Enantioselective Hydrosilylation of Imines Catalyzed by Chiral Zinc Acetate Complexes

Article abstract of DOI:10.1021/acs.joc.5b02613

A series of zinc acetate complexes with optically pure diphenylethanediamine (DPEDA)-derived ligands have been employed as enantioselective catalyst for the hydrosilylation of various imines. High control of stereoselectivity (up to 97% ee) and excellent yields (up to 96%) were gained for a broad range of N-phosphinoylimines by using (R,R)-N,N′-dibenzyl-1,2-diphenylethane-1,2-diamine. This is the first successful application of an air-stable and environmentally friendly chiral Zn(OAc)₂ complex instead of the previously used harmful diethylzinc in the asymmetric reduction of the C=N double bond.

Full text of DOI:10.1021/acs.joc.5b02613

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Note
Enantioselective Hydrosilylation of Imines
Catalyzed by Chiral Zinc Acetate Complexes
Agata Bez#ada, Marcin Szewczyk, and Jacek Mlynarski
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b02613 • Publication Date (Web): 14 Dec 2015
Downloaded from http://pubs.acs.org on December 16, 2015
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Enantioselective Hydrosilylation of Imines Catalyzed by Chiral Zinc Acetate Complexes
Agata Bezłada, Marcin Szewczyk and Jacek Mlynarski*
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Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland.
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Corresponding Author
*E-mail: jacek.mlynarski@gmail.com.
Table of Contents/Abstract Graphic
Abstract: A series of zinc acetate complexes with optically pure diphenylethanediamine
(DPEDA)-derived ligands have been employed as enantioselective catalyst for the
hydrosilylation of various imines. High control of stereoselectivity (up to 97% ee) and
excellent yields (up to 96%) were gained for broad range of N-phosphinoylimines by using
(R,R)-N’,N’-dibenzyl-1,2-diphenylethane-1,2-diamine. This is the first successful application
of air stable and environmentally friendly chiral Zn(OAc)2 complex instead of previously
used harmful diethtylzinc in the asymmetric reduction of C=N double bond.
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Chiral secondary amines are significant synthetic targets and useful synthons for
numerous drugs and natural products.1 Their exceptional importance for manufacture of
pharmaceuticals and agrochemicals stimulate intensive progress in development of methods
for laboratory and large-scale synthesis of enantiopure amines. Recently, several catalytic
methodologies for reduction of double C=N have been established with considerable
efficiency.2-4 In the number, asymmetric hydrosilylation provides an important alternative for
other methods because of mild conditions and application of safe, easy to handle, inexpensive
hydrosilanes.5
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While transition metal-catalyzed asymmetric hydrosilylation of ketones based on
noble metals such as Rh, Ru, Ir is well established,5 arsenal of know catalysts for
hydrosilylation of imines is far more limited. Previously proposed catalytic systems derived
from rhodium, ruthenium and iridium complexes with diferrocene or diphosphine ligands
afforded corresponding amines with moderate to low enantioselectivities only.6-8 More active
catalysts containing Ti-, Cu- and Rh-based chiral Lewis acids required stochiometric loading
of the Lewis acids, multistep synthesis of chiral ligands or special preparation of substrates.9-
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Therefore, the development of cheaper and general catalytic systems for the
enantioselective reduction of imines is still highly desired and deserves special attention.
One of the most promising yet still unexplored alternatives is application of chiral zinc
catalysts.12 Recently, environmentally benign and less expensive earth-abundant metals such
as Zn13 and Fe14 have been exhaustively explored in asymmetric synthesis. Replacing toxic
and expensive noble metals by zinc-based catalysts is one of the most important issues for
pharmaceutical research nowadays.
The use of Zn-catalyzed enantioselective hydrosilylation of aryl-alkyl ketones
developed by Mimoun15 and further extended to imines by Yun16 made a breakthrough in
practical hydrosilylation. The highly enantioselective reduction of imines was achieved by
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using Zn-diamine complexes in protic media (MeOH). However, according to this protocol a
typical catalyst system is formed in situ from equimolar amounts of hazardous and unstable
dialkylzinc and a secondary diamines.17
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The first application of zinc salt in asymmetric hydrosilylation of imines was reported
by Ireland et al. in 2004.18 Enantioselective reduction of imines by using
polymethylhydrosiloxane (PMHS) as a reducing agent in the presence of Zn(OTf)2 resulted in
the formation of chiral alkyl-aryl and cyclic amines with good yields albeit low ees (ca. 30%).
Driven by our interest in developing zinc-based catalysts for asymmetric transformations, we
reported recently efficient hydrosilylation of ketones by using chiral Zn(OAc)2-pybox
ligands.19 Now, we describe the first example of highly enantioselective hydrosilylation of
imines catalyzed by chiral catalyst composed of Zn(OAc)2 instead of expensive, dangerous
and inconvenient ZnEt2 with high impact on large-scale application in the industry (Scheme
1).
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Scheme 1. Diamiane Ligands Used in this Studies on Asymmetric Hydrosilylation
Initial optimization of the reaction conditions was carried out with catalytic system
composed of Zn(OAc)2 (5 mol%) and various diamine ligands in a mixture of THF/MeOH as
a solvent at room temperature in the presence of 3 equiv. of PMHS (Scheme 1). After many
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trials we selected derivatives of (R,R)-1,2-diphenylethane-1,2-diamine (DPEDA, L1) as the
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5
most promising candidates for further studies.
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Table 1. Asymmetric Hydrosilylation of Various Imines Catalyzed by Zinc Acetate with
Derivatives of (R,R)-DPEDA Ligands
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Yield
[%]a
ee
[%]b
Entry Substrate Ligand
1
2
3
4
5
6
7
8
78
68
96
1
1
1
1
1
2
3
4
L1
L2
L3
L4
L5
L2
L2
L2
80
n.d.
n.d.
40
-
-
98
75
-
90
n.d.
n.d.
-
Reaction conditions: Zn(OAc)2 (5 mol%) and
chiral ligand (L, 5 mol%) were stirred in 0.4 mL
of THF during 30 min. Then imine (0.44 mmol)
in 0.4 mL THF, silane and 0.2 mL of MeOH were
successively added. Reactions were carried out
a
for 24 h at rt under inert atmosphere. Isolated
yields after column chromatography. bDetermined
by HPLC.
To our delight, using (R,R)-L1 and zinc acetate as in situ generated catalyst and N-
phosphinoylimine (1) as a probe substrate resulted in a formation of desired amine (5) in good
yield and ee (68%, Table 1, entry 1). Data collected in Table 1 indicate that slight varying
ligand structure have tremendous impact on the reaction enantioselectivity. From among
tested benzyl and o-substituent benzyl derivatives of DPEDA (L2-L4), only ligand L2 gave
the desired product in very good isolated yield and high enantioselectivity (96%, entry 2).
Interestingly, application of o-substituted ligands (L3, L4) was unsuccessful. However,
application of more bulky ligand with tert-butyl substituents (L5) resulted in excellent
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enantioselectivity (up to 98% ee) but poor conversion, unfortunately. Despite the fact that
hydrosilylation of imine 1 to the corresponding amine 5 was not complete in 24 hours and
prolongation of the reaction time had only little effect on the conversion, this highly
promising attempt with diamine L2-Zn(OAc)2 encouraged us to further optimization of the
reaction conditions.
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The choice of the substituent attached to the imine nitrogen atom was then studied.
This revealed that only imines with diphenylphosphinoyl (1) and tosyl (2) groups showed
significant reactivity while the imines 3 and 4 were unreactive (Table 1, entries 7, 8).
The ratio of silane/substrate, used solvent as well as catalysts loading proved to be an
important parameters for the process optimization. Data collected in Table 2 show that the
application of ca. 3 equiv. of PMHS in a combination with 5 mol% of Zn-L2 catalyst in
THF/MeOH solvent gave the highest enantioselectivity (96%, entry 1). The amount of this
particular silane (PMHS, ca. 3 equiv.) seems to be crucial for the reaction yield as diminished
addition of reducing agent (ca. 2 equiv., entry 2) resulted in inferior efficiency. Similarly, the
decrease of the catalyst amount from 5 mol% to 2.5 mol% or 1 mol% lowered both yields and
ees (entries 3, 4). Surprisingly, increasing in the amount of the catalyst to 10 mol% prevented
formation of the product (entry 5). This tendency could be overcome by using additional
amount of PMHS (4-6 equiv., entry 6). This observation suggest that Zn(Ac)2-based catalyst
utilize polymethylhydroxysilane (PMHS) for the formation of zinc hydride, which is not
efficiently regenerated in the catalytic cycle.
Thus 5 mol% of catalyst loading was found to be the best catalyst loading under
elaborated reaction conditions with PMHS. Interestingly, in contrast to our predecessors,16,17
we observed similar reaction efficiency for the reduction performed in THF without any
additive of protic solvents (Table 1, entry 7). This interesting reaction condition was
maintained for the reduction performed in the presence of triethoxysilane leading to desired
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amine 5 with 89% ee (entry 11). We also tested other commercially available silanes to
perform the catalytic reduction of imines under milder reaction conditions. Results
summarized in Table 2 undoubtedly confirmed that the highest control of the stereoselectivity
is achieved only with PMHS in THF/MeOH solution (entries 1-6) or triethoxysilane in THF
(entry 11). The later reaction conditions were more efficient in the light of less amount of
used reducing agent (2 equiv. only). Thus, we can state that alcohol additives is not necessary
to enhance reaction rates of the zinc-catalyzed reduction of imines.
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Table 2. Asymmetric Hydrosilylation of N-phosphinoylimines with Zn(OAc)2: Studies on
Catalyst Loading and Solvent Effect
Entry
Silane
Solvent
Cat. [mol%]
Yield [%]a
ee [%]b
1
2
PMHS
PMHSc
THF/MeOH (4:1)
THF/MeOH (4:1)
THF/MeOH (4:1)
THF/MeOH (4:1)
THF/MeOH (4:1)
THF/MeOH (4:1)
THF
5
5
80
30
96
94
80
76
-
3
PMHS
2.5
1
60
4
PMHS
44
5
PMHS
PMHSd
10
10
5
trace
80
6
91
91
79
86
70
89
94
94
7
PMHS
70
8
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
THF/MeOH (4:1)
THF
5
65
9
5
52
10
11
12
13
THF/MeOH (4:1)
THF
5
36
5
95
THF
2.5
10
38
THF
70
Reaction conditions: Zn(OAc)2 (5 mol%) and L2 (5 mol%) were stirred in 0.4 mL of THF during 30 min. Then
imine (0.44 mmol,) in 0.4 mL THF, silane (0.08 mL ca. 3 equiv. of PMHS or 2 equiv. of (EtO)2MeSiH or (EtO)3SiH)
and 0.2 mL of MeOH were successively added. Reactions were carried out for 24 h at RT under inert atmosphere.
aIsolated yields after column chromatography. bDetermined by HPLC. c2 equiv. of silane was used. d6 equiv. of silane
was used.
With optimized reaction conditions, various N-phosphinoylimines were reduced by
using 5 mol% loading of catalyst, 3 equiv. of PMHS and THF/MeOH mixture (4:1) as a
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solvent (method A). Alternative reactions were also carried out with 2 equiv. of (EtO)3SiH in
dry THF (method B).
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In all cases, corresponding products were isolated with good to high yield and high
enantioselectivity. For most tested substrates, reactions performed in the presence of
(EtO)3SiH (Method B) resulted in the better yields than those with PMHS as hydrogenating
agent (Scheme 2). Reduction of phenylmethyl phosphinoylimine provided amine 5a with high
ee (up to 96%). Exchange of methyl group with ethyl decreased only the yield of the reaction
when PMHS was used as hydrogenating agent (product 5b). Substrates bearing electron-
withdrawing group attached to the aromatic ring were smoother reduced than imines
possessing electron-donating group, but ee values were high in most cases (entries 5c-i). Low
yields for N-phosphinoylimines having activated ring could be caused by much stronger Zn-N
bond due to higher electron density on the nitrogen atom. As a result in this particular case the
activity of the catalyst was decreased. Moreover, the Zn(OAc)2-L2 catalyst showed the same
effectiveness in the reduction of substrates possessing groups placed in ortho (5j-k), meta
(5h-i) and para (5c-g) positions. The hydrosilylation reaction of heteroaromatic and bicyclic
N-phosphinoylimines were also carried out with excellent ee values (5j-m).
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Scheme 2. Asymmetric Hydrosilylation of Various N-phosphinoylimines Catalyzed by
Zn(OAc)2-L2 Complex with PMHS in THF/MeOH (Method A) and (EtO)3SiH in THF
(Method B).
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Important application of our methodology would be its use in the synthesis of
pharmaceuticals or biologically active compounds. Several primary amines being drugs and
herbicide precursors can be easily obtained from corresponding imines by using elaborated
methodology. For example, the 3-hydroxy-substituted amine 5h is desired substrate for
Rivastigmine synthesis,20 4-chloro-substituted amine 5f is a key intermediate in the synthesis
of agricultural fungicide Capropamid.21 The 1-naphthalen structure of amine 5m can be found
in Cinacalcet,22 while amine 5l is useful intermediate in the synthesis of Rasagiline.23
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As was previously confirmed, application of protic solvent was not necessary for the
formation of amines with high efficiency and stereoselectivity. This observation is in contrast
to previously reported studies of the hydrosilylation reaction of N-phosphinoylimines where
diethylzinc-based catalysts required the presence of protic solvent, mostly THF/MeOH
(4:1).16 It was assumed that some additive of alcohol is responsible for the formation of
alkoxide zinc species which facilitate cleavage of the strong Zn-N bond during the catalytic
cycle. Based on the previously postulated mechanism,17 we propose possible catalytic cycle
for the zinc-catalyzed hydrosilylation of N-phosphinoylimine depicted at Scheme 3. Zinc
complex I is formed directly from diamine and zinc acetate in the first step. This, in turn,
undergoes the reaction with silane to provide the Zn-hydrido species II being the effective
reducing agent. When PMHS is used as a reducing agent (Scheme 2 and Scheme 3, Method
A) the reaction is more efficient with methanolic solution. Initially formed silane acetate
should be transformed into silane-OMe and facilitate the presumable turnover-limiting
catalyst regeneration step in MeOH. This is in accordance to the conclusion presented
previously by Yun16a and Kwit.17c From our results and in analogy to previously proposed
mechanism for hydrosilylation of imines we can assume that the first amount of silane is used
for the formation of the Zn-hydrido species II. This step utilize silane, and this explains why
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application of higher catalyst loading stops the reaction cycle (Table 2, entry 5) by
withdrawing silane which is necessary for the next reaction steps.
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The use of dry THF instead of the mixture of THF/MeOH as a solvent decreased yield
for the reaction performed with PMHS.24 Surprisingly application of monomeric
triethoxysialne was more efficient in THF in comparison to THF/MeOH media. Moreover,
the reduction using (EtO)3SiH gave nearly quantitative conversion in shorter time (24 hours).
This observation suggest, that the acetate ions in the reaction mixture play the same role as
methanol yet work more efficient, supporting release of the free N-phosphinoylated amine at
the end of the reaction. This can be described as follow, application of triethoxysilane does
not require methanol and Zn-hydrido species II can be efficiently regenerated in a catalytic
cycle based on acetate transfer from (EtO3)SiOAc to catalyst (Scheme 3, Method B). This
tendency was previously postulated by Shibasaki and used in a catalytic enantioselective
Mannich-type reaction of ketoimines.25 Finally, chiral (R,R)-ligand based II delivers hydride
from Re-face of imine substrate resulting in a formation of (S)-5.
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Scheme 3. Mechanistic Proposal for the Hydrosililation of N-phosphinoylimine
Catalyzed by chiral Zn-diamine Complex
Method B: silane = (EtO)3SiH/Zn(OAc)2/THF
Method A: silane = PMHS/Zn(OAc)2/THF-MeOH
R
Ph
NHR
Ph
Ph
NH
OAc
OAc
Zn(OAc)2
Zn
Ph
NHR
NH
R
I
silane-H
Ph
Ph
NHR
NHR
Zn(OAc)2
silane-OAc
(EtO)3Si-H
(EtO)3Si-OAc
silane-H
silane-OMe
R
R
R
R
Ph
Ph
NH
Ph
Ph
NH
Ph
Ph
NH
OAc
OAc
H
Ph
NH
OAc
OAc
Zn
Zn
Zn
Zn
OAc
OMe
NH
R
NH
R
H
NH
R
Ph
NH
R
II
IV
I
II
MeOH
(EtO)3Si-OAc
O
P
O
P
O
O
Ph2
Ph2
PPh2
PPh2
NH
R
NH
R
N
N
Ph
Ph
NH
Ph
NH
OAc
H
OAc
H
Zn
O
Zn
O
5
-( )
5 S
1
1
N
H
Ph
R
N
H
R
Ph
Ph
N
P
N
P
III
III
Ph Ph
Ph Ph
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To conclude we developed highly active zinc catalyst for efficient asymmetric
hydrosilylation of N-phosphinoylimines. The proposed methodology offers the application of
more stable Zn(OAc)2 instead of ZnEt2 together with safe and inexpensive hydrosilanes such
as PMHS or (EtO)3SiH without any loss in reaction yield and enantioselectivity. We also
confirmed that the usage of protic solvent is not required in the hydrosilylation reaction
catalyzed by zinc acetate. The proposed catalyst reduced various aromatic imines, derived
from acetophenone as well as heteroaromatic and bicyclic N-phosphinoylimines with
excellent ees.
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Experimental Section
General Information. All starting materials and reagents were purchased from commercial
sources and used without purification. Dry THF was distilled from potassium to prior to use.
TLC analysis of reaction mixtures was performed on silica gel 60F254 TLC plates.
1
Chromatography was carried out on 60 silica gel. H and 13C NMR spectra were recorded
1
with 600 MHz and referenced relative to tetramethylsilane (δ = 0 ppm) for H NMR, and
CDCl3 (δ = 77.16 ppm) for 13C NMR. Data are reported as follows: chemical shift,
multiplicity (br s = broad singlet, s = singlet, d = doublet, t = triplet, q = quartet, quint =
quintet, m = multiplet) and integration. IR spectra were recorded at ATR FTIR spectrometer.
Optical rotations were measured at room temperature with a digital polarimeter. HPLC
analysis were performed using AD-H and OD-H columns with UV detection at 206 nm. All
catalytic reactions were performed under inert atmosphere.
Synthesis of Ligands
Ligands L2-L5 were synthetized according to the literature procedure.26 (R,R)-1,2-
diphenylethylenediamine (1.0 equiv.) and K2CO3 (4.0 equiv.) were dissolved in 0.5 mL of dry
DMF. Next to the solution was added dropwise corresponding arylmethyl halide (2.0 equiv.)
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and the mixture was stirred at rt for 16 h. After this time a 5 mL of water was added and the
reaction mixture was extracted with diethyl ether (3×5 mL). The combined organic layers
were washed with water (3×5 mL) and dried over anhydrous Na2SO4. The solvent was
evaporated and the residue was purified by column chromatography on silica gel with hexane-
ethyl acetate as eluent.
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(1R,2R)-N,N’-dibenzyl-1,2-diphenylethane-1,2-diamine (L2)26
The product was obtained starting from benzyl chloride as a white solid after purification by
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column chromatography on silica gel Hx-AcOEt (4:1) (91 mg, 49%). H NMR (600 MHz,
CDCl3) δ 7.32 – 7.26 (m, 4H), 7.23 (m, 6H), 7.18 – 7.10 (m, 6H), 7.06 – 7.00 (m, 4H), 3.71
(s, 2H), 3.66 (d, J = 13.4 Hz, 2H), 3.49 (d, J = 13.4 Hz, 2H), 2.15 (s, 2H); 13C NMR (151
MHz, CDCl3) δ 141.2, 140.6, 128.3, 128.1, 128.0, 128.0, 126.9, 126.8, 68.4, 51.4; IR (ATR)
3312, 3085, 3062, 3028, 2887, 2827, 2750 cm-1; [α]D = –25.1 (c = 1.0, CHCl3); HRMS
20
[M+H]+ calcd for C28H29N2 393.2331, found 393.2317.
(1R,2R)-N,N’-bis(2-methoxybenzyl)-1,2-diphenylethane-1,2-diamine (L3)27
The product was obtained starting from o-methoxybenzyl chloride as a colorless oil after
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purification by column chromatography on silica gel Hx-AcOEt (4:1) (130 mg, 61%). H
NMR (600 MHz, CDCl3) δ 7.20 – 7.05 (m, 14H), 6.83 (t, J = 7.4, 2H), 6.75 (d, J = 8.0 Hz,
2H), 3.74 – 3.66 (m, 4H), 3.61 (s, 6H), 3.41 (d, J = 13.5 Hz, 2H), 2.45 – 1.99 (s, 2H); 13C
NMR (151 MHz, CDCl3) δ 157.6, 129.7, 128.0, 127.9, 126.7, 120.1, 110.0, 68.3, 55.0, 47.1;
IR (ATR) 3314, 3060, 3026, 2924, 2834, 1725 cm-1; [α]D = –38.0 (c = 1.0, CHCl3); HRMS
20
[M+H]+ calcd for C30H33N2O2 453.2542, found 453.2533.
(1R,2R)-N,N’-bis(2-phenylbenzyl)-1,2-diphenylethane-1,2-diamine (L4)16a
The product was obtained starting from 2-(chloromethyl)-1,1'-biphenyl as a colorless oil after
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purification by column chromatography on silica gel Hx-AcOEt (9:1) (167 mg, 65%). H
NMR (600 MHz, CDCl3) δ 7.34 – 7.27 (m, 6H), 7.27 – 7.15 (m, 12H), 7.10 – 7.03 (m, 6H),
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6.82 (m, 4H), 3.51 (d, J = 12.5 Hz, 2H), 3.43 (s, 2H), 3.30 (d, J = 12.5 Hz, 2H), 1.90 (s, 2H);
13C NMR (151 MHz, CDCl3) δ 142.1, 141.1, 141.1, 137.8, 130.1, 130.0, 129.0, 128.0, 127.9,
127.8, 127.3, 126.9, 126.8, 126.7, 68.8, 49.3; IR (ATR) 3311, 3058, 3024, 2952, 2922, 2850
9
cm-1; [α]D = –26.5 (c = 1.0, CHCl3); HRMS [M+H]+ calcd for C40H37N2 545.2957, found
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545.2952.
(1R,2R)-N,N’-bis(3,5-di-tert-butylbenzyl)-1,2-diphenylethane-1,2-diamine (L5)26
The product was obtained starting from 3,5-di-tert-butylbenzyl bromide as a white solid after
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purification by column chromatography on silica gel Hx-AcOEt (15:1) (187 mg, 65%). H
NMR (600 MHz, CDCl3) δ 7.28 (t, J = 1.7 Hz, 2H), 7.19 – 7.12 (m, 6H), 7.08 – 7.02 (m, 8H),
3.73 (s, 2H), 3.63 (d, J = 13.0 Hz, 2H), 3.49 (d, J = 13.0 Hz, 2H), 2.11 (s, 2H), 1.28 (s, 36H);
13C NMR (151 MHz, CDCl3) δ 150.6, 141.5, 139.7, 128.1, 127.9, 126.8, 122.4, 120.8, 68.48,
52.1, 34.8, 31.5; IR (ATR) 3359, 3023, 2965, 2949, 2901, 2862 cm-1; [α]D20 = –10.9 (c = 1.0,
CHCl3); HRMS [M+H]+ calcd for C44H61N2 617.4835, found 617.4842
Synthesis of N-phosphinoylimines
Preparation of ketoximes.: N-Phosphinoylimines were synthesized from the corresponding
oximes by the modification of the literature procedure.28 Suspension of hydroxylamine
hydrochloride (1.6 equiv), and NaOAc (2.0 equiv.) in 80% aqueous EtOH (20 mL) was stirred
at rt for 30 min. After this time ketone (1.0 equiv.) was added and the reaction mixture was
heated gently to reflux for 1 h. Following cooling to rt, the solvent was evaporated under
vacuum and the residue purified by column chromatography with hexane-ethyl acetate (9:1)
as eluent, to provide crystalline oxime with quantitative yield.
Preparation of N-phosphinoylimines.29 In the round bottle flask under inert atmosphere oxime
(1.0 equiv.) was dissolved in mixture of dry DCM/Hx (1:1) and cooled to –40°C. To the
reaction mixture TEA (1.1 equiv.) was added dropwise followed (after 10 min) by addition of
Ph2PCl (1.1 equiv). After adding was finished, reaction mixture was gradually warmed to
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room temperature and stirred overnight. When reaction was completed, contest of the flask
was poured over ice and the aqueous phase was extracted with DCM (3×15 mL). The
combined organic layers were washed with water, brine and dried over anhydrous
Na2SO4.The solvents were evaporated under reduced pressure to afford the crude ketimine,
which was purified by column chromatography on silica gel with DCM-Acetone (9:1 to 4:1)
as eluent.
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The spectral characteristics of N-phosphinoylimines 1a,29 1b,29 1c,29 1d,30 1e,30 1f,30
1g,11 1h,30 1i,30 1j,30 1k,31 1l,32 1m,30 1n,30 1o,30 were consistent with those reported previously
in the literature. Racemates of N-phosphinoylated amines were prepared by reduction of the
corresponding imines using NaBH4.
General Procedure for Asymmetric Hydrosilylation of N-phoshipnylimines: Method A
In the small round bottom flask with magnetic stirred bar under inert atmosphere were
dissolved in 0.4 mL of freshly distilled THF Zn(OAc)2 (4.1 mg; 0.022 mmol) and L2 (8.61
mg; 0.022 mmol). After 30 min solution of corresponding N-phosphinoylimine (0.44 mol; c =
0.44 mol/L), PMHS (80 µL, 3 equiv.) and 0.2 mL of dry methanol was added successively.
Reaction mixture was stirred at room temperature for 24 h. After indicated time, the reaction
mixture without any work-up was purified by column chromatography on silica gel with
DCM-acetone (9:1) as eluent.
General Procedure for Asymmetric Hydrosilylation of N-phoshipnylimines: Method B
In the small round bottom flask with magnetic stirred bar under inert atmosphere were
dissolved in 0.6 mL of freshly distilled THF Zn(OAc)2 (4.1 mg, 0.022 mmol) and L2 (8.61
mg; 0.022 mmol). After 30 min solution of corresponding N-phosphinoylimine (0.44 mol, c =
0.44mol/L), (EtO)3SiH (162 µL, 2 equiv.) was added successively. Reaction mixture was
stirred at room temperature for 24 h. After this time, the reaction mixture without any work-
up was purified by column chromatography on silica gel with DCM-acetone (9:1) as eluent.
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5a (S)-(-)-N-(1-phenylethyl)-diphenylphosphinamide33 was obtained from 1a29 as colorless
solid, 134 mg, yield 95%, ee 89% by HPLC Chiralcel OD-H Hexane-2-Propanol (90:10),
flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 15.5 min. (minor) (R), tS = 20.6 min. (major);
[α]D20 = –35.5 (c = 1.0, CHCl3); Lit.11 [α]D20 = +38.3 (for enantiomer R in EtOH).
5b (S)-(-)-N-(1-phenylpropyl)-diphenylphosphinamide33 was obtained from 1b29 as
colorless solid, 143 mg, yield 97 %, ee 87% by HPLC Chiralcel OD-H Hexane-2-Propanol
(90:10), flow 0.7 mL/min, UV=206 nm, 21 ºC, tR = 13.9 min. (minor), tS = 18.4 min. (major);
[α]D20 = –20.8 (c = 1.0, CHCl3).
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5c (S)-(-)-N-(1-(4-methoxylphenyl)-ethyl)-diphenylphosphinamide33 was obtained from
1c29 as light yellow solid, 85 mg, yield 55%, ee 93% by HPLC Chiralpak AD-H Hexane-2-
Propanol (90:10), flow 0.7 mL/min, UV=206 nm, 21 ºC, tS = 41.1 min. (major), tR = 46.3 min.
(minor); [α]D20 = –19.9 (c = 1.0, CHCl3); Lit.11 [α]D20 = –66.4 (in MeOH).
5d (S)-(-)-N-(1-(4-methylphenyl)ethyl)-diphenylphosphinamide33 was obtained from 1d30
as colorless solid, 108 mg, yield 73%, ee 95% by HPLC Chiralcel OD-H Hexane:2-
Propanol=90:10, flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 15.8 min. (minor), tS = 18.2 min.
(major); [α]D20 = –36.5 (c = 1.0, CHCl3); Lit.11 [α]D20 = –66.5 (in MeOH).
5e (S)-(-)-N-(1-(4-bromophenyl)ethyl)-diphenylphosphinamide16 was obtained from 1e30
as colorless solid, 150 mg, yield 85%, ee 91% by HPLC Chiralcel OD-H Hexane-2-Propanol
(90:10), flow 0.7 mL/min, UV=206 nm, 21 ºC, tR = 14.8 min. (minor), tS = 17.1 min. (major);
[α]D20 = –42.6 (c = 1.0, CHCl3).
5f (S)-(-)-N-(1-(4-chlorophenyl)ethyl)-diphenylphosphinamide33 was obtained from 1f30 as
colorless solid, 137 mg, yield 96%, ee 91% by HPLC Chiralcel OD-H Hexane-2-Propanol
(90:10), flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 27.4 min. (minor), tS = 32.6 min. (major);
[α]D20 = –55.7 (c = 1.0, CHCl3); Lit.11 [α]D20 = –73.4 (in MeOH).
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5g (S)-(-)-N-(1-(4-iodophenyl)ethyl)-diphenylphosphinamide11 was obtained from 1g11 as
light yellow solid, 148 mg, yield 76%, ee 91% by HPLC Chiralcel OD-H Hexane-2-Propanol
(90:10), flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 21.0 min. (minor), tS = 23.9 min. (major);
[α]D20 = –33.7 (c = 1.0, CHCl3).
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5h (S)-(-)-N-(1-(3-methoxylphenyl)ethyl)-diphenylphosphinamide33 was obtained from
1h30 as colorless solid, 115 mg, yield 75%, ee 93% by HPLC Chiralcel OD-H Hexane-2-
Propanol (90:10), flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 23.8 min. (minor), tS = 35.5 min.
(major); [α]D20 = –71.0 (c = 1.0, CHCl3); Lit.11 [α]D20 = –52.1 (in MeOH).
5i (S)-(-)-N-(1-(3-bromophenyl)ethyl)-diphenylphosphinamide was obtained from 1i30 as
colorless solid; 162 mg, yield 92%, ee 93% by HPLC Chiralcel OD-H Hexane-2-Propanol
(90:10), flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 18.6 min. (minor), tS = 24.7 min. (major);
[α]D20 = –39.5 (c = 1.0, CHCl3); 1H NMR (300 MHz, CDCl3): δ = 1.54 (d, 3H), 3.17-3.22 (m,
1H), 4.32-4.40 (m, 1H), 7.13-7.22 (m, 2H), 7.34-7.64 (m, 8H), 7.76–7.82 (m, 2H), 7.86–7.93
(m, 2H) ppm. 13C NMR (151 MHz, CDCl3): δ = 25.0, 122.7, 125.0, 128.5, 128.7,128.8,
129.3, 130.4, 132.1, 132.5, 133.9, 147.5 ppm. HRMS (ESI-TOF): calcd. for C20H19BrNNaPO
[M+Na]+: 422.0285 found 422.0279.
5j (R)-(+)-N-(1-(2-methoxylphenyl)ethyl)-diphenylphosphinamide34 was obtained from
1j30 as yellowish solid, 80mg, yield 52%, ee 88% by HPLC Chiralcel OD-H Hexane-2-
Propanol (90:10), flow 0.5 mL/min, UV=206 nm, 21 ºC, tS = 21.2 min. (minor), tR = 23.3 min.
(major); [α]D20 = +4.2 (c = 1.0, CHCl3), Lit.34 [α]D20 = –22.1 (for S enantiomer in MeOH).
5k (S)-(-)-N-(1-(2-chlorophenyl)ethyl)-diphenylphosphinamide35was obtained from 1k31 as
colorless solid, 98 mg, yield 69%, ee 92% by HPLC Chiralcel OD-H Hexane-2-Propanol
(90:10), flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 20.9 min. (minor), tS = 23.3 min. (major);
[α]D20 = -13.6 (c = 1.0, CHCl3).
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5l (S)-(-)-N-(1-indanamine)-diphenylphosphinamide16 was obtained from 1l32 as colorless
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solid, 136 mg, yield 93%, ee 93% by HPLC Chiralcel OD-H Hexane-2-Propanol (90:10),
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flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 22.8 min. (minor), tS = 38.1 min. (major); [α]D
=
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–47.7 (c = 1.0, CHCl3).
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5m (S)-N-(+)-(1-(1-Napthyl)ethyl)-diphenylphosphinamide33 was obtained from 1m30 as
colorless solid, 137 mg, yield 84%, ee 84% by HPLC Chiralcel OD-H Hexane-2-Propanol
(90:10), flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 22.9 min. (minor), tS = 29.4 min. (major);
[α]D20 = +56.9 (c = 1.0, CHCl3).
5n (S)-(-)-N-(1-(2-Napthyl)ethyl)-diphenylphosphinamide33 was obtained from 1n30 as
light yiellow solid, 155 mg, yield 95%, ee 89% by HPLC Chiralcel OD-H Hexane-2-
Propanol (90:10), flow 0.5 mL/min, UV=206 nm, 21 ºC, tR = 21.6 min. (minor), tS = 29.3 min.
(major); [α]D20 = –89.3 (c = 1.0, CHCl3); Lit.11 [α]D20 = –77.8 (in MeOH).
5o (S)-(-)-N-(1-Furylethyl)-diphenylphosphinamide33 was obtained from 1o30 as colorless
solid, 126 mg, yield 92%, ee 97% by HPLC Chiralpak AD-H Hexane-2-Propanol (90:10),
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flow 0.7 mL/min, UV=206 nm, 21 ºC, tR = 23.6 min (minor), tR = 26.0 min. (major); [α]D
=
–88.4 (c = 1.0, CHCl3); Lit.11 [α]D20 = –42.1 (in MeOH).
Acknowledgements
Financial support from the Polish National Science Centre (grant number NCN
2012/07/B/ST5/00909) is gratefully acknowledged.
Supporting Information. 1H and 13C NMR spectra and HPLC data. This material is available
free of charge via the Internet at http://pubs.acs.org.
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