Enantio- and Diastereoselective Synthesis of Chiral Allenes by Palladium-Catalyzed Asymmetric [3+2] Cycloaddition Reactions

Article abstract of DOI:10.1002/anie.201808345

A protocol for the asymmetric synthesis of highly substituted chiral allenes with control of point and axial chirality has been developed. A palladium-catalyzed [3+2] cycloaddition using readily available racemic allenes gives access to densely functional

Full text of DOI:10.1002/anie.201808345

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Accepted Article
Title: Enantio- and Diastereoselective Synthesis of Chiral Allenes by
Palladium-Catalyzed Asymmetric [3+2] Cycloaddition Reactions
Authors: Barry M. Trost, Daniel Zell, Guillaume Mata, Christoph Hohn,
and Autumn Maruniak
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808345
Angew. Chem. 10.1002/ange.201808345
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10.1002/anie.201808345
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Enantio- and Diastereoselective Synthesis of Chiral Allenes by
Palladium-Catalyzed Asymmetric [3+2] Cycloaddition Reactions
Barry M. Trost*, Daniel Zell, Christoph Hohn, Guillaume Mata, Autumn Maruniak
Abstract:
A
protocol for the asymmetric synthesis of highly
transfer catalysis,[15] Hayashi’s palladium-catalyzed SN2'
substitution of 2-bromo-1,3-butadienes,[24] and Bäckvall’s
chemoenzymatic approach (Fig. 1).[25] However, a significant
drawback of these methods is the lack of ability to build up more
molecular complexity at the allene’s substituents in a step
economic way.
substituted chiral allenes with control of point and axial chirality has
been developed. A palladium-catalyzed [3+2] cycloaddition using
readily available racemic allenes gives access to densely
functionalized chiral allenes with excellent yields and functional group
tolerance. The catalytic asymmetric protocol utilizes a broad range of
allenyl TMM donors to form cyclopentanes, pyrrolidines, and
spirocycles with very good control of regio-, enantio-, and
diastereoselectivity. The chiral allene moiety is shown to be a valuable
functional group for rapid elaboration towards complex targets.
In recent years, allenes have emerged as highly valuable building
blocks in synthetic chemistry. They found widespread
applications in marketed drugs (Enprostil,[1] Fenprostalene,[2] and
Prostalene), advanced molecular materials, redox switches,[3] as
chiral ligands[4] and exceptionally versatile substrates for
asymmetric transformations.[5] Additionally, there is a strong
demand for new efficient syntheses of allenes due to their
occurrence in many natural products.[6] Although allenes have
been synthetically accessible since Burton’s and von Pechmann’s
first preparation in 1887,[7] their correct structure remained elusive
until 1954. At that time, spectroscopic investigations revealed the
allene to be an orthogonal cumulative π-system,[8] which leads to
their unique reactivity profile and intrinsic chirality. Not surprisingly,
this allows allenes to participate in diverse chemical
transformations.[5a,5d,9] Thus, there has been a longstanding
interest in the development of novel synthetic methods for
generating enantiomerically pure allenes.[5d,10] Several traditional
approaches utilize common chemical transformations, such as
[3,3][11] and [2,3][12] sigmatropic rearrangements, SN2'
displacements,[13] and reductive transposition[14] of chiral
propargyl precursors to form enantiomerically enriched allenes.
More recent methods have employed traditional catalytic
asymmetric organocatalytic processes, such as a Mannich
reaction,[15] an alkynylogous Mukaiyama aldol reaction,[16] or a
three-step sequence including a Petasis reaction.[17] Furthermore,
metal-catalyzed asymmetric allene syntheses[13a, 18] have been
developed based on transformations of achiral dienes,[19] terminal
alkynes,[20] or vinyl triflates.[21] Noteworthy examples include
Alexakis’s synthesis of chloro allenes from propargyl
dichlorides,[22] Ma’s Pd-catalyzed synthesis of tri-substituted chiral
allenes from racemic propargyl carbonates,[23] Maruoka’s phase
Figure 1. Examples for palladium-catalyzed syntheses of racemic towards
chiral allenes.
In 2005, we reported an efficient and atom economical[26] method
to prepare enantiomerically pure allenes via the palladium-
catalyzed asymmetric allylic alkylation (AAA) of allenyl acetates
through a dynamic kinetic asymmetric transformation (DYKAT)
(Fig. 1).[27]
Herein, we report on the unprecedented implementation of a
DYKAT mechanism into the TMM reaction manifold,[28] giving
access to a wide variety of substituted chiral allenes from their
racemic counterparts (Fig. 1). This proof of concept could be
extended to novel TMM systems, thereby increasing the scope of
TMM cycloadditions to even more complex and diverse targets.
We initiated our optimization studies for the palladium-catalyzed
asymmetric [3+2] cycloaddition of the nitro-imino substituted
olefin 1 and the racemic tri-substituted allenyl TMM donor 2a by
comparing different solvents and reaction temperatures (Table 1).
Among a variety of solvents, toluene was the optimal choice
(entries 1–4), which delivered the product 3a in 88% yield with
very good diastereo- and enantioselectivity at only 23 °C,
[a]
Prof. Dr. B. M. Trost, Dr. D. Zell, Dr. C. Hohn, Dr. G. Mata, A.
Maruniak
Department of Chemistry
Stanford University
Stanford, CA 94305-5080 (USA)
E-mail: bmtrost@stanford.edu
whereas higher temperatures led to
a
decrease in
diastereoselectivity (entry 6). Furthermore, different bidentate
diamidophosphite ligands[29] L1–L4 were tested (entries 4 and 7–
9). A variation of the ligand’s backbone from methyl to ethyl
(entries 4 and 7) did not change the selectivities. However, the
stilbene-derived amido-backbone in ligands L1–L3 was superior
to the cyclohexyl-derived ligand L4 in terms of stereoselectivities
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201808345
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(entry 9). In contrast, a phosphoramidite ligand L5 was completely
unreactive.
Table 1. Optimization of reaction conditions.[a]
Yield / %
86
Entry
1
L
solvent
1,4-dioxane
CH2Cl2
THF
ee / %
94
---
dr
4:1
---
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
---
2
57
3
85
98
---
4:1
8:1
---
88
4
PhMe
5[b]
6[c]
7
PhMe
<5
78
PhMe
85
98
0
2:1
8:1
1:1
7:1
---
85
PhMe
84
8
PhMe
88
9
PhMe
82
---
---
10
PhMe
[a] Reaction conditions: 1 (0.10 mmol, 1.0 equiv), 2a (0.17 mmol, 1.7 equiv),
[CpPd(η3-C3H5)] (5.0 mol %), L (6.0 mol %), 23 °C, 20 h; isolated yields of
both diastereomers. dr values were determined by 1H-NMR analysis;
absolute ee values of major diastereomer by chiral HPLC. [b] at 4 °C. [c] at
60 °C.
With the optimized conditions in hand, the palladium-catalyzed
[3+2] cycloaddition with the -imino nitro olefin 1 and a broad
range of allenes 2 provided the expected cycloadducts 3 in overall
good to excellent yields (Scheme 1). The diastereoselectivity of
this process ranged from 4:1 to >20:1 with very good
enantioselectivities of 92–99%. The diastereoselectivity of the
products refers to the axial chirality of the allene with respect to
the trans-configured stereocenters of the cyclopentane moiety.
Generally, alkyl-substituted allenes (3a–d) were well tolerated
and delivered the corresponding products with excellent
selectivities. Even the methyl-substituted allene (3d) was
obtained in satisfactory 3.7:1 dr, which indicates a limited
Scheme 1. Scope of allene TMM donors. [a] reaction on 1.0 mmol scale: 90%
yield, same selectivities.
In addition, the reaction worked with a broad range of electrophilic
TMM acceptors. Thus, N-acyl pyrrole substituted olefins and
imineswere employed to access diversely substituted
cyclopentanes, pyrrolidines, and complex carbocycles in a step
economical way (Scheme 2). Generally, the α,β-unsaturated acyl
pyrroles (4a–c) and imines (4d–f) underwent the TMM reaction
highly regioselectively and generated the cycloadducts 5a–c and
the allenyl pyrrolidines (5d–f) with good yields and selectivities.
The reaction proceeded fully chemoselectively with tolerance of
susceptible halogen substituents (5d). Going forward, we
expanded the acceptor scope from tri- to tetra-substituted olefins,
building up two contiguous quaternary centers on the
cyclopentane ring in one step. With 3-isopropylidene oxindole
(4g) as the acceptor, the yield and diastereoselectivity for this
process remained good, albeit with slightly decreased
enantioselectivity. Interestingly, a previously established protocol
from our group[30] gave unsatisfactory results with tropone (4i) as
the acceptor. In striking contrast, the herein reported catalytic
system generates the meso-compound 5i with excellent yield.
influence
of
the
substituent’s
steric
demand
on
diastereoselectivity.
It
is worth noting that
higher
diastereoselectivities were obtained for aromatic substituents on
the allene (3e–3j). To our delight, alkyl groups bearing protected
heteroatoms (3k and 3l) were also well accepted, leading to α-
allenol and α-amino allene precursors with excellent levels of
diastereo- and enantioselectivity. Furthermore, the absolute
configuration of the allene cycloadduct (R,S,S)-3l was
unambiguously determined by X-ray analysis (Scheme 1).
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10.1002/anie.201808345
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(3.5:1 dr), which amounts to 68% of the kinetically preferred
allene enantiomer being converted (Scheme 4a). In addition, a
reaction between substrates 1 and 2l was stopped after 2.5 h and
only a small enantioenrichment of 10% ee of the reisolated allene
2l was observed (Scheme 4b), which can be due to a kinetic
resolution proceeding in parallel.
Scheme 4. Key mechanistic findings for the DYKAT mechanism.
Scheme 2. Scope of TMM acceptors. [a] reaction at 60 °C: 90% yield, 2.7:1 dr,
89% ee. [b] 2k (1.4 equiv). [c] 2a (3.0 equiv). [d] ee of minor diastereomer. [e]
2a (1.6 equiv).
Based on these mechanistic insights, the following catalytic cycle
was proposed (Scheme 5). First, ionization of the racemic donor
2 generates the cationic Pd-TMM complex A, which desilylates to
form B. The DYKAT process may either occur between A and A'
or B and B'. In either case, the interconversion of Pd-TMM
isomers by -- equilibration leads to the more stable
intermediates A or B via allene racemization. It places the bulky
palladium complex and the R-substituent of the allene on
opposing faces under DYKAT conditions. Subsequently, a
stepwise nucleophilic addition of intermediate B to an electron-
To date, studies on the mechanism of the TMM cycloadditions
have been inconclusive. In 1999, Singleton et al. obtained
evidence for a concerted mechanism by employing kinetic isotope
experiments.[31] Later, our group proposed a stepwise mechanism
for the TMM cycloaddition when using -trifluoromethyl styrenes
as acceptors.[28f] To further investigate the mechanism in detail,
we examined the reactions of TMM donor 2a with the cis- and
trans-stereoisomers
of
-methyl--nitrostyrene
(4j/4j',
deficient
acceptor
proceeds,
which
is
likely
the
respectively). Both reactions yielded a 2:1 mixture of syn- and
anti-diastereomers of cycloadduct 5j (Scheme 3). Under
otherwise identical conditions, a control experiment without the
TMM donor led to no isomerization of the -methyl--nitro
styrenes (4j/4j'). This result can be rationalized by a fast rotation
around the first newly formed -bond before final ring closure
occurs (Scheme 5, intermediate C) and supports a stepwise
cycloaddition mechanism to be operative.
enantiodetermining step. This process places the R-substituent
on the allene and the acceptor syn to each other and anti to the
electron withdrawing group. Finally, nucleophilic attack in
intermediate C occurs solely at C1, which accounts for the high
regioselectivity of this process.
Scheme 3. Reaction of trans-methyl-nitrostyrene 4j/4j' as evidence for a
stepwise mechanism. [a] anti-5j: 4:1 dr, 80% ee. [b] syn-5j: 10:1 dr, 84% ee.
Furthermore, compelling evidence was found for the involvement
of a DYKAT mechanism (Scheme 4). Thus, a reaction of allene
2d and acceptor 1 led to the formation of 88% of product 3d
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The axial chirality of the cycloadducts was furthermore transferred
to synthetically useful products in a stereoretentive manner. Thus,
the chiral allenes 5b and 5d were efficiently converted to the
spirocycles 12 and 15 by a gold-catalyzed cyclization reaction[35]
(Scheme 7a). The stereochemical assignments of the
cycloadducts 5 (Scheme 3) were verified by 2D-NMR studies of
the spirocycles 12 and 15 (see SI). Finally, a palladium-catalyzed
allylic alkylation of cycloadduct 3c successfully furnished product
14 with very good diastereoselectivity (Scheme 7b).[36]
Scheme 5. Proposed catalytic mechanism.
In addition, we showcase the chemoselective reactivity and utility
of the chiral allenes by rapidly transforming them to a variety of
molecular architectures. Thus, the ruthenium-catalyzed alkene-
allene addition[32] provides access to a synthetically important 1,3-
diene[33] 8 in a fully regioselective manner (Scheme 6a). Notably,
the allene 3c underwent a rhodium-catalyzed C–H activation
reaction[34] and delivered the densely functionalized
diastereomeric products 10a and 10b (Scheme 6b).
Scheme 7. Stereoselective transformations of the TMM cycloadducts.
In summary, recent developments of novel catalytic asymmetric
approaches to chiral allenes have increased the ability to access
these highly valuable chiral building blocks. Despite these
advances, syntheses of chiral allenes from readily available
starting materials using catalytic processes has yet to meet the
demand driven by their expanding applications.
Our methodology allows for the synthesis of structurally complex
chiral allenes from readily available racemic allenes. The reaction
proceeds via a catalyst-controlled generation of a diastereomeric
palladium/nucleophile complex and its addition to activated
achiral electrophiles, which leads to excellent levels of regio-,
diastereo-, and enantiocontrol. This one-step process rapidly
generates molecular complexity in a more atom- and step-
economical fashion. Furthermore, we were able to demonstrate
the involvement of a DYKAT process and a stepwise mechanism
for the cycloaddition event. The densely functionalized chiral
allenes were readily derivatized to a variety of useful molecules.
Scheme 6. a) Regioselective synthesis of 1,3-diene 8 by allene-alkene coupling,
b) rhodium-catalyzed C–H activation with allene 3c.
Acknowledgements
We thank the the DFG Research Fellowship (D.Z.), the Swiss
National Science Foundation (G.M.), and Evelyn Laing McBain
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Graduate Fellowship (A.M.). We acknowledge S. R. Lynch for
two-dimensional nuclear magnetic resonance analysis. We thank
the Stanford Nano Facilities for crystal structure analysis.
Keywords: asymmetric catalysis • chiral allenes • [3+2]
cycloaddition • DYKAT • mechanism
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COMMUNICATION
Barry M. Trost*, Daniel Zell, Christoph
Hohn, Guillaume Mata, Autumn
Maruniak
Page No. – Page No.
Title
[a]
Title(s), Initial(s), Surname(s) of Author(s) including Corres
Author(s)
Department
Institution
Address 1
E-mail:
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