Methylene-Bridged P-Chiral Diphosphines in Highly Enantioselective Reactions

Full text of DOI:10.1021/jo990131m

2
988
J . Org. Chem. 1999, 64, 2988-2989
Meth ylen e-Br id ged P -Ch ir a l Dip h osp h in es in
High ly En a n tioselective Rea ction s
Yoshinori Yamanoi and Tsuneo Imamoto*
F igu r e 1.
Department of Chemistry, Faculty of Science, Chiba University,
Yayoi-cho, Inage-ku, Chiba 263-8522, J apan
Received J anuary 26, 1999
Optically active diphosphines play a most important role
as the chiral bidentate ligands in transition metal-catalyzed
1
reactions. Although numerous chiral diphosphines have
been reported so far,1 the design and synthesis of new chiral
phosphine ligands are still a significant research subject in
the field of asymmetric catalysis. Described here is the
development of novel chiral diphosphines that are extremely
simple and small but exhibit excellent enantioselectivity in
representative catalytic asymmetric reactions.
,2
The newly designed chiral diphosphine ligands 1a -1d
3
(
abbreviated as MiniPHOS ) are shown in Figure 1. An
F igu r e 2. Top (A) and side (B) ORTEP drawings of [Rh((R,R)-t-
+
-
-
important feature of these ligands is that they are methyl-
ene-bridged P-chiral diphosphines possessing the smallest
Bu-MiniPHOS)2] PF6 . The PF6 anion and hydrogen atoms are
omitted for clarity. In the side view, one t-Bu-MiniPHOS is omitted
for clarity.
4
alkyl group (methyl group) and a bulky alkyl group at each
phosphorus atom. These ligands would form highly strained
four-membered C2-symmetric chelates with metal, and this
conformational rigidity together with the ideal asymmetric
environment might lead to high enantioselectivity.
Sch em e 1
These ligands were synthesized in three steps from
trichlorophosphine using phosphine-boranes as the inter-
5
mediates (Scheme 1). Thus, alkyldimethylphosphine-bo-
ranes 2a -2d were obtained from trichlorophosphine in good
to high yield. These compounds were reacted successively
with s-BuLi/(-)-sparteine, alkyldichlorophosphines, meth-
ylmagnesium bromide, and BH3-THF to afford optically
active phosphine-boranes 3a -3d and meso phosphine-
boranes in a ratio of ca. 1:1.6 The desired compounds
3
a -3d were easily obtained by recrystallization from metha-
nol or ethanol (13-28%). The boranato groups were removed
by the reaction with trifluoromethanesulfonic acid in tolu-
ene, followed by treatment with aqueous KOH, to provide
7
the MiniPHOS 1a -1d in almost quantitative yield.
(
1) For representative reviews, see the following: (a) Noyori, R. Asym-
metric Catalysis in Organic Synthesis; J ohn Wiley & Sons: New York, 1994.
b) Ojima, I., Ed. Catalytic Asymmetric Synthesis; VCH Publishers: Wein-
heim, 1993.
+
-
These ligands were allowed to react with [Rh(nbd)2] X
X ) BF4 or PF6) to afford the bischelate complexes
(
(
+
-
+
-
(2) For recently reported representative chiral diphosphines, see: (a) Pye,
[Rh(MiniPHOS)2] X , even with the use of [Rh(nbd)2] X
P. J .; Rossen, K.; Reamer, R. A.; Tsou, N. N.; Volante, R. P.; Reider, P. J . J .
Am. Chem. Soc. 1997, 119, 6207. (b) RajanBabu, T. V.; Ayers, T. A.; Halliday,
G. A.; You, K. K.; Calabrese, J . C. J . Org. Chem. 1997, 62, 6012. (c) Zhang,
F. Y.; Pai, C. C.; Chan, A. S. C. J . Am. Chem. Soc. 1998, 120, 5808. (d)
Qiao, S.; Fu, G. C. J . Org. Chem. 1998, 63, 4168. (e) J iang, Q.; J iang, Y.;
Xiao, D.; Cao, P.; Zhang, X. Angew. Chem., Int. Ed. Engl. 1998, 37, 1100.
and diphosphines in a 1:1 molar ratio. The molecular
structure of a rhodium complex [Rh((R,R)-t-Bu-MiniPHOS)2] -
+
-
8
PF6 was determined by single-crystal X-ray analysis. The
ORTEP drawing shown in Figure 2 clearly indicates the
expected C2-symmetric environment, where the bulky tert-
butyl groups effectively shield two diagonal quadrants and
(3) We abbreviate these chiral ligands as MiniPHOS, because they are
quite small when compared with all the chiral diphosphines reported so
far.
9
the methyl groups are placed at the other quadrants. This
(4) A few chiral 1,1-diphosphines have been described in the literature,
imposed asymmetric environment is expected to lead to high
enantioselectivity in asymmetric catalysis.
These rhodium complexes were used as catalyst precur-
and there has been only one report dealing with Rh-catalyzed asymmetric
hydrogenations with very low enantioselectivity. (a) Marinetti, A.; Menn,
C. L.; Ricard, L. Organometallics 1995, 14, 4983. (b) Babu, R. P. K.;
Krishnamurthy, S. S.; Nethaji, M. Tetrahedron: Asymmetry 1995, 6, 427.
(
c) Brunner, H.; Furst, J . Tetrahedron 1994, 50, 4303.
sors in asymmetric hydrogenation of various dehydroamino
acids and their methyl esters.1
,2,10
The results are sum-
(5) (a) Imamoto, T.; Oshiki, T.; Onozawa, T.; Kusumoto, T.; Sato, K. J .
Am. Chem. Soc. 1990, 112, 5244. (b) J ug e´ , S.; Stephane, M.; Laffitte, J . A.;
Gen eˆ t, J . P. Tetrahedron Lett. 1990, 31, 6357. (c) Ohff, M.; Holz, J .;
Quirmbach, M.; B o¨ rner, A. Synthesis 1998, 1391. (d) Carboni, B.; Monnier,
L. Tetrahedron 1999, 55, 1197 and references cited therein.
marized in Table 1. Almost complete enantioselectivity was
achieved for the hydrogenation of 2-acetamidoacrylic acid
(
6) Enantioselective deprotonation of aryl- or alkyldimethylphosphine-
boranes with (-)-sparteine/s-BuLi complex was reported. (a) Muci, A. R.;
Campos, K. R.; Evans, D. A. J . Am. Chem. Soc. 1995, 117, 9075. (b) Imamoto,
T.; Watanabe, J .; Wada, Y.; Masuda, H.; Yamada, H.; Tsuruta, H.;
Matsukawa, S.; Yamaguchi, K. J . Am. Chem. Soc. 1998, 120, 1635.
(8) Crystallographic Data for [Rh((R,R)-t-Bu-MiniPHOS)
2
]PF
6 52 6 5
: C22H F P -
-3
Rh; space group P4
3
; Z ) 4; D ) 1.356 g/cm ; cell constants a ) 11.113(4)
3
Å, c ) 27.301(5) Å, V ) 3371(1) Å ; temperature of data collection 293 K;
1619 unique reflections (I > 2.0σ(I)); R ) 0.067; Rw ) 0.089; GOF ) 1.41.
(9) Knowles, W. S. Acc. Chem. Res. 1983, 16, 106.
(
7) McKinstry, L.; Livinghouse, T. Tetrahedron Lett. 1994, 35, 9319.
1
0.1021/jo990131m CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/10/1999

Communications
J . Org. Chem., Vol. 64, No. 9, 1999 2989
Ta ble 1. Rh -Ca ta lyzed En a n tioselective Hyd r ogen a tion
of Deh yd r oa m in o Acid s a n d Th eir Meth yl Ester sa
Ta ble 3. Cu -Ca ta lyzed Asym m etr ic Mich a el Rea ction of
a
Dieth ylzin c to r,â-Un sa tu r a ted Keton es
entry
R1
H
R2
H
R3
H
MiniPHOS
H2 (atm)
ee (%)b
1
2
3
4
5
6
7
8
9
t-Bu
c-C6H11
i-Pr
Ph
t-Bu
c-C6H11
i-Pr
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
c-C6H11
i-Pr
t-Bu
c-C6H11
i-Pr
t-Bu
1
1
1
1
1
1
1
1
2
1
2
6
6
6
6
6
6
6
6
>99.9
99.1
98
26
H
H
Me
>99.9
98.9
98
97
95
98
95
97
94
83
94
90
88
87
85
a
All reactions were carried out in toluene at -80 °C with a
molar ratio Cu(OTf)2/MiniPHOS/R,â-unsaturated ketone/diethylz-
inc 1/1/100/110. These reactions were complete within 24 h.
H
H
H
H
Ph
Ar
Ph
Ar
H
H
Me
Me
Me
b
Isolated yield. c Determined by GC or HPLC using chiral col-
c
umns. d The absolute configurations were determined by compar-
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
e
c
ing the observed optical rotations with the literature values. The
absolute configurations were not determined.
-(CH2)5-
most of the chiral diphosphine ligands afforded only low to
moderate enantioselectivity in rhodium(I)-catalyzed hydrosi-
lylation of ketones. The use of the t-Bu-MiniPHOS-Rh
complex as a catalyst precursor in asymmetric hydrosilyla-
tion of simple ketones afforded very high enantioselectivities
-(CH2)4-
Me
Me
Me
Me
c-C6H11
(
Table 2). These results are comparable to the enantiose-
a
All reactions were carried out with a molar ratio of Rh-
lectivity obtained previously by use of the most effective
ligands.
MiniPHOS/dehydroamino acid derivatives 1/500. The reactions
were complete within 24-48 h. b Enantiomeric excesses were
determined by GC or HPLC using chiral columns, as described in
16
It was also found that these chiral diphosphines were
successfully used for catalytic asymmetric carbon-carbon
bond-forming reactions. Namely, the catalytic asymmetric
Michael reaction of diethylzinc to R,â-unsaturated ketones
in the presence of MiniPHOS-coordinated copper(II) triflate
afforded the corresponding addition products with high
enantiomeric excesses (Table 3).1
c
the Supporting Information. Ar ) 3-MeO-4-AcOC6H3.
Ta ble 2. Rh -Ca ta lyzed Asym m etr ic Hyd r osilyla tion of
a
Keton es
7,18
In conclusion, we have explored novel methylene-bridged
P-chiral diphosphines. These ligands exhibit excellent to
almost perfect levels of enantioselectivity in representative
catalytic asymmetric reactions, even though they are quite
simple and small in comparison with the previously reported
chiral diphosphines.
entry
R1
R2
temp (°C)
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
Ph
Me
Me
Me
Me
Me
Me
Et
-40
-40
-40
-40
0
-15
-20
-20
86
90
99
96
83
88
81
93
91
97
94
95
89
90
83
80
1-naphthyl
2-naphthyl
o-CH3C6H4
m-CH3C6H4
o-MeOC6H4
Ph
Ack n ow led gm en t . This work was supported by the
Research for the Future program of the J apan Society for
the Promotion of Science.
PhCH2CH2
Me
a
Su p p or tin g In for m a tion Ava ila ble: Synthetic procedures,
characterization data for new compounds, and enantiomeric excess
determinations. An X-ray crystallographic file, in CIF format, is
available through the Web only. This material is available free of
charge via the Internet at http://pubs.acs.org.
All reactions were carried out in THF with a molar ratio of
Rh-(R,R)-t-Bu-MiniPHOS/ketone/1-naphthylphenylsilane 1/100/
50. The reactions were complete with 3-4 days. Isolated yield.
Determined by GC or HPLC using chiral columns.
b
1
c
and its methyl ester (entries 1-3 and 5-7). It is noted that
the ligands having tert-butyl, cyclohexyl, and isopropyl
groups exhibited very high enantioselectivities, while a
ligand possessing phenyl groups leads to low selectivity
J O990131M
(11) Hoerrner et al. reported asymmetric hydrogenation of â,â-disubsti-
tuted dehydroamino acid derivatives to prepare â-methyltryptophan with
excellent enantiomeric excess. Hoerrner, R. S.; Askin, D.; Volante, R. P.;
Reider, P. J . Tetrahedron Lett. 1998, 39, 3455.
(entry 4). Excellent enantioselectivity was obtained for
several R-acetamidocinnamic acid derivatives (entries 8-11).
Moreover, these catalysts were also found to be effective for
the â,â-disubstituted enamides in the reduction of which it
(12) The following are the highest ee values for the enantioselective
10b
hydrogenations of â,â-disubstituted derivatives. Me-DuPHOS (99.4% ee);
10b
6b
13
Me-BPE (98.6% ee); BisP* (93.0% ee); BuTRAP (88% ee).
(13) Sawamura, M.; Kuwano, R.; Ito, Y. J . Am. Chem. Soc. 1995, 117,
9602.
14) The same rhodium catalysts were employed also for the asymmetric
has been notoriously difficult to achieve high enantioselec-
tivity (entries 12-19).11-15
(
To extend the utilization of the present catalyst precur-
sors, asymmetric hydrosilylation of simple ketones was
examined. Despite extensive experimentation in this area,
hydrogenation of itaconic acid. Almost perfect enantioselectivities (t-Bu-
MiniPHOS >99.9%; c-C H11-MiniPHOS >99.9%; i-Pr-MiniPHOS 98%) were
observed in this reaction.
6
(15) It is reasonable to consider that under the reaction conditions the
bischelate rhodium complexes dissociate to the free diphosphines and the
monochelate complexes which act as the actual catalyst species.
(16) Kuwano, R.; Uemura, T.; Saitoh, M.; Ito, Y. Tetrahedron Lett. 1999,
40, 1327 and references cited therein.
(17) (a) Rossiter, B. E.; Swingle, N. M. Chem. Rev. 1992, 92, 771. (b)
Krause, N. Angew. Chem., Int. Ed. Engl. 1998, 37, 283.
(18) (a) Feringa, B. L.; Pineschi, M.; Arnold, L. A.; Imbos, R.; de Vries,
A. H. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2620. (b) Kn o¨ bel, A. K. H.;
Escher, I. H.; Pfaltz, A. Synlett 1997, 1429.
(
10) Burk et al. have demonstrated that Rh and Ru catalysts bearing
DuPHOS or BPE exhibit very high enantioselectivities in various asym-
metric hydrogenations. (a) Burk, M. J .; Gross, M. F.; Harper, T. G. P.;
Kalberg, C. S.; Lee, J . R.; Martinez, J . P. Pure Appl. Chem. 1996, 68, 37.
b) Burk, M. J .; Wang, Y. M.; Lee, J . R. J . Am. Chem. Soc. 1996, 118, 5142.
c) Burk, M. J .; Kalberg, C. S.; Pizzano, A. J . Am. Chem. Soc. 1998, 120,
345. (d) Burk, M. J .; Bienewald, F.; Harris, M.; Zanotti-Gerosa, A. Angew.
Chem., Int. Ed. Engl. 1998, 37, 1931.
(
(
4