Direct Dearomatization of Pyridines via an Energy-Transfer-Catalyzed Intramolecular [4+2] Cycloaddition

Full text of DOI:10.1016/j.chempr.2019.10.016

Article
Direct Dearomatization of Pyridines via an
Energy-Transfer-Catalyzed Intramolecular
[4+2] Cycloaddition
Jiajia Ma, Felix Strieth-Kalthoff,
Toryn Dalton, ..., Klaus
Bergander, Constantin Daniliuc,
Frank Glorius
glorius@uni-muenster.de
HIGHLIGHTS
Catalytic dearomatization of
pyridines through a [4+2]
cycloaddition
Introduction of a recyclable
polymer-supported photocatalyst
Rapid construction of
isoquinuclidines
Photocatalysis enabled thermally
challenging organic
transformation
An energy-transfer-catalyzed dearomative [4+2] cycloaddition reaction of
pyridines is presented herein. Mechanistically, a ground-state alkene is readily
promoted to the corresponding triplet excited state. The resultant highly
energetic intermediate then undergoes an efficient dearomative cycloaddition to
a pyridine moiety, thus yielding an isoquinuclidine analog. The energy transfer
process is enabled by a recyclable polymer-immobilized, iridium-based
photocatalyst. This work demonstrates the contribution of visible light catalysis
toward enabling thermally challenging organic transformations.
Ma et al., Chem 5, 2854–2864
November 14, 2019 ª 2019 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2019.10.016

Article
Direct Dearomatization of Pyridines
via an Energy-Transfer-Catalyzed
Intramolecular [4+2] Cycloaddition
Jiajia Ma,1 Felix Strieth-Kalthoff,1,2 Toryn Dalton,1,2 Matthias Freitag,1 J. Luca Schwarz,1
Klaus Bergander,1 Constantin Daniliuc,1 and Frank Glorius1,3,
*
SUMMARY
The Bigger Picture
Driven by the demand for more
sustainable chemistry and
The catalytic dearomatization of pyridines, accessing medicinally relevant
N-heterocycles, is of high interest. Currently direct, dearomative strategies
rely generally on reduction or nucleophilic addition, thus limiting the architec-
ture of the dearomatized products to a six-membered ring. We herein introduce
a catalytic, dearomative cycloaddition reaction with pyridines using photoin-
duced energy transfer catalysis, thereby advancing dearomatization methodol-
ogy and increasing the topology of pyridine dearomatization products. This un-
precedented method features high yields, broad substrate scope (44 examples),
excellent functional group tolerance, and facile scalability. Furthermore, a recy-
clable and sustainable polymer immobilized photocatalyst was employed.
Computational and experimental investigations support a mechanism in which
a cinnamyl moiety is promoted to its corresponding excited triplet state
through visible-light-mediated energy transfer catalysis, followed by a regiose-
lective and dearomative [4+2] cycloaddition to pyridines. This work demon-
strates the contribution of visible light catalysis toward enabling thermally chal-
lenging organic transformations.
advances in synthetic
methodologies, visible light
photocatalysis has attracted great
attention from the synthetic
chemistry community over the
past decade. Among the
photochemical activation
processes, the photoinduced
energy transfer, which can
promote the ground state organic
molecule to its triplet excited
state, is relatively
underdeveloped compared to
photoinduced electron transfer.
Differing from the ground state
molecules, the accessible excited
state analog generally show
unique reactivity. As a
INTRODUCTION
Catalytic dearomatization reactions have experienced steady development owing
to their unique potential to convert common aromatics into various functional mol-
ecules in a straightforward manner.1–3 Among the aromatics investigated in this
field, pyridines have attracted considerable attention from the synthetic and
medicinal chemistry communities since dearomatized partially or fully saturated
N-heterocycles exhibit diverse inherent bioactivities or can be used as valuable
feedstock for organic synthesis.4
consequence, investigations of
visible-light-induced energy
transfer catalysis offer an exciting
opportunity for the discovery of
new synthetic transformations
under relatively mild conditions.
Currently disclosed methods on the direct, catalytic dearomatization of pyridines
generally require a sequential protocol involving initial electrophilic activation of
the nitrogen center followed by nucleophilic addition.5–9 Products obtained from
these methods are therefore limited to the architecture of six-membered rings
including dihydropyridine, tetrahydropyridine, and piperidine derivatives (Fig-
ure 1A). Methodologies allowing an increase in the topology of the products in
direct pyridine dearomatization have remained elusive. The rapid generation of
structural complexity from readily available feedstock constitutes one of the fore-
most challenges in organic synthesis. Cycloaddition reactions have been recognized
as a powerful protocol to achieve this goal, facilitating the construction of complex
architectures with corresponding atom and step economy and stereochemical
2854 Chem 5, 2854–2864, November 14, 2019 ª 2019 Elsevier Inc.

A
B
C
Figure 1. The Concept of Pyridine Dearomatization Chemistry
(A) The state of the art of direct catalytic pyridine dearomatization.
(B) The (dehydro)isoquinuclidine core among representative natural products and drugs and as feedstocks for the synthesis of pharmaceuticals.
(C) Our synthetic design of dearomative cycloaddition with pyridine. PS = photosensitizer. EnT = energy transfer.
specificity.10 To this end, merging dearomatizations of pyridines with cycloadditions
is undoubtedly of high interest, which could provide medicinally important N-hetero
bicyclic manifolds, such as isoquinuclidines, in a straightforward manner. The isoqui-
nuclidine (2-azabicyclo[2.2.2]octane) core, identified as a semi-rigid boat form of
piperidine, along with the related dehydrogenated derivatives, comprises the
core of various bioactive natural products and pharmaceuticals as well as feedstock
for synthesizing marketed drugs such as Tamiflu (Figure 1B).11,12
Direct catalytic dearomative cycloadditions of electron-rich aromatics such as naph-
thalenes, pyrroles, indoles, benzothiophenes, (benzo)furans, and others have been
well established; however, the pyridine manifold still presents a formidable chal-
lenge.13 This may be since (1) breaking the aromaticity requires harsh conditions
or an extremely reactive cycloaddend and (2) the potential adducts may undergo
thermally favored rearomatization. In this context, the search for non-thermal strate-
gies might lead to a potential solution. Indeed, under UV-mediated photochemical
conditions, direct dearomative cycloadditions of pyridine derivatives have been
known since the 1970s.14,15 Unfortunately, the multistep synthesis of specific pyri-
dine substrates, inferior yields of the dearomatization products, poor regioselectiv-
ity, and subsequently unpredictable side reactions enabled by UV light render each
of these methods of very limited utility in organic syntheses. The present renaissance
of visible light catalysis offers a new opportunity for dearomative cycloadditions
through visible-light-induced single electron transfer (SET),16–20 triplet-triplet en-
ergy transfer (EnT)21–25 catalysis, or in special cases, direct visible light excitation.26
1Organisch-Chemisches Institut, Westfalische
¨
For example, using a visible-light-excited arenophile, Sarlah and co-workers devel-
Wilhelms-Universitat Munster, Corrensstrasse 40,
¨
¨
Munster 48149, Germany
¨
oped an elegant dearomative cyclization-fragmentation cascade strategy for benze-
noid arenes, which has been incorporated into efficient syntheses of diverse natural
products.19,20 Very recently, the groups of You23 and Glorius24 independently
reported a dearomative cycloaddition strategy for indoles and naphthalenes
through visible-light-induced EnT catalysis. Mechanistically, naphthalene or indole
2These authors contributed equally
3Lead Contact
*Correspondence: glorius@uni-muenster.de
https://doi.org/10.1016/j.chempr.2019.10.016
Chem 5, 2854–2864, November 14, 2019 2855

Figure 2. Synthesis of an Immobilized Iridium Photocatalyst
derivatives, which exhibit triplet energies of 55–60 kcal/mol after accessed by a
visible-light-excited photocatalyst Ir(dF(CF3)ppy)2(dtbbpy)(PF6) ([Ir–F], 60.8 kcal/
mol), are readily promoted to the corresponding excited states through EnT, subse-
quently undergoing highly efficient dearomative cycloadditions. Nevertheless,
despite these advances, pyridine derivatives are not similarly amenable because
of their inert redox activity and high triplet energy (>70 kcal/mol), which render
them inaccessible to both SET and EnT. The [2+2] cycloaddition between unsatu-
rated moieties is thermally forbidden by the Woodward-Hoffmann rules, but is
commonly seen under photochemical conditions relying on the formation of a key
1,2-biradical intermediate.27–30 After consideration of these combined observa-
tions, we envisioned the 1,2-biradical intermediate could be employed to promote
direct pyridine dearomatization via a cycloaddition reaction under visible-light-
mediated conditions (Figure 1C). This proposal was judged to be reasonable and
applicable since (1) the 1,2-biradical cycloaddend is readily accessible from acti-
vated alkenes (48–62 kcal/mol) through visible-light-mediated EnT and displays
extremely high reactivity; (2) the dearomatized products were expected to be stable
under the mild conditions. We hereby report the realization of this concept, thereby
introducing a dearomative [4+2] cycloaddition reaction with pyridines and isoquino-
lines, furnishing numerous functionalized isoquinuclidine derivatives.
RESULTS AND DISCUSSION
Synthesis of an Immobilized Photocatalyst
Visible light catalysis has seen rapid development over the past decade, yet an over-
reliance on precious rare earth transition metal catalysts, such as iridium (Ir) and
ruthenium (Ru) complexes, along with trace presences of transition metal residues
in the target products, has hampered the application of visible light catalysis within
the chemical industry. Employment of organic dyes31 or semiconductors32 may be a
complementary solution, but not a fundamental substitution. A sustainable means
of using transition metal photocatalysts is therefore of high relevance. Inspired
by the development of heterogeneous catalysis,33–35 we, herein, demonstrated a
polymer immobilized, Ir-based photocatalyst of high recyclability and facile separa-
tion from the reaction mixture. The synthetic route is outlined in Figure 2. Accord-
ingly, a bis-cyclometalated iridium dimer 1 was treated with AgPF6 in acetonitrile,36
followed by the introduction of carboxylic acid based 2,20-bipyridine ligand 2. A
homogeneous iridium complex 3 was obtained in 90% yield over two steps (Fig-
ure 2). Oxalyl chloride mediated condensation of the dicarboxylic acid complex 3
with commercially available polymer (aminomethyl)polystyrene, provided the het-
erogeneous photocatalyst [Ir-F]@polymer as orange beads.
Dearomatization Reaction Evaluation
Inspired by the pioneering studies of UV-mediated benzenoid arenes dearomati-
zation, as introduced by Kohmoto and co-workers,37 we started our investi-
gation by examining the reactivity of pyridine-containing cinnamyl amides under
2856 Chem 5, 2854–2864, November 14, 2019

Table 1. Evaluation of the Pyridine Dearomatization Reactiona
Entry
Deviation from Standard Conditions
None
Yield (%)b
1
99 (95)c
2
CH2Cl2 as solvent
THF as solvent
CH3CN as solvent
DMF as solvent
1% H2O added
0.75 mol% catalyst
No catalyst
99
99
99
89
99
99
0
3
4
5
6
7
8
9
In the dark
0
10
Under air
0
aStandard conditions: 4a (0.10 mmol) and [Ir-F]@polymer (1.5 mol%) in acetone (1.0 mL, 0.1 M) were stirred
for 5 h under argon and irradiated with blue LEDs (lmax = 455 nm, latent temperature approx. 25–30ꢀC).
bYield was determined by crude 1H NMR analysis with CH2Br2 as internal standard.
cIsolated yield provided in parentheses. dr of products determined within a range of 1.1:1 to 2.0: 1. Ar =
argon. THF = tetrahydrofuran. DMF = dimethyl formamide.
visible-light-sensitized conditions. In the presence of the heterogeneous photocata-
lyst [Ir-F]@polymer (1.5 mol%), after irradiation for 5 h with blue LEDs (6 W, lmax
=
455 nm) in acetone, an N-cinnamoyl picolinamide 4a was fully converted to the
dearomative [4+2] cycloaddition product 5a in an excellent yield of 99% with 2:1
dr (Table 1, entry 1). Perfect regioselectivity was observed since no [2+2] or [3+2]
cycloaddition products were detected. This reaction displayed a high reaction
conditions tolerance: insensitivity to solvent polarity was observed over diverse sol-
vents such as CH2Cl2 (99% yield, entry 2), tetrahydrofuran (THF) (99% yield, entry 3),
CH3CN (99% yield, entry 4), and dimethyl formamide (DMF) (89% yield, entry 5), and
as well as in the presence of water (99% yield, entry 6). A decreased catalyst loading
of 0.75 mol% did not lead to any decrease of the yield of 5a (99% yield, entry 7). Con-
trol experiments verified that this [4+2] dearomatization reaction relied on the pho-
tocatalyst (entry 8), visible light (entry 9), and oxygen-free conditions (entry 10).
Scope and Limitation
The reaction scope is outlined in Figure 3. A variety of substituted picolinamides
were dearomatized smoothly under the photosensitized conditions. Alkylated
pyridines were converted to the corresponding products (5b and 5c) in excellent
yields of 93%–94% and diastereoselectivities of 1:1 to 1.3:1. Both electron-rich
and electron-deficient substituents were very well tolerated, providing correspond-
ing products (5d and 5e) in good yields (94% versus 86%) and diastereoselectivities
(3.5:1 versus 7:1). A halogen substituent was also compatible, giving 5f in 84%
yield and 6:1 dr. An imide-substituted pyridine was converted to the [4+2] cycload-
dition product (5g) in a good yield of 82% and with good diastereoselectivity of 5:1.
Nicotinamide derivatives also led to pleasing reaction outcomes (products 5h–5k,
52%–93% yield and 1.2:1 to >20:1 dr). Interestingly, by using CH2Cl2 instead of
acetone as solvent, excellent diastereoselectivities (>20:1) were obtained for 5h
Chem 5, 2854–2864, November 14, 2019 2857

Figure 3. Scope and Limitation
Dearomatization products were obtained under standard conditions A unless otherwise noted. For details, see Supplemental Information.
2858 Chem 5, 2854–2864, November 14, 2019

and 5j. A series of polycyclic dearomatized products (5l–5p) were obtained from
isoquinoline derivatives with excellent yields of 88%–97% and in high dr of 8:1
to >20:1. Both a naphthyl and a phenyl also underwent the dearomatization
smoothly to afford products (5q and 5r) in 92% yield for each and 7:1 to >20:1 dr.
Furthermore, substituents adjacent to the double bond displayed high tolerance
with respect to the electron-donating (products 5s–5u) and withdrawing (products
5v–5y) phenyl rings, a furanyl (product 5z), and thienyl substituents (products 6a
and 6b), thus providing the dearomatized products in 67%–97% yield and 1.4:1 to
5:1 dr. In contrast, when an n-propyl in place of the aromatics was used, no target
product (6c) was detected. The reaction shows excellent functional group tolerance,
including a nitrile (6g), a thioether (6h), an alkenyl substituent (6i), an acetal (6j), an
N-Boc-piperidine (6k), a tetrahydropyrane (6l), a cycloheptane (6m), a cyclodode-
cane (6n), and an additional pyridine (6o). Pyridines derived from drugs and natural
products including dopamine (6p), tryptamine (6q), mexiletine (6r), and leelamine
(6s), were all dearomatized efficiently under the visible-light-sensitized conditions.
Following conclusion of the scope exploration, a simple and commercially available
organic sensitizer benzil38 (Alfa Aesar, $19.80/100 g) was identified as being capable
of catalyzing the dearomatization reaction (products 5a, 5g, 6b, 6m, 6o, and 6r) in
similar efficiency. Although in the case of product 6p, the yield was dramatically
decreased to 22%.
To further demonstrate the practicality of the dearomatization protocol, sequential
gram scale reactions were performed using the recyclable photocatalyst [Ir-F]@poly-
mer. As outlined in Figure 4A, [4+2] cycloaddition products 6m, 6n, 6r, and 5a were
produced in 1.08 g (94% yield), 1.20 g (88% yield), 1.25 g (73% yield), and 0.94 g
(76% yield), respectively. Only one portion of photocatalyst [Ir-F]@polymer was em-
ployed for these four sequential gram-scale reactions. Notably, the heterogeneous
photocatalyst was separated from the reaction mixtures by simple filtration and
washing and then reused directly without further purification. The heterogeneous
photocatalyst could be recycled at least ten times for a standard dearomative
[4+2] cycloaddition reaction without a significant drop in the yields of dearomatiza-
tion products (Figure 4B, see Supplemental Information for details). The obtained
pyridine dearomatization products underwent facile reduction to afford diverse
(dehydro)isoquinuclidine derivatives. For example, compound 5a was converted
to dehydroisoquinuclidine
7 as a single diastereomer after treatment with
NaBH(OAc)3, followed by Ts-protection (Figure 4C). Notably, the minor isomer of
5a underwent a slower reduction than the major one and could thus be separated
by column chromatography. Starting from 5e, under Pd-catalyzed hydrogenation
condition, isoquinuclidine 8 was afforded in 96% yield and in a completely dia-
stereoselective fashion. A highly functionalized pyrrolidine-containing isoquinucli-
dine 9 was obtained in 97% yield after further treatment with LiAlH4. The polycyclic
structure of the major isomer of dehydroisoquinuclidine 11 was confirmed by X-ray
crystallographic analysis (Figure 4D).
Mechanistic Investigation
To investigate mechanistic aspects of the dearomative [4+2] cycloaddition, we
performed a variety of mechanistic experiments. Firstly, the kinetic profile of a
standard reaction mixture shows that no detectable amounts of [2+2] or [3+2] cyclo-
addition products are formed at any time, whereas the E to Z isomerization of sub-
strate 4a is detected39 and both isomers are ultimately converted to 5a (Figure 5A).
Secondly, Stern–Volmer analysis reveals that the luminescence emission of homoge-
neous photocatalyst [Ir-F] is quenched efficiently by 4a, whereas no quenching
was observed with compound 12 devoid of an olefin moiety (Figure 5B). The
Chem 5, 2854–2864, November 14, 2019 2859

A
C
B
D
Figure 4. Investigation and Application of the Developed Catalytic System
(A) Sequential gram scale reactions with catalyst recycling.
(B) Evaluation of recycled catalyst performance.
(C) Synthetic transformations to (dehydro)isoquinuclidines.
(D) Structural confirmation by X-ray analysis (see also Figure S3). For detailed reaction conditions, see the Supplemental Information.
Stern–Volmer constant (KSV) of 4a is determined to be 0.525 mMꢁ1. Thirdly, Z-4a
could be isolated and again subjected to the standard reaction conditions to afford
product 5a with the same outcome in comparison to the use of E-4a (Figure 5E).
These observations indicate both E and Z-4a were converted to the common 1,2-bir-
adical intermediate 34a via triplet EnT. Fourthly, intermediate 34a could be trapped
by adding 2,3-dimethylbuta-1,3-diene (10 equiv.) or styrene (10 equiv.) to a standard
reaction mixture, thus affording the corresponding [2+2] cycloaddition products 13
or 14 in 60% and 4:1 dr or 25% yield and 2:1 dr, respectively (Figure 5F). Finally,
further evidence for triplet EnT was gained by comparing different photocatalysts.
As a result, yields of the dearomatization product 5a are correlated to the intrinsic
2860 Chem 5, 2854–2864, November 14, 2019

B
A
C
D
F
E
Ph
Ph
Figure 5. Mechanistic Investigations
(A) Kinetic profile of photoinduced dearomatization reaction using E-4a as substrate with [Ir-F]@polymer.
(B) Stern-Volmer plots of homogeneous catalyst Ir(dF(CF3)ppy)2(dtbbpy)(PF6) using E-4a and 12 as quenchers.
(C) Measurement of initial rates of photoinduced dearomatization reactions under different conditions.
(D) Comparison of different photocatalysts.
(E) Stereochemical information loss experiments.
(F) Biradical trapping through photoinduced [2+2] cycloadditions. For more details, see Figures S1 and S2. PC = photocatalyst. EnT = energy transfer.
Conc. = concentration. Eq. = equivalent.
triplet energy of Ir-based (I, II, and III) or Ru-based (IV) photocatalysts, while unre-
lated to their redox properties (Figure 5D).
To rationalize the observed regio- and diastereoselectivity, DFT calculations were
conducted and the result is partially outlined in Figure 6 (see Supplemental
Information for details).40 Accordingly, the triplet energy transfer from photoexcited
[Ir-F]@polymer (ET = ꢂ60.8 kcal/mol) to substrate E-4ad (calculated as ET = 45.8 kcal/
mol), as well as its rotamer E-4ad0 (calculated as ET = 46.5 kcal/mol), is
Chem 5, 2854–2864, November 14, 2019 2861

Figure 6. Computational Analysis
Computational method: (U)CAM-B3LYP / def2-TZVPP / CPCM (MeCN). For more details, see Figures S4–S9. TS = transition state. INT = intermediate.
thermodynamically feasible. From the resulting excited state, relaxation to the
ground state would lead to a statistical mixture of Z- and E-configured starting
material 4ad or 4ad0, which is in excellent agreement with the observed E4Z isom-
erization under the photosensitized conditions (Figure 5A). Notably, 34ad is the ma-
3
jor intermediate in comparison to its rotamer 4ad0 because of its higher stability
(DG = ꢁ1.2 kcal/mol) and lower triplet excitation energy (DET = ꢁ0.7 kcal/mol).
3
3
Taking 4ad as an example (for computations concerning 4ad0, see Supplemental
Information), within this 1,2-biradical intermediate, the electrophilic a-carbonyl
radical undergoes 5-exo-trig cyclization (DG+ = +11.2 kcal/mol) to the pyridine
moiety, giving rise to a 1,6-biradical intermediate 3INT1 (DG = ꢁ8.3 kcal/mol).
Subsequent thermoneutral intersystem crossing (ISC) into the open-shell 1INT1
(DG = ꢁ7.9 kcal/mol) enables radical-radical recombination to occur and afford
the [4+2] cycloaddition product 6d. The selectivity of [4+2] over [2+2] cycloaddition
can be rationalized by the strong thermodynamical instability of the [2+2]-product
(DG = 45.9 kcal/mol) and further supported by analysis of the spin population of
intermediate 1INT1, in which the spin density is preferentially located on para-car-
bon atom (cs = 0.38) over the ortho-carbon atoms (cs = 0.30).
In summary, we hereby disclose a direct dearomative [4+2] cycloaddition reaction of
pyridines. This protocol allows rapid access to a variety of highly functionalized
isoquinuclidine analogs in high yields, excellent functional group tolerance and
facile scalability. A polymer immobilized, Ir-based photocatalyst was introduced
and could be recycled by simple filtration and washing, highlighting the sustainabil-
ity of this method. Mechanistic experiments, kinetic analysis and DFT calculations
support the occurrence of a visible-light-mediated energy transfer event. Overall,
efficient production of a highly reactive 1,2-biradical cycloaddend under mild
photosensitized conditions and the sufficient stability of the corresponding adduct
under these mild conditions are key to the success of this work.
EXPERIMENTAL PROCEDURES
A dried 5 mL Schlenk tube was charged with the substrate 4, the heterogeneous
catalyst [Ir-F]@polymer (1.5 mol %) or the organic sensitizer benzil (20 mol %), and
2862 Chem 5, 2854–2864, November 14, 2019

acetone or CH2Cl2 (0.1 M). The reaction mixture was degassed via freeze-pump-
thaw for two cycles. After the mixture was thoroughly degassed and filled with
argon, the Schlenk tube was sealed tightly and stirred under irradiation with 6 or
30 W blue LEDs for the indicated time (monitored by TLC). The organic solution
was concentrated under reduced pressure. The crude material was purified by flash
chromatography on silica gel (n-pentane/EtOAc) to afford the [4+2] cycloaddition
products.
DATA AND CODE AVAILABILITY
The crystal structure data of compound 11 (CCDC 1913256) has been deposited in
the Cambridge Structural Database.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2019.10.016.
ACKNOWLEDGMENTS
We are grateful for generous financial support from the Deutsche Forschungsge-
meinschaft (Leibniz Award, SPP2102 [F.S.-K.] and SFB858 [T.D.]) and the Alfried
Krupp von Bohlen and Halbach Foundation (J.L.S.). We thank Michael Holtkamp,
Dr. Huan-Ming Huang, Dr. Michael J. James, Dr. Jun Li, Dr. Zackaria Nairoukh, Dr.
Michael Teders, and Dr. Jian-Heng Ye (all WWU) for helpful discussions and exper-
imental contributions.
AUTHOR CONTRIBUTIONS
J.M., T.D., J.L.S., and F.G. designed, performed, and analyzed experiments. F.S.-K.
performed the DFT calculations. M.F. synthesized the ligand 2. K.B. performed and
analyzed the 2D NMR. C.D. performed the X-ray analysis. J.M., F.S.-K., T.D., and
F.G. prepared the manuscript with contributions from all authors.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: July 24, 2019
Revised: October 3, 2019
Accepted: October 18, 2019
Published: November 14, 2019
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