Contra-Thermodynamic, Photocatalytic E→Z Isomerization of Styrenyl Boron Species: Vectors to Facilitate Exploration of Two-Dimensional Chemical Space

Article abstract of DOI:10.1002/anie.201800286

Designing strategies to access stereodefined olefinic organoboron species is an important synthetic challenge. Despite significant advances, there is a striking paucity of routes to Z-α-substituted styrenyl organoborons. Herein, this strategic imbalance is redressed by exploiting the polarity of the C(sp²)?B bond to activate the neighboring π system, thus enabling a mild, traceless photocatalytic isomerization of readily accessible E-α-substituted styrenyl BPins to generate the corresponding Z-isomers with high fidelity. Preliminary validation of this contra-thermodynamic E→Z isomerization is demonstrated in a series of stereoretentive transformations to generate Z-configured trisubstituted alkenes, as well as in a concise synthesis of the anti-tumor agent Combretastatin A4.

Full text of DOI:10.1002/anie.201800286

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Accepted Article
Title: Contra-Thermodynamic, Photocatalytic E→Z Isomerization of
Styrenyl Boron Species: Vectors to Facilitate Exploration of 2D
Chemical Space
Authors: John Molloy, Jan Metternich, Allan Watson, and Ryan Gilmour
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800286
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Contra-Thermodynamic, Photocatalytic E→Z Isomerization of Styrenyl
Boron Species: Vectors to Facilitate Exploration of 2D Chemical Space
John J. Molloy,§[a] Jan B. Metternich,§†[b] Constantin G. Daniliuc,‡[b] Allan J. B. Watson,*[a,c] Ryan Gilmour*[b]
Dedicated to Prof. Dr. Jack D. Dunitz FRS on the occasion of his 95th birthday
Abstract: The high efficiency and stereospecificity of cross-coupling
technologies utilizing olefinic organoboron nucleophiles has been a key
development in the interrogation of 2D and, following stereoselective
elaboration, 3D chemical space. Accordingly, designing strategies to
access stereodefined olefinic organoboron species is an important synthetic
challenge. Despite significant advances, there is a striking paucity of routes
to Z-α-substituted styrenyl organoborons. Herein, we redress this strategic
imbalance by exploiting the polarity of the C(sp2)-B bond to activate the
halides/pseudo-halides.[5] This conservation of stereochemical
information allows the outcome of the reaction to be predicted a priori
such that the alkenyl species can be considered as a masked vector to
explore the desired 2D space. Naturally, this feature has the caveat that
the chemical space exploration is directly linked to the geometry of the
starting material and the ease with which it can be prepared: Advances
in the hydro- and cupro-boration of alkynes continues to provide
expedient access to trans[6] and trans-α-substituted[7] styrenyl boron
species, respectively (Scheme 1a). This directly identifies cis-α-
substituted styrenyl systems as a strategic limitation. Whilst elegant
strategies do exist to prepare unsubstituted styrenes,[8] installation of
the -substituent can be laborious (Scheme 1b).[9] Consequently, a
method that would allow access to the cis isomer from the more readily
accessible trans isomer would address this limitation, simplify access,
neighboring π-system: This enables
a mild, traceless photocatalytic
isomerization of readily accessible E-α-substituted styrenyl BPins to
generate the corresponding Z-isomers with high fidelity. Preliminary
validation of this contra-thermodynamic E→Z isomerization is
demonstrated via a series of stereoretentive transformations to generate Z-
configured tri-substituted alkenes, as well as in a concise synthesis of the
anti-tumor agent Combretastatin A-4.
and permit divergence from
a common and easily accessed
intermediate. Herein, we disclose an operationally simple,
photocatalytic isomerization of trans-α-substituted styrenyl boronic
acid, pinacol esters (BPins) to the corresponding cis isomers with high
fidelity (Scheme 1c). The synthetic utility of the method is
demonstrated in a range of subsequent transformations to generate Z-
configured alkenes, as well as in a concise synthesis of anti-tumor
agent combretastatin A4.[10]
The C(sp2)-B bond is an inimitable vector for the exploration of
chemical space.[1] This prominence is particularly evident in the
pharmaceutical industry, where organoboron species retain a privileged
position in drug discovery. The strategic importance of this bond is a
consequence of stability, simplicity of installation, and versatility in
subsequent transformations.[1,2] Of the multitude of transition-based
platforms that embrace organoboron species for cross-coupling, the
Suzuki-Miyaura reaction may be credited with having had the greatest
impact on reshaping the pharmaceutical landscape. Indeed, over 40%
of all C-C bond forming reactions in medicinal chemistry can be
attributed to this process.[3] The unique blend of reproducibility,
scalability and structural tolerance has inspired numerous laboratories
to rigorously investigate the mechanistic intricacies of this remarkable
transformation.[1,2] The last decade has witnessed key breakthroughs:[4]
In particular, seminal investigations by Denmark and coworkers have
uncovered the elusive pre-transmetalation intermediate, which is a key
event in the oxo-palladium pathway. [4d,4e]
From a molecular design perspective, a notable advantage of this
venerable transformation is the associated stereospecificity observed
when using substituted alkenyl organoboron species or alkenyl
[a]
[b]
M. Sc. J. J. Molloy, Dr. A. J. B. Watson
WESTCHEM, Department of Pure and Applied Chemistry,
University of Strathclyde, 295 Cathedral Street Glasgow, UK.
Dr. J. B. Metternich, Dr. C. G. Daniliuc, Prof. Dr. R. Gilmour
Organisch Chemisches Institut, Westfälische Wilhelms-Universität
Münster, Corrensstraße 40, 48149 Münster, Germany.
E-mail: ryan.gilmour@uni-muenster.de
Scheme 1. Formation of styrenyl BPins. a) Cuproboration to form trans-α-
substituted styrenyl BPins; b) Synthesis of cis-α-styrenyl BPins; c) Proposed
photocatalytic E→Z isomerization of trans-α-substituted styrenyl BPins (BPins
= boronic acid, pinacol esters).
Homepage: http://www.uni-muenster.de/Chemie.oc/gilmour/
Dr. A. J. B. Watson
EASTCHEM School of Chemistry, University of St Andrews
North Haugh, St Andrews, Fife, UK.
[c]
E-mail: aw260@st-andrews.ac.uk
§
†
These authors contributed equally.
Spatio-temporal control of pre-existing alkene geometry remains
a multifaceted challenge in synthesis.[11] Consequently, the contra-
thermodynamic E→Z isomerization of alkenes continues to lack
Current address: Department of Chemistry and Chemical Biology
Harvard University, 12 Oxford Street Cambridge, MA 02138, USA.
X-ray Crystallographer.
‡
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generality. This is noteworthy given the prominence of vinyl C(sp2)-X
precursors in stereospecific cross-coupling chemistry. Fortunately,
constraints such as microscopic reversibility can be circumvented by
photochemical activation. Seminal studies by Hammond, Arai, and
others[12] provide a platform for selective geometric isomerization by
photosensitization.[13] In recent years, this concept has translated to
more challenging substrate classes. Pertinent examples include the
E→Z isomerization of allylic alcohols and amines by Weaver,[14] and
the application of (–)-riboflavin to invoke E→Z isomerization of
polarized olefins.[15] In this latter scenario, a conjugated π-system can
be activated by the excited state of the photocatalyst via an energy
transfer manifold (ET). Subsequent rotation of the central bond in the
transient intermediate can give rise to both the trans and cis isomers.
However, developing allylic strain in the cis form deconjugates the π-
systems and thus reactivation is inefficient. This process thereby
induces an intrinsic directionality in favor of the contra-
thermodynamic process (E→Z, Scheme 1c). Since, trigonal planar
boron molecules contain an empty Lewis acidic p-orbital, thereby
increasing electron deficiency, we reasoned that a photocatalytic
isomerization via an energy transfer enabled by a suitable sensitizer
would constitute a simple and elegant solution this problem. Based on
this working hypothesis, we sought to explore several photocatalysts in
a benchmark transformation. To that end, the geometric isomerization
of E-1 to Z-1 was explored (Table 1).
Weaver[14] proved to be highly effective for this process. Indeed, with a
significantly reduced catalyst loading (1 mol%) and a shorter reaction
time of 16 h (entries 6-8). Performing the reaction under an inert
atmosphere of argon not only increased reaction efficiency, but it
implicates a triplet energy transfer manifold (ET) as being operational
(Entry 9).[16] Additional support for this proposal derived from an
experiment performed under an oxygen atmosphere in which the
selectivity was notably diminished (entry 10, Z:E 39:61).
Having established a set of general conditions to achieve the
efficient E→Z isomerization of styrenyl BPins, scope and limitations
were established (Table 2).
Table 2. Establishing Substrate Scope.[a]
Table 1. Optimization of the isomerization E-1 → Z-1.
Entry
Catalyst
Catalyst
loading
(mol%)
Reaction
Time
Irradiation
wavelength
(nm)
Z/E
ratio[a]
(h)
1
2
-
-
24
24
24
24
24
24
24
16
16
16
450
400
365
400
365
450
450
450
450
450
12:88
75:25
73:27
48:52
75:25
90:10
86:14
79:21
94:6[b]
39:61[c]
(–)-riboflavin
Anthracene
Fluorenone
Benzophenone
Ir(ppy)3
5
5
5
5
5
1
1
1
1
3
4
5
6
7
Ir(ppy)3
[a]
Reactions were performed in MeCN on a 0.1 mmol scale at ambient
temperature under an argon atmosphere. Z:E ratio was determined by 1H
NMR spectroscopy.
8
Ir(ppy)3
[b]
Anthracene (5 mol%) and UV-light (365 nm) were
9
Ir(ppy)3
required for isomerization. N.B. The isomerized product 12 is E-configured,
reflecting the higher IUPAC priority of F than C. CCDC-1815285 contains the
supplementary crystallographic data for E-16. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
10
Ir(ppy)3
[a]
Reactions were performed in MeCN on a 0.1 mmol scale at ambient
temperature. Z:E selectivity was determined by 1H NMR spectroscopy.
Reaction was performed under an argon atmosphere.[c] Reaction was
performed under an oxygen atmosphere.
[b]
Pleasingly, the protocol was transferable to a range of substrates
including extended α-substituents such as ethyl and propyl units, Z-2
and Z-3 respectively. The importance of the α-substituent is reflected
in Z-4, where a reduction in selectivity is observed. Moreover, varied
functionalities on each position of the aryl ring were well tolerated,
Whilst common organic sensitizers provided proof of principle (entries
2-5 cf entry 1) at 5 mol% catalyst loadings, unsatisfactory levels of
selectivity were observed after 24 h irradiation (up to Z:E 75:25).
Consequently, the triplet sensitizer Ir(ppy)3 that had been reported by
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including electron deficient halides (Z-5, Z-6, Z-9, and Z-11) and
electron rich motifs (Z-7 and Z-8) with all of the aforementioned
examples proceeding with high levels of selectivity favouring the Z-
isomer. Interestingly, a reduction in selectivity was observed with
ortho-substituents: This may be attributable to the deconjugation of the
starting material (A1,3-strain) that compromises the efficiency of the
initial excitation of the trans styenyl BPin (Z-9 and Z-10).[15] Electron
Miyaura coupling in medicinal chemistry, Z-1 was processed to the
cis-stilbene 17 80% yield as a single geometric isomer. The reaction
with methyl acrylate under oxidative Heck conditions also proceeded
smoothly to generate the retinal mimic 18 in 80% yield. The cis vector
was also utilised in the elegant homologation developed by Aggarwal
and co-workers21 to generate the Z-configured allylic BPin 19. Ligand
exchange on boron also proved facile to deliver the corresponding
deficient α-substituents proved to be more challenging (Z-12 and Z-13), BF3K species 20.22 Reversing the nucleophilic character of the cis
although the bis-BPin species Z-13 does provide a platform for
subsequent bidirectional coupling.[17] A particular strength identified
during this investigation was the tolerance to variation in the
organoboron residue itself (Z-14, Z-15, and Z-16). Cognisant of the
fact that boron protecting group orthogonality is essential to ensure
chemoselectivity in subsequent transformations, the BDAN[18] (Z-15)
and BMIDA[19] (Z-16) systems were also subjected to our general
conditions. Recent application of these motifs in the iterative synthesis
of important polyketides further exemplifies the synthetic value of
these building blocks.[18,19]
vector Z-1 to an electrophilic species was also enabled through
iodination of the boron residue (21).23 This simple polarity reversal
enabled a union of Z-1 and 21 to generate the Z,Z-diene 22 in 88%
yield. Gratifyingly, it was possible to isolate crystals that were suitable
for X-ray analysis (Scheme 2, lower left, CCDC 1815286). This
valuable structural insight clearly demonstrates the importance of 1,3-
allylic strain in deconjugating the aryl ring from the π-system. This
destabilising, non-covalent interaction is a critical feature in placing the
high Z-selectivities observed in this transformation on a structural basis.
Finally, encouraged by the efficient conversion of Z-1 to the cis-
stilbene 17 via Suzuki-Miyaura coupling, the synthesis of the anti-
tumour natural product combretastatin A-4 was realized (Scheme 3,
83% yield).[8,24] Fragment Z-23 not only provides a direct access to the
natural product in a single step, but also allows for the rapid generation
of libraries and complements the existing metathesis.[8]
Finally, to provide preliminary validation of the synthetic value of
this protocol in accessing new areas of 2D space, vector Z-1 was
exposed to a selection of stereospecific transformations (Scheme 2).
Scheme 3. Concise synthesis of Combretastatin A-4 utilising fragment Z-
23.
In summary, we have developed an efficient and operationally
simple method for the preparation of cis-α-substituted styrenyl BPins.
A straightforward photocatalytic isomerization of the readily available
trans-isomer allows direct access to the significantly more challenging
cis-isomer with high stereoselectivity. As valuable fragments for the
exploration of 2D space, and for the subsequent elaboration into 3D
space, this method now allows the facile investigation of vectors that
were previously troublesome to interrogate. A series of representative,
stereospecific transformations constitute preliminary validation of this
strategy in the synthesis of Z-configured trisubstituted olefins, and
underscore the value of vectors such as Z-1 in molecular design.
Scheme 2. Stereospecific transformations employing vector Z-1: (a) Ethyl 4-
bromobenzoate (1.2 eq.), Pd(OAc)2 (5 mol%), SPhos (10 mol%), K3PO4 (3.0
eq.), H2O (5.0 eq.), 1,4-dioxane, 80 C; (b) methyl acrylate (6.0 eq.), Pd(OAc)2
(5 mol%), O2, DMA, 50 C; c) (3-Phenylpropyl) diisopropylcarbamate (1 eq.),
(+)-sparteine (1.2 eq.), sBuLi (1.1 eq.), MgBr2OEt2 (2.0 eq.), Et2O, -78 C; d)
Aq. KHF2 (4.5 M, 5 eq.), MeOH, rt; e) Aq. NaOH (3 M, 3.0 eq.), iodine (2.0 eq.),
THF, rt; f) 21 (1 eq.), Pd(OAc)2 (5 mol%), SPhos (10 mol%), K3PO4 (3.0 eq.),
H2O (5.0 eq.), 1,4-dioxane, 80 C. CCDC-1815286 contains the
supplementary crystallographic data for 22. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
We acknowledge financial support from the WWU Münster,
Deutsche Forschungsgemeinschaft (Excellence Cluster EXC
1003) and Fonds der Chemischen Industrie (Fellowship to JBM).
JJM was supported by a PEER/PECRE award by WestChem
and the Scottish Funding Council.
Keywords: alkenes • catalysis • conformation • geometric
isomerization • medicinal chemistry
Significant quantities of Z-1 were required and so the
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Entry for the Table of Contents
John J. Molloy, Jan B. Metternich,
Constantin G. Daniliuc, Allan J. B.
Watson,* and Ryan Gilmour*
Page No. – Page No.
Contra-Thermodynamic,
Photocatalytic E→Z Isomerization of
Styrenyl Boron Species: Vectors to
Facilitate Exploration of 2D Chemical
Space
Text for Table of Contents
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