A stereospecific ruthenium-catalyzed allylic alkylation

Article abstract of DOI:10.1002/1521-3773(20020315)41:6<1059::AID-ANIE1059>3.0.CO;2-5

Good regioselectivity and chirality transfer for aryl-substituted allyl units is achieved in allylic alkylations with a wide range of nucleophiles by using the highly active ruthenium catalyst 1. This method provides a route to antidepressants such as (-)-fluoxetine from (S)-ephedrine (see scheme; Cp* = η⁵-C₅Me₅, TBAT = tetrabutylammonium triphenyldifluorosilicate).

Full text of DOI:10.1002/1521-3773(20020315)41:6<1059::AID-ANIE1059>3.0.CO;2-5

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[4] a) For an early study, see: T. Rosen, C. H. Heathcock, J. Am. Chem.
Soc. 1985, 107, 3731 3733; b) for an overview and leading references,
see Ref. [1a].
[5] a) For an early study, see: J. Hiratake, Y. Yamamoto, J. Oda, J. Chem.
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8063 8066. After the submission of our manuscript, Rovis described
one example of a catalytic asymmetric desymmetrization of an
anhydride by ZnEt2 (79% ee): E. A. Bercot, T. Rovis, J. Am. Chem.
Soc. 2002, 124, 174 175.
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Chem. Int. Ed. Engl. 1997, 36, 2282 2316.
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alkylations normally favor nucleophilic addition to the less
substituted allyl terminus although ligands can influence this
selectivity.[1] Early results demonstrate the effectiveness of
Mo[3] and W[4] catalysts and, more recently, their ability to
induce enantioselectivity.[5] Such catalysts normally do not
work with heteroatom nucleophiles. Iridium catalysts have
been reported to favor attack on the more substituted carbon
with a carbon–and most recently nitrogen–nucleophile, but
the use of an oxygen nucleophile like phenol has not been
reported.[6] Rhodium catalysis has proven to be very interest-
ing in that the regioselectivity of the substitution is deter-
mined by the position of the leaving group.[7] Herein, we
report our preliminary observations that the ruthenium-
catalyzed reaction favors attack at the more substituted
carbon atom regardless of the regioisomeric nature of the
substrate and does so with complete retention of enantiomeric
purity when a chiral scalemic substrate is employed. This
study has led to a facile synthetic strategy to antidepressants
like fluoxetine,[8] the active ingredient of prozac, from
ephedrine.
Pioneering work in ruthenium-catalyzed allylic alkylation
by Watanabe et al. with the [Ru(cod)(cot)] (cod ˆ 1,5-cyclo-
octadiene; cot ˆ 1,3,5-cyclooctatriene) complex has indicated
a bias for attack at the more substituted terminus with some
nucleophiles, although only a 50:50 regioisomeric mixture was
obtained by using a cinnamyl carbonate and malonate
anion.[9] [CpRu(cod)Cl][10] (1) and [CpRu(PPh3)2Cl][11] (2)
have been employed together with heteroatom nucleophiles,
but the regioselectivity has not been satisfactorily addressed.
Our recent work on cyclocondensations of allenes and vinyl
ketones using [CpRu(NCCH3)3]PF6 (3) induced us to examine
this complex as a catalyst for regioselective allylic alkylation.
We chose the reaction shown in Equation (1) as a standard.
In contrast to the earlier reports in which 1 or 2 were used and
which required elevated temperatures, complex 3 effected the
[12] a) Kise and Yoshida have provided a single example of the addition of
an RLi/(À)-sparteine/MgBr2 mixture to an aldehyde: N. Kise, T. Urai,
J.-i. Yoshida, Tetrahedron: Asymmetry 1998, 9, 3125 3128; b) Oka-
moto and Yuki have applied RMgBr/(À)-sparteine to asymmetric
polymerization reactions: Y. Okamoto, K. Suzuki, T. Kitayama, H.
Yuki, H. Kageyama, K. Miki, N. Tanaka, N. Kasai, J. Am. Chem. Soc.
1982, 104, 4618 4624.
[13] For an NMR study of dineopentylmagnesium/(À)-sparteine, see: G.
Fraenkel, B. Appleman, J. G. Ray, J. Am. Chem. Soc. 1974, 96, 5113
5119.
A Stereospecific Ruthenium-Catalyzed Allylic
Alkylation**
Ph
LG
4
CH3O2C
Ph
CO2CH3
Barry M. Trost,* Pierre L. Fraisse, and Zachary T. Ball
CO2CH3
CO2CH3
Ph
(1)
NaCH(CO2CH3)2
Metal-catalyzed allylic alkylations provide a powerful tool
for the construction of complex molecules. One of the benefits
of such substitutions is the prospect that the regioselectivity
with unsymmetrical allyl substrates can be controlled by the
catalyst rather than the position of the allylic substituent
serving as the leaving group. Palladium-catalyzed allylic
6
5
a: LG = OCO2tBu
b: LG = OCO2CH3
c: LG = Cl
reaction of carbonate 4a at ambient temperature in DMF to
give a 1:2ratio of 5:6 in nearly quantitative yield. Attempts to
increase this selectivity by varying the reaction conditions
failed and thus we examined changes in the ligand. We
reasoned that a more sterically demanding catalyst might
favor the monosubstituted olefin adduct initially formed from
™branched∫ attack to afford 5. Under identical conditions with
carbonate 4a, [Cp*Ru(NCCH3)3]PF6 (7) (Cp* ˆ h5-C5Me5)
gave a 9:1 ratio of 5:6 (96% yield) in 2h. In acetone, the
selectivity increased to 19:1 (quantitative yield). Reactions of
the methyl carbonate 4b are generally faster. Indeed, in DMF
within 30 min, a quantitative yield of alkylation products was
obtained in a 14:1 ratio of 5:6 with only 1 mol% of catalyst 7.
[*] Prof. B. M. Trost, P. L. Fraisse, Z. T. Ball
Department of Chemistry
Stanford University
Stanford, CA 94305-5080 (USA)
Fax : (‡1)650-725-0002
E-mail: bmtrost@stanford.edu
[**] We thank the National Science Foundation and the National Institutes
of Health, and General Medical Sciences, for their generous support
of our programs. Mass spectra were provided by the Mass Spectrom-
etry Facility of the University of California San Francisco, supported
by the NIH Division of Research Resources.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
Angew. Chem. Int. Ed. 2002, 41, No. 6
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
1433-7851/02/4106-1059 $ 17.50+.50/0
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Cinnamyl chlorides (4c) are also effective electrophiles.
Although poor regioselectivity is observed at ambient tem-
perature, at À408C in DMF, a quantitative yield of virtually
pure 5 (5:6 110:1) was obtained.
path a
NaH, DMF, RT
4a
30%
b/l 1.6:1.0
The regioselectivity as a function of the regioisomeric
allylic carbonates 4a and 8 was examined with a series of
malonates [Eq. (2); b ˆ branched, l ˆ linear]. Very similar
O
O
NC
NC
(4)
Ph
X = H
OMOM
99% b/l
99% b/l
12/1
9/1
1 mol% 7
12
NHBoc
98% b/l
4.5/1
87%
8
(n-C4H9)3N, neat
path b
Ph
OCO2tBu
4a
b/l 22/1
1 mol% 7
X
CH3O2C
Ph
CO2CH3
1 mol% 7
X
O
OBoc
O
CH3O2C
CO2CH3
(n-C4H9)3N
X
CN
X
(2)
NaH, DMF
(5)
NC
9
n
n
13
a: X = H (5)
14
OCO2R
b: X = OMOM
c: X = NHBoc
a: X = O
b: X = S
n
X
%
b/l
Ph
8
O
O
1
2
86
85
70/1
16/1
X = H
OMOM
98% b/l
12/1
8/1
a: R = tBu
99% b/l
93% b/l
b: R = CH3
NHBoc
4/1
S
S
1
2
98
88
100/1
13/1
results were obtained with three different malonates, whereby
the branched product 9 was strongly favored. To a first
approximation these results support the postulate of a p-
allylruthenium intermediate. Catalyst-controlled regioselec-
tivity is further demonstrated for an unsymmetrical, disub-
stituted allyl unit [Eq. (3)], for which excellent preference for
OCO2t-Bu
H
CO2CH3
(S)-8a
H
CO2CH3
X = H
(R)-9a
CO2CH3
CO2CH3
(6)
1 mol% 7
X
MeO2C
MeO2C
CH3O2C
CO2CH3
NaH, DMF, RT
OCO2Me
CO2CH3
NHBoc
(3)
Ph
X = NHBoc
Ph
11
H
NaH, DMF
91% regioisomer ratio = 20/1
CO2CH3
10
(R)-9c
OCO2CH3
H
(R)-8b
the benzylic position is observed for an allylic carbonate of
opposite regiochemistry. Unfortunately, direct extension to a-
cyanocyclopentanone led to a sluggish reaction (30% con-
version after 4 h) and poor regioselectivity (b/l 1.6:1) [Eq. (4),
path a]. This problem was resolved in a most gratifying
fashion–by simply running the reaction without solvent using
tri-n-butylamine as base and with 8 as the substrate [Eq. (4),
path b]. These neat conditions proved general for bulkier
nucleophiles, and Equation (5) illustrates the applicability of
heteroaryl substrates 13. Notably, diastereoselection has not
been observed for the unsymmetrical b-keto ester nucleo-
philes. Yields given are for an approximately 1:1 mixture of
(sometimes separable) diastereomers.
Having established the involvement of a functional equiv-
alent of a p-allylruthenium intermediate, we examined the
stereochemistry of the substitution by using chiral substrate 8
of high enantiopurity [Eq. (6)]. Starting with (S)-8a of
94% ee gave (R)-9a of 94% ee, therefore constituting com-
plete chirality transfer (CT). The absolute configuration was
established by comparison with a known sample. Similarly,
starting with the enantiomeric substrate (R)-8b of 99% ee
with the amidomalonate gave the epimeric product (R)-9c of
99% ee, again constituting 100% CT. Thus, ionization occurs
to generate cleanly only one enantiomeric p-allylruthenium
complex that does not equilibrate under the reaction con-
ditions. We next sought to extend the scope of our enantio-
specific alkylation to O-nucleophiles in the context of the
synthesis of the antidepressants (R)-fluoxetine[8] and (R)-
tomoxetine from (S)-ephedrine as shown in Scheme 1.[12] The
starting allyl alcohol was available by Hoffmann elimination
of the quaternary ammonium salt derived from ephedrine
using strong base.[13] Both enantiomers of the allyl alcohol are
readily available from the corresponding commercial ephe-
drine. The free phenol was not a satisfactory nucleophile, but
its silyl ether in the presence of a catalytic amount of a
1060
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1433-7851/02/4106-1060 $ 17.50+.50/0
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COMMUNICATIONS
D. L. VanVranken, Chem. Rev. 1996, 96, 395;
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[2] B. M. Trost, Acc. Chem. Res. 1996, 29, 355.
[3] B. M. Trost, M. Lautens, J. Am. Chem. Soc.
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[4] J. Lehmann, G. C. Lloyd-Jones, Tetrahedron
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[5] For enantioselective substitution reactions
with Mo: B. M. Trost, I. Hachiya, J. Am.
Chem. Soc. 1998, 120, 1104; B. M. Trost, S.
Hildbrand, K. Dogra, J. Am. Chem. Soc. 1999,
121, 10416; for asymmetric W: G. C. Lloyd-
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[6] R. Takeuchi, N. Ve, K. Tanabe, K. Yamashita,
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123, 1234.
Scheme 1. Synthesis of antidepressants. a) CH3I, K2CO3, CH3OH, RT; b) LDA, THF, HMPA, À50
to 08C; c) n-C4H9Li, ClCO2CH3, THF, À78 to 08C; d) 1 mol% 7, p-CF3C6H4OTMS, 1 mol% TBAT,
neat, 358C; e) 1 mol% 7, o-CH3C6H4OTMS, 5 mol% (n-C4H9)4NCl, 10 mol% (C2H5)3N, neat, 358C;
f) 9-BBN-H, THF, 358C then KOH, H2O2, 08C; g) CH3SO2Cl, (C2H5)3N, CH2Cl2, 08C then CH3NH2,
CH3OH, 708C. LDA ˆ lithium diisopropylamide, HMPA ˆ hexamethylphosphoramide, 9-BBN-
H ˆ 9-borabicyclo[3.3.1]nonane.
[8] D. W. Robertson, N. D. Jones, J. K. Swartzen-
druber, K. S. Yang, D. T. Wong, J. Med. Chem.
1988, 31, 185. For the biological activity of the
single-enantiomer drug: D. W. Robertson, J. H. Krushinski, R. W.
Fuller, J. D. Leander, J. Med. Chem. 1988, 31, 1412.
desilylating agent (tetrabutylammonium triphenyldifluorosil-
icate (TBAT) or tetrabutylammonium chloride (TBAC)) with
1 mol% of 7 under neat conditions effected the desired
reaction. The reaction of the trimethylsilyl ether of p-
trifluorocresol gave a 7:1 b/l ratio from which 15a was
isolated in 81% yield.
Standard hydroboration, oxidation, and substitution pro-
vided (À)-fluoxetine (17). Switching to the o-cresol system
similarly gave a 6:1 b/l ratio from which 15b was isolated in
81% yield. In both cases, excellent chirality transfer is
observed, demonstrating the effectiveness of our catalyst for
both carbon and heteroatom nucleophiles.
The ruthenium-catalyzed allylic alkylation differs from
previous catalytic systems in several important respects. Like
Pd, Mo, and W- but unlike Rh–regioselectivity is not highly
dependent on the nature of the starting carbonate. Unlike Pd
and Mo, but like Rh and W, substitution of the scalemic chiral
substrate occurs with high net retention of configuration.
Heteroatom nucleophiles which fail with Mo and W succeed
with this system. Thus, this catalyst nicely complements the
previously developed systems. Importantly, reactions proceed
extremely well under mild conditions with 1 mol% of catalyst.
The solvent-free conditions are commensurate with the ideals
of green chemistry. The practicality of the methodology is
illustrated by a novel synthesis of (R)-fluoxetine and (R)-
tomoxetine starting from inexpensive ephedrine using a
heretofore unknown elimination to generate the allyl alcohol
of high enantiopurity.
[9] S. W. Zhang, T. Mitsudo, T. Kondo, Y. Watanabe, J. Organomet. Chem.
1995, 485, 55; S. W. Zhang, T. Mitsudo, T. Kondo, Y. J. Watanabe,
Organomet. Chem. 1993, 450, 197. A report has appeared very recently
using a planar chiral ruthenium complex for the enantioselective
alkylation of a symmetric meso-type allyl unit: Y. Matsushima, K.
Onitsuka, T. Kondo, T. Mitsudo, S. J. Takahashi, J. Am. Chem. Soc.
2001, 123, 10405.
[10] T. Kondo, Y. Morisaki, S. Uenoyama, K. Wada, T. Mitsudo, J. Am.
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[11] S. K. Kang, D. Y. Kim, R. K. Hong, P. S. Ho, Synth. Commun. 1996, 26,
3225.
[12] For synthetic efforts towards both drugs, see the following and
references therein: H. L. Liu, B. H. Ho, T. Anthonsen, J. Chem. Soc.
Perkin Trans. 1 2000, 11, 1767.
[13] A. Hoffman, Chem. Ber. 1881, 14, 166; K. Hayakawa, I Fuji, J.
Kanematsu, J. Org. Chem. 1983, 48, 166.
[14] F. Bracher, T. Litz, Bioorg. Med. Chem. 1996, 4, 877.
[15] R. Chenevert, G. Fortier, R. B. Rhlid, Tetrahedron 1992, 48, 6769.
Received: January 8, 2002 [Z18496]
[1] For general reviews of Pd-catalyzed allylic substitution: S. A. God-
leski in Comprehensive Organic Synthesis, Vol. 4 (Eds.: B. M. Trost, I.
Fleming, M. F. Semmelhack), Pergamon, Oxford, 1991; B. M. Trost,
Angew. Chem. Int. Ed. 2002, 41, No. 6
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
1433-7851/02/4106-1061 $ 17.50+.50/0
1061