A new chiral P,N-ligand derived from 1-phenylphospholane-2-carboxylic acid (phenyl-P-proline) for palladium-catalyzed asymmetric allylic substitution reactions

Article abstract of DOI:10.1002/adsc.200505234

New types of P,N-ligands, cis- and trans-3, containing a tetrahydroisoquinoline skeleton as an N-donor were synthesized from (1R,2S)-1-phenylphospholane-2-carboxylic acid (phenyl-P-proline, 1). The cis isomer, cis-3, was found to act as an excellent ligan

Full text of DOI:10.1002/adsc.200505234

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DOI: 10.1002/adsc.200505234
A New Chiral P,N-Ligand Derived from
1-Phenylphospholane-2-carboxylic Acid (Phenyl-P-proline) for
Palladium-Catalyzed Asymmetric Allylic Substitution Reactions
Xiang-Min Sun, Masatoshi Koizumi, Kei Manabe, Shu¯ Kobayashi*
Graduate School of Pharmaceutical Sciences, The University of Tokyo, The HFRE Division, ERATO, Japan Science and
Technology Agency (JST), Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Fax: (þ81)-3-5684-0634; e-mail: skobayas@mol.f.u-tokyo.ac.jp
Received: June 3, 2005; Accepted: September 15, 2005
Abstract: New types of P,N-ligands, cis- and trans-3,
containing a tetrahydroisoquinoline skeleton as an
N-donor were synthesized from (1R,2S)-1-phenyl-
phospholane-2-carboxylic acid (phenyl-P-proline, 1).
The cis isomer, cis-3, was found to act as an excel-
lent ligand in palladium-catalyzed asymmetric allylic
substitution reactions. The reactions of 1,3-diphenyl-
2-propenyl acetate (5) with several nucleophiles in
the presence of [Pd(p-allyl)Cl]2, cis-3 (Pd:ligand¼
1:2), and a base afforded the desired products in
high yields with high enantioselectivity. It was sug-
gested that these ligands did not serve as P,N-biden-
tate ligands but as P-monodentate ligands in these
reactions.
and consequently, exhibited good enantioselectivity in
a palladium-catalyzed asymmetric allylic substitution
reaction. To obtain higher enantioselectivity, a new class
of P,N hybrid ligand 2 has been designed (Scheme 1).
Herein, we report the synthesis of 2 and its use in palla-
dium-catalyzed allylic substitution reactions.
Scheme 1. Design of a new type of P,N-ligand 2.
Keywords: allylic substitution; asymmetric catalysis;
palladium; P,N-ligands
New types of hybrid P,N-ligands 3 containing a tetra-
hydroisoquinoline skeleton as an N-donor were synthe-
sized from 1 via intermediate amides 4 as shown in
Scheme 2. The synthesis of the intermediate 4 was ach-
ieved by amidation of 1 with 1,2,3,4-tetrahydroisoquino-
line in the presence of 1-hydroxy-7-azabenzotriazole
The design and synthesis of chiral phosphine ligands (HOAt),[7] and a mixture of isomers, trans-4 and cis-4,
have played a significant role in the development of was obtained in high yield with a ratio of 52:48, respec-
transition metal-catalyzed asymmetric reactions.[1] tively. These isomers could be easily separated by col-
Among them, P-stereogenic tertiary phosphines pro- umn chromatography on silica gel. When the reaction
vide one of the most important and promising ligands.[2] was conducted in the absence of HOAt at 08C, trans-4
On the other hand, chiral bidentate ligands with two dif- was obtained selectively in good yield (trans-4:cis-4¼
ferent donor atoms (so-called ꢀhybrid ligandsꢁ) also give 95:5). Borane reduction of the amide group of 4 fol-
effective asymmetric environments, and the catalytic lowed by deboronation[8] of the phosphorus by treat-
ability of metal complexes with these ligands has been ment with pyrrolidine afforded trans-3 and cis-3 in
well documented.[3] One advantage of this type of chiral good yields, respectively.
ligand lies in the difference in trans influence between
The P,N hybrid ligands thus prepared were first evalu-
two donor atoms.[4] Furthermore, coordination ability ated in the palladium-catalyzed asymmetric allylic sub-
of the donor atoms may affect reactivity and stereoselec- stitution of allyl acetate. [Pd(p-allyl)Cl]2 (2 mol % Pd)
tivity in asymmetric catalysis.[5] Recently, we have de- and BSA (150 mol %)–NaOAc (5 mol %) were used
signed
a new type of P-stereogenic phosphine, as a palladium source and a base, respectively. It was sur-
(1R,2S)-1-phenylphospholane-2-carboxylic acid (phe- prising to find that no desired product was obtained
nyl-P-proline, 1), and established an efficient synthetic when 2 mol % of trans-3 (Pd:ligand¼1:1) was used
route to it.[6] The P-stereogenic phosphine constructed (Table 1, entry 4). The desired product was obtained in
an asymmetric environment near the palladium center, low yield with 45% ee when 2.5 mol % of trans-3
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Scheme 2. Preparation of P,N-ligands 3.
(Pd:ligand¼1:1.25) was used (Table 1, entry 5). The was obtained. These peaks may correspond to those of
same tendency was observed, albeit with higher ee, the complexes of Pd:cis-3¼1:1, in which cis-3 coordi-
when cis-3 was used as a ligand (entry 14). To clarify nates to the palladium in P,N-bidentate and P-monoden-
these interesting observations the reaction was carefully tate fashions. Furthermore, when another 0.5 equivalent
investigated by changing the molar ratios of Pd to the li- of the ligand was added to the mixture, a new peak at
gand and the reaction time. It was revealed that no de- 28.2 ppm appeared. This is the only peak observed in
sired product was obtained when the molar ratio of the the 31P NMR spectrum of a 1:2 mixture of Pd and cis-3
ligand to Pd was less than 1. On the other hand, when in toluene-d8 at ꢀ608C. In the case of trans-3, the
the molar ratio was more than 1, the desired product same tendency was observed. These results indicate
was obtained (entries 5–9, 14–18). Interestingly, the that the real catalyst species is the complex of
enantioselectivities obtained in these cases were inde- Pd:trans-3 or cis-3¼1:2, in which trans-3 or cis-3 does
pendent of the molar ratios of the ligand to Pd and the not serve as a P,N-bidentate ligand but as a P-monoden-
reaction time. These results strongly indicate that tate ligand.[5a]
trans-3 and cis-3 did not serve as P,N-bidentate ligands
but as P-monodentate ligands.
Next, to optimize the reaction conditions, the effect of
solvents, BSA activators, temperature, and palladium
The following experiments were performed to probe sources were examined. While no significant effect of
the composition of the catalyst mixtures at various li- solvents was observed (Table 2, entries 1–5), the highest
gand-to-metal ratios. When Pd was mixed with cis-3 in ee was obtained when toluene was used as the solvent
toluene-d8 at ꢀ608C in a ratio of 1:1, a 31P NMR spec- (entry 5). As for BSA activators LiOAc, NaOAc,
trum with two major peaks at 27.0 ppm and 27.8 ppm KOAc, and CsF gave high yields and enantioselectivities
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Table 1. Allylic substitution reactions using catalyst mixtures of various ligand-to-metal ratios.
.
Entry
Ligand
Pd :ligand
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
trans-3
trans-3
trans-3
trans-3
trans-3
trans-3
trans-3
trans-3
trans-3
cis-3
cis-3
cis-3
cis-3
cis-3
1: 0.5
1: 0.75
1: 0.9
1: 1
1: 1.25
1: 1.5
1: 2
1: 2.5
1: 3
1: 0.5
1: 0.75
1: 0.9
1: 1
0 (0)[a]
0 (0)[a]
0 (0)[a]
0 (0)[a]
1
– (ꢀ)[a]
– (ꢀ)[a]
– (ꢀ)[a]
– (ꢀ)[a]
45
4 (93)[a]
16 (96)[a]
48 (95)[a]
59
45 (45)[a]
45 (45)[a]
45 (45)[a]
46
0 (0)[b]
0 (0)[b]
0 (0)[b]
0 (0)[b]
3 (50)[b]
12 (93)[b]
42 (97)[b]
42 (99)[b]
32
– (ꢀ)[b]
– (ꢀ)[b]
– (ꢀ)[b]
– (ꢀ)[b]
93 (94)[b]
94 (94)[b]
94 (94)[b]
94 (94)[b]
94
1: 1.25
1: 1.5
1: 2
1: 2.5
1: 3
cis-3
cis-3
cis-3
cis-3
[a]
The reaction time was 4 h.
The reaction time was 6 h.
[b]
(entries 5–10). In particular, the reaction was accelerat-
Finally, other nucleophiles were tested [Eqs. (1)–(3)].
ed when KOAc was used as a BSA activator (entries 7 1,3-Diketones and benzylamine reacted with 5 under
and 8).[9] While the enantioselectivity was slightly im- the standard reaction conditions to afford the desired
proved at lower temperature (entry 8), the yield was sig- products in high yields with high enantiomeric excesses.
nificantly decreased when Pd2(dba)3 ·CHCl3 was used as
In conclusion, new types of P,N-ligands, cis-3and trans-3,
a palladium source instead of [Pd(p-allyl)Cl]2 (entry 11). derived from phenyl-P-proline were synthesized, and li-
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Table 2. Optimization of the reaction conditions.
Entry
Solvent
Temp. [ 8C]
Activator
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
CH2Cl2
CH3CN
DMF
0
0
0
0
0
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
LiOAc
KOAc
KOAc
CsF
93
98
98
86
80
80
98
99
98
90
19
87
89
89
90
94
90
94
96
94
96
95
THF
toluene
toluene
toluene
toluene
toluene
toluene
toluene
0
0
ꢀ20
0
10
ꢀ20
ꢀ20
CsF
KOAc
11[a]
[a]
Pd2(dba)3 ·CHCl3 was used as a palladium source.
cis-4: white solid; mp 113.9–114.08C; [a]2D7: ꢀ150.58 (c 1.0,
gand cis-3achieved high enantioselectivity in Pd-catalyzed
allylic substitution reactions. It is suggested that the ligands
do not serve as P,N-bidentate ligands but as P-monoden-
tate ligands inthese reactions. Further investigations touti-
lize P,N-ligands derived from phenyl-P-proline in asym-
metric catalysis are in progress.
1
EtOH); H NMR (600 MHz, CDCl3): d¼1.00 (brq, 3H, J¼
111 Hz), 1.73–4.78 (m, 13H), 6.86–7.76 (m, 9H); 13C NMR
(150 MHz, CDCl3): d¼26.56, 26.73, 27.12, 27.38, 27.66, 27.68,
29.16, 31.28, 31.54, 39.85, 43.26, 43.34, 43.48, 43.65, 44.39,
47.04, 125.41 (d, J¼44.5 Hz), 126.23 (d, J¼44.5 Hz), 126.19,
126.30, 126.45, 128.00, 128.10 (d, J¼10.1 Hz), 128.32 (d, J¼
10.1 Hz), 128.57, 131.53, 131.89, 132.08, 132.68, 132.81,
132.87, 132.92, 133.96, 134.32, 167.15 (d, J¼4.3 Hz), 167.69
(d, J¼4.3 Hz); 31P NMR (243 MHz, CDCl3): d¼37.37 (d, J¼
72.4 Hz), 37.67 (d, J¼72.4 Hz); IR (KBr): n¼2927, 2368,
2333, 1646, 1434 cmꢀ1; ESI-MS m/z¼360 [MþNa]; anal.
calcd. for C20H25BNOP: C 71.24, H 7.47, N 4.15; found: C
70.98, H 7.52, N 4.00.
Experimental Section
Synthesis of trans- and cis-4
To a mixture of 1[6] (1.554 g, 7.0 mmol) and HOAt (1.063 g,
7.7 mmol) in CH2Cl2 (16 mL), 1,2,3,4-tetrahydroisoquinoline
(1.6 mL, 14.0 mmol) was added. After stirring for 10 min at
08C, EDC·HCl (2.684 g, 14.0 mmol) was added to the mixture.
The reaction mixture was then warmed to room temperature
and stirred for 4 h. The reaction was quenched by adding a
1.0 N aqueous solution of HCl, and the aqueous solution was
extracted with CH2Cl2. The organic layer was dried with
Na2SO4 and the solvents were evaporated to dryness. The
crude product was purified by column chromatography on sili-
ca gel (eluent: hexane/AcOEt, 2:1) affording trans-4 in 45%
yield and cis-4 in 42% yield, respectively.
Synthesis of the P,N-Type Ligand 3
To a solution of cis-4 (2.004 g, 5.9 mmol) in THF (12 mL), BH3 ·
THF in THF (1.8 M, 33 mL, 59.4 mmol) was added. The reac-
tion mixture was then stirred for 24 h at 408C. The mixture was
quenched with a 1.0 N aqueous solution of HCl at 08C, neutral-
ized with a1.0 N aqueous solution of NaOH, and then the aque-
ous solution was extracted with CH2Cl2. The organic layer was
dried with Na2SO4 and the solvents were evaporated to dry-
ness. The crude intermediate was dissolved in Et2O, and fil-
tered through silica gel to remove an insoluble by-product. A
degassed pyrrolidine (30 mL) solution of the intermediate
was stirred for 5 h at 408C under argon. Pyrrolidine was evapo-
rated and the residue was treated with a 1.0 N aqueous solution
of HCl (pH¼3), then neutralized by a 1.0 N aqueous solution
of NaOH. The product was extracted with CH2Cl2. The organic
layer was dried with Na2SO4 and the solvents were evaporated
to dryness. The crude product was purified by column chroma-
tography on silica gel (eluent: hexane/AcOEt 2:1) affording
cis-3.
trans-4: colorless viscous oil; [a]2D8: ꢀ52.58 (c 1.0, EtOH); 1H
NMR (600 MHz, CDCl3): d¼0.71 (brq, 3H, J¼149.8 Hz),
2.11–6.39 (m, 13H), 7.03–7.86 (m, 9H); 13C NMR (150 MHz,
CDCl3): d¼25.76 (d, J¼4.3 Hz), 28.22 (d, J¼7.2 Hz), 28.45,
28.50, 29.43, 31.67 (d, J¼4.3 Hz), 31.73 (d, J¼4.3 Hz), 40.31,
43.35, 43.54, 44.19, 44.36, 44.87, 47.79, 125.70, 125.97, 126.41,
126.54, 126.68, 126.72, 127.91, 128.82, 129.22, 129.29, 130.68
(d, J¼17.2 Hz), 130.96 (d, J¼17.2 Hz), 131.67, 131.73,
131.81, 131.87, 131.93, 133.28, 133.96, 135.15, 167.57, 168.06;
31P NMR (243 MHz, CDCl3): d¼39.44, 41.55; IR (KBr): n¼
2929, 2375, 1645, 1437 cmꢀ1; ESI-MS: m/z¼360 [MþNa];
anal. calcd. for C20H25BNOP: C 71.24, H 7.47, N 4.15, found:
C 70.98, H 7.49, N 4.07.
cis-3: yield: 90%; white solid; mp 64.1–64.28C; [a]2D9:
1
ꢀ113.68 (c 0.60, EtOH); H NMR (600 MHz, CDCl3): d¼
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Chiral P,N-Ligand Derived from 1-Phenylphospholane-2-carboxylic Acid
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1.36–1.41 (m, 1H), 1.72–1.76 (m, 1H), 1.96–2.38 (m, 7H),
(R)-(E)-1-Benzylamino-1,3-diphenyl-2-propene (9)
¼
2.55–2.66 (m, 2H), 2.88 (m, 2H), 3.43 (d, 1H, J 14.4 Hz),
To a mixture of 5 (126 mg, 0.5 mmol), [Pd(p-allyl)Cl]2 (1.8 mg,
0.005 mmol), cis-3 (6.2 mg, 0.02 mmol) in toluene (1 mL), ben-
zylamine (0.164 mL, 1.5 mmol) was added at 08C. After being
stirred for 3 h at 08C, H2O and CH2Cl2 were added to the reac-
tion mixture. After extracting with CH2Cl2, the organic layer
was dried with Na2SO4 and the solvents were evaporated to
dryness. The crude product was purified by PTLC on silica
gel (eluent: hexane/AcOEt, 6:1) affording (R)-(E)-1-benzyl-
amino-1,3-diphenyl-2-propene (9); yield: 98%; 80% ee; [a]2D3:
ꢀ21.38 (c 1.5, CHCl3). The enantiomeric excess of 9 was deter-
mined by chiral HPLC (column, chiralcel OJ; eluent, 2-propa-
nol/hexane, 1:19; flow rate, 1.0 mL/min; detection, 254-nm
light). The absolute configuration of 9 was determined by the
optical rotation according to the literature.[12]
3.61 (d, 1H, J¼14.4 Hz), 6.98–7.31 (m, 7H), 7.51 (m, 2H);
13C NMR (100 MHz, CDCl3): d¼23.03 (d, J¼11.4 Hz), 26.37
(d, J¼2.9 Hz), 28.82, 31.85 (d, J¼4.3 Hz), 40.36 (d, J¼
12.4 Hz), 50.67, 55.83, 58.30, 125.58, 126.05, 126.56, 127.72 (d,
J¼6.7 Hz), 128.44, 128.57, 133.84, 134.02, 134.34, 136.00; 31P
NMR (162 MHz, CDCl3): d¼9.82; IR (KBr): n¼3064, 2935,
2861, 2786, 2753, 1425, 1375, 744 cmꢀ1; ESI-MS (HR): m/z¼
310.1706 [MþH]; calcd. for C20H25NP: 310.1719.
trans-3: yield: 79%; colorless oil; [a]2D0: ꢀ52.38 (c 1.0,
1
EtOH); H NMR (400 MHz, CDCl3): d¼1.46–1.56 (m, 1H),
1.73–1.85 (m, 1H), 1.87–2.11 (m, 4H), 2.52–2.61 (m, 1H),
2.67–2.89 (m, 6H), 3.62 (d, 1H, J¼15.1 Hz), 3.74 (d, 1H, J¼
15.1 Hz), 6.97–7.14 (m, 4H), 7.22–7.30 (m, 3H), 7.44–7.48
(m, 2H) ppm; 13C NMR (100 MHz, CDCl3): d¼26.03 (d, J¼
11.4 Hz), 27.88 (d, J¼2.9 Hz), 28.91, 33.35, 42.99 (d, J¼
10.49 Hz), 50.83, 56.32, 62.68 (d, J¼36.2 Hz), 126.02, 126.56,
127.50, 128.28 (d, J¼5.9 Hz), 128.59, 130.73, 130.89, 134.44,
141.84 (d, J¼21.9 Hz) ppm; 31P NMR (243 MHz, CDCl3):
d¼4.18; IR (neat): n¼3068, 2929, 2795, 1585, 1497, 1430,
1372, 1193, 740 cmꢀ1; ESI-MS (HR): m/z¼310.1718 [MþH];
calcd. for C20H25NP: 310.1719.
Acknowledgements
This work was partially supported byGrand-in-Aid for Scientif-
ic Research from Japan Society of the Promotion of Sciences
(JSPS). We thank Mr. Nobuyuki Shiraishi for his contribution
to the early stage of this work.
References and Notes
General Procedure for Palladium-Catalyzed Allylic
Substitution of 5 using cis-3
[1] a) E. N. Jacobsen, A. Pfaltz, H. Yamamoto (Eds.), Com-
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Chem. Rev. 1981, 81, 365. The general trends for the trans
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R2NH>OR2.
To a mixture of 1,3-diphenyl-2-propenyl acetate (126 mg,
0.5 mmol), [Pd(p-allyl)Cl]2 (1.8 mg, 0.005 mmol), cis-3
(6.2 mg, 0.02 mmol), and NaOAc (2.1 mg, 0.025 mmol) in tol-
uene (1 mL), dimethyl malonate (60 mL, 0.525 mmol) was add-
ed. The mixture was stirred for 5 min at room temperature,
cooled to 08C, and stirred for 10 min. BSA (185 mL,
0.75 mmol) was added and the mixture was then stirred for
1 h at 08C. H2O and CH2Cl2 were added to the reaction mix-
ture. After extracting with CH2Cl2, the organic layer was dried
with Na2SO4 and the solvents were evaporated to dryness. The
crude product was purified by PTLC on silica gel (eluent: hex-
ane/AcOEt, 6:1) affording (S)-(E)-1,3-diphenyl-2-methoxy-
carbonyl-4-penpenoate (6); yield: 98% with 94% ee. The enan-
tiomeric excess (ee) of 6 was determined by chiral HPLC (col-
umn, chiralpak AD; eluent, 2-propanol/hexane, 1:19; flow
rate, 1.0 mL/min; detection, 254-nm light). The absolute con-
figuration of the product was determined by comparison of
its chiral HPLC behavior with that described in the litera-
ture.[10]
(S)-(E)-3-Acetyl-4,6-diphenyl-5-hexen-2-one (7): yield:
96%; 93% ee (S); [a]2D3: þ5.98 (c 1.3, EtOH). The enantiomeric
excess was determined by chiral HPLC analysis (column, chiral-
cel OJ; eluent, 2-propanol/hexane, 1:9; flow rate, 1.0 mL/min;
detection, 254-nm light). Theabsolute configuration of7 wasde-
termined by the optical rotation according to the literature.[3c]
(R)-(E)-3-Acetyl-3-methyl-4,6-diphenyl-5-hexen-2-one
(8): yield: 88%; 88% ee (R); [a]2D3: þ25.08 (c 1.8, EtOH). The
enantiomeric excess was determined by chiral HPLC analysis
(column, chiralcel OJ; eluent, 2-propanol/hexane, 1:9; flow
rate, 1.0 mL/min; detection, 254-nm light). The absolute con-
figuration of 8 was determined by the optical rotation accord-
ing to the literature.[11]
[5] a) A. M. Porte, J. Reibenspies, K. Burgess, J. Am. Chem.
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abe, W. W.-L. Lam, N. Shiraishi, J. Kobayashi, M. Shiro,
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[7] L. A. Carpino, J. Am. Chem. Soc. 1993, 115, 4397.
[8] T. Imamoto, T. Oshiki, T. Onozawa, T. Kusumoto, K.
Sato, J. Am. Chem. Soc. 1990, 112, 5244. The deborona-
tion of a P-stereogenic phosphine borane complex has
been proven to proceed in a stereospecific manner with
retention of the configuration. Furthermore, 13C and
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