Arene-Free Ruthenium(II/IV)-Catalyzed Bifurcated Arylation for Oxidative C?H/C?H Functionalizations

Article abstract of DOI:10.1002/anie.201909457

Experimental and computational studies provide detailed insight into the selectivity- and reactivity-controlling factors in bifurcated ruthenium-catalyzed direct C?H arylations and dehydrogenative C?H/C?H functionalizations. Thorough investigations revealed the importance of arene-ligand-free complexes for the formation of biscyclometalated intermediates within a ruthenium(II/IV/II) mechanistic manifold.

Full text of DOI:10.1002/anie.201909457

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201909457
German Edition: DOI: 10.1002/ange.201909457
À
C H Activation
Arene-Free Ruthenium(II/IV)-Catalyzed Bifurcated Arylation for
À
À
Oxidative C H/C H Functionalizations
Torben Rogge and Lutz Ackermann*
Abstract: Experimental and computational studies provide
detailed insight into the selectivity- and reactivity-controlling
À
factors in bifurcated ruthenium-catalyzed direct C H aryla-
À
À
tions and dehydrogenative C H/C H functionalizations.
Thorough investigations revealed the importance of arene-
ligand-free complexes for the formation of biscyclometalated
intermediates within a ruthenium(II/IV/II) mechanistic mani-
fold.
À
À
Scheme 1. Bifurcated ruthenium(II/IV)-catalyzed C H/C H arylations.
Bottom left: Structure of key transition state.
À
O
ver the last decade, catalyzed C H activation has gained
considerable momentum and has emerged as a powerful tool
in molecular synthesis[1] with applications in the pharmaceut-
ical industry,[2] materials science,[3] and for the synthesis of
agrochemicals.[4] Among the commonly used transition-metal
catalysts, ruthenium[5] offers an effective, economically
attractive alternative to more costly palladium, platinum, or
iridium catalysts,[6] often proceeding through mechanistically
4) An energetically favorable ruthenium(II/IV/II) catalytic
manifold.
We initiated our studies by probing the effect exerted by
various aryl halides 2 in the carboxylate-assisted C H
À
À
distinct pathways. Thus, protocols for the direct C H
activation of arene 1 (Figure 1). Aryl halide 2e devoid of
a substituent in the ortho-position and 2-fluoro-substituted 2 f
furnished the expected arylated products 3 in excellent yield
and selectivity. In sharp contrast, aryl halides 2h–m, which
bear an electron-withdrawing substituent in the 2-position,
led to bifurcation in terms of a preferential formation of the
arylation under ruthenium catalysis have been developed,[7]
with major contributions by Oi/Inoue,[8] Dixneuf,[9] and our
group,[10] among others.[11] Despite the wealth of viable
canonical, isohypsic arylations, studies on more challenging
À
À
ruthenium-catalyzed dehydrogenative C H/C H activation
À
À
À
for the formation of C C bonds continue to be largely limited
C H/C H activation product 4. Aryl chlorides were found to
be less reactive, while bromides and iodides showed com-
to the use of costly metal salts as terminal oxidants.[12] In
contrast, exploiting the potential of mild aryl halides as
oxidants through a unique manifold has been significantly
hampered by scarce mechanistic insights.[13] Within our
studies on ruthenium-catalyzed C H activation,[14] we
À
obtained key insights into the working mode of bifurcated
À
À
À
C H arylations and oxidative C H/C H activations, on
which we report herein. Notable results of this study comprise
(Scheme 1):
1) Chemoselectivity control through the judicious choice of
stereoelectronic properties.
2) Decoordination of p-cymene for the in situ formation of
an arene-ligand-free active catalyst.
3) Detailed mechanistic insights by DFT calculations,[15]
providing strong support for a turnover-limiting oxidative
addition event.
[*] T. Rogge, Prof. Dr. L. Ackermann
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Gçttingen
Tammannstraß e 2, 37077 Gçttingen (Germany)
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Homepage: http://www.ackermann.chemie.uni-goettingen.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201909457.
À
Figure 1. Bifurcated C H arylations with halides 2. Yields of isolated
products are given. Mes=2,4,6-trimethylphenyl.
15640
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 15640 –15645

Angewandte
Communications
Chemie
Table 1: Influence of the reaction medium.[a]
parable reactivities, with sterically congested 2,6-disubstitu-
tion being less well tolerated.
Variation of the arylpyridine 1 revealed a strong influence
of its substitution pattern on the bifurcated course of the
reaction (Figure 2). Thus, substrates devoid of ortho-substitu-
ents and heteroatom-substituted compounds 1b and 1d
À
exclusively furnished the C H arylated products 3, while
substrates 1a and 1g,h containing alkyl or benzyl groups in
Entry Deviation from the standard conditions
3al [%] 4a [%]
22 51
1
2
3
4
–
À
À
the 2-position underwent the oxidative C H/C H activation
process with high levels of selectivity.
without [Ru]
without MesCO2H
[Ru(O2CMes)2(p-cymene)] (5.0 mol%), without 24
–
–
8[b]
8[b]
57
MesCO2H
5
6
7
dioxane as solvent
MeCN as solvent
NMP as solvent
25
trace
13
40
78
18
8
9
10
DMSO as solvent
DMF as solvent
p-cymene (1.0 equiv) added
–
–
11[b]
22
18[b]
54
11
22
55
(5.0 mol%)
[a] Reaction conditions: 1a (0.50 mmol), 2l (0.75 mmol), [RuCl2(p-
cymene)]2 (2.5 mol%), MesCO2H (30 mol%), PhMe (2.0 mL), 1208C,
20 h. Yields of isolated products are given. [b] Conversion determined by
1H NMR analysis using 4-(MeO2C)2C6H4 as the internal standard.
Figure 2. Influence of the phenylpyridine substitution pattern. Yield of
isolated products are given. [a] Bis-arylated product was obtained.
Ar=4-F-C6H4.
Subsequently, the influence of the reaction medium on the
chemoselectivity was studied. The use of apolar solvents as
the reaction medium resulted in a mixture of products with
comparable performances (Table 1, entries 1–5). Interest-
ingly, when polar, aprotic MeCN was employed as the
solvent, product 4a was obtained as the sole product in high
yield (entry 6), while other polar solvents fell short in
providing comparable efficacies (entries 7–9). Furthermore,
the essential roles of the ruthenium catalyst and the
MesCO2H additive were confirmed (entries 2 and 3), sub-
À
À
Scheme 2. Ruthenium-catalyzed dehydrogenative C H/C H arylation.
moiety efficiently underwent the desired transformation,
delivering 2,2’-binaphthyl 4 f in high yield.[17]
Intrigued by the bifurcation behavior of the ruthenium-
catalyzed C H arylation manifold, we became interested in
unraveling the catalystꢀs mode of action. Kinetic isotope
effect (KIE) studies with the two best performing aryl
bromides revealed KIE values of kH/kD ꢀ 2.2 and 1.8,
respectively (Scheme 3a). A reaction in the presence of
isotopically labeled D2O as the cosolvent led to significant H/
D scrambling in the ortho-position of recovered substrate 1a,
À
stantiating the importance of carboxylate assistance.[16]
A
cyclometalated complex failed to provide improved results
(entry 11).
À
À
The developed C H/C H activation strategy allowed the
rapid construction of densely substituted biaryls in good
yields and excellent levels of regioselectivity (Scheme 2). It is
noteworthy that compound 1 f containing a valuable naphthyl
À
indicating a reversible C H cleavage (Scheme 3b). Careful
analysis of the reaction mixture resulted in the detection of
Angew. Chem. Int. Ed. 2019, 58, 15640 –15645
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15641

Angewandte
Communications
Chemie
Scheme 3. Key mechanistic findings.
74% of PhCF3 as byproduct, confirming aryl bromide 2 as the
terminal oxidant (Scheme 3c).
Furthermore, during the course of the reaction, a substan-
tial amount of p-cymene decoordination from the ruthenium
precatalyst was observed. Hence, after a reaction time of
3 min, 26% of free p-cymene was detected, with this amount
further increasing to more than 70%. These findings are
indicative of an arene-ligand-free complex being catalytically
competent (Figure 3).[18] Similar results were obtained irre-
spective of the nature of the employed substrates 1 and 2
(Figures S4–S6 in the Supporting Information).
Figure 4. Kinetic analysis.
reaction order was found with respect to the catalyst
concentration (Figure 4c).[19]
Motivated by our findings we became interested in further
investigating the reaction mechanism by means of density
functional theory (DFT) calculations at the PBE0-D3(BJ)/
def2-TZVP + SMD(MeCN)//TPSS-D3(BJ)/def2-TZVP level
Detailed kinetic analysis revealed a reaction order of
1 with respect to the phenylpyridine concentration for the C
of theory.[20] C H ruthenation on one out of two coordinated
À
À
À
À
H/C H activation as well as for the C H arylation process
(Figures 4a and S7). No dependence on the aryl halide
concentration was observed (Figure 4b), while a positive
arylpyridine motifs occurs through formation of agostic
complex B, generating ruthenacyclic intermediate
C
À
(Figure 5). Then, C H ruthenation on the second substrate
leads to the generation of the biscyclometalated complex E,
which was calculated to be À0.6 kcalmolÀ1 more stable than
adduct A. In the presence of a nitrile either as solvent or
reagent, ligand exchange can occur on intermediate C, which
stabilizes the complex and reduces the energy barrier for the
À
second C H ruthenation process. Subsequently, a turnover-
limiting oxidative addition of the aryl halide forms
ruthenium(IV) intermediate H, providing support for an
oxidation-induced reductive elimination event.[21] Employing
2-bromobenzonitrile as the oxidant further reduces the
energy of the oxidative addition elementary step by 3.8 kcal
molÀ1 (Figure S10). It is also noteworthy that a ruthenium(II/
0/II) manifold as well as higher spin states were shown to be
unlikely to be operative (Figure S11 and Table S13).
The oxidative addition of the parent bromobenzene 2e
facilitates the reductive elimination between the aryl ligands,
leading to the preferential formation of the arylated com-
pound 3ae. In sharp contrast, the use of 2-bromobenzonitrile
(2l) as the aryl halide led to a distinct change in the
À
À
chemoselectivity, favoring formation of the C H/C H acti-
Figure 3. Detection of free p-cymene.
vated product 4a with DDG° values of 3.9 kcalmolÀ1
TS6–TS7
15642 www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 15640 –15645

Angewandte
Communications
Chemie
Figure 5. Gibbs free energy diagram for the formation of biscyclometalated complex E and oxidative addition at the PBE0-D3(BJ)/def2-
TZVP+SMD(MeCN)//TPSS-D3(BJ)/def2-TZVP level of theory. Ar=o-tolyl.
and À5.7 kcalmolÀ1, respectively, for bromobenzene and 2-
bromobenzonitrile, with the aryl moiety acting as an electron-
withdrawing spectator ligand (Figure 6).[22] Thereafter, proto-
demetalation liberates the desired product and regenerates
the catalytically active complexes A and C, respectively
(Schemes S1–S3).
À
A detailed bond order analysis of the relevant C H
ruthenation steps within a More OꢀFerral–Jencks plot is
suggestive of a base-assisted internal electrophilic substitu-
tion (BIES)[23] rather than a conventional concerted metal-
ation/deprotonation (CMD)[24] C H cleavage event (Fig-
À
ures 7 and S12).[20]
In conclusion, we have reported on detailed insight into
À
À
bifurcated ruthenium-catalyzed oxidative C H/C H activa-
tions. Combined experimental and theoretical mechanistic
À
Figure 7. More O’Ferral–Jencks plot for the C H ruthenation step.
À
studies revealed the bifurcated C H arylation manifold
through an arene-ligand-free ruthenium complex as the
catalytically competent species. In this context, comprehen-
sive DFT computations provided strong support for the
formation of a biscyclometalated complex as well as an
oxidation-induced reductive elimination process within
a ruthenium(II/IV/II) manifold.
Acknowledgements
Generous support by the DFG (SPP1807 and Gottfried-
Wilhelm-Leibniz award to L.A.) is gratefully acknowledged.
Figure 6. Gibbs free energy diagram for the selectivity-determining
À
reductive elimination for the bifurcated C H arylation.
Angew. Chem. Int. Ed. 2019, 58, 15640 –15645
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15643

Angewandte
Communications
Chemie
Ackermann, Chem. Commun. 2016, 52, 13171 –13174; c) L.
Ackermann, A. V. Lygin, Org. Lett. 2011, 13, 3332 –3335; d) L.
Ackermann, R. Vicente, H. K. Potukuchi, V. Pirovano, Org. Lett.
2010, 12, 5032 –5035; e) L. Ackermann, R. Born, P. lvarez-
Bercedo, Angew. Chem. Int. Ed. 2007, 46, 6364 –6367; Angew.
Chem. 2007, 119, 6482 –6485; f) L. Ackermann, A. Althammer,
R. Born, Angew. Chem. Int. Ed. 2006, 45, 2619 –2622; Angew.
Chem. 2006, 118, 2681 –2685; g) L. Ackermann, Org. Lett. 2005,
7, 3123 –3125.
Conflict of interest
The authors declare no conflict of interest.
À
Keywords: C H activation ·density functional calculations ·
oxidative catalysis ·reaction mechanisms ·ruthenium
How to cite: Angew. Chem. Int. Ed. 2019, 58, 15640–15645
Angew. Chem. 2019, 131, 15787–15792
[11] a) Y. Koseki, K. Kitazawa, M. Miyake, T. Kochi, F. Kakiuchi, J.
Org. Chem. 2017, 82, 6503 –6510; b) C. J. Teskey, S. M. A. Sohel,
D. L. Bunting, S. G. Modha, M. F. Greaney, Angew. Chem. Int.
Ed. 2017, 56, 5263 –5266; Angew. Chem. 2017, 129, 5347 –5350;
c) A. Biafora, T. Krause, D. Hackenberger, F. Belitz, L. J.
Gooßen, Angew. Chem. Int. Ed. 2016, 55, 14752 –14755; Angew.
Chem. 2016, 128, 14972 –14975; d) L. Huang, D. J. Weix, Org.
Lett. 2016, 18, 5432 –5435; e) R. K. Chinnagolla, M. Jeganmo-
han, Chem. Commun. 2014, 50, 2442 –2444; f) Y. Aihara, N.
Chatani, Chem. Sci. 2013, 4, 664 –670; g) N. Dastbaravardeh, M.
Schnürch, M. D. Mihovilovic, Org. Lett. 2012, 14, 1930 –1933;
h) F. Kakiuchi, S. Kan, K. Igi, N. Chatani, S. Murai, J. Am. Chem.
Soc. 2003, 125, 1698 –1699.
[12] For a review, see: a) C.-J. Li, Acc. Chem. Res. 2009, 42, 335 –344;
for selected examples, see: b) S. Dana, D. Chowdhury, A.
Mandal, F. A. S. Chipem, M. Baidya, ACS Catal. 2018, 8, 10173 –
10179; c) D. Srimani, Y. Ben-David, D. Milstein, Angew. Chem.
Int. Ed. 2013, 52, 4012 –4015; Angew. Chem. 2013, 125, 4104 –
4107; d) J. Dong, Z. Long, F. Song, N. Wu, Q. Guo, J. Lan, J. You,
Angew. Chem. Int. Ed. 2013, 52, 580 –584; Angew. Chem. 2013,
125, 608 –612; e) X. Guo, G. Deng, C.-J. Li, Adv. Synth. Catal.
2009, 351, 2071 –2074; f) S. Oi, H. Sato, S. Sugawara, Y. Inoue,
Org. Lett. 2008, 10, 1823 –1826; g) G. Deng, L. Zhao, C.-J. Li,
Angew. Chem. Int. Ed. 2008, 47, 6278 –6282; Angew. Chem.
2008, 120, 6374 –6378.
[13] L. Ackermann, P. Novµk, R. Vicente, V. Pirovano, H. K.
Potukuchi, Synthesis 2010, 2245 –2253.
[14] L. Ackermann, Acc. Chem. Res. 2014, 47, 281 –295.
[15] a) D. L. Davies, S. A. Macgregor, C. L. McMullin, Chem. Rev.
2017, 117, 8649 –8709; b) D. Balcells, E. Clot, O. Eisenstein,
Chem. Rev. 2010, 110, 749 –823.
[16] L. Ackermann, Chem. Rev. 2011, 111, 1315 –1345.
[17] CCDC 1942734 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
[1] For recent reviews, see: a) S. M. Khake, N. Chatani, Trends
Chem. 2019, 1, 524 –539; b) P. Gandeepan, T. Müller, D. Zell, G.
Cera, S. Warratz, L. Ackermann, Chem. Rev. 2019, 119, 2192 –
2452; c) P. H. Dixneuf, J. F. Soule, Chem. Rev. 2018, 118, 7532 –
7585; d) P. Gandeepan, L. Ackermann, Chem 2018, 4, 199 –222;
e) C. S. Wang, J. C. K. Chu, T. Rovis, Angew. Chem. Int. Ed.
2018, 57, 62 –101; Angew. Chem. 2018, 130, 64 –105; f) K.
Murakami, S. Yamada, T. Kaneda, K. Itami, Chem. Rev. 2017,
117, 9302 –9332; g) Y. Park, Y. Kim, S. Chang, Chem. Rev. 2017,
117, 9247 –9301; h) J. He, M. Wasa, K. S. L. Chan, Q. Shao, J.-Q.
Yu, Chem. Rev. 2017, 117, 8754 –8786; i) J. F. Hartwig, M. A.
Larsen, ACS Cent. Sci. 2016, 2, 281 –292; j) L. McMurray, F.
OꢀHara, M. J. Gaunt, Chem. Soc. Rev. 2011, 40, 1885 –1898, and
references therein.
[2] a) T. Cernak, K. D. Dykstra, S. Tyagarajan, P. Vachal, S. W.
Krska, Chem. Soc. Rev. 2016, 45, 546 –576; b) M. Seki, Org.
Process Res. Dev. 2016, 20, 867 –877; c) L. Ackermann, Org.
Process Res. Dev. 2015, 19, 260 –269.
[3] a) T. Bura, J. T. Blaskovits, M. Leclerc, J. Am. Chem. Soc. 2016,
138, 10056 –10071; b) D. J. Schipper, K. Fagnou, Chem. Mater.
2011, 23, 1594 –1600.
[4] J. Hubrich, T. Himmler, L. Rodefeld, L. Ackermann, ACS Catal.
2015, 5, 4089 –4093.
[5] For selected reviews, see: a) P. Nareddy, F. Jordan, M. Szostak,
ACS Catal. 2017, 7, 5721 –5745; b) J. A. Leitch, C. G. Frost,
Chem. Soc. Rev. 2017, 46, 7145 –7153; c) C. Bruneau, P. H.
Dixneuf, Top. Organomet. Chem. 2015, 55, 137 –188; d) C.
Bruneau, Top. Organomet. Chem. 2014, 48, 195 –236; e) P. B.
Arockiam, C. Bruneau, P. H. Dixneuf, Chem. Rev. 2012, 112,
5879 –5918; f) N. Chatani, S. Inoue, K. Yokota, H. Tatamidani,
Y. Fukumoto, Pure Appl. Chem. 2010, 82, 1443 –1451; g) L.
Ackermann, R. Vicente, Top. Curr. Chem. 2010, 292, 211 –229;
h) L. Ackermann, R. Vicente, A. R. Kapdi, Angew. Chem. Int.
Ed. 2009, 48, 9792 –9826; Angew. Chem. 2009, 121, 9976 –10011.
[6] On July 1, 2019, the prices of platinum, rhodium, iridium,
palladium, and ruthenium were 837, 3350, 1480, 1549, and
253 US$ per troy oz, respectively. See: http://www.platinum.
matthey.com/prices/price-charts.
[18] a) M. Simonetti, D. M. Cannas, X. Just-Baringo, I. J. Vitorica-
Yrezabal, I. Larrosa, Nat. Chem. 2018, 10, 724 –731; b) J.
McIntyre, I. Mayoral-Soler, P. Salvador, A. Poater, D. J.
Nelson, Catal. Sci. Technol. 2018, 8, 3174 –3182; c) M. Simonetti,
G. J. Perry, X. C. Cambeiro, F. Julia-Hernandez, J. N. Arokiana-
thar, I. Larrosa, J. Am. Chem. Soc. 2016, 138, 3596 –3606; d) P.
MarcØ, A. J. Paterson, M. F. Mahon, C. G. Frost, Catal. Sci.
Technol. 2016, 6, 7068 –7076; e) H. H. Al Mamari, E. Diers, L.
Ackermann, Chem. Eur. J. 2014, 20, 9739 –9743; f) L. Acker-
mann, A. Althammer, R. Born, Tetrahedron 2008, 64, 6115 –
6124; g) L. Ackermann, A. Althammer, R. Born, Synlett 2007,
2833 –2836.
[7] B. Li, P. H. Dixneuf, Chem. Soc. Rev. 2013, 42, 5744 –5767.
[8] a) S. Oi, E. Aizawa, Y. Ogino, Y. Inoue, J. Org. Chem. 2005, 70,
3113 –3119; b) S. Oi, S. Fukita, N. Hirata, N. Watanuki, S.
Miyano, Y. Inoue, Org. Lett. 2001, 3, 2579 –2581.
[9] a) B. Li, C. Darcel, P. H. Dixneuf, ChemCatChem 2014, 6, 127 –
130; b) B. Li, C. B. Bheeter, C. Darcel, P. Dixneuf, Top. Catal.
2014, 57, 833 –842; c) K. S. Singh, P. H. Dixneuf, ChemCatChem
2013, 5, 1313 –1316; d) E. Ferrer Flegeau, C. Bruneau, P. H.
Dixneuf, A. Jutand, J. Am. Chem. Soc. 2011, 133, 10161 –10170;
e) B. Li, C. B. Bheeter, C. Darcel, P. H. Dixneuf, ACS Catal.
2011, 1, 1221 –1224; f) P. B. Arockiam, C. Fischmeister, C.
Bruneau, P. H. Dixneuf, Angew. Chem. Int. Ed. 2010, 49, 6629 –
6632; Angew. Chem. 2010, 122, 6779 –6782; g) I. Özdemir, S.
Demir, B. C¸etinkaya, C. Gourlaouen, F. Maseras, C. Bruneau,
P. H. Dixneuf, J. Am. Chem. Soc. 2008, 130, 1156 –1157.
[10] a) S. R. Yetra, T. Rogge, S. Warratz, J. Struwe, W. Peng, P. Vana,
L. Ackermann, Angew. Chem. Int. Ed. 2019, 58, 7490 –7494;
Angew. Chem. 2019, 131, 7569 –7573; b) R. Mei, C. Zhu, L.
[19] It is noteworthy that a complex kinetic behavior was observed
when 2-bromobenzonitrile was employed as a reagent, indicat-
ing a distinct change in mechanism (Figure S9).
[20] For details, see the Supporting Information.
[21] a) J. Kim, K. Shin, S. Jin, D. Kim, S. Chang, J. Am. Chem. Soc.
2019, 141, 4137 –4146; b) S. Y. Hong, Y. Park, Y. Hwang, Y. B.
Kim, M.-H. Baik, S. Chang, Science 2018, 359, 1016 –1021; c) K.
Shin, Y. Park, M.-H. Baik, S. Chang, Nat. Chem. 2017, 10, 218 –
224.
[22] E. Zuidema, P. W. N. M. van Leeuwen, C. Bo, Organometallics
2005, 24, 3703 –3710.
15644 www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 15640 –15645

Angewandte
Communications
Chemie
[23] a) K. Naksomboon, J. Poater, F. M. Bickelhaupt, M. . Fernµn-
dez-IbµÇez, J. Am. Chem. Soc. 2019, 141, 6719 –6725; b) D. Zell,
M. Bursch, V. Müller, S. Grimme, L. Ackermann, Angew. Chem.
Int. Ed. 2017, 56, 10378 –10382; Angew. Chem. 2017, 129, 10514 –
10518; c) W. Ma, R. Mei, G. Tenti, L. Ackermann, Chem. Eur. J.
2014, 20, 15248 –15251.
[24] S. I. Gorelsky, D. Lapointe, K. Fagnou, J. Am. Chem. Soc. 2008,
130, 10848 –10849.
Manuscript received: July 27, 2019
Revised manuscript received: August 20, 2019
Accepted manuscript online: September 2, 2019
Version of record online: September 20, 2019
Angew. Chem. Int. Ed. 2019, 58, 15640 –15645
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15645