remodeling the catalytic asymmetric hydro-
phosphination reaction an efficient manner to develop the corresponding hydroarsination reaction?
Herein, a chiral PCP-Pd(II) pincer complex adept at generating enantioenriched phosphines was examined
in the asymmetric hydroarsination reaction. Under distinct conditions, tertiary phosphines and arsines
were generated in excellent yields (P: 96%, As: 91%) and ees (P: 90%, As: 85%). While secondary arsine
reagents were not direct substitutes for the analogous phosphines, important parameters were identified
which increased yield and ee of the hydroarsination reaction. Unlike the PCP-PdOAc pincer complex
commonly used for hydrophosphinations, hydroarsination reactions involved a PCP-PdCl catalyst with
10 equiv. of CsF for optimal performance. Notable differences between the two reactions and their
workup procedures were highlighted to guide further developments in the field. Lastly, respective mecha-
nisms were proposed and contrasted for the activation of HEPh2 (E = P, As).
Received 16th January 2019,
Accepted 5th March 2019
DOI: 10.1039/c9dt00221a
rsc.li/dalton
arsine compounds. Despite being perceived as a close phos-
phine analogue, organoarsine developments have lagged
Introduction
As chemical syntheses incline towards greener technologies, behind and preparations still rely primarily on traditional
catalytic asymmetric hydrofunctionalization reactions have methods involving harsh reagents/conditions.12 In addition,
steadily gained attention as an atom-economical C–heteroatom arsine compounds, widely perceived as toxic, may benefit from
bond formation strategy.1 Not only are reactants completely streamlined syntheses which can minimize health, environ-
incorporated into the desired product, chemical waste (solvents, mental and energy costs incurred.13 Catalytic asymmetric
auxiliaries, resolving agents etc.) is also minimized with the use hydrofunctionalizations often proceed under mild conditions
of stereoselective catalysts reducing the need for optical resolu- and hold great potential in minimizing wastage of chemicals14
tion (Scheme 1). Particularly for air-sensitive phosphines which or optical resolving agents encountered in arsine syntheses
suffer from cumbersome handling and workup procedures,2
heightened synthetic convenience has stimulated a wave of
applications across several fields such as pharmaceuticals,
agrochemicals, transition metal catalysis and organocatalysis.3
Notably, several arsine analogues have displayed superior
performance over the corresponding phosphines in indust-
rially-important cross-coupling (Suzuki,4 Hiyama,5 Stille,6
Negishi7), Heck,8 cyclization,9 hydroformylation10 and
carbonylation11 reactions. However, catalytic asymmetric
hydrofunctionalizations remain poorly adapted in furnishing
Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link,
637371 Singapore. E-mail: pakhing@ntu.edu.sg
†Electronic supplementary information (ESI) available. CCDC 1849918 1849920.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9dt00221a
Scheme 1 Strategies for chiral compound syntheses.
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today.15 Synthetic challenges have limited progress and the loading, base addition, ancillary ligand, temperature etc.) to
catalytic asymmetric hydroarsination methodology remains reaction conditions. Mildly-activated α,β-unsaturated ketones
virtually unexplored.16 In a pioneering work, it was observed were employed to broaden the substrate scope beyond highly
that ligand design was instrumental to good catalytic activity electron-deficient nitroalkenes.17 The catalytic activation of
and a Pd(II)-pincer complex (an established asymmetric hydro- secondary phosphine and arsine reagents revealed insights
phosphination catalyst) could catalyze the hydroarsination of which assisted in overcoming synthetic challenges of the
nitrostyrene albeit with modest ees.17 Correspondingly, this hydroarsination protocol. This study sets the ground to further
suggests that suitable catalysts may already exist in the array of utilise hydroarsination reactions for the efficient synthesis of
well-established Pd-based pincer type hydrophosphination cat- chiral arsines applicable in a variety of applications.
alysts.18 These complexes arguably have yet to be systematically
investigated, adapted and utilized for the catalytic asymetric
hydroarsination reaction (Fig. 1).
Results and discussion
Catalyst activity in hydrophosphination
Phosphines are generally regarded as more nucleophilic
than the corresponding arsine,19 which may be useful in pre-
The catalytic activity and stereoselectivity of PCP Pd(II)-pincer
dicting the catalytic activity observed for nucleophilic phospa-
complex 2 was examined with α,β-unsaturated ketone 1a
and arsa-Michael reactions. Activation of the H–E (E = P, As)
(Table 1). Acetone was selected as the solvent based on pre-
bond is also of significant interest in understanding such
vious optimization studies.18g The PdOAc-complex 2b was con-
sistent in achieving good ee (81%) in generating phosphine
sulfide 3 even at RT (entry 2). By lowering the temperature to
−40 °C, phosphine sulfide 3 could be isolated in 96% yield
with 90% ee (entry 4). A single recrystallization allowed the iso-
lation of enantiopure phosphine sulfide (S)-3 (Fig. 2), illustrat-
hydrofunctionalization reactions. With the lower pKa of
arsines (pKa of HPPh2 = 21.7, pKa of HAsPh2 = 20.3),20 it may
also be reasonable to predict that hydroarsination reactions
can be more challenging. However, factors such as ionization
potentials, bond energies and energies of solvation that
combine to determine the reactivity of nucleophiles are
complex.21 The presence of a transition metal catalyst further
reduces the predictability of how nucleophilicity of the
reagents influence the reaction, leaving experimental obser-
vations a crucial part of the investigation.
Table 1 Pd-Catalyzed asymmetric hydrophosphination reactiona
Herein, a Pd(II) hydrophosphination catalyst was applied in
the hydroarsination reaction after modifications (reagent
Entry
Cat.
Solvent
T (°C)
t (h)
Yieldb (%)
eec (%)
1
2
3
4
5
2a
2b
2b
2b
2b
Acetone
Acetone
Acetone
Acetone
MeOH
25
25
−25
−40
RT
24
8
18
30
15
4
ND
81
90
95
96
88
86
90 (>99)(S)d
11
a Reaction conditions: Enone 1a (12.1 mg, 0.05 mmol, 1.0 eq.), HPPh2
(13.96 mg, 0.08 mmol, 1.5 eq.), cat. (R,R)-2 (2.50 μmol, 5 mol%),
solvent (2 mL). b Isolated yield. c Determined by chiral HPLC of pure
phosphine sulfide 3. ee in parentheses obtained after a single recrystal-
lization. ND = not determined. d Absolute stereochemical configuration
determined by X-ray crystallographic analysis.
Fig. 1 Pd pincer complexes as catalysts in hydrophosphination
reactions.18
Fig. 2 Molecular structure of chiral phosphine sulfide (S)-3.
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ing the simplicity of deriving enantiopure compounds with this stereoselectivities were attempted. Firstly, reducing the reac-
protocol. Notably, the poor catalytic activity of PdCl-complex 2a tion temperature to 0 °C increased ee to 49% albeit with lower
in the hydrophosphination of enone 1a suggested a stark influ- yield of 28% (entry 6). Evidently, lower temperatures could not
ence of ancillary ligands on catalytic activity (entry 1).
singlehandedly improve catalytic performance as no catalytic
activity was observed below −40 °C (entry 7). Secondly, con-
ducting the reaction at 35 °C resulted in diminished yield of
16% (entry 8) even though higher temperatures typically
improve reactivities. Lastly, increasing the catalyst loading to
15 mol% decreased yield to 23% (entry 9). Despite these dis-
heartening results, Pd-catalyst 2b remained more favourable
than Pt- and Ni-complexes 5 and 6 bearing the same PCP
ligand.23
During the hydroarsination reaction catalyzed by PdOAc-
complex 2b, the homocoupling of diphenylarsine to form tet-
raphenyldiarsine 7 was observed and confirmed by X-ray crys-
tallography (Scheme 2). From a series of controlled experi-
ments, it was noted that diarsine 7 could also be obtained
from diphenylarsine with (1) heating at 50 °C in MeOH/H2O
(10% v/v), (2) PdOAc-complex 2b (5 mol%) or (3) DIPEA
(1.1 equiv.).24 This correlates with the lower yields observed
when the reaction was conducted at higher temperature (entry
8) or with increased catalyst loading (entry 9). Several other
syntheses of tetraphenyldiarsine 7 has been reported in litera-
ture,25 and other oligoarsines bearing As–As bonds have been
used as useful arsine precursors as well.12c,d,26 In this instance,
it is worth mentioning that no formation of arsine 4a was
observed when diarsine 7 was used as an alternative arsine
precursor in the presence of catalyst 2b.
Catalyst activity in hydroarsination
From the promising results obtained in Table 1, PdOAc-
complex 2b was further investigated for its efficiency in the
corresponding hydroarsination reaction (Table 2). It was
hypothesized that while the relatively weaker arsine nucleo-
phile would lead to poorer reactivity in an arsa-Michael reac-
tion,19,21 reactivity trends would still resemble the corres-
ponding hydrophosphination reaction. However, no arsine
adduct 4a was obtained from the hydroarsination of enone 1a
conducted in acetone (entry 1). Results of the solvent screening
did not resemble the Ni-catalyzed hydroarsination of nitro-
styrene either.17 Even though MeOH was previously the solvent
of choice for the Ni-catalyzed hydroarsination,17 it did not
afford phosphine adduct 3 (Table 1, entry 5) nor arsine adduct
4a (Table 2, entry 2) in good yields. While the Ni-catalyzed
hydroarsination benefited from the controlled addition of
water (10% v/v) which decreased reaction duration by up to
twelve-folds (60 min to 5 min) with a concomitant increase in
yield,17 the addition of 5–15% (v/v) of water in this instance
did not improve yields (26–34%) and ees (29–42%) signifi-
cantly (entries 3–5). The conjecture that reactivity was related
to solvent dielectric constant was also invalidated upon observ-
ing poor catalytic activity in solvents with similar dielectric
constants such as EtOH, acetonitrile, DMSO and nitro-
methane.22,23 Other common laboratory solvents were also
screened to no avail.
Effect of base on hydroarsination
Subsequently, the use of water as an additive, high tempera-
tures and PdOAc-complex 2b were avoided. In consideration
that diarsine 7 was obtained in the presence of DIPEA, a base
could directly influence the outcome of the reaction. It is par-
ticularly noteworthy that the modest pKa of diphenylarsine in
THF (pKa = 20.3)20 suggests that deprotonation may pose a
challenge, although no pKa values of HAsPh2 in MeOH were
available to the best of our knowledge. To complement the use
of an external base, PdCl-complex 2a bearing a non-basic
counteranion was considered and optimized for activity and
stereoselectivity (Table 3). Several bases were observed to have
varying degree of effects on reactivity and stereoselectivity
(entries 2–5).23 Overall, CsF (10 equiv.) afforded arsine 4a in
the highest yield (72%) and ee (85%) (entry 9). Several loadings
of CsF were screened (1, 2, 5, 10 and 20 equiv., entries 6–10)
which revealed a positive relationship between CsF loadings,
With 10% v/v MeOH/H2O (entry 4) being the best-perform-
ing solvent thus far, three strategies to improve yields and
Table 2 Optimization of hydroarsination conditionsa
Entry
Solvent
T (°C)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9d
Acetone
MeOH
25
25
25
25
25
0
−40
35
25
0
16
26
33
34
28
0
—
21
32
42
29
49
—
40
41
MeOH/H2O (5%)
MeOH/H2O (10%)
MeOH/H2O (15%)
MeOH/H2O (10%)
MeOH/H2O (10%)
MeOH/H2O (10%)
MeOH/H2O (10%)
16
23
a Reaction conditions: Enone 1a (12.1 mg, 0.05 mmol, 1.0 eq.), HAsPh2
b
(17.26 mg, 0.08 mmol, 1.5 eq.), cat. (5 mol%), solvent (2 mL). 1H
NMR yield with respect to enone 1. c Determined by chiral HPLC of the
crude reaction mixture. d Cat. (R,R)-2b (6.74 mg, 7.50 μmol, 15 mol%)
was used instead.
Scheme 2 Formation of diarsine species.
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Table 3 Optimization of conditions for PCP Pd–Cl catalyst 2aa
Table 4 Substrate scope of Pd–Cl cat. 2aa
Entry
4
Ar
Yieldb (%)
eec (%)
Entry
Base
T (°C)
Yieldb (%)
eec (%)
1
2
3
4
5
4a
4b
4c
4d
4e
4-ClC6H4
C6H5
4-OCH3C6H4
4-CH3C6H4
4-CF3C6H4
72
45
34
36
73
85
78
72
81
84
1
2
3
4
5
6
7
8
—
K2CO3
DIPEA
Pyridine
NaSO3
CsF
CsF (2 equiv.)
CsF (5 equiv.)
CsF (10 equiv.)
CsF (20 equiv.)
CsF (10 equiv.)
CsF (10 equiv.)
CsF (10 equiv.)
CsF (10 equiv.)
CsF (10 equiv.)
25
25
25
25
25
25
25
25
25
25
0
0
55
36
16
49
18
40
54
72
73
54
23
63
55
91
—
62
11
31
71
56
71
74
85
81
80
65
70
81
85
a Reaction conditions: Enone
1 (0.05 mmol, 1.0 eq.), HAsPh2
(17.26 mg, 0.08 mmol, 1.5 eq.), cat. (R,R)-2a (2.19 mg, 2.50 μmol,
5 mol%), CsF (76.0 mg, 0.50 mmol, 10.0 eq.), solvent (2 mL). 1H
b
9
NMR yield with respect to enone 1. c Determined by chiral HPLC of the
10
11
12
13
14d
15e
crude reaction mixture.
−20
35
25
25
significantly and relatively electron-rich enones 1b–d corre-
sponded to the lower yields of arsine adduct 4 (34–45%)
(entries 2–4). Conversely, comparable yields were observed for
enones with 4-chloro (4a, 72% yield) and 4-trifluoromethyl (4e,
73% yield) substituents (entries 1 and 5). These observations
were not surprising given the nucleophilic nature of the sec-
ondary arsine reagent in the hydroarsination reaction. With
electronic and stereochemical limitations imposed by PdCl-
catalyst 2a, an existing repository of other pincer complexes
bearing structurally dissimilar ligands (Fig. 1) may be
advantageous.
a Reaction conditions: Enone 1a (12.1 mg, 0.05 mmol, 1.0 eq.), HAsPh2
(17.26 mg, 0.08 mmol, 1.5 eq.), cat. (R,R)-2a (2.12 mg, 2.50 μmol,
5 mol%), base (0.05 mmol, 1.0 eq.), solvent (2 mL). 1H NMR yield
b
with respect to enone 1. c Determined by chiral HPLC of the crude reac-
tion mixture. d Reaction was stirred for 50
h
instead. e HAsPh2
(36.82 mg, 0.16 mmol, 3 equiv.) was used instead.
yields and stereoselectivities (Fig. 3).27 Gratifyingly, no compli-
cation arose from the plausible formation of diarsine 7 even
with a large excess of CsF. As expected, low and high tempera-
tures were less favourable for yields (23–63%) and ees
(65–80%) (entries 11–13). Longer reaction times of up to 50 h
were unsuccessful in raising reaction yields (entry 14). Instead,
yield could be increased to 91% without compromising stereo-
selectivities (85% ee) by using 3 equiv. of diphenylarsine
(entry 15).
Characteristics of arsine adducts
It may be relevant to briefly discuss differences in handling
these phosphine and arsine derivatives of enone 1 as a refer-
ence for future work (Table 5). Typically, free hydrophosphina-
tion adducts are protected prior to work-up for ease of hand-
ling such as in Table 1. Reagents chemically react with
18h,f,28
(H2O2,18a–c,e,f S8
etc.) or coordinate to (BH3,18d transition
Electronic effect of substituents on hydroarsination
metals29 etc.) the free tertiary phosphines, converting them to
bench-stable compounds tolerant of common purification pro-
cesses (e.g. extraction, column chromatography). However,
such reagents were incompatible (H2O2, transition metals) or
unreactive (S8, BH3) with arsines 4 and degraded products
such as enone 1 were observed. In fact, free arsines 4 (unlike
the tertiary phosphine analogue) were resistant to oxidation in
solid and solution states under ambient conditions for a pro-
longed period of time. Aside from workup procedures, other
reactivity differences were observed between secondary phos-
phines and arsines. For example, no formation of tetraphenyl-
diphosphine was observed under conditions that rapidly form
tetraphenyldiarsine 7 (Scheme 2). Consequently, higher load-
ings of arsine (3 equiv., Table 3, entry 15) may be required for
comparable yields to the analogous hydrophosphination reac-
tion. With respect to chiral catalyst 2, the hydrophosphination
of enone 1a was slightly more stereoselective than the corres-
ponding hydroarsination under respective optimized con-
ditions (P: 90% ee, As: 85% ee). The lower stereoselectivity of
Under optimized conditions, PdCl-catalyst 2a was screened to
investigate the electronic effect of substituents on the hydro-
arsination reaction (Table 4). Para-substituted enones 1 were
selected to minimize the steric influence of substituents on
the reactive site. Generally, stereoselectivities were relatively
consistent at 72–85% ee (entries 1–5). However, yields varied
Fig. 3 Relationship between [CsF], yield and ee.
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Table 5 Summary of differences
E = As
E = P
Stability of E(III) state
Higher
Lower
Common workup procedure
Byproduct formation
No protection
Ph2As–AsPh2
Lower
Protection of lone pair (e.g. with H2O2, S8, BH3)
Ph2P–PPh2 (slow)
Higher
Stereoselectivity of hydrofunctionalizationa
Scope of chiral ligands
Limited (e.g. PCP)
More (e.g. PCP, NCN, PCN, NCS, CP, CN)
a With respect to PCP Pd catalyst 2 used.
the hydroarsination reaction under optimized conditions tiary phosphine adduct. While the mechanism for Pd-catalyzed
could be partially attributed to the larger size of As reducing hydrophosphinations is generally well-understood, there is a
the effectiveness of stereo-induction from chiral catalyst 2. lack of evidence to suggest that these mechanisms are immedi-
Lastly, several other potential catalysts screened decomposed ately applicable to the hydroarsination reaction especially
under an excess of diphenylarsine even though they thrived with notable differences observed during solvent screening
under hydrophosphination conditions (Fig. 4). Coordinatively (Table 2).23 To investigate the interactions between HAsPh2
robust tridentate pincer complexes30 may have an advantage and PdCl-complex 2a in hydroarsination, a series of 31P{1H}
over other common catalyst classes such as bidentate cyclome- NMR studies were conducted in MeOD to replicate the solvent
talated complexes (e.g. complex 8,29b
9
29c) or even other pincer environment under optimized conditions. The key findings are
complexes (mono- or bi-metallic) with weaker donor atoms summarized in Scheme 3.31 Two pathways (a) and (b) were
(e.g. complex 10,18h 11 18f).
responsible for generating the active intermediate A. In the
absence of a base, HAsPh2 could still generate intermediate A,
albeit to a smaller extent, either directly or via intermediate
B.31 The presence of an external base (e.g. DIPEA) afforded a
relatively larger population of the active intermediate A. This
was achieved by generating a more reactive arsenide nucleo-
phile that displaced the chloride anion to form intermediate A
(pathway a),32 or to deprotonate the acidified H–As bond on
intermediate B (pathway b). Despite our best efforts, a single
crystal of intermediate A could not be obtained and was
instead only observed via 31P{1H} NMR spectroscopy.31 Other
terminal arsenido complexes of Pd have been reported before,
but are rarely isolated due to their instability and the
potential high reactivity at the arsenido and metal centres.33
Subsequently, the formation of hydroarsination adducts 4 are
proposed to resemble the hydrophosphination pathway given
Deprotonation of HEPh2 (E = P, As)
From experimental observations, it was noted that PdOAc-
complex 2b was an efficient catalyst in hydrophosphination
while PdCl-complex 2a was better suited for hydroarsination. It
is evident that the ancillary ligand on Pd-complex 2 has a
direct influence on the catalyst reactivity in hydroarsination
(Tables 2 and 3). This might indicate different activation path-
ways of HEPh2 (E = P, As) involving complexes 2a and 2b under
their respective optimized conditions. The proposed hydropho-
sphination mechanism catalyzed by PCP PdOAc complexes has
been independently reported by the groups of Duan18c and
Leung.29a Generally, the Pd-catalyzed hydrophosphination
reaction entails four main steps: (1) coordination of HPPh2
to the PdOAc-complex, (2) deprotonation of HPPh2 by the
internal acetate base, (3) intermolecular 1,4-nucleophilic
attack on the substrate, and (4) protonation to afford the ter-
Scheme 3 Proposed interactions of catalyst 2a and HEPh2 under
Fig. 4 Other Pd(II) complexes investigated in this work.
respective optimized reaction conditions.
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the similar stereo- and regio-selectivities observed in Pd- (2 mL) and brought to the desired temperature. Enone 1a
complex 2-catalyzed hydrophosphination and hydroarsination (12.1 mg, 0.05 mmol, 1.0 equiv.) was subsequently added and
reactions (vide supra). While a single crystal of arsine 4 suitable the reaction was stirred at the stated temperature. Reaction
for X-ray diffraction analyses could not be obtained after progress was monitored with 31P{1H} NMR spectroscopy and
repeated attempts, the absolute chirality of arsine adduct 4 upon complete conversion, S8 (3.21 mg, 0.10 mmol, 2.0 equiv.)
was assigned to be (S) based on phosphine (S)-3 (Fig. 2) in was added and the solution was allowed to RT. After stirring
view of similar C–E (E = As, P) bond formation steps.18g,h,34 for 30 min at RT, volatiles were removed and the crude product
Importantly, it is to be noted that the key step of acidifying the was purified via silica gel chromatography (5 hexane : 1 EA) to
H–E bond for both the hydrophosphination and hydroarsina- afford the pure phosphine sulfide 3 as a white solid. The ee
tion reactions occur upon interaction with Pd catalyst 2. was determined on a Daicel Chiralpak IC column with
Notable differences in the two reactions (under their respective n-hexane/2-propanol = 97/3, flow = 0.8 mL min−1, wavelength =
optimized condition) still exist regarding the activation of 220 nm. Retention times: 9.5 min (major, S isomer), 12.1 min
HEPh2 (E = As, P). For HPPh2, initial coordination to PdOAc (minor,
R isomer). [α]D = −226.0 (c 1.00, DCM). Mp:
catalyst 2b was required before the phosphido-intermediate C 117.1–118.2 °C. 1H NMR (CDCl3, 400 MHz): δ 8.18–8.12 (m,
could be generated.18c,29a With PdCl-complex 2a, no chloride 2H, Ar), 7.86–7.84 (m, 2H, Ar), 7.55–7.50 (m, 6H, Ar), 7.42–7.40
displacement was observed by HPPh2 to generate intermedi- (m, 3H, Ar), 7.28–7.25 (m, 4H, Ar), 7.08–7.06 (m, 2H, Ar), 4.83
ates B or C (pathways c and d).35 This was clearly demon- (ddd, 1H, 3JPH = 10.2 Hz, 3JHH = 10.2 Hz, 3JHH = 2.4 Hz, PCCH),
2
3
2
strated when the hydrophosphination of enone 1a did not 4.06 (ddd, 1H, JPH = 18.3 Hz, JHH = 10.5 Hz, JHH = 5.24 Hz,
2
3
2
proceed in the presence of PdCl complex 2a in acetone as the PCH), 3.30 (ddd, 1H, JPH = 18.0 Hz, JHH = 11.6 Hz, JHH = 2.4
optimized solvent of choice (Table 1, entry 1). On the other Hz, PCH); 13C NMR (CDCl3, 100 MHz): δ 196.8 (s, 1C, CvO),
1
hand because of the highly polar environment favoured for the 133.8–128.3 (12C, Ar), 40.8 (s, 1C, C(O)CH), 39.8 (d, 1C, JPC
=
hydroarsination reaction, trace quantities of −AsPh2 ions also 3.9 Hz, PC); 31P{1H} NMR (CDCl3, 162 MHz): δ 51.0. HRMS
exist under reaction conditions. This partially contributes to (+ESI) m/z: (M + H)+ calcd for C27H23ClOPS, 461.0896; found,
the formation of intermediate A, which is made more signifi- 461.0892.
cant in the presence of weak bases.32 In essence, the choice of
General procedure for catalytic asymmetric hydroarsination.
solvent directly influences the reaction in terms of generating HAsPh2 (17.26 mg, 0.08 mmol, 1.5 equiv.) was charged to a
the active catalytic intermediates A and C.
pre-weighed Schlenk vessel under N2. Catalyst (2.50 μmol,
5 mol%) and base (0.50 mmol, 10.0 equiv.) was added, washed
down with the stated solvent (2 mL) and brought to the
desired temperature. Enone 1 (0.05 mmol, 1.0 equiv.) was sub-
sequently added and the reaction was stirred at the stated
temperature. After 24 h, volatiles were removed and two drops
of the crude reaction mixture was withdrawn from the flask
and diluted with IPA (1 mL) to prepare the HPLC sample.
Arsine adduct 4a could be crystallized from DEE to afford the
pure product 4a as white crystalline needles.
Experimental
Materials and methods
All reactions were carried out under a positive pressure of
nitrogen using standard Schlenk techniques. Solvents were
purchased from their respective companies (ACN, MeOH,
DCM: VWR Chemicals, DEE: Merck, toluene, n-hexane
Avantor, acetone: Sigma-Aldrich, THF: Tedia) and used as sup-
plied. DCM and THF were dried and distilled. Where necess-
ary, solvents were degassed prior to use. A Low Temp
Pairstirrer PSL-1400 was used for controlling low temperature
reactions. Column chromatography was done on Silica gel 60
(Merck). Melting points were measured using SRS Optimelt
Automated Point System SRS MPA100. Optical rotations were
measured with JASCO P-1030 Polarimeter in the specified
solvent in a 0.1 dm cell at 22.0 °C. NMR spectra were recorded
on Bruker AV 300, AV 400 and AV 500 spectrometers. Chemical
shifts were reported in ppm and referenced to an internal
SiMe4 standard (0 ppm) for 1H NMR, chloroform-d
(77.23 ppm) for 13C NMR, and an external 85% H3PO4 for 31P
{1H} NMR.
4a White solid. 72% yield. The ee was determined on a
Daicel Chiralpak IF column with n-hexane/2-propanol = 98/2,
flow = 1.0 mL min−1, wavelength = 230 nm. Retention times:
10.2 min (major), 11.2 min (minor). [α]D = −171.8 (c 4.70, DEE)
1
(measured for Table 4 entry 1). Mp: 137.2–138.2 °C. H NMR
(CDCl3, 400 MHz): δ 7.76–7.73 (m, 2H, Ar), 7.63–7.61 (m, 2H,
Ar), 7.52–7.48 (m, 1H, Ar), 7.42–7.35 (m, 5H, Ar), 7.23–7.15 (m,
3H, Ar), 7.11–7.08 (m, 4H, Ar), 7.06–7.02 (m, 2H, Ar), 4.19 (dd,
1H, 3JHH = 11.2 Hz, 2JHH = 3.2 Hz, AsCCH), 3.67 (dd, 1H, 3JHH
=
3
3
17.2 Hz, JHH = 11.2 Hz, AsCH), 3.22 (dd, 1H, JHH = 17.2 Hz,
2JHH = 3.2 Hz, AsCCH); 13C NMR (CDCl3, 100 MHz): δ 198.2 (s,
1C, CvO), 134.1–128.2 (12C, Ar), 42.2 (s, 1C, AsC), 40.2 (s, 1C,
C(O)CH). HRMS (+ESI) m/z: (M + H)+ calcd for C27H23AsClO,
473.0653; found, 473.0648.
4b White solid. The ee was determined on a Daicel
Chiralpak IF column with n-hexane/2-propanol = 99/1, flow =
Synthetic procedures
General procedure for catalytic asymmetric hydrophosphina- 0.8 mL min−1, wavelength = 230 nm. Retention times:
tion. HPPh2 (13.96 mg, 0.08 mmol, 1.5 equiv.) was charged to 18.5 min (major), 20.1 min (minor). [α]D = −38.6 (c 4.90, DEE)
1
a pre-weighed Schlenk vessel under N2. Catalyst 2 (2.50 μmol, (measured for Table 4 entry 2). Mp: 126.3–127.2 °C. H NMR
5 mol%) was added, washed down with the stated solvent (CDCl3, 400 MHz): δ 7.75–7.73 (m, 2H, Ar), 7.65–7.63 (m, 2H,
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Ar), 7.50–7.46 (m, 1H, Ar), 7.41–7.34 (m, 5H, Ar), 7.22–7.18 (m, and ees of the hydroarsination reaction. The hydrophosphina-
1H, Ar), 7.15–7.12 (m, 6H, Ar), 7.08–7.05 (m, 3H, Ar), 4.23 (dd, tion reaction conducted with PCP PdOAc complex 2a at −40 °C
1H, 3JHH = 11.2 Hz, 2JHH = 3.2 Hz, AsCCH), 3.73 (dd, 1H, 3JHH
=
without the addition of an external base afforded phosphine
3
3
17.2 Hz, JHH = 11.2 Hz, AsCH), 3.21 (dd, 1H, JHH = 16.8 Hz, sulfide 3 in 96% yield with 90% ee. In contrast, PCP PdCl
2JHH = 3.2 Hz, AsCCH); 13C NMR (CDCl3, 100 MHz): δ 198.5 (s, complex 2b afforded arsine 4a in 91% yield with 85% ee only
1C, CvO), 135.3–126.3 (12C, Ar), 42.4 (s, 1C, AsC), 40.9 (s, 1C, under an excess of HAsPh2 and 10 equiv. of CsF at RT. These
C(O)CH). HRMS (+ESI) m/z: (M + H)+ calcd for C27H24AsO, conditions were rationalized upon identifying the formation of
439.1043; found, 439.1043.
tetraphenyldiarsine 7 via homocoupling of HAsPh2 (Scheme 2).
4c White solid. The ee was determined on a Daicel Experimental evidence suggests that secondary arsine reagents
Chiralpak IF column with n-hexane/2-propanol = 97/3, flow = (HAsPh2) could not be directly used in place of the analogous
1.0 mL min−1, wavelength = 254 nm. Retention times: phosphine (HPPh2) while expecting similar reactivities and
15.5 min (major), 17.8 min (minor). [α]D = −23.7 (c 3.80, DEE) stereoselectivities. Nevertheless, capitalizing on the existing
(measured for Table 4 entry 3). 1H NMR (CDCl3, 400 MHz): selection of established hydrophosphination catalysts is argu-
δ 7.74–7.72 (m, 2H, Ar), 7.64–7.62 (m, 2H, Ar), 7.50–7.46 (m, ably the most efficient way forward for furthering the corres-
1H, Ar), 7.42–7.33 (m, 5H, Ar), 7.20–7.13 (m, 3H, Ar), 7.09–7.04 ponding hydroarsination reaction. To that effect, differences
3
(m, 4H, Ar), 6.70–6.68 (m, 2H, Ar), 4.18 (dd, 1H, JHH
=
encountered for these two reactions (and subsequent workup
2
3
11.1 Hz, JHH = 3.0 Hz, AsCCH), 3.67 (dd, 1H, JHH = 16.9 Hz, procedures) were summarized to aid the handling of arsine
3
2
3JHH = 11.3 Hz, AsCH), 3.17 (dd, 1H, JHH = 17.0 Hz, JHH
3.1 Hz, AsCCH). HRMS (+ESI) m/z: (M + H)+ calcd for activation of HPPh2 and HAsPh2 were discussed. Clearly, cata-
C28H26AsO2, 469.1149; found, 469.1148. lytic asymmetric hydroarsinations remain a fertile field for
=
compounds. In addition, crucial differences between catalyst
4d White solid. The ee was determined on a Daicel development in terms of (1) catalyst scope beyond hom-
Chiralpak IF column with n-hexane/2-propanol = 98/2, flow = ogenous platinum-group pincer complexes and (2) substrate
1.0 mL min−1, wavelength = 254 nm. Retention times: scope (e.g. unactivated CvC bonds, CvX bonds). With substi-
11.9 min (major), 13.4 min (minor). [α]D = −48.8 (c 3.70, DEE) tuted secondary arsines, this methodology may even directly
(measured for Table 4 entry 4). 1H NMR (CDCl3, 400 MHz): access organoarsines bearing chirality on the As atom for use
δ 7.80–7.78 (m, 2H, Ar), 7.66–7.63 (m, 3H, Ar), 7.49–7.38 (m, in coordination chemistry.36
6H, Ar), 7.16–7.12 (m, 6H, Ar), 7.07–7.05 (m, 2H, Ar), 4.22 (dd,
1H, 3JHH = 11.1 Hz, 2JHH = 3.2 Hz, AsCCH), 3.70 (dd, 1H, 3JHH
=
3
3
17.0 Hz, JHH = 11.1 Hz, AsCH), 3.18 (dd, 1H, JHH = 16.9 Hz, Conflicts of interest
2JHH = 3.2 Hz, AsCCH), 2.35 (s, 3H, CH3). HRMS (+ESI) m/z:
(M + H)+ calcd for C28H26AsO, 453.1200; found, 453.1198.
There are no conflicts to declare.
4e White solid. The ee was determined on a Daicel
Chiralpak IF column with n-hexane/2-propanol = 99/1, flow =
0.8 mL min−1, wavelength = 260 nm. Retention times:
14.8 min (major), 16.2 min (minor). [α]D = −56.6 (c 6.80, DEE)
(measured for Table 4 entry 5). 1H NMR (CDCl3, 400 MHz):
δ 7.80–7.76 (m, 3H, Ar), 7.64–7.62 (m, 2H, Ar), 7.58–7.47 (m,
4H, Ar), 7.42–7.36 (m, 6H, Ar), 7.21–7.14 (m, 3H, Ar), 7.01–7.05
Acknowledgements
W. S. T. acknowledges NTU for the award of a research scholar-
ship. The MOE Tier 1 grant (M4011537 RG108/15) is acknowl-
edged for funding this work. We are also grateful to Prof. S. B.
Wild of the Australian National University for his insightful
discussions which contributed to this work.
3
2
(m, 1H, Ar), 4.28 (dd, 1H, JHH = 11.2 Hz, JHH = 3.2 Hz,
3
3
AsCCH), 3.75 (dd, 1H, JHH = 17.4 Hz, JHH = 11.2 Hz, AsCH),
3
2
3.29 (dd, 1H, JHH = 17.5 Hz, JHH = 3.3 Hz, AsCCH); 19F NMR
(CDCl3, 377 MHz): δ −62.4 (s). HRMS (+ESI) m/z: (M + H)+
calcd for C28H23AsF3O, 507.0917; found, 507.0911.
Notes and references
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In conclusion, a PCP Pd(II) pincer catalyst was demonstrated to
successfully catalyze the asymmetric hydroarsination of func-
tionalized enones in excellent yields (up to 91%) and ees (up
to 85%). As a catalyst originally optimized for the hydropho-
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