Asymmetric synthesis of N-(diphenylphosphinyl)amines promoted by chiral carbosilane dendritic ligands in the enantioselective addition of dialkylzinc compounds to N-(diphenylphosphinyl)imines

Article abstract of DOI:10.1002/1099-0690(200209)2002:18<3115::AID-EJOC3115>3.0.CO;2-9

The enantioselective addition of dialkylzinc compounds to N- (diphenylphosphinyl)imines by using chiral dendrimers with flexible carbosilane backbones gives enantiomerically enriched N-(diphenylphosphinyl)amines with up to 92% ee. Wiley-VCH Verlag GmbH, 6

Full text of DOI:10.1002/1099-0690(200209)2002:18<3115::AID-EJOC3115>3.0.CO;2-9

SHORT COMMUNICATION
Asymmetric Synthesis of N-(Diphenylphosphinyl)amines Promoted by Chiral
Carbosilane Dendritic Ligands in The Enantioselective Addition of Dialkylzinc
Compounds to N-(Diphenylphosphinyl)imines
Itaru Sato,[a] Kenji Hosoi,[a] Ryo Kodaka,[a] and Kenso Soai*[a]
Keywords: Amines / Alkylation / Asymmetric synthesis / Dendrimers / Imines
The enantioselective addition of dialkylzinc compounds to N-
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
(diphenylphosphinyl)imines by using chiral dendrimers with 2002)
flexible carbosilane backbones gives enantiomerically en-
riched N-(diphenylphosphinyl)amines with up to 92% ee.
Introduction
metallic reagents. When chiral dendrimers with flexible
backbones are used as chiral ligands, the flexibility of the
backbones may cause unfavorable interactions between the
chiral sites and may reduce enantioselectivity.[10a] Thus,
highly enantioselective synthesis utilizing chiral dendrimers
with flexible backbones has been a challenge. We report
here a highly enantioselective addition of dialkylzinc com-
pounds to N-(diphenylphosphinyl)imines by using dendritic
chiral ligands composed of flexible carbosilane backbones
bearing four or twelve chiral β-amino alcohols (Figure 1).
The enantioselective addition of organometallic reagents
to imines is a useful method to obtain chiral amines.[1]
However, compared to the corresponding addition reaction
to aldehydes,[2] much less attention has been paid to the
enantioselective addition of dialkylzinc reagents to imines.
We have developed an enantioselective addition of dial-
kylzinc compounds to N-(diphenylphosphinyl)imines in the
presence of chiral β-amino alcohols for the formation of
enantiomerically enriched N-(diphenylphosphinyl)am-
ines.[3,4] Subsequent removal of the diphenylphosphinyl
group by an acid hydrolysis[5] affords enantiomerically en-
riched secondary amines.
Results and Discussion
The enantioselective addition of dialkylzinc compounds
to N-(diphenylphosphinyl)imines was examined in the pres-
ence of chiral dendritic ligands 1, 2, and a chiral dimer 3
(Scheme 1 and Table 1). When the chiral dendrimer 1,[11b]
containing four chiral sites, was used as the chiral ligand,
the enantioselective addition of Et2Zn to N-(diphenylphos-
phinyl)imine (4a) gave (R)-N-(diphenylphosphinyl)amine
(5a) with 92% ee in 78% yield (Table 1, entry 1).[13] The
yield and enantioselectivity with chiral dendrimer 1 are sim-
ilar to those of the chiral poly(phenylethyne) dendrimers
with rigid backbones.[10b] Reaction with N-(diphenylphos-
phinyl)imine 4b affords N-(diphenylphosphinyl)amine 5b
with 90% ee (entry 2). Addition of iPr2Zn to N-(di-
phenylphosphinyl)imine 4c proceeded with slightly higher
enantioselectivity — to give (R)-N-(diphenylphosphinyl)-
amine 6 with 89% ee — than the corresponding addition
of diethylzinc to 4c, giving N-(diphenylphosphinyl)amine 5c
with 82% ee (entries 3 and 4). On the other hand, N-(di-
phenylphosphinyl)imines 4d gave chiral N-(diphenylphos-
phinyl)amines 5d (88% ee, entry 5). The chiral dendrimer 1
can be recovered and reused.[14] Recovered 1 gave N-(di-
phenylphosphinyl)amine 5a with 91% ee without any loss
of enantioselectivity (entry 6). Chiral dendrimer 2, with 12
The synthetic application of dendrimers, which are poly-
mers with particular molecular weights and structures, are
of current interest.[6] Hyperbranched chain ends are able to
load chiral functionalities such as β-amino alcohols, and
the resulting chiral macromolecule works as a dendritic
chiral ligand in asymmetric synthesis.[7,8] Compared to
other polymer-bound chiral ligands,[9] dendritic chiral li-
gands possess superior applicabilities in: i) high activities of
every chiral sites due to their location at the periphery; ii)
high enantioselectivity induced by approximately the same
chiral circumstances of each chiral site. We have developed
PAMAM-based chiral dendrimers[10a] and chiral poly-
(phenylethyne) dendrimers.[10b,11a] Chiral poly(phenyl-
ethyne) dendrimers with rigid backbones act as chiral
ligands for the enantioselective addition of diethylzinc to
N-(diphenylphosphinyl)imines.
On the other hand, the backbone of carbosilane dendri-
mers[12] is flexible and it hardly coordinates with organo-
[a]
Department of Applied Chemistry, Faculty of Science, Tokyo
University of Science,
Kagurazaka, Shinjuku-ku, Tokyo, 162Ϫ8601, Japan
Fax: (internat.) ϩ81-3/3235-2214
E-mail: ksoai@ch.kagu.sut.ac.jp
Eur. J. Org. Chem. 2002, 3115Ϫ3118  WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ193X/02/0918Ϫ3115 $ 20.00ϩ.50/0
3115

I. Sato, K. Hosoi, R. Kodaka, K. Soai
SHORT COMMUNICATION
l)amine 5a was obtained with 92% and 90% ee in the reac-
tions with 0.13 and 0.083 molar equiv. of chiral dendrimer
2, respectively (entries 7 and 8). The enantioselectivity of
chiral dendrimer 2 is comparable to that of chiral den-
drimer 1 (entry 1) and chiral dimer 3 (entry 12). Addition
of Et2Zn to N-(diphenylphosphinyl)imines 4b and 4d using
chiral dendrimer 2 proceeded with high enantioselectivity
to give chiral N-(diphenylphosphinyl)amines 5b (84% ee)
and 5d (85% ee), respectively (entries 9 and 10). Isopropyl-
ation of N-(diphenylphosphinyl)imine 4c afforded N-(di-
phenylphosphinyl)amine 6 with 86% ee in 70% yield
(entry 11).
In summary, chiral dendrimers 1 and 2 with flexible
backbone are effective chiral ligands in the enantioselective
synthesis of chiral N-(diphenylphosphinyl)amines by the
addition of dialkylzinc reagents to N-(diphenylphosphinyl)-
imines.
Experimental Section
According to the literature,[11b] chiral dendrimer 1 was synthesized
by the reaction between lithiated (1R,2S)-N-(4-bromobenzyl)-O-
(tert-butyldimethylsilyl)ephedrine[11b] and tetrakis[3-(chlorodime-
thylsilyl)propyl]silane[12a] followed by the cleavage of the silyl
ether.[11b] Chiral dendrimer 2 was prepared from tetrakis[3-{tris[3-
(chlorodimethylsilyl)propyl]silyl}propyl]silane[12a] and (1R,2S)-N-
(4-bromobenzyl)-O-(trimethylsilyl)ephedrine.[11b] Chiral dimer 3
was derived from commarcially available 1,3-(chlorodimethylsilyl)e-
thane and (1R,2S)-N-(4-bromobenzyl)-O-(tert-butyldimethylsilyl)e-
phedrine.[11b] Compounds 4a,[3a,15] 4b,[3a,10a] 4c[3a,15] and 4d[15] were
prepared according to a literature procedure.[15] Compounds
5a,[3a,16] 5b,[3a][10a] 5c,[3a,16] 5d[4a] and 6[17] are known compounds.
Dendrimer 1: Colorless oil. [α]2D5 ϭ Ϫ26.8 (c ϭ 1.00, CHCl3). 1H
NMR (300 MHz, CDCl3): δ ϭ 0.26 (s, 24 H), 0.54 (m, 8 H), 0.80
(m, 8 H), 0.99 (d, J ϭ 6.8 Hz, 12 H), 1.34 (m, 8 H,), 2.20 (s, 12
H,), 2.93 (qd, J ϭ 6.8, 5.4 Hz, 4 H), 3.60 (d, J ϭ 13.5 Hz, 4 H),
3.63 (d, J ϭ 13.5 Hz, 4 H), 4.90 (d, J ϭ 5.4 Hz, 4 H), 7.2Ϫ7.5 (m,
36 H) ppm. 13C NMR (75 MHz, CDCl3):δ ϭ Ϫ2.8, 9.9, 17.4, 18.5,
20.6, 38.7, 59.1, 63.4, 73.5, 126.1, 126.9, 128.0, 128.1, 133.5, 138.2,
140.0, 142.5 ppm. FT-IR (neat): ν˜ ϭ 3432 cmϪ1, 2792, 1450, 1249.
HR MS (FABϩ) calcd. for C88H129N4O4Si5 [M ϩ H]: 1445.886;
found 1445.886.
Dendrimer 2: Colorless oil. [α]2D2 ϭ Ϫ21.5 (c ϭ 1.00, CHCl3). 1H
NMR (300 MHz, CDCl3): δ ϭ 0.23 (s, 72 H), 0.58 (m, 40 H), 0.80
(m, 24 H), 0.97 (d, J ϭ 6.6 Hz, 36 H), 1.21 (m, 32 H), 2.17 (s, 36
H), 2.90 (m, 12 H), 3.58 (d, J ϭ 13.9 Hz, 12 H), 3.62 (d, J ϭ
13.9 Hz, 12 H), 4.89 (d, J ϭ 4.0 Hz, 12 H), 7.2Ϫ7.5 (m, 108 H)
ppm. 13C NMR (75 MHz, CDCl3):δ ϭ Ϫ2.8, 9.8, 17.4, 18.6, 20.6,
38.7, 59.1, 63.6, 73.4, 126.1, 126.9, 128.0, 128.1, 133.5, 138.2, 139.8,
142.5 ppm. FT-IR (neat): ν˜ ϭ 3382 cmϪ1, 2915, 1450, 1250.
MALDI-TOF MS (α-CHCA): m/z ϭ 4564.7 [M ϩ H]ϩ.
Figure 1. Chiral dendrimers 1, 2 and chiral dimer 3
Dimer 3: Colorless powder; m.p. 78 °C (hexane). [α]2D5 ϭ Ϫ36.9 (c ϭ
1.00, CHCl3). 1H NMR (300 MHz, CDCl3): δ ϭ 0.26 (s, 12 H),
0.68 (s, 4 H), 1.01 (d, J ϭ 6.8 Hz, 6 H), 2.22 (s, 6 H), 2.95 (qd, J ϭ
6.8, 4.8 Hz, 2 H), 3.63 (d, J ϭ 13.6 Hz, 2 H), 3.64 (d, J ϭ 13.6 Hz,
2 H), 4.90 (d, J ϭ 4.8 Hz, 2 H), 7.2Ϫ7.5 (m, 18 H) ppm. 13C NMR
(75 MHz, CDCl3):δ ϭ Ϫ3.5, 7.8, 9.9, 38.8, 59.1, 63.4, 73.5, 126.2,
Scheme 1
chiral sites,[11b] also works as a highly enantioselective chiral
ligand in the enantioselective addition of Et2Zn to N-(di-
phenylphosphinyl)imine (4a). Thus, N-(diphenylphosphiny- 126.9, 128.0, 128.0, 133.7, 137.8, 140.0, 142.4 ppm. FT-IR (neat):
3116 Eur. J. Org. Chem. 2002, 3115Ϫ3118

Asymmetric Synthesis of N-(Diphenylphosphinyl)amines
SHORT COMMUNICATION
Table 1. Highly enantioselective synthesis of N-(diphenylphosphinyl)amines by the addition of dialkylzinc compounds to N-(diphenylphos-
phinyl)imines using chiral dendrimers 1, 2 or chiral dimer 3
N-(diphenylphosphinyl)imine
Chiral dendrimer
(mol. equiv.)
N-(diphenylphosphinyl)amine
Entry[a]
R1
R2
yield [%][b]
ee [%][c]
1
2
3
4
5
6
2-naphthyl
p-tolyl
phenyl
phenyl
4-ClC6H4
2-naphthyl
4a
4b
4c
4c
4d
4a
1
1
1
1
1
1
(0.25)
Et
Et
iPr
Et
Et
Et
5a
5b
6
5c
5d
5a
78
71
79
81
72
77
92
90
89
82
88
91
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)[d]
7
8
9
10
11
2-naphthyl
2-naphthyl
p-tolyl
4-ClC6H4
phenyl
4a
4a
4b
4d
4c
2
2
2
2
2
(0.13)
(0.083)
(0.13)
(0.13)
(0.13)
Et
Et
Et
Et
iPr
5a
5a
5b
5d
6
70
70
71
74
70
92
90
84
85
86
12
13
14
2-naphthyl
phenyl
phenyl
4a
4c
4c
3
3
3
(0.50)
(0.50)
(0.50)
Et
iPr
Et
5a
6
5c
71
81
80
94
90
86
[a]
[b]
Reactions were run in toluene at 0 °C for 48 h using 3 molar equiv. of dialkylzinc compounds.
Isolated yields.[c] Determined by
[d]
HPLC analysis on a chiral stationary phase.
Recovered chiral dendrimer was used.
ν˜ ϭ 3394 cmϪ1, 2951, 2842, 2788, 1454, 1392. HR MS (FABϩ) N-(1-Phenylpropyl)-P,P-diphenylphosphinamide (5c):[3a,16] HPLC
calcd. for C40H57N2O2Si2 [M ϩ H]: 653.3959; found 653.3962.
conditions: Chiralcel OD; 4.6 ϫ 250 mm; 254 nm UV detector; elu-
ent: 3% 2-propanol in hexane; flow rate: 1.0 mL/min; retention
time: 15 min for R-isomer, 24 min for S-isomer.
N-(4-Methybenzylidene)-P,P-diphenylphosphinamide (4b): Colorless
powder; m.p. 154 °C (hexane/toluene). 1H NMR (300 MHz,
CDCl3): δ ϭ 2.46 (s, 3 H), 7.30 (m, 2 H), 7.4Ϫ7.5 (m, 6 H), 7.8Ϫ7.9
(m, 6 H), 9.38 (d, J ϭ 34.8 Hz, 1 H) ppm. FT-IR (KBr): ν˜ ϭ
1630 cmϪ1, 1195. HR-MS calcd. for C22H24ONP [Mϩ]: 319.1126;
found 319.1117.
N-[1-(4-Chlorophenyl)propyl]-P,P-diphenylphosphinamide (5d):[4a]
HPLC conditions: Chiralpak AD; 4.6 ϫ 250 mm; 254 nm UV de-
tector; eluent: 10% 2-propanol in hexane; flow rate: 1.0 mL/min;
retention time: 20 min for R-isomer, 24 min for S-isomer.
(؉)-N-(2-Methyl-1-phenylpropyl)-P,P-diphenylphosphinamide
(6):[17] Colorless needles; m.p. 155 °C (hexane/dichloromethane,
87% ee). [α]2D5 ϭ ϩ35.3 (c ϭ 1.00, MeOH, 87% ee). HPLC condi-
tions: Chiralcel OD; 4.6 ϫ 250 mm; 254 nm UV detector; eluent:
2% 2-propanol in hexane; flow rate: 1.0 mL/ min; retention time:
24 min for R-isomer, 34 min for S-isomer.
Representative Procedure for the Enantioselective Addition of Dial-
kylzinc Compounds to N-(diphenylphosphinyl)imine 4a in the Pres-
ence of Chiral Dendrimer 3: A 1  toluene solution of Et2Zn (0.3
mL, 0.3 mmol) at 0 °C was added to a mixture of N-(diphenylphos-
phinyl)imine 4a (35.5 mg, 0.1 mmol) and dendritic chiral ligand 3
(38.0 mg, 0.0083 mmol) in toluene (2 mL). After stirring the mix-
ture for 48 h at 0 °C, the reaction was quenched by adding sat. aq.
ammonium chloride (3 mL). The mixture was filtered through cel-
ite and the separated aqueous layer was extracted with dichlorome-
thane. The combined organic layer was dried over anhydrous so-
dium sulfate and the solvents evaporated. Purification of the res-
idue by thin layer chromatography (eluent: dichloromethane/acet-
one, 5:1) gave N-(diphenylphosphinyl)amine 5a (27.1 mg, 70%).
The ee value was determined to be 90% by HPLC analysis using a
chiral stationary phase (Chiralcel OD; 4.6 ϫ 250 mm; 254 nm UV
detector; eluent: 3% 2-propanol in hexane; flow rate: 1.0 mL/min;
retention time: 27 min for R-isomer, 44 min for S-isomer).
Acknowledgments
This work was supported by a Grant-in-Aid from the Ministry of
Education, Sports, Culture, Science and Technology.
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A
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[17b]
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