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Asymmetric Cycloisomerization of 1 6- and 1 7-Enynes by Transition-Metal Catalysts.

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Highlights
Synthetic Methods
Asymmetric Cycloisomerization of 1,6- and 1,7-Enynes
by Transition-Metal Catalysts
Ian J. S. Fairlamb*
Keywords:
atom economy · cyclization · enantioselectivity ·
palladium · rhodium · ruthenium
Introduction
An abundance of optically active
molecules found in nature contain highly functionalized carbo- and heterocycles. These important structural features, which are often absolutely essential for biological activity and function,
provide an impetus for synthetic chemists to design and develop new and more
efficient methods for the synthesis of
such compounds. Transition-metal-catalyzed cyclizations are arguably some of
the most useful methods, and high
selectivity and efficiency are often realized. It is somewhat surprising to find
that asymmetric transition-metal-catalyzed cyclization reactions are generally
quite limited, particularly when compared to other asymmetric reactions,
such as hydrogenation or allylic alkylation, often referred to as the benchmark
asymmetric reactions for testing new
chiral transition-metal catalysts. Clearly,
asymmetric cyclization reactions have
great potential to facilitate access to
both simple and structurally complex
cyclic compounds in an efficient, but
enantioselective fashion. Over the past
twenty years, the transition-metal-catalyzed (Ti, Ru, Co, Rh, Ni–Cr, Pd, and
Pt) “Alder-ene” cycloisomerization reaction of 1,n-enynes (n = 6, 7)[1] has
emerged as an extremely powerful,
atom-economic, and environmentally
friendly method for the rapid assembly
[*] Dr. I. J. S. Fairlamb
Department of Chemistry
University of York
Heslington, York, YO10 5DD
(United Kingdom)
Fax: (+ 44) 1904-432-516
E-mail: ijsf1@york.ac.uk
1048
of complex carbo- and heterocyclic frameworks in a highly
regio-, diastereo-, and chemoselective way [Eq. (1), 1!2].[2]
Trost is without doubt a pioneer in this area, particularly in
the development of catalysts and
Trost was first to report an asymmetric protocol. It was found that a
combination of [Pd2(dba)3]·CHCl3 (dba =
trans,trans-dibenzylideneacetone) plus a
carboxylic acid, such as acetic acid,
effectively catalyzed the cycloisomerization of 1,6-enynes [Eq. (2), 3!4].[3] The
catalytic intermediate was proposed to
be a palladium(ii) hydride species
“HPdOAc”, which is the oxidativeaddition product of acetic acid and Pd0.
This was extended to the use of enantiomerically pure carboxylic acids, such
as Mosher's acid, ((S)-()-2-methoxy-2trifluoromethylphenyl acetic acid) and
(S)-()-binaphthoic acid, which facilitated the asymmetric cycloisomerization
reaction, although with modest, but
encouraging enantioselectivity (33 % ee
was the highest).
Asymmetric induction was improved by the employment of amidodiphosphane ligands (Trost modular ligands) in combination with a tartrate
chiral auxiliary contained within the 1,6or 1,7-enyne, allowing a double stereodifferentiation to occur [Eq. (3), 5 a!
6 a].[4] It was found that the chiral ligand
operates independently of the chirality
of the tartrate auxiliary. The distant
carboxylic acid unit in the substrate is
essential for asymmetric induction, but
the presence of the chiral tartrate is not.
This was proven by comparison with
reactions with the ethyl and methyl
esters, which gave poor selectivity. A
model was proposed where the carboxylic acid rigidifies the substrate by
coordination to Pd in the chiral discriminating step (carbopalladation).
More efficient bidentate ligands for
palladium were introduced by Ito and
co-workers, namely the trans-coordinating bisferrocenyldiphosphanes based on
(S,S)-(R,R)-trap [Eq. (4)].[5] Phosphane
ligands reduce the rate of cyclization,
although they are crucial for smooth
conversion, minimizing side reactions,
DOI: 10.1002/anie.200301699
Angew. Chem. Int. Ed. 2004, 43, 1048 –1052
their employment in the total syntheses
of natural products, where the scope and
limitations of the methodology have
been studied extensively. Over the past
few years there has been a resurgence of
interest in the development of the
asymmetric variant of the cycloisomerization reaction, particularly of 1,6enynes. Here, these developments, as
well as the initial investigations on the
asymmetric reaction, are highlighted.
Asymmetric Pd-Catalyzed Cycloisomerization
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angewandte
Chemie
and prolonging the lifetime of the catalytic species. In this reaction, Ha elimination is the favored process to give 1,4diene 8 a as the major product (Hb
elimination gives the 1,3-diene 8 b).
Increasing the electron-withdrawing
ability on the P–aryl substituents results
in higher enantioselectivities and reactivities (up to 95 % ee), but this appears
to be substrate sensitive (nine examples,
34–76 % ee). Alteration of the alkene
configuration from Z to E resulted
in a reversal of the observed configuration of the newly formed stereogenic
center in the product (E!R) to
(Z!S).
In addition to these results, it was
shown that other optically active cischelating phosphanes, such as chiraphos
and diop, amongst others, gave poor
selectivities (6–15 % ee), whereas for
binap and the electron-donating
ettrap no conversion occurred even at
80 8C.
Angew. Chem. Int. Ed. 2004, 43, 1048 –1052
Mikami et al. found that the cycloisomerization of 1,6-enyne 9!10, in
benzene at 100 8C, with typical palladium catalysts (Pd(OAc)2, [Pd2(dba)3]·
www.angewandte.org
CHCl3/AcOH, and [Pd2(dba)3]·CHCl3/
F3CCO2H) and (R)-binap gave higher
enantioselectivity (up to 84 % ee), although with insufficient catalytic activity (< 25 % yield) [Eq. (5)].[6]
On switching to Pd(OCOCF3)2 an
improvement in yield (> 99 %) and
enantioselectivity (93 % ee) was seen.
Use of the related binap ligands (S)-H8binap and (R)-segphos increased enantioselectivity to > 99 % ee. For these
ligands, changing the solvent to dimethyl sulfoxide (DMSO) resulted in improved reaction rates but with a slight
reduction in enantioselectivity (ca. 90–
96 % ee). It is proposed that the
CF3COO ion is released associatively
in DMSO and gives up one coordination
site on a square-planar cationic complex
[TS-1 in Eq. (5)]. In the nonpolar solvent, the counterion remains coordinated to the PdII species, where the olefin is
forced to coordinate the fully squareplanar PdII, producing a relatively unfavorable five-coordinate intermediate,
which is proposed to slow down the
reaction [TS-2 in Eq. (5)].
Mikami and Hatano extended this
work to the asymmetric cycloisomerization of 1,7-enynes to provide six-membered rings, which are more difficult to
form than the five-membered-ring systems.[7] In particular, a highly efficient
asymmetric synthesis of quinoline derivatives bearing a quaternary carbon
center (11!12) or a spiro ring (13!14)
was achieved using the catalyst combination (S)-binap/[Pd(MeCN)4](BF4)2 in
the presence of formic acid in DMSO at
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1049
Highlights
100 8C [Eq. (6)]. Interestingly, these reactions can be monitored by following
changes in the color of the solution. The
reaction solution is light yellow initially
and becomes a deep green clear solution
after only 2 min at 100 8C. The color
lightens as the reaction proceeds, finally
turning to a yellow-orange color when
the reaction is complete.
It was subsequently found that simply preparing the catalysts in situ led to a
significant improvement in reactivity
and enantioselectivity for several prototypical substrates containing hetero-
atom tether linkages. The catalyst prepared from binap, in particular, gave
excellent asymmetric induction and
demonstrated good reactivity [Eq. (8),
17!18].[9]
Substrate scope has been broadened
by application of the new protocol to the
highly enantioselective cyclosiomerization of enyne amides to give functionalized lactams in extraordinarily high
enantioselectivities [Eq. (9), 19!20].[10]
These reactions demonstrate broad
functional-group tolerance, but notably,
functionalized vinyl derivatives such as
vinyl ether, vinyl acetate, vinyl allylic
ether, and enamides containing lactams
are accessible. This further offers a new
asymmetric approach to kainic acid
analogues [Eq. (10), 21!22]. A limitation is in the use of (E)-olefins, where
poor conversion and low enantioselectivities are observed. This is possibly
Asymmetric Rh-Catalyzed
Cycloisomerization
Zhang and co-workers reported the
first asymmetric Rh-catalyzed 1,6enyne cyclosiomerization reactions employing precursor catalysts containing
chiral bidendate phosphanes and phosphinites [Eq. (7), 15!16].[8] Surprisingly no reactivity was observed with the
Rh/binap system, whereas Me-duphos,
bicp, and bicpo were very useful. Generally, the catalyst system proved quite
responsive to subtle changes in the
electronic nature of the substrate and
ligand—a limiting feature with respect
to broader substrate scope.
attributable to weaker coordination of E
substrates to RhI. A highly stereoselective kinetic resolution of 1,6-enynes and
a RhI-catalyzed intramolecular cycloisomerization reaction have also been developed, allowing access to polyfunctional tetrahydrofurans and 1,6-enynes
with two stereogenic centers with high
enantioselectivity.[11]
Zhang and his team have employed
this catalyst system in an asymmetric
“formal” synthesis of (+)-pilocarpine
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2004, 43, 1048 –1052
Angewandte
Chemie
[Eq. (11), 23!24].[12] Functionalized amethylene-g-butyrolactones are accessible in high yields (90–98 %) and enantioselectivities (all examples given are
> 99 % ee!). A number of these reactions were complete in < 5 min! Clearly
this catalyst system is extremely powerful.
A catalytic cycle for the Rh-catalyzed cycloisomerization reaction has
been proposed (Scheme 1).[10] This consists of bidentate coordination of the
enyne with a RhI species to form a
coordinated intermediate II. Depending
on the substituent R2, either oxidative
cyclization of the enyne or oxidative
addition could be expected. Here, only
oxidative cyclization occurs, affording
metallacyclopentenes III. A regiospecific b-H elimination of metallacyclopentene III-A provides the RhH species
IV, which then undergoes reductive
elimination to provide the expected
1,4-diene product. The E geometry of
the double bond is expected, as b-H
elimination is favored from the less
sterically hindered metallacyclopentene
III-A, as opposed to the more sterically
congested metallacyclopentene III-B,
which provides the Z isomer. Under
Rh catalysis, the cycloisomerization reaction tolerates the presence of allylic
acetate and carbonate groups, which
generally show a high reactivity towards
oxidative addition with low-valent transition metals. It is clear that in this
system that oxidative cyclization is
strongly favored over oxidative addition.
Scheme 1. The proposed mechanism for the Rh-catalyzed cycloisomerization of enynes.
Angew. Chem. Int. Ed. 2004, 43, 1048 –1052
www.angewandte.org
In a variation of the cycloisomerization reaction, Widenhoefer et al. have
developed the first asymmetric 1,6enyne cyclization/hydrosilylation reaction using rhodium catalysts containing
(R)-biphemp [Eq. (12), 25!26].[13]
Various 1,6-enynes gave functionalized silylated alkylidenecyclopentanes
in generally good yields and enantioselectivities of up to 92 % ee. These results
are very encouraging, given that they
present the first highly enantioselective
transformations of malonate-type enyne
substrates.
Summary and Outlook
The future perspectives for asymmetric cycloisomerization reactions, catalyzed by transition metals, are very
positive and encouraging. The rhodiumcatalyzed reaction is without doubt the
current leader in terms of substrate
scope (including (Z)-olefins), enantioselectivity, and catalyst reactivity. The
ruthenium-catalyzed asymmetric variant represents a new challenge. Trost
has reported that [CpRu(CH3CN)3]PF6
is an excellent catalyst for enyne cycloisomerization, which is proposed to
proceed via a ruthenacyclopentene intermediate (RuIV), formed by oxidative
cyclometalation, in the catalytic cycle.[14]
This catalyst possibly precludes the use
of chiral phosphanes,[15] and alternative
strategies may be required if this versatile metal is to enjoy similar success as
rhodium and palladium in catalyzed
asymmetric reactions. It should be noted
that the cycloisomerization reactions
catalyzed by PtII, similar to those with
RuII, also proceed by an oxidative cyclometalation process. [16] Asymmetric variants of the Pt-catalyzed reaction remain
to be developed.
The emergence of efficient protocols
for the asymmetric cycloisomerization
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1051
Highlights
of enynes to provide nonracemic carboand heterocyclic compounds has been
demonstrated. Further application of
these protocols to the synthesis of other
natural products and molecules of biological importance and interest is expected over the coming years.
[1] For comprehensive reviews, see: a) C.
Aubert, O. Buisine, M. Malacria, Chem.
Rev. 2002, 102, 813; b) B. M. Trost, M. J.
Krische, Synlett 1998, 1; c) B. M. Trost,
Acc. Chem. Res. 1990, 23, 34; d) For a
mechanistic overview, see: G. C. LloydJones, Org. Biomol. Chem. 2003, 1, 215.
[2] The thermal Alder reaction, in the
absence of a transition metal, has had
few applications in organic synthesis,
due to limitations in scope, particularly
as extremely high temperatures are
required.
[3] B. M. Trost, D. C. Lee, F. Rise, Tetrahedron Lett. 1989, 30, 651.
[4] B. M. Trost, B. A. Czeskis, Tetrahedron
Lett. 1994, 35, 211.
[5] A. Goeke, M. Sawamura, R. Kuwano, Y.
Ito, Angew. Chem. 1996, 108, 686; Angew. Chem. Int. Ed. Engl. 1996, 35, 662
1052
[6]
[7]
[8]
[9]
[10]
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[trap = 2,2’’-bis[1-(diarylphosphanyl)ethyl]-1,1’’-biferrocene].
M. Hatano, M. Terada, K. Mikami,
Angew. Chem. 2001, 113, 255; Angew.
Chem. Int. Ed. 2001, 40, 249 [binap =
2,2’-bis(diphenylphosphanyl)-1,1’-binaphthyl), chiraphos = 2,3-bis(diphenylphosphanyl)butane, diop = 2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphanyl)butane].
M. Hatano, K. Mikami, J. Am. Chem.
Soc. 2003, 125, 4704.
P. Cao, X. Zhang, Angew. Chem. 2000,
112, 4270; Angew. Chem. Int. Ed. 2000,
39, 4104 [duphos = 1,2-bis(phospholano)benzene,
bicp = 2,2’-bis(diphenylphosphanyl)-1,1’-dicyclopentane,
bicpo = 2,2’-bis(diphenylphosphinite)-1,1’dicyclopentane].
A. Lei, M. He, S. Wu, X. Zhang, Angew.
Chem. 2002, 114, 3607; Angew. Chem.
Int. Ed. 2002, 41, 4104.
A. Lei, J. P. Waldkirch, M. He, X. Zhang,
Angew. Chem. 2002, 114, 4708; Angew.
Chem. Int. Ed. 2002, 41, 4526. Pdcatalyzed asymmetric cyclization of similar enynes initiated by acetoxypalladation results in b-deacetoxypalladation
(elimination). This protocol is very useful for the asymmetric synthesis of
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[11]
[12]
[13]
[14]
[15]
[16]
substituted g-butyrolactones, see: a) Q.
Zhang, X. Lu, X. Han, J. Org. Chem.
2001, 66, 7676; b) Q. Zhang, X. Lu, J.
Am. Chem. Soc. 2000, 122, 7604.
A. Lei, M. He, X. Zhang, J. Am. Chem.
Soc. 2003, 125, 11472.
A. Lei, M. He, X. Zhang, J. Am. Chem.
Soc. 2002, 124, 8198.
H. Chakrapani, C. Liu, R. A. Widenhoefer, Org. Lett. 2003, 5, 157. It was
found that Rh/binap complexes were
not effective catalysts for cyclization/
hydrosilylation. [biphemp = 6,6’-bis-(diphenylphosphanyl)-2,2’-dimethyl-biphenyl].
a) B. M. Trost, F. D. Toste, J. Am. Chem.
Soc. 1999, 121, 9728; b) B. M. Trost, F. D.
Toste, J. Am. Chem. Soc. 2000, 122, 714;
c) B. M. Trost, F. D. Toste, J. Am. Chem.
Soc. 2002, 124, 5025; for a related article,
see: d) M. Mori, N. Saito, D. Tanaka, M.
Takimoto, Y. Sato, J. Am. Chem. Soc.
2003, 125, 5606 and references therein.
The presence of phosphane ligands with
this catalyst might slow down the rate of
cycloisomerization.
M. MQndez, M. P. MuRuz, C. Nevado,
D. J. CSrdenas, A. M. Echavarren, J.
Am. Chem. Soc. 2001, 123, 10511.
Angew. Chem. Int. Ed. 2004, 43, 1048 –1052
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