Chiral phosphapalladacycles as efficient catalysts for the asymmetric hydrophosphination of substituted methylidenemalonate esters: Direct access to functionalized tertiary chiral phosphines

Article abstract of DOI:10.1021/om201115n

A chiral palladacycle-promoted enantioselective asymmetric hydrophosphination of substituted methylidenemalonate esters using diphenylphosphine that provides direct access to chiral tertiary phosphines is reported. Screening of three easily accessible C,N and C,P palladacycles as catalysts for this synthetic scenario provided insights into critical factors in catalyst design that influence the activation and stereochemistry in Pd(II)-catalyzed asymmetric P-H addition reactions involving such activated substrates.

Full text of DOI:10.1021/om201115n

Article
pubs.acs.org/Organometallics
Chiral Phosphapalladacycles as Efficient Catalysts for the
Asymmetric Hydrophosphination of Substituted
Methylidenemalonate Esters: Direct Access to Functionalized
Tertiary Chiral Phosphines
Chang Xu,† Gan Jun Hao Kennard,† Felix Hennersdorf,‡ Yongxin Li,† Sumod A. Pullarkat,*,†
and Pak-Hing Leung*,†
†Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637616, Singapore
‡Institute of Inorganic Chemistry, Faculty of Chemistry and Mineralogy, University of Leipzig, Johannisalle 29, D-04103, Leipzig,
Germany
S
* Supporting Information
ABSTRACT: A chiral palladacycle-promoted enantioselective
asymmetric hydrophosphination of substituted methylidene-
malonate esters using diphenylphosphine that provides direct
access to chiral tertiary phosphines is reported. Screening of
three easily accessible C,N and C,P palladacycles as catalysts
for this synthetic scenario provided insights into critical factors
in catalyst design that influence the activation and stereochemistry in Pd(II)-catalyzed asymmetric P−H addition reactions
involving such activated substrates.
functionalized phosphine products were obtained in low ee's.5
This is indeed a drawback when considering the vast range of
functionalities tolerated in traditional methods.
INTRODUCTION
Functionalized enantiopure phosphines are widely utilized as
efficacious ligands toward a range of metal-catalyzed industrially
■
relevant asymmetric transformations such as hydrogenations,1
Herein we report the investigations into the palladacycle-
cross-coupling reactions,2 and hydroformylations.3 However
the employment of stoichiometric promoters and resolving
agents has been the dominant approach in their synthesis.4
Metal-catalyzed asymmetric addition of the P−H moiety to
unsaturated functionalized substrates offers an alternative atom-
efficient protocol for accessing these compounds. Among the
few reports in the literature of such a protocol are the
pioneering work by Glueck et al. involving the Pt0-(Me-
Duphos)- and Pt0-(diphos)-catalyzed hydrophosphination of
activated olefins,5 organocatalyst-catalyzed hydrophosphination
of nitroalkenes6 and α,β-unsaturated aldehydes,7 and chiral
P,C,P and P,C,N pincer complex catalyzed hydrophosphination
of enones and α,β-N-acylpyrroles, generating phosphine
oxides.8 We have previously disclosed the extensive utilization
of stoichiometric amounts of the chiral palladacycle (R/S)-
{[Pd[Me2NCH(Me)C10H6](μ-Cl)}2 and its derivatives toward
the synthesis of a wide range of chiral tertiary phosphine ligand
motifs.9 Recently, we had also extended their use to the
catalytic asymmetric hydrophosphination of aromatic enones,
thus directly generating chiral keto-functionalized tertiary
phosphines in good yields and enantioselectivities.10a−c
However, catalytic asymmetric hydrophosphination of sub-
strates other than those carrying cyano, keto, nitro, aldehyde,
and pyrrole functionalities has been conspicuously absent in the
literature except for the few instances wherein ester-
promoted asymmetric P−H bond addition of ester to
functionalized unsaturated moieties and present the results
from our studies, which involve the first instance in the
literature of catalytic addition of secondary phosphines to esters
leading to the efficient generation of the corresponding chiral
tertiary phosphines in good yields and high enantioselectivities.
The analysis of three P,C- and N,C-based palladacycles in their
role as catalysts gave us crucial insights into factors that
influence the design of efficient catalysts in such synthetic
scenarios.
RESULTS AND DISCUSSION
■
The three palladacycles employed in this study (Figure 1) were
prepared by treating the corresponding chloro-bridged dimeric
palladium compounds11a−c with silver perchlorate in acetoni-
trile via a procedure reported previously for (R)-110a and (R)-
3.11d In our hands, these complexes and their derivatives have
repeatedly proven their efficacy in asymmetric C−C,12a C−
N,13c C−P,9a−e,12c and C−As12b,13a,b bond formation scenarios.
A key structural feature in these palladacyles is that the
organometallic ring is locked into the static δ conformation in
solution (for the R isomer).14 This in turn fixes the position of
Received: November 11, 2011
Published: March 20, 2012
© 2012 American Chemical Society
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complexes. It was evident from our initial results that the
activation in these two instances does not meet the require-
ments for a metal-catalyzed P−H addition across the CC
bond. We have previously reported that this class of
cyclopalladated complexes can indeed recognize the subtle
electronic differences between the keto and ester functional
groups. For instance, they activate the CC bonds in vinyl
ketones efficiently toward cycloaddition reactions16 but provide
only poor activation to the CC bonds in acrylates.17 We
therefore proceeded to examine the addition of diphenylphos-
phine to diethyl benzylidenemalonate. With this diester-
substituted substrate, the hydrophosphination proceeded in a
facile manner, requiring only 1.5 h for full product conversion
at room temperature (RT). The 31P{1H} NMR spectrum of the
product in CDCl3 displayed a singlet at δ 2.7, indicating a
highly regioselective addition.
Figure 1. Palladacycles used in the study.
the prochiral NMe/PPh groups into nonequivalent axial and
equatorial positions, whereby they can control the stereo-
chemistry at the neighboring coordination sites. The electronic
properties of the N and P heteroatoms and the comparatively
stronger π-accepting aromatic carbon of the organopalladium
ring allow the complex to possess unique molecular recognition
ability with respect to incoming substrates.15
The three chosen complexes thus provided us an opportunity
to evaluate the effect of variation in key electronic factors (N vs
P), steric tuning of substituents on the heteroatom (Me2 vs
Ph2), and different moieties at the site adjacent to the Pd−C
bond on the aromatic ring (H vs Me) while retaining the key
features that have made them highly effective chiral auxiliaries.
With these catalysts in hand we proceeded to test their efficacy
in the asymmetric P−H addition to unsaturated esters (Table
1).
An efficient and reliable procedure for the analysis of ee of
such tertiary phosphines has been previously reported by us10
and is shown in Scheme 1.
Scheme 1. Protocol Adopted for the Determination of ee by
Coordination of the Tertiary Phosphine to (R)-
{[Pd[Me2NCH(Me)C10H6](μ-Cl)}2
Our preliminary investigation started with the P−H addition
of diphenylphosphine to ethyl cinnamate. The reactions were
performed in the presence of 5 mol % of (R)-1, (S)-2, and (R)-
3, respectively, under conditions that have been established
from our previous studies on enones.10a−c However all three
complexes failed to generate any monophosphine product even
after 2 weeks at room temperature. The reaction of diethyl
allylmalonate with diphenylphosphine also did not proceed
under similar conditions.
The adequate activation of the CC bond by adjacent
electron-withdrawing moieties has been established to be a key
factor that aids such P−H addition protocols using metal
a
Table 1. Screening of Reaction Conditions
b
c
entry
cat
solvent
base
NEt3
temp (°C)
reactivity (h)
conversion (%)
ee (%)
1
(R)-1
(R)-1
(S)-2
(R)-3
(S)-1
(R)-1
(R)-1
(R)-1
(R)-1
(R)-1
(R)-1
(R)-1
(R)-1
(R)-1
DCM
DCM
DCM
DCM
DCM
DCM
acetone
MeCN
RT
−80
−80
−80
−80
−40
−80
−40
−40
−80
RT
1.5
3.5
24
99
99
63
96
2
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
DBU
t-BuONa
3
99
65
4
7
99
86
5
3.5
2.5
>6
4
99
96
6
99
93
7
99
95
8
99
81
9
chloroform
24
99
85
10
11
12
13
14
THF
NIL
10
NIL
83
NIL
18
1,4-dioxane/water, 10:1
toluene
DCM
DCM
−80
−80
−40
>24
24
NIL
35
NIL
NIL
36
d
5.5
99
a
Conditions: HPPh2 (50.0 mg, 2.69 × 10−4 mol), 5 mol % cat, 1.2 equiv of diethyl benzylidenemalonate (80.2 mg, 3.23 × 10−4 mol), 1 equiv of base
b
(2.69 × 10−4 mol), and 6 mL of degassed dichloromethane were reacted at the indicated temperature. Conversion was calculated from 31P{1H}
c
NMR. ee was determined from 31P{1H} NMR integration of the respective signals via the use of a chiral derivatizing agent. The detailed procedure
d
t
is described in S5. Solid BuONa was used.
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This procedure was used to determine the ee of 63% by
31P{1H} NMR spectroscopy for the asymmetric P−H bond
addition to diethyl benzylmalonate at RT (Table 1, entry 1). It
needs to be noted that the use of this procedure usually allows
us to obtain single crystals of the adducts of the tertiary
phosphine on the palladacycle directly, thus allowing the
assignment of absolute stereochemistry from X-ray diffraction
data. Interestingly, for the current diester-substituted phosphine
complexes, it is necessary to replace the chloro ligand with iodo
in order to generate single crystals that are suitable for
structural investigations (Figure 2).
the change of catalyst from (R)-1 to its equally accessible
enantiomeric counterpart (S)-1 resulted in the generation of
the opposite enantiomeric form of the tertiary phosphine
product in exactly the same ee (Table 1, entries 2 and 5).
Besides providing definitive evidence of the influence of the
chiral catalyst on stereocontrol, this experiment also provides
access to both enantiomers of the product in high optical
purity.
Under the optimum conditions mentioned above, with (R)-1
as the catalyst, various solvents, temperatures, and bases were
also subsequently examined (Table 1, entries 6−14). Dichloro-
methane uniformly gave the highest conversions (99%) and ee
values (63−96%) (Table 1, entries 1, 2, 5, 6). Although similar
selectivity was observed with acetone as solvent (Table 1, entry
7), it was accompanied by a concommitent decrease of
reactivity. Appreciable selectivities were also recorded with
acetonitrile and chloroform, but these were not comparable
with those obtained using dicloromethane as solvent (Table 1,
entries 8, 9). No desired phosphine product was obtained when
the hydrophosphination was carried out in THF (Table 1, entry
10), extremely poor reactivity was observed with toluene
(Table 1, entry 12), and poor selectivity was attained with 1,4-
dioxane/water = 10:1 (Table 1, entry 11). The results of
screening of bases showed that Et3N offered the most
appropriate basicity, compared with tBuONa, which was
associated with a low ee (36%) and DBU, which caused
lowering of reactivity (Table 1, entries 13, 14). In the absence
of base, no product formation was observed even after 1 week
at room temperature. A variety of temperatures were also
examined using dichlormethane as solvent. The analysis (Table
1, entries 1, 2, 6) revealed a general trend: an increase in
temperature leads to an increase in reactivity, coinciding with a
decrease in enantioselectivity. When the temperature was
increased from −80 to −40 °C, the reaction time decreased by
1 h with a slight decrease of ee (3%). When the temperature
was raised from −40 °C to room temperature, the reaction time
again decreased by 1 h, but a marked 30% drop in ee was
observed.
Figure 2. Molecular structure and absolute stereochemistry of the
tertiary phosphine (major isomer) coordinated to the palladacycle
complex (Scheme 1) with 50% probability thermal ellipsoids shown
(Cl atom on Pd replaced with I to aid in crystallization).
The asymmetric hydrophosphination reaction of diethyl
benzylidenemalonate was then screened as a model reaction in
order to examine the effect of the palladacycles as catalysts. The
palladacycle complex 1 is the most effective among the three
(Table 1, entries 2 to 5) and was selected as the catalyst. The
reaction, when performed in the presence of 5 mol % 1,
generated the desired product in 99% yield with 96% ee (Table
1, entries 2 and 5) within 3.5 h and at −80 °C. It was seen that
With the optimized conditions established, i.e., (R)-1 as the
catalyst, dichloromethane as solvent, and NEt3 as base at −80
°C, various substituted methylidenemalonate esters were
screened, and the results are summarized in Table 2. The
results show that the catalyst is highly efficient for the P−H
Table 2. Substrate Scope for the Phospalladacycle-Catalyzed P−H Addition of Diphenylphosphine to Substituted
a
Methylidenemalonate Esters
b
c
entry
R
conversion (%)
ee (%)
1
2
3
4
5
6
7
Ph
99
99
99
99
99
99
NIL
96
OEt
Me
74
ca. 66
94
2-furan
4-MeOC6H4
(E)-PhCHCH
NH2
ca. 99
>99
NIL
a
Conditions: HPPh2 (50.0 mg, 2.69 × 10−4 mol), 5 mol % cat (R)-1 (8.4 mg, 1.34 × 10−5 mol, 5 mol % of HPPh2), 1.2 equiv of substituted
methylidenemalonate ester (3.23 × 10−4 mol), 1 equiv of NEt3 (27.1 mg, 2.69 × 10−4 mol), and 6 mL of degassed DCM were reacted at −80 °C.
b
c
Conversion was calculated from 31P{1H} NMR. ee was determined from 31P{1H} NMR integration of the respective signals via the use of a chiral
derivatizing agent. The detailed procedure is described in S5.
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addition process (99% conversion; Table 2, entries 1−6),
allowing the transformation of substrates to the corresponding
C-chiral monophosphine adducts (ee value 66−99%; Table 2,
entries 1−6). The substituents at the β-position of the
substituted methylidenemalonate esters also had a large
influence on the ee value of the products. Substrates bearing
an aromatic phenyl group and a substituted phenyl group
showed higher stereoselectivities (ee: 96, 99%; Table 2, entries
1, 5) than their aliphatic counterparts (ee: 74, 66% ; Table 2,
entries 2, 3).
Interestingly, the asymmetric hydrophosphination of diethyl
cinnamylidenemalonate (Table 2, entry 6) also proceeded with
excellent regio- and enantioselectivities. Diphenylphosphide
addition occurred exclusively at the more electron-deficient
CC bond, yielding a vinyl phosphine adduct with an ee value
of >99%. Addition of 2 equiv of diphenylphosphine to the
substrate under similar conditions also resulted in the
generation of the same monophosphine product with no
trace of diphosphines observed in the crude 31P{1H} NMR
spectrum. Substrates bearing a furan-2-yl heterocyclic beta-
substituent also showed a rather high reactivity and stereo-
selectivity (ee, 94%, Table 2, entry 4). The lack of reactivity
seen in the case of diethyl 2-(aminomethylene)malonate (Table
2, entry 7) is attributed to the potential chelation impeding the
availability of catalyst vacant sites in the catalyst (vide infra).
We have previously discussed the possible mechanism in
place for such metal-catalyzed hydrophosphination scenarios
using palladacycles.10a−c We believe that a similar mechanism is
in action for this particular synthetic scenario (Scheme 2).
cycle).15 It needs to be noted that the coordination of
diphenylphoshine to palladium and the subsequent P−H
activation are crucial in generating the nucleophile prior to
the P−C bond formation, as shown in Scheme 2.
The enhanced reactivity of the phosphapalladacycle (R)-1
when compared to the naphthylamine-based palladacycle (R)-3
(Table 1, entries 2, 4, 5) can be also attributed to the fact that
the phosphorus of the metallacycle ring (by virtue of its ability
to accommodate extensive back-donation from the metal
center) makes the activation of the diphenylphosphine moiety
and its subsequent nucleophilic attack on the diester more
facile. The relatively slower reactivity of the C,N complex (R)-3
can also be attributed to the classic features that trans N−Pd−P
bonds are more stable than trans P−Pd−P bonds. Thus the
product elimination process will be slower when (R)-3 is
involved. The enhanced stereoselectivity exhibited by (R)-1 and
(S)-1 can also be attributed to the fact that the conformational
rigidity of the palladacycle fixes the positions of the phenyl
groups on P (as compared to Me on (R)-3) into axial and
equatorial positions, from wherein they can project their greater
stereochemical influence on the adjacent reaction site.
On the basis of the same mechanism, it is also easy to
understand why diethyl 2-(aminomethylene)malonate failed to
undergo P−H addition (Table 2, entry 7). The product from
the initial hydrophosphination reaction in this instance will be a
P,N (N from the amino moiety) chelate on palladium, which
can then potentially block the access of both catalytic sites on
the metal to incoming substrates, thus effectively terminating
the reaction once 5% of the catalyst is rendered inactive via
chelation.
Scheme 2. Proposed Mechanism for the Pd-Catalyzed
Hydrophosphination of Substituted Methylidenemalonate
Esters
CONCLUSION
■
In summary, we have developed an efficient protocol for the
palladacycle-catalyzed asymmetric P−H additions to substi-
tuted methylidenemalonate esters. This approach thus provides
a practical direct route toward optical pure functionalized
tertiary chiral phosphines in high yields and enantioselectivities,
circumventing the need for elaborate protection and depro-
tection protocols as well as avoiding the need to deal with
phosphine oxides, which can in some instances lead to
complications.18 This body of work has also provided insights
into the factors that need to be considered when designing
catalysts for such asymmetric P−H additions. Further studies
are currently under way to explore the applicability of this
protocol to other functionalized substrates.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: (+65) 6791-1961. E-mail: sumod@ntu.edu.sg; pakhing@
ntu.edu.sg.
■
However the screening of the three palladacycles in this study
provided us further insights into the key catalyst design features
that are of consequence in the catalytic cycle. For instance, the
hindered reactivity of (S)-2 when compared to (R/S)-1 (Table
1, entries 2, 3, 5) is a clear indication that the presence of the
Me group at the site adjacent to the Pd−C bond on the
aromatic ring in (S)-2 prevents the easy accessibility of the
bulky phosphine moieties involved to the electronically
preferred coordination site (trans to the P of the pallada-
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge the Ministry of Education,
Singapore, for financial support (MOE2009-T2-2-065) and
■
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