Modulation of the coordination geometries of NCN and NCNC Rh complexes for ambidextrous chiral catalysts

Article abstract of DOI:10.1039/c9cc06520b

A chirality switch between novel NCN pincer Rh complexes and a related double cyclometalated NCNC Rh complex containing secondary amino groups is described. Their catalytic abilities were determined in asymmetric alkynylation of ethyl trifluoropyruvate, a

Full text of DOI:10.1039/c9cc06520b

Volume 55 Number 85 4 November 2019 Pages 12725–12882
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Jun-ichi Ito, Hisao Nishiyama et al.
Modulation of the coordination geometries of NCN and
NCNC Rh complexes for ambidextrous chiral catalysts

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Modulation of the coordination geometries of
NCN and NCNC Rh complexes for ambidextrous
chiral catalysts†
Cite this: Chem. Commun., 2019,
55, 12765
Received 22nd August 2019,
Accepted 18th September 2019
Jun-ichi Ito, *a Takahiro Ishihara,b Takaki Fukuoka,b Siti Rokiah Binti Mat Napi,b
c
Hajime Kameo
and Hisao Nishiyama*b
DOI: 10.1039/c9cc06520b
rsc.li/chemcomm
A chirality switch between novel NCN pincer Rh complexes and ligands containing two N–H bonds can form the anti and syn
a related double cyclometalated NCNC Rh complex containing conformational isomers (Scheme 1a).9 These isomers should
secondary amino groups is described. Their catalytic abilities were possess different chiral recognition abilities. Therefore, struc-
determined in asymmetric alkynylation of ethyl trifluoropyruvate, tural interconversion between isomers might provide chiral
and the change in the coordination geometry of the Rh catalysts switching functionality in asymmetric catalysis. However,
affected the stereochemistry of the products.
bis(amino) ligands generally adopt one of the two conformations
according to their structure (i.e., acyclic or macrocyclic).10,11 To
Artificial systems inspired by natural substances, such as induce the switching function, an additional coordinating group
allosteric proteins and metalloenzymes, have been studied to that governs interconversion and immobilization must be intro-
help fabricate switchable catalysts triggered by external stimuli, duced. The present study examined metal–carbon covalent bonds
such as irradiation, pH changes, oxidation, and the addition of as a trigger for structural changes (Scheme 1b). Cyclometalation
other substances.1 Structural changes in the active site of a results in an additional metal–carbon bond, which fixes the
catalyst are critical to the switching function of the active and coordination geometry.12 Therefore, the formation and cleavage
inactive forms.2 Switchable catalysts are also an attractive tool of a metal–carbon bond were hypothesized to alter the coordina-
for chirality control in asymmetric reactions.3,4 This strategy tion pattern upon interconversion of the anti and syn conforma-
enables the synthesis of both enantiomers through structural tions of the N–H bonds.
changes in catalysts containing a single chiral source. Recently,
Thus, the present study focused on an NCN pincer ligand
Feringa and co-workers demonstrated a photo- and thermo- with secondary amino tethers as the flexible coordination sites
responsive organocatalyst,5 and Canary and co-workers described in the rigid pincer scaffold.13 Construction of a double cyclo-
chiral switching in a redox responsive system involving a Cu atom.6 metalated NCNC complex was also achieved by C–H bond activation
In those systems, a structural change involving a hydrogen-bonding of the NCN complex. The catalytic activities of the Rh complexes
group, such as an N–H bond, was important for substrate were evaluated in asymmetric alkynylation of a ketoester with an
recognition.
alkyne; the structures of the complexes significantly affected the
Transition-metal catalysts containing an N–H bond adjacent absolute configurations of the products.
to a metal center are potential synergistic systems due to
Ligand precursors 1a–c were prepared by substitution of
hydrogen bonding.7,8 In systems with chiral ligands, multiple 1,5-bis(chloromethyl)-2,4-dimethylbenzene with the corresponding
N–H bonds confer geometric diversity. For example, bis(amino)
a Department of Molecular and Macromolecular Chemistry, Graduate School of
Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
E-mail: jito@chembio.nagoya-u.ac.jp
b Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
E-mail: hnishi@chembio.nagoya-u.ac.jp
c Department of Chemistry, Graduate School of Science, Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
† Electronic supplementary information (ESI) available: Experimental and crys-
Scheme 1 (a) Bisamino ligand systems with anti and syn conformations.
(b) Interconversion of the anti and syn conformations by cyclometalation.
tallographic details. CCDC 1507453–1507456. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c9cc06520b
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Scheme 3 Preparation of NCNC-Rh complex 4.
Scheme 2 Preparation of NCN-Rh complexes.
activation mediated by the acetate ligands.18,19 Heating 3a with
NaHCO3 in benzene resulted in the formation of double
cyclometalated complex 4 in 65% yield (Scheme 3). The phenyl
carbons of 4 attached to Rh at the pendant and center positions
appeared at 128.7 (1JRhC = 21.9 Hz) and 151.9 ppm (1JRhC = 37.6 Hz),
respectively, in the 13C NMR spectrum. These types of double
cyclometalated complexes are quite rare.20
The molecular structure of 4 was dinuclear with a bridging
H2O ligand (Fig. 2). Each Rh fragment possessed an octahedral
geometry, with NCNC tetradentate coordination. A significant
feature was the syn arrangement of the N–H bonds. Accord-
ingly, the configurations of the N1 and N2 atoms were R and S,
respectively. Thus, the stereochemistry at the N1 atom was
inverted during cyclometalation. This result clearly indicates
that NCN and NCNC complexes provided complementary
reaction fields stemming from the anti- and syn-substitution
patterns of the N–H bonds.
After establishing the geometric control of the ancillary
ligands, the catalytic activities of the NCN and NCNC ligated
Rh complexes were evaluated in the asymmetric alkynylation of
ethyl trifluoropyruvate 6 with phenylacetylene 5a as a model
reaction (Table 1).21 When NCN-Rh complex 3a was used as a
catalyst, (R)-propargyl alcohol (R)-7a was obtained in 72% yield
with 63% ee (entry 1). Complex 3b containing bulky substituents
increased the enantioselectivity, while 3c was inactive (entries 2
and 3). In contrast, 4 resulted in the inversion of enantioselec-
tivity, affording the S-enantiomer (S)-7a in 81% yield with 36% ee
(entry 4). The enantioselectivity improved upon a decrease in
temperature (entry 5). These results confirm the concept of the
chiral amines.14 The desired NCN-Rh complexes 2a–c were prepared
by cyclometalation of 1a–c with RhCl3Á3H2O in diisopropylamine
1
with heating (Scheme 2). The H NMR spectra of 2a–c indicated
C2-symmetric geometry in solution. An N–H signal was observed at d
4.75–5.10 ppm. In the 13C NMR spectra, the Rh–C signal appeared
as a doublet at d 145.7–147.2 ppm (1JRhC = 30.8 Hz). Further reaction
of 2a–c with silver acetate or silver pivalate gave the corresponding
carboxylate complexes 3a–c in 75–90% yields. The N–H signals of
1
3a–c appeared at d 7.64–7.78 ppm in the H NMR spectra, which
were shifted downfield compared to those of 2a–c due to hydrogen
bonding with the acetate ligands.
X-ray analysis revealed that 2a and 3a possessed C2-symmetry with
anti-arrangement of the N–H moieties (Fig. 1). The sterically hindered
phenyl groups were oriented outside toward the Rh center. The
absolute configuration of the N atoms attached to the Rh center
was determined to be S. In 3a, the N1–O3 and N2–O4 distances
(2.75 and 2.77 Å, respectively) as well as 1H NMR data suggested the
presence of hydrogen bonding between the acetate ligands and N–H
groups.15 Another structural feature was the square pyramidal geo-
metry of Rh(III), in which the C1 atom of the phenyl fragment was
located in the apical position.16 This is in marked contrast to other
NCN pincer Rh complexes, in which Rh centers adopt an octahedral
geometry upon H2O ligation.17 The DFT calculations indicated that
the unusual geometry of 2a was due to the unexpectedly high LUMO
level and high steric bulkiness of the nitrogen substituents.14
The aromatic rings at the flexible tethers of the amino
groups were thought to undergo intramolecular C–H bond
Fig. 1 ORTEP diagrams of (a) 2a and (b) 3a with 50% probability. Selected
bond lengths (Å) of 2a: Rh1–C1 1.909(8), Rh1–N1 2.076(4), Rh1–Cl1 2.3117(10),
3a: Rh1–C1 1.904(3), Rh1–N1 2.085(3), Rh1–N2 2.072(3), Rh1–O1 2.037(2),
Rh1–O3 2.030(2).
Fig. 2 ORTEP diagram of 4 with 50% probability. Selected bond lengths
(Å): Rh1–C1 1.919(3), Rh1–C22 1.985(3), Rh1–N1 2.138(3), Rh1–N2 2.104(3),
Rh1–O1 2.234(2), Rh1–O3 2.2807(6).
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Table 1 Asymmetric alkynylation of 6 with 5aa
Entry
Catalyst
Temp.
Yield (%)
eeb (%)
1
2
3
4
5
6c
3a
3b
3c
4
4
4
30
30
30
30
0
72
75
3
81
57
63
63 (R)
69 (R)
40 (R)
36 (S)
66 (S)
80 (R)
Scheme 4 (a) Reaction of 4 with 5a. (b) Catalytic reaction of 5a with 6 by
using 8 as a catalyst.
0
a
Reaction conditions: 5a (0.3 mmol), 6 (0.2 mmol), Rh catalyst
b
c
To obtain additional information about the catalytic reac-
tion, the reactivity of 4 with alkyne 5a was examined. The
reaction proceeded smoothly at room temperature within 1 h
to give NCN-Rh complex 8 in 61% yield (Scheme 4a). The 13C
NMR spectrum of 8 contained signals corresponding to the
(3 mol%), Et2O (2 mL), 30 1C, 24 h. Determined by HPLC. Pretreat-
ment of 4 with 5a at room temperature for 1 h before addition of 6.
catalyst design, as 3a and 4 gave 7a with opposite absolute
configurations.
alkynyl ligand attached to the Rh center at d 98.9 (1JRhC
58.1 Hz) and 105.2 (2JRhC = 11.4 Hz) ppm. The signal for the ipso
carbon in the NCN ligand was observed at 154.6 ppm (1JRhC
=
The experimental procedure also affected the chirality of
the products. When 4 was treated with PhCCH at room tem-
perature for 1 h prior to addition of 6, the R-enantiomer (R)-7a
was obtained with 80% ee (Table 1, entry 6). The stereochemistry
of the product was comparable to that obtained with NCN Rh
complex 3a. Overall, this method provides a very simple way to
control product chirality starting from 4.
The chiral switching functions of 3a and 4 in asymmetric
alkynylation with different alkynes are illustrated in Table 2.
The reaction of alkynes 5b and 5c in the presence of 3a gave the
corresponding alcohols in 85 and 79% ee, respectively (entries 1
and 3). In contrast, use of 4 provided the corresponding
products with the inversion of stereochemistry in 46 and 79%
ee (entries 2 and 4, respectively). 9-Ethynylphenanthrene 5d
showed a similar tendency (entries 5 and 6). The switching
function of 4 triggered upon addition of an alkyne was also
examined (entries 7–10). Catalysts generated by pretreating 4
with alkynes 5e and 5f gave the corresponding products in
66 and 75% ee, respectively. However, the use of 4 in the reaction
afforded enantiomers in 69 and 57% ee, respectively.
=
35.3 Hz). However, the signal of another ipso carbon observed
at 128.7 ppm in 4 disappeared. Thus, 8 was identified as an
alkynyl complex with NCN tridentate coordination. The catalytic
reaction between 5a and 6 using 8 as a catalyst similarly afforded
R-enantiomer 7a with 60% ee (Scheme 4b). This result implies
that the NCN-Rh complexes act as catalysts for the formation of
R-enantiomers.
A reaction pathway is proposed in Scheme 5. Based on the
reactivity of the Rh acetate complex with alkynes,21,22 an alkynyl
intermediate, A1, is generated upon C–H activation of PhCCH
(5a) with 4. Subsequent insertion of a ketone into the Rh–C
bond of the alkynyl ligand gives (R)-7a with a R-stereocenter.
The NCNC-Rh system is thought to undergo the same
mechanism via B1, giving (S)-7a as a major enantiomer. In this
case, the NCNC tetradentate coordination could be maintained
during the catalytic cycle. Although the details of the enantiomeric
discrimination and turnover-limiting step are unknown,23 the anti
and syn configurations of the two N–H bonds determined by NCN
and NCNC ligand systems could control product chirality. The
NCNC-Rh complex 4 also served as a catalyst for the R-product
when an alkyne was treated before addition of a ketone. B1 is
relatively unstable in the presence of a proton source, such as
acetic acid, which is generated by C–H bond activation of an
Table 2 Asymmetric alkynylation of 6 with 5a
Entry
Alkyne (Ar)
Catalyst
Yield (%)
eeb (%)
1
4-CF3C6H4 (5b)
4-CF3C6H4 (5b)
4-BrC6H4 (5c)
3a
4
3a
4
3a
4
4
4
4
4
58
22
72
8
67
31
54
50
64
23
85 (À)
46 (+)
79 (À)
79 (+)
68 (À)
21 (+)
66 (À)
69 (+)
75 (À)
57 (+)
2c
3
4c
4-BrC6H4 (5c)
5
9-Phenanthrene (5d)
9-Phenanthrene (5d)
4-CH3C6H4 (5e)
4-CH3C6H4 (5e)
4-CH3OC6H4 (5f)
4-CH3OC6H4 (5f)
6c
7d,e
8f
9d,e
10d
a
Reaction conditions: 5 (0.3 mmol), 6 (0.2 mmol), Rh catalyst (3 mol%),
b
Et2O (2 mL), 30 1C, 24 h. Determined by HPLC and polarimetry.
c
d
e
À20 1C, 48 h. 0 1C, 24 h. Pretreatment of 4 with 5 at room
f
Scheme 5 Proposed reaction pathway for catalytic reaction.
Chem. Commun., 2019, 55, 12765--12768 | 12767
temperature for 12 h before addition of 6. 0 1C, 45 h.
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7 (a) R. Noyori and T. Ohkuma, Angew. Chem., Int. Ed., 2001, 40, 40;
(b) Transition Metals for Organic Synthesis, ed. M. Beller and C. Bolm,
Wiley-VCH, Weinheim, Germany, 2nd edn, 2004; (c) W. Tang and
X. Zhang, Chem. Rev., 2003, 103, 3029; (d) T. Ikariya and J. Blacker,
Acc. Chem. Res., 2007, 40, 1300; (e) T. Ikariya, Top. Organomet. Chem.,
2011, 37, 31; ( f ) S. E. Clapham, A. Hadzovic and R. H. Morris, Coord.
Chem. Rev., 2004, 248, 2201.
8 N. V. Belkova, L. M. Epstein, O. A. Filippov and E. S. Shubina, Chem.
Rev., 2016, 116, 8545.
9 C. Gemel, K. Folting and K. G. Caulton, Inorg. Chem., 2000, 39, 1593.
10 (a) K. Hiraki, J. Tsutsumida and Y. Fuchita, Chem. Lett., 1986, 337;
(b) K. Hiraki, Y. Fuchita, Y. Ohta, J. Tsutsumida and K. I. Hardcastle,
J. Chem. Soc., Dalton Trans., 1992, 833; (c) X. Ribas, D. A. Jacson,
alkyne. Consequently, active species switched from B1 to A1 upon
protonation of the Rh–C bond in the NCNC ligand.
In conclusion, new NCN pincer Rh complexes containing
secondary amino groups as tethered coordinating groups were
prepared. The molecular structures of NCN pincer complexes 2
and 3 included five-coordinated square planar geometries
with the anti configuration of the N–H bonds and the S
absolute configuration at the nitrogen atoms. Acetate complex
3 underwent intermolecular C–H bond activation to give double
cyclometalated complex 4, which contained unique NCNC
tetradentate coordination with the syn configuration of the
N–H bonds. In asymmetric alkynylation of terminal alkynes
with b-ketoesters, the NCN and NCNC Rh complexes acted as
chiral switchable catalysts for the formation of both enantio-
mers. This switching system based on the NCN and NCNC Rh
complexes is not yet fully reversible and requires different
reactions such as heating reaction and addition of an alkyne.
This research was supported by a Grant-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
(No. 26410114 and 15H03808).
´
B. Donnadieu, J. Mahıa, T. Parella, R. Xifra, B. Hedman,
K. O. Hodgson, A. Llobet and T. D. P. Stack, Angew. Chem., Int.
Ed., 2002, 41, 2991; (d) R. Xifra, X. Ribas, A. Llobet, A. Poater,
`
M. Duran, M. Sola, T. D. P. Stack, J. Benet-Buchholz, B. Donnadieu,
´
J. Mahıa and T. Parella, Chem. – Eur. J., 2005, 11, 5146; (e) A. Casitas,
`
M. Canta, M. Sola, M. Costas and X. Ribas, J. Am. Chem. Soc., 2011,
´
133, 19386; ( f ) X. Ribas, C. Calle, A. Poater, A. Casitas, L. Gomez,
R. Xifra, T. Parella, J. Benet-Buchholz, A. Schweiger, G. Mitrikas,
`
M. Sola, A. Llobet and T. D. P. Stack, J. Am. Chem. Soc., 2010,
132, 12299.
11 (a) T. Arai, T. Moribatake and H. Masu, Chem. – Eur. J., 2015,
21, 10671; (b) T. Arai, I. Oka, T. Morihata, A. Awata and H. Masu,
Chem. – Eur. J., 2013, 19, 1554.
12 M. Albrecht, Chem. Rev., 2010, 110, 576.
13 (a) M. Albrecht and G. van Koten, Angew. Chem., Int. Ed., 2001,
40, 3750; (b) M. E. van der Boom and D. Milstein, Chem. Rev., 2003,
103, 1759; (c) J. Choi, A. H. R. MacArthur, M. Brookhart and
A. S. Goldman, Chem. Rev., 2011, 111, 1761; (d) N. Selander and
K. J. Szabo, Chem. Rev., 2011, 111, 2048.
Conflicts of interest
There are no conflicts to declare.
14 See the ESI†.
15 The charge-transfer (CT) stabilization energy was analysed by the
second-order perturbation with the natural bond orbitals (NBOs),
and the presence of significant LP(O)-s*(N–H) CT interactions (22.2
kcal molÀ1) was confirmed.
Notes and references
1 (a) M. Barboiu and J.-M. Lehn, Proc. Natl. Acad. Sci. U. S. A., 2002,
99, 5201; (b) M. Marchivie, P. Guionneau, J. A. K. Howard,
16 (a) M. Gerisch, J. R. Krumper, R. G. Bergman and T. D. Tilley, J. Am.
Chem. Soc., 2001, 123, 5818; (b) A. A. H. van der Zeijden, G. van Koten,
J. M. Ernsting, C. J. Elsevier, B. Krijnen and C. H. Stam, J. Chem. Soc.,
Dalton Trans., 1989, 317; (c) W. J. Hoogervorst, K. Goubitz, J. Fraanje,
M. Lutz, A. L. Spek, J. M. Ernsting and C. J. Elsevier, Organometallics,
2004, 23, 4550; (d) B. Rybtchinski, A. Vigalok, Y. Ben-David and
D. Milstein, J. Am. Chem. Soc., 1996, 118, 12406.
17 (a) A. A. H. van der Zeijden, G. van Koten, R. Luijk, K. Vrieze, C. Slob,
H. Krabbendam and A. L. Spek, Inorg. Chem., 1998, 27, 1014;
(b) Y. Motoyama, M. Okano, H. Narusawa, N. Makihara, K. Aoki
and H. Nishiyama, Organometallics, 2001, 20, 1580.
´
G. Chastanet, J.-F. Letard, A. E. Goeta and D. Chasseau, J. Am. Chem.
Soc., 2002, 124, 194; (c) L. R. Holloway, H. H. McGarraugh,
M. C. Young, W. Sontising, G. J. O. Beran and R. J. Hooley, Chem.
Sci., 2016, 7, 4423; (d) M. Su¨ßner and H. Plenio, Angew. Chem., Int.
Ed., 2005, 44, 6885; (e) G.-H. Ouyang, Y.-M. He, Y. Li, J.-F. Xiang and
Q.-H. Fan, Angew. Chem., Int. Ed., 2015, 54, 4334; ( f ) M. Galli,
J. E. M. Lewis and S. M. Goldup, Angew. Chem., Int. Ed., 2015,
54, 13545; (g) S. Semwal and J. Choudhury, ACS Catal., 2016, 6, 2424.
2 (a) U. Lu¨ning, Angew. Chem., Int. Ed., 2012, 51, 8163; (b) E. R. Kay,
D. A. Leigh and F. Zerbetto, Angew. Chem., Int. Ed., 2007, 46, 72;
(c) V. Blanco, D. A. Leigh and V. Marcos, Chem. Soc. Rev., 2015, 44, 5341.
18 (a) L. Ackermann, Chem. Rev., 2011, 111, 1315; (b) D. H. Ess,
R. J. Nielsen, W. A. Goddard III and R. A. Periana, J. Am. Chem.
Soc., 2009, 131, 11686.
´
3 (a) Y. H. Kim, Acc. Chem. Res., 2001, 34, 955; (b) M. Bartok, Chem.
Rev., 2010, 110, 1663; (c) J. Escorihuela, M. I. Burguete and S. V. Luis,
Chem. Soc. Rev., 2013, 42, 5595.
19 (a) J. Ito and H. Nishiyama, Eur. J. Inorg. Chem., 2007, 1114; (b) J. Ito,
T. Kaneda and H. Nishiyama, Organometallics, 2012, 31, 4442.
20 P. Scherl, H. Wadepohl and L. H. Gade, Organometallics, 2013,
32, 4409.
21 (a) T. Ohshima, T. Kawabata, Y. Takeuchi, T. Kakinuma, T. Iwasaki,
T. Yonezawa, H. Murakami, H. Nishiyama and K. Mashima, Angew.
Chem., Int. Ed., 2011, 50, 6296; (b) T. Wang, J.-L. Niu, S.-L. Liu,
J.-J. Huang, J.-F. Gong and M.-P. Song, Adv. Synth. Catal., 2013,
355, 927; (c) K. Morisaki, M. Sawa, R. Yonesaki, H. Morimoto,
K. Mashima and T. Ohshima, J. Am. Chem. Soc., 2016, 138, 6194.
22 J. Ito, M. Kitase and N. Nishiyama, Organometallics, 2007, 26, 6412.
23 Mechanistic studies of catalytic alkynylation of a-ketiminoesters
suggested that coordination of alkynes could be involved as the
turnover-limiting step.21c
4 (a) Y. Nagata, T. Kuroda, K. Takagia and M. Suginome, Chem. Sci.,
2014, 5, 4953; (b) M. Nagamoto, D. Yamauchi and T. Nishimura,
Chem. Commun., 2016, 52, 5876; (c) R. Doran, M. P. Carroll, R. Akula,
B. F. Hogan, M. Martins, S. Fanning and P. J. Guiry, Chem. – Eur. J.,
2014, 20, 15354; (d) T. Inagaki, A. Ito, J. Ito and H. Nishiyama, Angew.
Chem., Int. Ed., 2010, 49, 9384; (e) Y. Sohtome, S. Tanaka, K. Takada,
T. Yamaguchi and K. Nagasawa, Angew. Chem., Int. Ed., 2010,
¨
49, 9254; ( f ) A. Frolander and C. Moberg, Org. Lett., 2007, 9, 1371;
(g) H. Y. Kim, H.-J. Shih, W. E. Knabe and K. Oh, Angew. Chem., Int.
Ed., 2009, 48, 7420.
5 (a) J. Wang and B. L. Feringa, Science, 2011, 331, 1429; (b) D. Zhao,
T. M. Neubauer and B. L. Feringa, Nat. Commun., 2015, 6, 6652.
6 S. Mortezaei, N. R. Catarineu and J. W. Canary, J. Am. Chem. Soc.,
2012, 134, 8054.
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