Syntheses of Novel Chiral Monophosphines, 2,5-Dialkyl-7-phenyl-7-phosphabicycloheptanes, and Their Application in Highly Enantioselective Pd-Catalyzed Allylic Alkylation

Full text of DOI:10.1021/jo9623548

J . Org. Chem. 1997, 62, 4521-4523
4521
respectively. The synthesis employed Birch reduction,10
followed by asymmetric hydroboration11 and recrystalli-
zation to 100% ee. Conversion of the optically pure diols
to the corresponding mesylates proceeds cleanly. Nu-
cleophilic substitution by Li2PPh on the chiral dimesy-
lates 3 and 4 generated the corresponding bicyclic
phosphines, which were trapped by BH3‚THF to form the
air-stable boron-protected monophosphines 5 and 6,
respectively. Deprotection with a strong acid12 produces
the desired products (7, (1R,2S,4R,5S)-(+)-2,5-dimethyl-
7-phenyl-7-phosphabicyclo[2.2.1]heptane; 8, (1R,2R,4R,5R)-
(+)-2,5-diisopropyl-7-phenyl-7-phosphabicyclo[2.2.1]-
heptane) in high yields.
Syn th eses of Novel Ch ir a l
Mon op h osp h in es,
2,5-Dia lk yl-7-p h en yl-7-p h osp h a bicyclo-
[2.2.1]h ep ta n es, a n d Th eir Ap p lica tion in
High ly En a n tioselective P d -Ca ta lyzed
Allylic Alk yla tion s
Zhaogen Chen, Qiongzhong J iang, Guoxin Zhu,
Dengming Xiao, Ping Cao, Cheng Guo, and
Xumu Zhang*
Department of Chemistry, 152 Davey Laboratory,
The Pennsylvania State University,
University Park, Pennsylvania 16802
We chose Pd-catalyzed allylic alkylation to test the
effectiveness of these new monophosphines as chiral
ligands. Although many palladium complexes of multi-
dentate phosphine and nitrogen ligands are excellent
catalysts for this reaction,13,14 palladium complexes of
simple chiral monophosphines are normally not effec-
tive.15 We were delighted to find that Pd-catalyzed allylic
Received December 16, 1996X
Design and synthesis of chiral phosphines have played
a significant role in the development of transition metal
catalyzed asymmetric reactions.1 Many excellent chiral
bidentate phosphines such as DIPAMP,2 DIOP,3 Chira-
phos,4 and BINAP5 have been developed for a variety of
catalytic reactions. Recent additions to this family of
ligands include the Duphos and BPE species of Burk and
co-workers.6 The most significant characteristics of
Duphos and BPE are (1) they contain several alkyl groups
attached to the phosphine and thus are more electron
rich than many related chiral arylphosphine ligands and
(2) the steric environment can be varied by changing the
substituents on the chiral carbon centers. Highly enan-
tioselective reactions have been reported with these
ligands.6 Despite these successes, one potential problem
is the conformational flexibility that exists in these
ligands which may limit their applicability to other types
of reactions. It is well-known that rapid interconversion
of the envelope and half-chair conformations can occur
in five-membered rings. We now report new chiral
phosphines with rigid fused bicyclic rings which do not
possess the conformational flexibility associated with the
five-membered rings present in the Duphos and BPE
ligands (Figure 1).
The rigid fused bicyclic [2.2.1] structure represents a
new motif in chiral ligand design. Analogous to Burk’s
systems, changes in the size of the R group on the ring
system can modulate the asymmetric induction and high
enantioselectivities can be achieved. Herein, we report
the syntheses of chiral monophosphines with this fused
bicyclic ring structure (Figure 2)7 and their application
in Pd-catalyzed asymmetric allylic alkylations.
The ligand synthesis depends on the availability of
enantiomerically pure cyclic 1,4-diols. Halterman8 and
Vollhardt9 have previously prepared chiral cyclopenta-
diene derivatives from the chiral diols.8,9 Halterman8 has
synthesized chiral diols 1 and 2 from the inexpensive
starting materials p-xylene and p-diisopropylbenzene,
(7) Upon completion of the work reported here, a related chiral fused
bicyclic phosphine, 2,6-dimethyl-9-phenyl-9-phosphabicyclo[3.3.1]-
nonane, was reported for Pd-catalyzed allylic alkylations (Hamada,
Y.; Seto, N. Ohmori, H.; Hatano, K. Tetrahedron Lett. 1996, 37, 7565).
Compared with this ligand, 7 and 8 are more rigid (one less methylene
group) and the chiral environment can be varied by changing the R
group. We expect that these attributes will be important for fine-tuning
activity and selectivity of phosphines like 7 and 8 in various asym-
metric catalytic reactions.
(8) (a) Chen, Z.; Halterman, R., L. Synlett 1990, 103. (b) Chen, Z.;
Eriks, K.; Halterman, R. L. Organometallics 1991, 10, 3449. (c) Chen,
Z.; Halterman, R., L. J . Am. Chem. Soc. 1992, 114, 2276. (d) Halter-
man, R. L.; Chen, Z.; Khan, M. Organometallics 1996, 15, 3957.
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Boese, R. J . Am. Chem. Soc. 1987, 109, 8105.
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47, 5074.
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96, 395. (b) Pfaltz, A. Acc. Chem. Res. 1993, 26, 339. (c) Hayashi.; T.
In Catalytic Asymmetric Synthesis; Ojima, I., Ed; VCH Publishers:
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Chem. Soc. 1996, 118, 235. (b) Pen˜a-Cabrera, E.; Norrby, P.-O.; Sjo¨gren,
M.; Vitagliono, A.; De Felice, V.; Oslob, J .; Ishii, S.; O’Neil D.;
A° kermark, B.; Helquest, P. J . Am. Chem. Soc. 1996, 118, 4299. (c)
Barbaro, P.; Curro, A.; Hermann, J .; Nesper, R.; Pregosin, P. S.;
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P.; Helmchen, G. J . Chem. Soc., Chem. Commun. 1995, 1845. (e) Rieck,
J .; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 2687. (f)
Andersson, P. G.; Harden, A.; Tanner, D.; Norrby, P. Chem. Eur. J .
1995, 1, 12. (g) von Matt, P.; Lloyd-J ones, G. L.; Minidis, A. B. E.;
Pfaltz, A.; Macko, L.; Neuburger, M.; Zehnder, M.; Ru¨egger, H.;
Pregosin, P. S. Helv. Chim. Acta 1995, 78, 265. (h) Trost, B. M.; Organ,
M. G.; O’Doherty, G. A. J . Am. Chem. Soc. 1995, 117, 9662. (i) Gamez,
P.; Dunjic, B.; Fache, F.; Lemaire, M. J . Chem. Soc., Chem. Commun.
1994, 1417. (j) Brown, J . M.; Hulmes, D. I.; Guiry, P. J . Tetrahedron
1994, 50, 4493. (k) Togni, A.; Breutel, C.; Schnyder, A.; Spindler, F.;
Landert, H.; Tijani, A. A. J . Am. Chem. Soc. 1994, 116, 4062. (l)
Hayashi, T.; Iwamura, H.; Naito, M.; Matsumoto, Y.; Uozumi, Y.; Miki,
M.; Yanagi, K. J . Am. Chem. Soc. 1994, 116, 775. (m) Trost, B. M.;
Bunt, R. C. J . Am. Chem. Soc. 1994, 116, 4089. (n) Pregosin, P. S.;
Ru¨egger, H.; Salzmann, R.; Albinati, A.; Lianza, F.; Kunz, R. W.
Organometallics 1994, 13, 83. (o) Sprinz, J .; Kiefer, Tetrahedron Lett.
1994, 35, 1523. (p) Allen, J . V.; Coote, S. J .; Dawson, G. J .; Frost, C.
G.; Martin, C. J .; Williams, J . M. J . J . Chem. Soc., Perkin Trans. 1
1993, 2065. (q) von Matt, P.; Pfaltz, A. Angew. Chem., Int. Ed. Engl.
1993, 32, 566. (r) Trost, B. M.; Van Vranken, D. L.; Bingel, C. J . Am.
Chem. Soc. 1992, 114, 9327. (s) Trost, B. M.; Van Vranken, D. L.
Angew. Chem., Int. Ed. Engl. 1992, 31, 328. (t) Trost, B. M.; Murphy,
D. J . Organometallics 1985, 4, 1143. (u) Auburn, P. R.; Mackenzie, P.
B., Bosnich, B. J . Am. Chem. Soc. 1985, 107, 2033. (v) Trost, B. M.;
Strege, P. E. J . Am. Chem. Soc. 1977, 99, 1649.
X Abstract published in Advance ACS Abstracts, J une 1, 1997.
(1) (a) Ojima, I., Ed. Catalytic Asymmetric Synthesis; VCH: New
York, 1993. (b) Sheldon, R. A. Chirotechnology; Marcel Dekker: New
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J ohn Wiley & Sons: New York, 1994.
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(5) (a) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345. (b)
Noyori, R. Science 1990, 248, 1194.
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1990, 9, 2653. (b) Burk, M. J . J . Am. Chem. Soc. 1991, 113, 8518. (c)
Burk, M. J .; Feaster, J . E.; Nugent, W. A.; Harlow, R. L. J . Am. Chem.
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S0022-3263(96)02354-7 CCC: $14.00 © 1997 American Chemical Society

4522 J . Org. Chem., Vol. 62, No. 13, 1997
Notes
F igu r e 1.
F igu r e 2.
Ta ble 1. P a lla d iu m -Ca ta lyzed Asym m etr ic Allylic Alk yla tion w ith Ch ir a l Mon op h osp h in esa
entry
L*
[Pd]
[Pd]:L*
Nu
additive
time (h)
yield (%)
% eeb
1
2
3
4
5
6
7
8
9
7
7
7
7
7
7
7
7
7
8
Pd2(dba)3
Pd(OAc)2
1:2.2
1:2.2
1:1.1
1:2.2
1:3.3
1:2.2
1:2.2
1:2.2
1:2.2
1:2.2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH(NHAc)(CO2Et)2
CH2(CO2Me)2
1.5
4.0
5.0
2.0
1.5
1.0
2.0
2.0
2.0
3.5
96
98
97
93
96
80
95
99
95
99
74 (R)
72 (R)
60 (R)
95 (R)
96 (R)
[Pd(C3H5)Cl]2
[Pd(C3H5)Cl]2
[Pd(C3H5)Cl]2
[Pd(C3H5)Cl]2
[Pd(C3H5)Cl]2
[Pd(C3H5)Cl]2
[Pd(C3H5)Cl]2
[Pd(C3H5)Cl]2
2.8% AgBF4
2.8% LiCl
97 (R)
96 (R)
>97c (R)
>99.5d (S)
78 (R)
10
a
The reaction was carried out under N2 using 1,3-diphenyl-2-propenyl acetate, Nu (nucleophile) (300 mol %), BSA (bis(trimethylsily-
b
l)acetamide) (300 mol %), KOAc (2 mol %), toluene, [Pd] 1.4 mol % and L*. % ee was measured by HPLC using a Chiralcel OD column,
and the absolute configuration was determined by comparing the optical rotation with literature values.14q c % ee was measured by
comparing the optical rotation with literature values.14q % ee was measured by HPLC using a Chiralcel OJ column.
d
alkylation with the new monophosphine 7 gave excellent
enantioselectivities and conversions (Table 1), compa-
rable to the best results (99% ee) reported to date.13
placed with pent-3-en-2-yl actetate (CH3CHdCHCH-
(OAc)CH3), only 34% ee was obtained in the product 2-(1-
methylbut-2-enyl)malonic acid dimethyl ester under the
same conditions as in entry 4. This is due to the low
steric effect of the methyl groups compared to that of the
phenyl groups. Finally, altering the size of the substit-
uents on the chiral monophosphine influences the enan-
tioselectivity. Compared to the result with dimethyl
ligand 7 (entry 4, 95% ee), lower enantioselectivity was
observed with the more sterically demanding diisopropyl
ligand 8 (entry 10, 78% ee).
In conclusion, we have developed a family of chiral
phosphines with a unique fused bicyclic [2.2.1] ring
structure. Pd-catalyzed allylic alkylations with mono-
phosphine 7 gives excellent enantioselectivities. Mecha-
nistic aspects of this reaction are being investigated as
are the use of structurally related compounds for other
asymmetric catalytic reactions.
In our initial experiments, 1,3-diphenyl-2-propenyl
acetate and dimethyl malonate were used as substrates.
The enantioselectivity of Pd-catalyzed allylic alkylation
strongly depends on the nature of the palladium precur-
sor (entries 1, 2, and 4). The ratio of phosphine to
palladium is also important. Our experimental results
(entries 3, 4, and 5) showed that 2 equiv of phosphine is
required for highly enantioselective allylic alkylation. We
have also removed chloride ion using AgBF4 (entry 6) and
have added excess chloride (entry 7), but the enantiose-
lectivity remains about the same under both of these sets
of conditions. These observations suggest that chloride
does not remain in the coordination sphere of the pal-
ladium, perhaps due to the strong coordinating ability
of the electron-donating monophosphine 7. In the cata-
lytic reaction, it is likely that two monophosphines
coordinate with one palladium. Changing the nucleo-
phile from dimethyl malonate to 2,4-pentanedione (entry
8) or diethyl acetamidomalonate (entry 9) has some effect
on the enantioselectivity. Greater than 99.5% ee was
achieved in the alkylation with diethyl acetamidoma-
lonate. When 1,3-diphenyl-2-propenyl acetate was re-
Exp er im en ta l Section
Gen er a l P r oced u r e. THF and toluene were distilled under
nitrogen from a sodium/benzophenone ketyl. Methylene chloride
was distilled under nitrogen from CaH2. Compounds 1-4 were
prepared according to the literature procedure.8
(1R ,2S ,4R ,5S )-(+)-2,5-Dim e t h yl-7-p h e n yl-7-p h osp h a -
bicyclo[2.2.1]h ep ta n e-Bor a n e (5). To phenylphosphine (3.0

Notes
J . Org. Chem., Vol. 62, No. 13, 1997 4523
mL, 27.3 mmol) in THF (200 mL) was added n-BuLi (34.5 mL
of a 1.6 M solution in hexane, 55 mmol) via syringe at -78 °C
over 20 min. Then the orange solution was warmed up to rt
and stirred for 1 h at rt. To the resulting orange-yellow
suspension was added a solution of (1S,2S,4S,5S)-2,5-dimeth-
ylcyclohexane-1,4-diol bis(methanesulfonate) (3, 8.25 g, 27.5
mmol) in THF (100 mL) over 15 min. After the mixture was
stirred overnight at rt, the pale-yellow suspension was hydro-
washed with brine, followed by water, and then dried over Na2-
SO4. Evaporation of the solvent gave a pure phosphine product,
which was confirmed by NMR. Yield: 0.9 g (96%). [R]25
)
D
+92.5° (c 2.3, toluene). 1H NMR (CDCl3, 360 MHz): δ 7.38-
7.34 (m, 2H), 7.26-7.21 (m, 2H), 7.19-7.16 (m, 1H), 2.60-2.54
(m, 2H), 1.89-1.62 (m, 5H), 1.44-1.42 (m, 1H), 1.16 (d, J ) 6.12
Hz, 3H), 0.55 (d, J ) 6.95 Hz, 3H). 13C NMR (CDCl3): δ 138.7
(d, J ) 29.3 Hz), 131.4 (d, J ) 13.0 Hz), 127.9 (d, J ) 2.35 Hz),
126.6 (s), 47.3 (d, J ) 13.5 Hz), 45.3 (d, J ) 10.2 Hz), 39.2 (d, J
) 6.7 Hz), 39.2 (d, J ) 5.3 Hz), 38.7 (d, J ) 6.7 Hz), 34.7 (d, J
) 17.2 Hz), 22.4 (d, J ) 7.8 Hz), 21.5 (s). 31P NMR(CDCl3): δ
-7.29.
lyzed with
a saturated NH4Cl solution. The mixture was
extracted with ether (2 × 50 mL), and the combined organic
solution was dried over anhydrous sodium sulfate. After filtra-
tion, the solvents were removed under reduced pressure. The
residue was dissolved in methylene chloride (100 mL) and
treated with BH3‚THF (40 mL of a 1.0 M solution in THF, 40
mmol), and the mixture was stirred overnight. It was then
poured into a saturated NH4Cl solution and extracted with CH2-
Cl2 (3 × 50 mL). The combined organic solution was dried over
anhydrous Na2SO4 and filtered, and the solvent was removed
under reduced pressure. The residue was subjected to chroma-
tography on a silica gel column and eluted with hexanes/CH2-
Cl2 (4:1), affording the product as a white solid. Yield: 1.95 g
(31%). [R]25D ) + 59.5° (c 1.07, CHCl3).1H NMR (CDCl3)SPCLN
δ 7.60-7.30 (m, 5 H), 2.60-2.40 (m, 2 H,), 2.15-2.05 (m, 1 H),
2.04-1.80 (m, 4 H), 1.65-1.50 (m, 1 H), 1.32 (d, 3J (HH) ) 6.5
(1R ,2R ,4R ,5R )-(+)-2,5-Diiso p r o p y l-7-p h e n yl-7-p h os-
p h a bicyclo[2.2.1]h ep ta n e (8). The same procedure as in the
preparation of 7 was used. Yield: 1.0 g (95.5%). [R]25D ) +43.9°
(c 1.2, toluene). 1H NMR (CDCl3, 360 MHz): δ 7.35-7.30 (m,
2H), 7.24-7.14 (m, 3H), 2.94-2.85 (m, 2H), 1.76-1.53 (m, 5H),
1.25-1.14 (m, 2H), 1.06 (d, J ) 7.77 Hz, 3H), 0.95-08.0 (m, 1H),
0.87 (dd, J ) 3.77 Hz, 7.89 Hz, 6 H), 0.49 (d, J ) 9.30 Hz, 3H).
13C NMR (CDCl3): δ 138.8 (d, J ) 30.49 Hz), 130.7 (d, J ) 12.2
Hz), 127.7 (d, J ) 2.87 Hz), 126.5 (s), 53.4 (d, J ) 6.34 Hz), 48.6
(d, J ) 17.06 Hz), 42.0 (d, J ) 13.43 Hz), 40.5 (d, J ) 9.96 Hz),
37.6 (d, J ) 11.09 Hz), 37.4 (d, J ) 9.74 Hz), 33.0 (d, J ) 6.11
Hz), 31.9 (s), 21.9 (s), 21.8 (s), 21.2 (s), 20.4 (s). 31P NMR
(CDCl3): δ -7.49.
En a n tioselective Allylic Alk yla tion . The procedures are
exemplified by the experiments carried out with ligand 7 in
toluene. To a stirring solution of [Pd2(η3-C3H5)2Cl2] (3.0 mg,
0.008 mmol) in toluene (1.5 mL) was added ligand 7 (0.36 mL
of 0.1 M solution in toluene, 0.036 mmol) under a nitrogen
atmosphere. After 30 min, racemic 1,3-diphenyl-1-acetoxypro-
pene (150 mg, 0.60 mmol) was added. Then the solution was
allowed to be stirred for 30 min. N,O-Bis(trimethylsiyl)acet-
amide (0.44 mL, 1.8 mmol), dimethyl malonate (0.21 mL, 1.8
mmol), and potassium acetate (3 mg, 0.03 mmol) were added in
this order. The reaction was monitored by TLC (eluent: hexane/
ethyl acetate ) 10/1). After 1.5 h, TLC showed the reaction was
over. After the solvent was evaporated in vacuo, column
chromatography on silica gel (eluent: hexane/ethyl acetate )
10/1) of the residue yielded the pure product: yield 190 mg,
97.7% . The optical purity was determined to be 95.5% ee by
HPLC (Daicel Chiralcel OD column, 1 mL/min, hexane/2-
propanol ) 99/1).
3
Hz, 3 H), 0.59 (d, J (HH) ) 6.7 Hz, 3 H), 1.6-0.2 (br); 13C NMR
(CDCl3) δ 131.7 (d, 2J (PC) ) 7.3 Hz), 130.6 (d, 1J (PC) ) 43.9
Hz), 129.9 (d, 4J (PC) ) 2.0 Hz), 128.4 (d, 3J (PC) ) 8.6 Hz), 43.07
1
1
(d, J (PC) ) 30.5 Hz), 40.9 (d, J (PC) ) 31.6 Hz), 36.3, 36.7 (d,
3J (PC) ) 13.5 Hz), 35.9 (d, 2J (PC) ) 3.5 Hz), 34.7 (d, 2J (PC) )
9.8 Hz), 20.8, 20.5; 31P NMR (CDCl3): δ 36.3 (d, broad, 1J (PB)
) 58.8 Hz); MS: m/z 232 (M+, 0.42), 218 (M+ - BH3, 100), 203
(7.41), 176 (14.60), 136 (9.81), 109 (16.67), 91 (6.59), 77 (5.51),
65 (3.71); HRMS: calcd for C14H22BP 232.1552 (M+), found
232.1578; calcd for
218.1233.
C14H19P
218.1224 (M+ - BH3), found
(1R ,2R ,4R ,5R )-(+)-2,5-Diiso p r o p y l-7-p h e n y l-7-p h os-
p h a bicyclo[2.2.1]h ep ta n e-bor a n e (6). The same procedure
as in the preparation of 5 was used. Yield: 0.33 g (50%). [R]25
D
) +25.5° (c 1.02, CHCl3).1H NMR (CDCl3): δ 7.55-7.30 (m, 5
H), 2.85-2.70 (m, 2 H), 2.30-2.20 (m, 1 H), 2.18-2.00 (m, 1 H),
1.95-1.65 (m, 4 H), 1.40-1.20 (m, 2 H), 1.03 (d, 3J (PH) ) 6.5,
3
3
3H), 0.87 (d, J (PH) ) 6.7, 3H), 0.85 (d, J (PH) ) 7.4, 3H), 0.53
(s, broad, 3 H), 1.5-0.2 (br). 13C NMR (CDCl3): δ 131.2 (d,
2J (PC) ) 8.3 Hz), 130.7 (d, 1J (PC) ) 45.2 Hz), 130.0 (d, 4J (PC)
3
2
) 2.5 Hz), 128.5 (d, J (PC) ) 9.5 Hz), 50.3 (d, J (PC) ) 2.1 Hz),
48.8 (d, 2J (PC) ) 9.7 Hz), 38.3 (d, 1J (PC) ) 30.5 Hz), 36.8 (CH2),
36.7 (d, 1J (PC) ) 31.5 Hz), 34.7 (d, 3J (PC) ) 13.7 Hz), 31.9, 31.1,
22.4, 21.6, 20.7, 20.1. 31P NMR (CDCl3): δ 36.8 (d, broad, 1J (PB)
) 51.4 Hz). MS: m/z 288 (M+, 0.49), 274 (M+ - BH3, 100), 259
(9.96), 231 (34.54), 190 (8.15), 163 (9.80), 136 (12.91), 109 (16.19),
Ack n ow led gm en t. This work was supported by a
Camille and Henry Dreyfus New Faculty Award, an
ONR young investigator award, a DuPont Young Fac-
ulty Award, and Hoechst Celanese Corporation. We
acknowledge a generous loan of precious metals from
J ohnson Matthey Inc. We thank Supelco for the gift of
a â-DEX GC column.
77 (6.33), 65 (3.89), 43 (22.82). HRMS: calcd for C18H27
P
274.1850, (M+ - BH3), found 274.1855.
(1R ,2S ,4R ,5S )-(+)-2,5-Dim e t h yl-7-p h e n yl-7-p h osp h a -
bicyclo[2.2.1]h ep ta n e (7). To a solution of the corresponding
borane complex of the phosphine (5, 1.0 g, 4.31 mmol) in CH2-
Cl2 (22 mL) was added tetrafluoroboric acid-dimethyl ether
complex (2.63 mL, 21.6 mmol) dropwise via a syringe at -5 °C.
After the addition, the reaction mixture was allowed to warm
up slowly and stirred at rt. After 20 h, 31P NMR showed the
reaction was over, and it was diluted by CH2Cl2 and neutralized
by a saturated NaHCO3 aqueous solution. The aqueous layer
was extracted with CH2Cl2. The combined organic solution was
Su p p or tin g In for m a tion Ava ila ble: 1H NMR, 13C NMR,
and 31P NMR spectra for compounds 5-8 (14 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, can
be ordered from the ACS; see any current masthead page for
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J O9623548