Synthesis and enantioselectivity of P-chiral phosphine ligands with alkynyl groups

Article abstract of DOI:10.1002/anie.200702513

(Formula Presented) A range of asymmetric transformations are catalyzed by transition-metal complexes of the title ligands with excellent enantioselectivities. The ligands, which were applied successively in Rh-catalyzed hydrogenation and Rh- or Pd-cataly

Full text of DOI:10.1002/anie.200702513

Communications
DOI: 10.1002/anie.200702513
Asymmetric Catalysis
Synthesis and Enantioselectivity of P-Chiral Phosphine Ligands with
Alkynyl Groups**
Tsuneo Imamoto,* Youichi Saitoh, Aya Koide, Tomokazu Ogura, and Kazuhiro Yoshida
In memory of Yoshihiko Ito
Stereospecific substitution reactions at stereogenic phospho-
rus atoms provide a variety of optically active organophos-
phorus compounds.[1] Such reactions are particularly useful
for the preparation of key intermediates for the synthesis of
P-chiral phosphine ligands.[2] We previously studied the
stereochemistry of electrophilic substitution reactions of
optically active secondary phosphinoboranes to demonstrate
the utility of such transformations for the synthesis of P-chiral
phosphine ligands.[3] Optically active phosphinoboranes with
a halogen substituent on the phosphorus atom are also
potentially useful as precursors to nonracemic organophos-
phorus compounds because they can undergo stereospecific
nucleophilic substitution reactions. JugØ and co-workers have
studied extensively the synthesis and reactivity of chloro-
phosphanylboranes for the preparation of various P-chiral
phosphine ligands,[4] whereas Stankevic and Pietrusiewicz
Scheme 1. Generation and substitution reactions of the optically active
halophosphanylboranes 2a and 2b.
have described the reactivity of optically active phosphinous
acid–boranes and related substrates.[5] However, these studies
were limited to substrates with at least one aryl group on the
phosphorus atom, and the stereospecificity of the substitution
reactions was not always high. We envisaged that halophos-
phanylboranes with two different alkyl groups would react
with various nucleophiles to give substitution products that
are difficult to synthesize by existing methods.
All alkynyl lithium reagents tested reacted with 2a to give
the expected substitution product 3 in high yield and almost
exclusively with inversion of configuration (Table 1, entries 1
and 3–5).[7] Although there have been many reports on
substitution reactions at chiral phosphorus atoms,[1,2,4,5,8] to
our knowledge, there has been no previous example of
To investigate the validity of this approach, we subjected
enantiomerically pure (S)-(tert-butylmethylphosphanyl)bor-
ane (1)[3f,6] to deprotonation with nBuLi at À788C followed
by halogenation with 1,2-dibromoethane or 1,2-diiodoethane
to give the halophosphanylboranes 2a,b as crystalline solids in
high yields. These compounds, however, were configuration-
ally unstable and racemized gradually at room temperature;
hence, they were not isolated but treated directly with
organometallic reagents to give a range of substitution
products (Scheme 1).
Table 1: Substitution reactions of the halophosphanylboranes 2a,b.[a]
Entry
2
Reagent
Product Yield [%][b] ee [%][c]
1
2
3
4
5
2a PhCCLi
2b PhCCLi
2a tBuCCLi
2a iPr3SiCCLi
2a Me3SiCCLi
2a PhMgBr
2b PhMgBr
3a
3a
3b
3c
3d
3e
3e
83
12
82
86
81
28
97 (S) inversion
32 (S) inversion
98 (S) inversion
98 (S) inversion
99 (S) inversion
83 (S) inversion
–
6
7
0[d]
8
9
10
11
12
13
2a o-MeOC6H4MgBr 3 f
68
63
79
94
92
80
93 (S) inversion
91 (S) retention
89 (S) retention
96 (S) retention
93 (S) retention
99 (S) retention
[*] Prof. Dr. T. Imamoto, Y. Saitoh, A. Koide, T. Ogura,
Prof. Dr. K. Yoshida
2a nBuLi
4a
4a
4b
4b
4b
2b nBuLi
Department of Chemistry
2a PhCH2MgCl
2b PhCH2MgCl
2a 1. 2tBuLi,
2. PhCH2Br
Graduate School of Science, Chiba University
Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
Fax: (+81)43-290-2791
E-mail: imamoto@faculty.chiba-u.jp
[**] This research was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (Advanced Molecular Transformations of
Carbon Resources) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
[a] All reactions were carried out in diethyl ether. The reagents were
combined at À788C, and the reaction mixture was allowed to warm to
room temperature (see details in the Supporting Information). [b] Yield
of the isolated product. [c] The configuration of the major enantiomer is
given in parentheses. [d] (S)-tert-Butylmethylphosphanylborane was
isolated in high yield.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
8636
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Angewandte
Chemie
stereospecific substitution by alkynyl nucleophiles. The treat-
ment of 2a with phenyl and o-methoxyphenyl Grignard
reagents under similar conditions also provided inversion
products (Table 1, entries 6 and 8). In sharp contrast, the
treatment of 2a,b with n-butyllithium and benzylmagnesium
chloride gave substitution products with retention of config-
uration (Table 1, entries 9–12).[9] This stereochemical out-
come may result from metal–halogen exchange followed by
the reaction of the phosphoranyl anion and halobutane or
benzyl halide thus generated to give 4a or 4b. In fact, the
successive treatment of 2a with tert-butyllithium (2 equiv)
and benzyl bromide resulted in the formation of (S)-4b with
99% ee (Table 1, entry 13).
Scheme 2. Synthesis of the alkynyl phosphine ligands 5a–e.
DABCO=1,4-diazabicyclo[2.2.2]octane.
Among the substitution products, we were especially
interested in alkynylated phosphinoboranes. The alkynyl
groups considered in their entirety are much larger than a
methyl group; however, the carbon–carbon triple bond
moiety is slim owing to the linearity of the arrangement of
atoms around sp-hybridized carbon atoms,[10] and these
groups seem to behave like small alkyl groups, such as the
methyl group. This idea led us to synthesize new ligands in the
form of the (S,S)-1,2-bis(alkynyl(tert-butyl)phosphanyl)-
ethanes 5a–e. Although these ligands are structurally similar
Scheme 3. Rhodium-catalyzed hydrogenation of methyl (Z)-a-acetami-
docinnamate in the presence of ligands 5a–e. nbd=norbornadiene.
the alkynyl groups (ethynyl and 1-propynyl) appear to behave
as very small substituents.
The new ligands were also tested in the Rh-catalyzed
asymmetric 1,4-addition of aryl boronic acids to a,b-unsatu-
rated carbonyl compounds (Table 2). The use of 5a and 5e in
to 1,2-bis(tert-butylmethylphosphanyl)ethane (tBu-BisP*),[11]
we believed that the large difference in the steric bulk of the
tert-butyl and alkynyl groups on the stereogenic phosphorus
atom might induce outstanding enantioselectivities in tran-
sition-metal-catalyzed asymmetric reactions. Furthermore,
the electron-withdrawing nature of the alkynyl group
decreases the electron density on the phosphorus atom and
should lead to a difference in the catalytic activities of ligands
5 with respect to those observed for BisP*.
Table 2: Asymmetric 1,4-addition of aryl boronic acids to a,b-unsatu-
rated carbonyl compounds under the catalysis of RhI/5.
The alkynylated phosphinoboranes in hand were con-
verted into the target ligands. The deprotonation of com-
pounds 3a–c with sBuLi or tBuLi and subsequent oxidative
coupling produced compounds 6a–c, which were deprotected
to furnish the desired diphosphine ligands 5a–c. Compound
6d was formed from 6c by desilylation, and subsequent
methylation gave 6e. Finally, the deprotection of 6d and 6e
led to 5d and 5e, respectively (Scheme 2).
The enantioinduction ability of ligands 5a–e was tested in
a typical Rh-catalyzed hydrogenation reaction with methyl
(Z)-a-acetamidocinnamate (Scheme 3).[12] Ligands 5a–c with
phenyl, tert-butyl, or triisopropylsilyl groups at the alkyne
termini were found to induce relatively high enantioselectiv-
ities (88–92% ee) in the range expected for this general type
of ligand: Apparently the asymmetric reaction center is not
affected significantly by the spatial effects of the alkyne
substituents. However, almost perfect enantioselectivity was
observed when 5d and 5e were used. In the case of 5d and 5e,
Entry
5
Enone
Ar
t [h]
Yield [%][a]
ee [%][b]
1[c]
2[c,d]
3[c,e]
4[c,g]
5
5a
5b
5c
5d
5e
5a
5a
5a
5a
5e
5e
5e
5e
7
7
7
7
7
8
9
7
7
8
9
7
7
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2
2
12
12
3
2
2
2
2
3
3
3
3
93
94
99.4 (R)
46 (R)
68 (R)
64[f]
trace
91
–
99.1 (R)
91 (R)
97.2 (R)
96.0 (R)
98.4 (R)
93 (R)
98.7 (R)
97.6 (R)
99.5 (R)
6[c]
7[c]
8[c]
9[c]
10
11
12
13
90
91
96
99
92
88
97
97
4-CF3C6H4
4-MeOC6H4
Ph
Ph
4-CF3C6H4
4-MeOC6H4
[a] Yield of the isolated product. [b] The configuration of the major
enantiomer is given in parentheses. [c] A quantity of 1 mol% of the
catalyst was used. [d] The reaction was carried out at 508C. [e] The
reaction was carried out at 708C. [f] 2-Cyclohexen-1-one was recovered in
31% yield. [g] The reaction was carried out at 908C.
Angew. Chem. Int. Ed. 2007, 46, 8636 –8639ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
a variety of substrate–reagent combinations revealed their
Keywords: asymmetric catalysis · boranes · hydrogenation ·
nucleophilic substitution · phosphane ligands
.
exceedingly high enantioinduction ability. The addition
products were formed with 99.1% ee or higher in the best
cases (Table 2, entries 1, 5, and 13). The use of ligands 5b and
5c resulted in significantly lower selectivities (46 and 68% ee,
respectively; Table 2, entries 2 and 3), and the addition
reaction proceeded sluggishly when 5d was used (Table 2,
entry 4), probably because the terminal alkyne inhibits the
Rh-catalyzed reaction under these basic conditions. The
results obtained with 5e are comparable or superior to
those obtained with binap,[13] QuinoxP*,[3f] or tBu-BisP*.[14]
The high utility of ligands 5d and 5e was also demon-
strated in a Pd-catalyzed asymmetric ring-opening reaction,
[1] For representative reviews, see: a) A. Grabulosa, J. Granell, G.
Muller, Coord. Chem. Rev. 2007, 251, 25 – 90; b) B. Carboni, L.
Monnier, Tetrahedron 1999, 55, 1197 – 1248; c) M. Ohff, J. Holz,
M. Quirmbach, A. Börner, Synthesis 1998, 1391 – 1415; d) J. M.
Brunel, B. Faure, M. Maffei, Coord. Chem. Rev. 1998, 178–180,
665 – 698; e) O. I. Kolodiazhnyi, Tetrahedron: Asymmetry 1998,
9, 1279 – 1332; f) H. B. Kagan, M. Sasaki in The Chemistry of
Organophosphorus Compounds, Vol. 1 (Ed.: F. R. Hartley),
Wiley, New York, 1990, chap. 3; g) D. Valentine, Jr. in Asym-
metric Synthesis, Vol. 4 (Eds.: J. D. Morrison, J. W. Scott),
Academic Press, New York, 1984, chap. 3, p. 263; h) T. Imamoto
in Handbook of Organophosphorus Chemistry (Ed.: R. Engel),
Marcel Dekker, New York, 1992, chap. 1, pp. 1 – 52.
[2] For representative reviews, see: a) P.-H. Leung, Acc. Chem. Res.
2004, 37, 169 – 177; b) W. Tang, X. Zhang, Chem. Rev. 2003, 103,
3029 – 3069; c) K. V. L. CrØpy, T. Imamoto, Adv. Synth. Catal.
2003, 345, 79 – 101; d) K. V. L. CrØpy, T. Imamoto, Top. Curr.
Chem. 2003, 229, 1 – 40; e) K. M. Pietrusiewicz, M. Zablocka,
Chem. Rev. 1994, 94, 1375 – 1411.
[3] a) T. Imamoto, T. Oshiki, T. Onozawa, T. Kusumoto, K. Sato, J.
Am. Chem. Soc. 1990, 112, 5244 – 5252; b) T. Oshiki, T. Imamoto,
J. Am. Chem. Soc. 1992, 114, 3975 – 3977; c) T. Imamoto, T.
Oshiki, T. Onozawa, M. Matsuo, T. Hikosaka, M. Yanagawa,
Heteroat. Chem. 1992, 3, 563 – 575; d) K. V. L. CrØpy, T. Ima-
moto, Tetrahedron Lett. 2002, 43, 7735 – 7737; e) Y. Wada, T.
Imamoto, H. Tsuruta, K. Yamaguchi, I. D. Gridnev, Adv. Synth.
Catal. 2004, 346, 777 – 788; f) T. Imamoto, K. Sugita, K. Yoshida,
J. Am. Chem. Soc. 2005, 127, 11934 – 11935 (tBu-QuinoxP*= 2,3-
bis(tert-butylmethylphosphanyl)quinoxaline).
[4] a) S. JugØ, M. Stephan, J. A. Laffitte, J.-P. GenÞt, Tetrahedron
Lett. 1990, 31, 6357 – 6360; b) E. B. Kaloun, R. Merd›s, G.-P.
GenÞt, J. Uziel, S. JugØ, J. Organomet. Chem. 1997, 529, 455 –
463; c) D. Moulin, S. Bago, C. Bauduin, C. Darcel, S. JugØ,
Tetrahedron: Asymmetry 2000, 11, 3939 – 3956; d) C. Bauduin,
D. Moulin, E. B. Kaloun, C. Darcel, S. JugØ, J. Org. Chem. 2003,
68, 4293 – 4301; e) C. Darcel, D. Moulin, J.-C. Henry, M.
Lagrelette, P. Richard, P. D. Harvey, S. JugØ, Eur. J. Org.
Chem. 2007, 2078 – 2090.
[15]
which proceeded rapidly at room temperature in the
presence of catalysts prepared from [PdCl2(cod)] and the
ligands to give the desired products in high yields and with
outstanding enantioselectivities of up to 99.9% ee (Table 3,
entries 4–6 and 9). These enantioselectivities are higher than
those observed upon the use of previously reported P-chiral
phosphine ligands (tBu-QuinoxP*: up to 97.6% ee;[3f] Ad-
QuinoxP*: 98.5% ee;[16] tBu-BisP*: 94% ee[17]) and are the
highest reported to date for this class of reactions.[15]
Table 3: Asymmetric alkylative ring opening catalyzed by [PdCl2(cod)]/5.
Entry
5
R1
R2
t [h]
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
8
9
5a
5b
5c
5d
5e
5d
5d
5d
5e
5e
5e
H
H
H
H
H
H
Me
Me
Me
Me
Me
Et
Me
Et
Et
1.5
1.5
1.5
2
2
6
4
8
6
4
92
90
92
93
94
90
92
89
89
93
92
95.8
96.3
95.8
99.9
99.8
99.9
94
98.4
99.2
94
MeOCH2O
MeOCH2O
H
MeOCH2O
MeOCH2O
[5] M. Stankevic, K. M. Pietrusiewicz, J. Org. Chem. 2007, 72, 816 –
822, and references therein.
[6] K. Nagata, S. Matsukawa, T. Imamoto, J. Org. Chem. 2000, 65,
4185 – 4188.
[7] The stereochemical course of the reaction was confirmed on the
basis of the absolute configuration of the substitution product
3a. The absolute configuration of compound 6a, which is
derived from 3a, was determined to be S,S by single-crystal X-
ray analysis. Hence, 3a has the S configuration, which indicates
that the substitution reaction occurred with inversion of config-
uration (see the Supporting Information).
10
11
Me
Et
8
97.8
[a] Yield of the isolated product. [b] Configuration of the major
enantiomer: 1S,2S. cod=cyclooctadiene.
In conclusion, we found that (bromo(tert-butyl)methyl-
phosphanyl)borane undergoes stereospecific nucleophilic
substitution reactions with alkynyl lithium reagents to give
alkynylated products in high yields with excellent enantiose-
lectivities. The products were converted into the P-chiral
phosphine ligands 5a–e, which induced high to almost perfect
enantioselectivity in representative transition-metal-cata-
lyzed asymmetric reactions. These ligands, particularly 5d
and 5e, have potential for use in other asymmetric catalytic
reactions.
[8] R. M. Stoop, A. Mezzetti, F. Spindler, Organometallics 1998, 17,
668 – 675.
[9] The
chlorinated
compound
(R)-(tert-butyl-
(chloro)methylphosphanyl)borane was prepared by treating 1
with nBuLi and hexachloroethane. It exists as a stable crystalline
solid and hardly reacted at all with benzylmagnesium chloride at
room temperature.
[10] Gold(I) complexes of triethynylphosphine ligands with bulky
substituents at the alkyne termini show unique catalytic activity:
A. Ochida, H. Ito, M. Sawamura, J. Am. Chem. Soc. 2006, 128,
16486 – 16487; A. Ochida, M. Sawamura, Chem. Asian J. 2007, 2,
609 – 618.
Received: June 9, 2007
Revised: July 31, 2007
Published online: October 5, 2007
[11] a) T. Imamoto, J. Watanabe, Y. Wada, H. Masuda, H. Yamada,
H. Tsuruta, S. Matsukawa, K. Yamaguchi, J. Am. Chem. Soc.
8638
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Angewandte
Chemie
1998, 120, 1635 – 1636; b) I. D. Gridnev, Y. Yamanoi, N. Higashi,
H. Tsuruta, M. Yasutake, T. Imamoto, Adv. Synth. Catal. 2001,
343, 118 – 136.
[14] The catalytic efficiency of the structurally analogous and
electron-rich phosphine ligand tBu-BisP* was tested in the
reaction of 2-cyclohexenone with phenylboronic acid under the
same conditions as those described in Table 2. The reaction at
408C for 2 h afforded the corresponding 1,4-addition product
with 20% ee in 37% yield.
[12] To examine the possibility that the alkynyl group of these ligands
could be hydrogenated under the reaction conditions, a 1:1
mixture of 5a and [Rh(nbd)2]BF4 in methanol was stirred under
1 atm of hydrogen pressure at room temperature for 3 h. The
1H NMR spectra of the reaction mixture were very complicated,
and we could not clarify whether the alkynyl group was
hydrogenated or not. On the other hand, 6a was recovered
unchanged after being stirred as a solution in methanol in the
presence of 5a/[Rh(nbd)2]BF4 (5 mol%) under 3 atm of hydro-
gen pressure at room temperature for 5 h.
[13] a) Y. Takaya, M. Ogasawara, T. Hayashi, M. Sakai, N. Miyaura,
J. Am. Chem. Soc. 1998, 120, 5579 – 5580; b) T. Hayashi, M.
Takahashi, Y. Takaya, M. Ogasawara, J. Am. Chem. Soc. 2002,
124, 5052 – 5058; c) T. Hayashi, K. Ueyama, N. Tokunaga, K.
Yoshida, J. Am. Chem. Soc. 2003, 125, 11508 – 11509; d) T.
Hayashi, Pure Appl. Chem. 2004, 76, 465 – 475; e) T. Hayashi, K.
Yamazaki, Chem. Rev. 2003, 103, 2829 – 2844; f) K. Fagnou, M.
Lautens, Chem. Rev. 2003, 103, 169 – 196, and references therein.
[15] a) M. Lautens, J.-L. Renaud, S. Hiebert, J. Am. Chem. Soc. 2000,
122, 1804 – 1805; b) M. Lautens, K. Fagnou, S. Hiebert, Acc.
Chem. Res. 2003, 36, 48 – 58; c) M. Lautens, S. Hiebert, J. Am.
Chem. Soc. 2004, 126, 1437 – 1447; d) S. Cabrera, R. G. Arrayꢀs,
J. C. Carretero, Angew. Chem. 2004, 116, 4034 – 4037; Angew.
Chem. Int. Ed. 2004, 43, 3944 – 3947; e) M. Li, X.-X. Yan, W.
Hong, X.-Z. Zhu, B.-X. Cao, J. Sun, X.-L. Hou, Org. Lett. 2004, 6,
2833 – 2835.
[16] T. Imamoto, A. Kumada, K. Yoshida, Chem. Lett. 2007, 36, 500 –
501 (Ad-QuinoxP* = 2,3-bis(adamantylmethylphosphanyl)qui-
noxaline).
[17] Oxabenzonorbornadiene was treated with diethylzinc in the
presence of [PdCl2(cod)]/tBu-BisP* (2 mol%) at room temper-
ature for 6 h to give (1S,2S)-2-ethyl-1,2-dihydronaphthalen-1-ol
in 93% yield with 94% ee.
Angew. Chem. Int. Ed. 2007, 46, 8636 –8639ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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