Synthesis of 1,5-P,N-phosphino-sulfoximines through phospha-Michael reaction of alkenyl sulfoximines and their evaluation as ligands in palladium-catalyzed allylic alkylation

Article abstract of DOI:10.1016/j.tetlet.2007.10.006

We describe a modular synthesis of cyclic and acyclic 1,5-P,N-phosphino-sulfoximines by using a phospha-Michael reaction of the corresponding alkenyl sulfoximines with HPPh₂/KOtBu as key step, which proceeds with medium diastereoselectivity. Th

Full text of DOI:10.1016/j.tetlet.2007.10.006

Available online at www.sciencedirect.com
Tetrahedron Letters 48 (2007) 8752–8756
Synthesis of 1,5-P,N-phosphino-sulfoximines through phospha-
Michael reaction of alkenyl sulfoximines and their evaluation
as ligands in palladium-catalyzed allylic alkylation
Fabien Lemasson, Hans-Joachim Gais* and Gerhardt Raabe
Institut fu¨r Organische Chemie der Rheinisch-Westfa¨lischen Technischen Hochschule (RWTH) Aachen,
Landoltweg 1, 52074 Aachen, Germany
Received 19 September 2007; accepted 1 October 2007
Available online 5 October 2007
Abstract—We describe a modular synthesis of cyclic and acyclic 1,5-P,N-phosphino-sulfoximines by using a phospha-Michael reac-
tion of the corresponding alkenyl sulfoximines with HPPh2/KOtBu as key step, which proceeds with medium diastereoselectivity.
The palladium-catalyzed allylic alkylation of racemic 1,3-diphenyl allyl acetate with malonate in the presence of a SSRCRC-config-
ured N-benzyl-substituted cyclic phosphino-sulfoximine gave the corresponding alkene with 97% ee in 98% yield. A comparative
study of N-substituted phosphino-sulfoximines showed the selectivity of the Pd(0)-catalyst to be dependent not only on the chiral
backbone of the ligand but also on the N-substituent and configuration of the sulfoximine group.
Ó 2007 Elsevier Ltd. All rights reserved.
The palladium(0)-catalyzed allylic alkylation has
emerged as a valuable method in asymmetric synthesis.1
Of the various chiral ligands that have been developed
for the Pd-atom,1 catalysts containing bidentate P,N-
ligand2 as, for example, 1,5-P,N-phosphino-oxazolines
III3 show particularly high selectivity and activity.
Despite the availability of several classes of P,N-ligands
there is still a quest for new ligands of this type.4
Because of our investigations of the Pd-catalyzed asym-
metric allylation of S-5 and O-nucleophiles6 and
application of sulfoximines in asymmetric synthesis,7
we became interested in the synthesis and evaluation
of cyclic and acyclic phosphino-sulfoximines of types I
and II, respectively, as ligands (Fig. 1). Phosphino-
sulfoximines I and II, having besides the chiral sulfoxi-
mine group also a chiral backbone, are expected to func-
tion as bidentate 1,5-P,N-ligands and form 6-membered
ring chelates, the sulfoximine group of which is embed-
ded in the ring. The variation of the substituents R2 and
R3 of I and the ring size of II should provide a means to
influence the efficiency of the ligands. Particularly yield-
ing in this regard could be a variation of the substituent
R1 because of its close proximity to the Pd-atom.
Recently, 1,4-P,N-phosphino-sulfoximines IV8 and V9
featuring a PCCNS connectivity had been described,
which gave high enantioselectivities in the Pd-catalyzed
reaction of 1,3-diphenyl-2-propenyl acetate with malon-
ate.8a,9 However, these P,N-ligands are in contrast to I
and II not capable to form with the Pd(0)-atom N,P-
chelates, the sulfoximine group of which is incorporated
into the ring. Here we describe a modular synthesis of
phosphino-sulfoximines I and II through a base-cata-
lyzed phospha-Michael reaction of the corresponding
alkenyl sulfoximines with HPPh2 and their evaluation
in Pd-catalyzed allylic alkylation.
R
R1
R1
N
S
N
S
PPh2
PPh2
R3
N
PPh2
O
O
O
Ph
Ph
R2
n
I
II
III
R
Me
Ar
S
N
PPh2
Ar
S
N
iPr
PPh2
O
O
V
IV
*
Corresponding author. Tel.: +49 2418094686; fax: +49
2418092665; e-mail: gais@rwth-aachen.de
Figure 1. P,N-Phosphino-sulfoximines and P,N-phosphinooxazolines.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.006

F. Lemasson et al. / Tetrahedron Letters 48 (2007) 8752–8756
8753
sulfoximine.9 Treatment of the alkenyl sulfoximines
2a–d with 1.1 equiv of HPPh2 and 10 mol % of KOtBu
in absolute THF gave phosphino-sulfoximines 6a–d as
mixtures of diastereomers (Scheme 2, Table 2), which
were isolated after treatment with BH3–THF as borane
adducts 5a–d in good overall yields. The diastereomers
of 5a–d were separated by column chromatography.
Similarly, treatment of cycloalkenyl sulfoximines 4a
and 4b with 1.1 equiv of HPPh2 and 10 mol % of KOtBu
in absolute THF followed by the addition of BH3–THF
gave phosphino-sulfoximines 7a and 7b, respectively, as
mixtures of only two diastereomers (Table 3), which
according to NMR spectroscopy have the trans-configu-
ration. The direct application of KPPh2ÆBH3 in the
phospha Michael reaction9 of 4 was not suitable because
of a partial hydroboration of the cycloalkenyl sulfoxi-
mine (Scheme 3).
−
1. n-BuLi, THF, 78 °C
2. PhCHO, 78 °C
O
O
RN
Ph
RN
−
S
S
3. ClCO2Me, −78 °C to rt
Me
Ph
Ph
−
4. DBU, 78 °C to rt
1a−d
−
2a d
−
n-BuLi, THF, 78 °C
1.
a: R = Me, b: R = Bn
c: R = Ts, d: R = SitBuPh2
−
2. Br(CH2)4CHO, 78 °C
3. ClCO2Me, −78 °C to rt
−
4. DBU, 78 °C to rt
RN
Ph
O
O
RN
LDA, THF
S
S
−
78 °C to rt
Ph
Br
4a,b
3a,b
Scheme 1. Synthesis of alkenyl and cycloalkenyl sulfoximines.
Acyclic alkenyl sulfoximines 2a–d10 were synthesized by
a one pot addition–elimination route11 starting from the
enantiopure N-substituted S-methyl sulfoximines 1a–
d.12 Sulfoximines 1a–d in turn are readily accessible
from S-methyl-S-phenylsulfoximine13 through reaction
with CH2O/HCO2H, PhCH2Br, TsCl, and ClSitBuPh2,
respectively. Thus, the successive treatment of sulfoxi-
mines 1a–d with n-BuLi, PhCHO, ClCO2Me, and 1,8-
diaza-bicyclo[5.4.0]-undec-7-ene (DBU) gave the alkenyl
sulfoximines 2a–d in very good overall yields as single
E-isomers (Scheme 1, Table 1).
The absolute configuration of phosphine-borane
adducts RSSC-5a (Fig. 2) and SSRCRC-7b (Fig. 3) was
determined by X-ray crystal structure analysis.16 NMR
spectroscopy of the diastereomers of 5a–d, 7a, and 7b
RN
Ph
O
S
Ph
−
2a d
a
b
1. HPPh2,cat . KOtBu, rt
: R = Me, : R = Bn
c: R = Ts, d: R = Si
t
BuPh2 2. BH3-THF,0 °C to rt
3. 1N HCl, 0 °C, pH 5
H3B
Cycloalkenyl sulfoximines 4a,b were synthesized by a
two-step route. The successive treatment of sulfoximines
1a and 1b with n-BuLi, 5-bromopentanal, ClCO2Me,
and DBU furnished the methyl- and benzyl-substituted
acyclic x-bromo-alkenyl sulfoximines 3a and 3b, respec-
tively. Treatment of 3a and 3b with lithium diisoprop-
ylamide (LDA) led to a lithiation at the a-position14
H3B
O
RN
Ph
RN
O
PPh2
PPh2
S
S
+
Ph
Ph
Ph
−
SSRC-5a d
5a−d
-
SSSC
DABCO, toluene,40 °C
followed by
a cyclization of the corresponding
x-bromo-a-lithio alkenyl sulfoximines and gave cyclo-
alkenyl sulfoximines 4a and 4b, respectively, in good
overall yields.15
O
RN
Ph
O
PPh2
Ph
RN
Ph
PPh2
S
S
+
Ph
6a−d
-
SSRC
−
SSSC-6a d
We envisioned a synthesis of phosphino-sulfoximines
6a–d, 8a, and 8b through a phospha-Michael reaction
of KPPh2 to alkenyl and cycloalkenyl sulfoximines 2a–d,
4a, and 4b, respectively. Although this route should
result in the formation of mixtures of diastereomers, this
would allow a determination of the enantioselectivity of
the catalyst in dependence of the configuration of the
ligand. The feasibility of a phospha-Michael reaction of
alkenyl sulfoximines had been previously demonstrated
in the case of the addition of KPPh2ÆBH3 to a vinyl
Scheme 2. Synthesis of acyclic phosphino-sulfoximines.
Table 2. Synthesis of acyclic phosphino-sulfoximines
Compound
R
SSRC:SSSC
Yielda (%)
5a
5b
5c
5d
Me
Bn
Ts
SitBuPh2
78:22
64:36
27:73
58:42b
78
78
70
80
a Overall yields based on sulfoximines 2a–d.
b Configuration has not been assigned.
Table 1. Synthesis of alkenyl and cycloalkenyl sulfoximines
Compound
R
Yielda (%)
2a
2b
2c
2d
4a
4b
Me
Bn
Ts
SitBuPh2
Me
88
91
92
85
56
70
Table 3. Synthesis of cyclic phosphino-sulfoximines
Compound
R
SSRCRC:SSSCSC:SSRCSC:SSSCRC Yield (%)a
7a
7b
Me 50:50:0:0
Bn 50:50:0:0
80
80
Bn
a Overall yields based on sulfoximines 1a–d.
a Overall yields based on sulfoximines 4a and 4b.

8754
F. Lemasson et al. / Tetrahedron Letters 48 (2007) 8752–8756
O
RN
Ph
S
4a,b
1. HPPh2,cat . KO
t
Bu, rt
a
b
: R = Me, : R = Bn
2. BH3-THF,0 °C tort
3. 1N HCl,0 °C, pH5
H3B
H3B
RN
Ph
O
PPh2
RN
Ph
O
PPh2
S
S
+
7a,b
-
SSSCSC
7a,b
-
SSRCRC
DABCO, toluene,40 °C
RN
Ph
O
RN
Ph
O
PPh2
PPh2
Figure 3. Structure of the phosphino-sulfoximine borane adduct
SSRCRC-7b in the crystal. Selected bonding parameters: S–O
1.434(4), S–N 1.492(4), C–S 1.830(4), P–B 1.927(6), C–P 1.841(4).
S
S
+
8a,b
SSRCRC
-
SSSCSC-8a,b
3 mol % of the ligand, which was heated at 40 °C under
argon for 2 h, was treated with rac-9, N,O-bis(trimethyl-
silyl)acetamide (BSA), and a catalytic amount of LiOAc.
Degassed CH2Cl2 was used as solvent since a screening
of several solvents had shown it to be the best one in
terms of activity and enantioselectivity.
Scheme 3. Synthesis of cyclic phosphino-sulfoximines.
Acyclic ligands 6a–d were first tested. The reaction in the
presence of the R-configured N-methyl-substituted
ligand SSRC-6a gave the allyl derivative R-10 with 65%
ee (Table 4). Interestingly, the use of the S-configured
ligand SSSC-6a afforded S-10 with only 10% ee. This
shows that not only the stereogenic C-atom but also
the stereogenic S-atom plays an important role in the
selectivity and activity of ligand 6a. The substituent at
the N-atom has also a significant effect on the selectivity
of the catalyst. While the reaction in the presence of the
N-methyl-substituted ligand SSRC-6a gave R-10 with
65% ee, that with the N-benzyl substituted ligand
SSRC-6b furnished R-10 with 82% ee. Interestingly,
experiments with the N-silyl- and N-tosyl-substituted
sulfoximines 6c and SSRC-6d, respectively, revealed very
Figure 2. Structure of the phosphino-sulfoximine borane adduct
RSSC-5a in the crystal. Selected bonding parameters: S–O 1.455(2),
S–N 1.521(4), C–S 1.786(7), P–B 1.923(3), C–P 1.853(5).
revealed characteristic chemical shift differences, which
allowed, with the exception of the N-silyl derivatives
5d, in combination with the results of the crystal struc-
ture analysis a structural assignment.
Table 4. Acyclic 1,5-P,N-phosphino-sulfoximines in Pd-catalyzed
alkylation
MeO2C
CO2Me
1.5mol% Pd2(dba)3.CHCl3
3 mol% 6, CH2Cl2
OAc
Ph
CH (CO Me) ,
Ph
Ph
Ph
2
2
2
Phosphanes 6a–d, 8a, and 8b were obtained by treat-
ment of the borane adducts 5a–d, 7a, and 7b, respec-
tively, with 1,4-diazabicyclo[2.2.2]octane (DABCO) in
absolute toluene at 40 °C followed by a column filtra-
tion of the crude reaction mixtures under argon atmo-
sphere in almost quantitative yields. In solution
phosphanes 6a–d, 8a, and 8b are sensitive toward
oxygen.
BSA, LiOAc
rac-
9
10
10
Yield (%) eea (%) Confn
6
R
Confn T (°C)
a
a
Me
Me
SSRC
SSSC
rt
rt
90
40
65
10
R
S
c
Ts
SSSC
rt
rt
17
5
5
15
S
b
d
SitBuPh2 SSRC
R
The alkylation of malonate with racemic 1,3-diphenyl-2-
propenyl acetate rac-9 was selected as test reaction for
ligands 6a–d, 7a, and 7b. A mixture of 1.5 mol %
of Pd2(dba)3ÆCHCl3 (dba = dibenzylideneacetone) and
a Determined by HPLC: Chiralpack-IA, n-heptane/isopropanol, 95:5,
0.75 ml/min, 35 bar.
b Configuration has not been assigned.

F. Lemasson et al. / Tetrahedron Letters 48 (2007) 8752–8756
8755
low enantioselectivities and activities of these catalysts.
It is proposed that the striking difference between 6a,b
and 6c,d is due to a different coordination of the
Pd-atom by the ligands. While the N-alkyl-substituted
sulfoximines 6a and 6b most likely act as bidentate
P,N-ligands, the N-tosyl/silyl-substituted sulfoximines
6c and 6d function as monodentate P-ligands. Support
for this notion comes from the reactivity of sulfoximines
toward electrophiles and Lewis acids. While N-alkyl
sulfoximines easily react with both at the N-atom, N-
silyl and N-tosyl sulfoximines do not engage in such
reactions.17,18
N
Nu
S
X
P
Pd
Ph
O
VI
Ph
Ph
N
P
Ph
Pd
X
S
O
Nu
Having obtained promising results with the acyclic
phosphino-sulfoximines, we investigated the Pd-cata-
lyzed reaction of rac-9 with malonate by using the cyclic
phosphino-sulfoximines. The reaction of rac-9 in the
presence of the N-methyl-substituted ligand SSRCRC-
8a gave R-10 with 86% ee (Table 5). With diastereomer
SSSCSC-8a, having the opposite configuration at the C-
atoms, the reaction showed a reduced enantioselectivity
and afforded the S-configured alkene S-10 with only
73%. Thus, the catalysts containing cyclic ligands
SSRCRC-8a and SSSCSC-8a exhibit a higher enantio-
selectivity than those derived from the acyclic ligands
perhaps because of a higher rigidity. These results show
furthermore that not only the chiral backbone but also
the stereogenic S-atom of the cyclic ligands has a pro-
found influence upon the selectivity.
VII
Figure 4. Alleged structures and dynamics of p-allyl-Pd-complexes
containing SSRCRC-8b as ligand.
not known, formation of the diastereomeric chelate
complexes VI and VII, featuring a P,N-coordination
of the Pd-atom, may be assumed (Fig. 4). The proposal
of a formation of chelate rather than mono-dentate
complexes between Pd(0) and 1,5-P,N-phosphino-
sulfoximines 6a, 6b, 8a, and 8b is based on the following
observations. First, the Pd-catalyzed reaction of rac-9
in the presence of phosphino-sulfoximines 6c and 6d,
the N-atoms of which have a low Lewis basicity, pro-
ceeded only with low enantioselectivities. Second, the
enantioselectivity of the Pd-catalyzed reaction of rac-9
in the presence of cyclic phosphino sulfoximine
SSRCRC-8a (86% ee) remained the same upon changing
the ligand Pd(0) ratio from 2:1 to 1:1. Reaction of VI
could be faster than that of VII because of a steric inter-
action between the allyl- and P-phenyl groups as well as
between the benzyl and the allyl-phenyl group in the
latter complex. This model is based on the assumption
that the attack of the nucleophile at VI trans to the
P-atom is kinetically preferred. Such a preference for a
trans-P-attack had been observed in p-allyl-Pd-
complexes containing phosphino-oxazoline ligand III19
and other P,N-ligands.20
The N-benzyl-substituted sulfoximine SSRCRC-8b
turned out to be the best ligand in terms of selectivity
and activity of the catalyst. The Pd-catalyzed reaction
of rac-9 (0.5 mmol) with malonate in the presence of
SSRCRC-8b (3 mol %) gave R-10 with 97% ee in 98%
yield within 50 min. The corresponding reaction of
rac-9 in the presence of diastereomer SSSCSC-8b affor-
ded the S-configured alkene S-10 with only 79% ee.
The in situ generation of SSSCSC-8b from borane adduct
SSSCSC-7b with DABCO did not diminish enantioselec-
tivity of the Pd-catalyzed alkylation of rac-9.
In summary, we have developed a modular synthesis of
acyclic and cyclic phosphino-sulfoximines of types I and
II. The catalyst containing cyclic ligand SSRCRC-8b
showed high enantioselectivity. The asymmetric induc-
tion mainly stems from the chiral backbone of the ligand
and to some extend also from the configuration of the
sulfoximine group and the structure of its N-substituent.
Because of the route chosen for the synthesis of phos-
phino-sulfoximines 6 and 8, various derivatives should
be accessible for further testing. A subject of further
studies will be the enhancement of the diastereoselectiv-
ity of the phospha-Michael addition of alkenyl sulfoxi-
mines and the determination of the structure of the
Pd(0)/8 complexes.
Although the structure and dynamics of the p-allyl-Pd
complex(es) derived from rac-9 and SSRCRC-8b are
Table 5. Cyclic 1,5-P,N-phosphino-sulfoximines in Pd-catalyzed
alkylation
MeO2C
CO2Me
Ph
1.5mol%Pd2 (dba)3.CHCl3
3 mol% 8, CH2Cl2
OAc
Ph
CH (CO Me) ,
Ph
Ph
2
2
2
BSA,LiOAc
rac-
9
10
8
R
Confn
T (°C)
10
Yield (%)
eea (%)
Confn
a
a
Me
Me
SSRCRC
SSSCSC
rt
rt
98
95
86
73
R
S
b
bb
Bn
Bn
SSSCSC
SSSCSC
rt
rt
96
96
79
79
S
S
Acknowledgments
a Determined by HPLC: Chiralpack-IA, n-heptane/isopropanol, 95:5,
0.75 ml/min, 35 bar.
b Prepared in situ from its borane adduct with DABCO.
Financial support of this work by the Deutsche Fors-
chungsgemeinschaft (SFB 380 and GK 440) is gratefully
acknowledged. We thank Dr. J. Runsink for NMR

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F. Lemasson et al. / Tetrahedron Letters 48 (2007) 8752–8756
10. Compound 2a: (a) Johnson, C. R.; Lockard, J. P.;
Kennedy, E. R. J. Org. Chem. 1980, 45, 264–271;
Compound 2c: (b) Edelmeier, I.; Gais, H.-J. Tetrahedron
Lett. 1985, 26, 4359–4362.
experiments, Dr. C. Lehmann, MPI Mulheim, for
¨
collecting the data set of RSSC-5a, and Dr. C. Wittland,
Grunenthal, Aachen, for ESI-HRMS measurements.
¨
11. Gais, H.-J.; Hainz, R.; Muller, H.; Bruns, P. R.; Giesen,
¨
N.; Raabe, G.; Runsink, J.; Nienstedt, S.; Decker, J.;
Schleusner, M.; Hachtel, J.; Loo, R.; Woo, C.-W.; Das, P.
Eur. J. Org. Chem. 2000, 3973–4009.
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