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H-Bond-Directing Organocatalyst for Enantioselective [4+2] Cycloadditions via Dienamine Catalysis§
Shoulei Wang,a Carles Rodriguez-Escricha and Miquel A. Pericàsa,b*
a

Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
b
Departament de Química Orgànica, Universitat de Barcelona, 08080, Barcelona (Spain)
Supporting Information Placeholder
H-bond-directing
dienamine catalysis

cat.

HO

R2

O

Ph
N N

H

R2

O

+

R1

N
H

OH

(10 mol %)

17 examples
up to 89% yield
13:1 dr, 99% ee

R3

R1

N
N
Ph

R3
O
CHO

ABSTRACT: An efficient, highly regio- and stereoselective [4+2] cycloaddition reaction to generate tetrahydropyranopyrazole frameworks has been developed. To this end, a dienamine-based catalytic strategy that relies on the H-bonddirecting effect of the hydroxy group of a dinaphthylprolinol-type aminocatalyst has been used. This enables the synthesis
of multifunctionalized heterocyclic derivatives with three contiguous stereocenters in good yields and excellent enantioselectivities.

Optically active pyrazolones and their derivatives are widely occurring scaffolds that can be found in many natural products
and medicinally active molecules.1 As a consequence, the catalytic asymmetric synthesis of pyrazolones has been the
subject of several studies,2 mainly involving three different strategies: (a) the deprotonation of the relatively acidic α-protons
to generate a transient nucleophile;3 (b) the exploitation of the inherent nucleophilicity of the pyrazole nitrogens;4 and (c) the
use of exocyclic alkylidene pyrazolones as Michael acceptors,5 which can lead to cascade processes for the construction
of spirocycles.
R
Me
N
N
O
O
H
anti-inflammatory

N

Me

O
CN

N
Ph

N

HN
O

antiplatelet

O

N
NH2
O
antibacterial

Figure 1. Some biologically active pyranopyrazol(on)es.

Tetrahydropyranopyrazoles6 (THPPs) and dihydropyrano-pirazolones7 (DHPPOs) are formal derivatives of pyrazolones in
which this heterocyclic moiety is fused to a pyran or pyranone ring, respectively. Several members of this subclass present
interesting pharmaceutical and biological properties, which make them appealing synthetic targets (Figure 1). Nevertheless,
the catalytic enantioselective transformation of pyrazolone derivatives into functionalized THPPs has received scant attention. In 2012, Enders disclosed an aminocatalytic 3-component reaction that provides THPPs with two stereocenters.8 One
year later, Wang and co-workers reported the bifunctional thiourea-catalyzed asymmetric [4+2] cycloaddition of α,βunsaturated γ-butyrolactams with exocyclic alkylidene pyrazolones (Scheme 1, top).9 As for the related DHPPOs, the Ye
and Biju groups independently reported NHC-catalyzed enantioselective annulations for the synthesis of these scaffolds.10
However, the catalytic enantioselective construction of THPP scaffolds bearing three contiguous stereocenters is still a
challenging endeavour.

Scheme 1. [4+2] Cycloaddition with alkylidene pyrazolones.
O
NBoc

N N

Boc
N

N

O

N
H-Bond-directing
(bifunctional thiourea) Ph

Ph

O

Jiang, Wang et al.
2013

R2

R2

O

R1

R2

R3

R1

O

H-Bond-directing Ph
(dienamine catalysis)

R1

N
N

R3

this work

O
CHO

Cycloadditions play a pivotal role in the construction of diverse heterocyclic skeletons.11 Of special interest is the application of aminocatalytic [4+2] processes that involve dienamine intermediates. These can play a dual role as dienes or dienophiles12 and their nucleophilicity allows inverse-electron-demand hetero-Diels-Alder (IEDHDA) reactions to be carried out
when they are combined with Michael acceptors. In 2012, Jørgensen et al. pioneered the H-bond-directing dienamine
strategy in an excellent report showing a distal regio- and stereoselective IEDHDA reaction.13 More recently, the Chen
group developed a [4+2] cycloaddition reaction of 1-aza-1,3-butadienes with α,β-unsaturated aldehydes by employing
dienamine catalysis.14 Inspired by previous works on H-bond-directing dienamine-mediated strategies,13,15 we report herein
an IEDHDA reaction between enals and alkylidene pyrazolones. This dienamine-mediated process gives rise to THPPs with
three contiguous stereocenters in excellent stereoselectivities. We began to study this reaction with pyrrolidine-derived
aminocatalysts, given their success in dienamine-mediated processes.16 Alkylidene pyrazolone 2a and α,β-unsaturated
aldehyde 3a were chosen as model substrates and a range of solvents were screened at room temperature (Table 1).
Table 1. Optimization of reaction conditions. a

N N

H

+

O
Ph

cat. 1 (10 mol %)
solvent, rt

Ph

Ar
N
H
1a

HO

Ar
N
N
R
H
H
1g
1b: R = OTMS, Ar = C6H5
1c: R = OTMS, Ar = 3,5-(CF3)2C6H3
1d: R = OH, Ar = C6H5
1e: R = OH, Ar = 3,5-(CF3)2C6H3
1f: R = OH, Ar = 2-naphthyl

COOH

Ar

O

Ar
OH

N
H
1h: Ar = C6H5
1i: Ar = 2-naphthyl

N
H

O

N
H

N
H

1j

solvent

1a

t

yield
b

drc

ee
(%)d

70

1:1

11

1b

CH2Cl2

36

78

4:1

24

1c

CH2Cl2

48

76

4:1

43

1d

CH2Cl2

48

72

6:1

67

1e

CH2Cl2

60

61

6:1

66

1f

CH2Cl2

72

63

6:1

74

7

1g

CH2Cl2

6

77

4:1

20

8

1h

CH2Cl2

48

80

7:1

78

9

1i

CH2Cl2

48

76

7:1

81

10

1j

CH2Cl2

48

65

4:1

–38

11

1i

DCE

48

73

7:1

83

12

1i

CHCl3

48

72

7:1

80

13

1i

MeCN

48

75

7:1

80

14

1i

Et2O

48

69

6:1

68

15

1i

toluene

48

80

8:1

90

16

1i

mesitylene

48

77

8:1

89

17

CF3

12

6

CF3

CH2Cl2

5

N N
N N
H

(%)

4

O

(h)

3

Ph

4a O

3a

cat.

1

Ph

N
N

Ph
2a

entry

2

O

Ph

1i

DMF

48

>10

nd

nd

a

Reactions performed on a 0.1 mmol scale (see Supporting Information).
c
1
d
Isolated yield. Determined by H NMR spectroscopy. Determined by
chiral HPLC after reduction to the corresponding alcohol.
b

2

We first examined the [4+2] cycloaddition reaction catalyzed by proline, which gave the desired product in excellent regioselectivity and yield, but with poor dr and ee (Table 1, entry 1). When α,α-diarylprolinol silyl ethers were employed, both dr
and ee increased slightly (entries 2 and 3). Subsequently, screening of various α,α-diarylprolinols, gave a remarkable increase in stereoselectivity (entries 4-6). We tentatively attribute this to the H-bonding ability of the free hydroxy group. Notably, we found that tetrazole 1g could enhance the reactivity dramatically (6 h) but without any improvement in ee (entry
7). Pleasingly, compared to 1d, catalyst 1h (bearing an extra hydroxy group in C417) allowed to improve the yield with much
higher ee and dr (entry 8). Based on this, we firstly prepared the more hindered 4-hydroxydinaphthylprolinol catalyst 1i,
which proved beneficial in terms of ee (entry 9). Pyrrolidine-squaramide catalyst 1j failed to improve the catalytic activity
and stereoselectivity (entry 10). Solvent screening (entries 11-17) indicated that toluene was the best option: in these conditions, using catalyst 1i, the THPP 4a could be isolated in 80% yield, 90% ee and 8:1 dr (entry 15). With the optimized
conditions, the substrate scope of the asymmetric cycloaddition reaction was undertaken and a series of chiral THPPs
were synthesized (Scheme 2).
In general, alkylidene pyrazolones bearing different electron-withdrawing and electron-donating substituents were well
tolerated. Specifically, all of the para-substituted substrates (4b-4h) were obtained in good yields (75-89%), high diastereoselectivities (8:1-10:1 dr) and excellent enantioselectivities (90-99% ee). In contrast to these, the meta-substituted 4i and
4j gave a little decrease in the diastereo- and enantioselectivity, while maintaining the high yields. Importantly, the system
also admitted introduction of double substitution on the ortho-ortho´ position, providing 4k in excellent diastereoselectivity
(13:1 dr), comparable enantioselectivity (87% ee) and slightly lower yield. Furthermore, 4l bearing a bulkier 2-naphthyl
group was formed in moderate diastereoselectivity (4:1 dr) but with good yield (73%) and excellent enantioselectivity (93%
ee).
Scheme 2. Scope of alkylidene pyrazolones a
Ph

O
N N

O

H

+
R1

cat. 1i (10 mol %)
toluene, rt

Ph

R1

N
N

Ph
O

Ph
2

4

3a
Br

Cl

O
F

4a
48 h, 80% yield
8:1 dr, 90% ee
Me

4b
72 h, 78% yield
9:1 dr, 95% ee
OMe

4c
72 h, 75% yield
10:1 dr, 93% ee
CF3

4d
72 h, 81% yield
9:1 dr, 90% ee

4e
48 h, 77% yield
8:1 dr, 93% ee

4f
72 h, 82% yield
8:1 dr, 94% ee
Me

4g
48 h, 85% yield
10:1 dr, 93% ee

4h
48 h, 89% yield
10:1 dr, 99% ee

OMe

CN

Me MeO
OMe

4i b
48 h, 73% yield
5:1 dr, 87% ee
a

4j
48 h, 81% yield
6:1 dr, 87% ee

4k
72 h, 56% yield
13:1 dr, 87% ee

4l
72 h, 73% yield
4:1 dr, 93% ee
b

Reactions performed on a 0.15 mmol scale (see the Supporting Information). Performed at 0 ºC.

3

R2
Ph

O
N N
H cat. 1i (10 mol %)

R2 +

O

Ph

Ph

N
N

R3
O

toluene, rt
Ph

R3
2

R2
R3

4

3
R2

= Me

=

R3

Me

R2

= Me

=

OMe
4n OMe
72 h, 74% yield
8:1 dr, 84% ee

4m
48 h, 72% yield
8:1 dr, 86% ee

=

= Me

=

CF3

4o
18 h, 82% yield
8:1 dr, 86% ee

R2 = Me
R3

R3

O

R2 = n-Pr
R3

F

=
4q
72 h, 88% yield
7:1 dr, 90% ee

4p
48 h, 83% yield
8:1 dr, 88% ee

Scheme 3. Scope of various enal reaction partners. a
a

Reactions performed on a 0.15 mmol scale (see Supporting Information).

Next, we set our sights on the performance of various α,β-unsaturated aldehydes. As presented in Scheme 3, the [4+2]
cycloaddition reactions proceeded smoothly when enals with diverse electronic properties were tested. The corresponding
chiral tetrahydropyranopyrazoles 4m-4p were obtained with high enantioselectivities (84-88%), good diastereoselectivities
(8:1 dr) and good yields ranging from 72-83%. Gratifyingly, an alkylidene pyrazolone bearing a linear aliphatic substituent
afforded product 4q in 88% yield and high stereoselectivity (7:1 dr, 90% ee). As for alkyl-substituted enals, we have only
tested crotonaldehyde, which gave good yields but very poor stereoselectivities.
The absolute configuration of tosylate 4b’’ (prepared from 4b’ as shown in Scheme 4) was unambiguously determined by
X-ray single-crystal analysis.18 The rest of the cycloadducts have been assigned by analogy to 4b.
Scheme 4. Synthesis and X-ray structure of 4b’’.
Br

Br

Ph

Ph

Et3N N
N
CH2Cl2

N
N
Ph

TsCl
DMAP

O

Ph

4b'

OH

O

4b''

OTs

The model we propose to account for the formation of the major product is depicted in Scheme 5. The regioselectivity of
the attack can be easily inferred from the structure of the final product and it also matches with the expected course of
action given the polarization of the dienamine and the alkylidene pyrazolone. Regarding the stereoselectivity, whereas the
pyrazolone geometry is fixed, the dienamine can adopt several conformations. However, as demonstrated by Jørgensen
and co-workers on the basis of DFT calculations,16a the (E,s-trans,E)-dienamine is the most stable conformer. Thus, out of
the four possible approaches (see Supporting Information for details) only the one shown in Scheme 5 leads to the major
stereoisomer. The preference for such an exo approach is justified in terms of (a) H-bonding between the hydroxy group of
the diarylprolinol and the carbonyl of the pyrazolone and (b) the repulsion between the aromatic group in the pyrazolone
(slightly tilted out-of-plane) and the dienamine, which would take place in an alternative endo approach.
Scheme 5. Model proposed to explain the regio - and stereoselectivity.
H-bond

N

Nap

O H

Nap
H

N

O
H

OH

N

R2
H
R1 R3
H

R2

R1

N

N
Ph

O

R3
CHO

exo approach

4

In summary, we have disclosed the first asymmetric [4+2] cycloaddition between alkylidene pyrazolones and enals through
H-bond-directing dienamine catalysis. The reaction, promoted by a bifunctional 4-hydroxydinaphthylprolinol species, proceeds smoothly with excellent regioselectivity and high stereoselectivity (up to 89% yield, 13:1 dr, and 99% ee). Thus, we
have disclosed a new and efficient protocol for the synthesis of enantioenriched, multifunctionalized tetrahydropyranopyrazole derivatives containing three contiguous stereocenters. Further application of this method and biological evaluation of
the THPPs is currently underway.

ASSOCIATED CONTENT
Supporting Information
Experimental procedures, characterization data and copies of NMR spectra and HPLC chromatograms (PDF).
CIF file for 4b’’ (CIF).

AUTHOR INFORMATION
Corresponding Author
* E-mail: mapericas@iciq.es

Notes
The authors declare no competing financial interest.

ACKNOWLEDGMENT
Financial support from the Institute of Chemical Research of Catalonia (ICIQ) Foundation, MINECO (grant CTQ2012–38594-C02–
01 and Severo Ochoa Excellence Accreditation 2014-2018: SEV-2013-0319) and DEC Generalitat de Catalunya (Grant
2014SGR827) is acknowledged. The CELLEX Foundation is also gratefully acknowledged for financing the High-Throughput
Experimentation (HTE) laboratory.

REFERENCES
§

Dedicated to Prof. Karl Anker Jørgensen on his 60th anniversary.

(1) (a) Fustero, S.; Sánchez-Roselló, M.; Barrio, P.; Simón-Fuentes , A., Chem. Rev. 2011, 111, 6984-7034; (b) Sujatha, K.; Shanthi, G.;
Selvam, N. P.; Manoharan, S.; Perumal, P. T.; Rajendran, M., Bioorg. Med. Chem. Lett. 2009, 19, 4501-4503; (c) Bondock, S.; Rabie,
R.; Etman, H. A.; Fadda, A. A., Eur. J. Med. Chem. 2008, 43, 2122-2129; (d) Kimata, A.; Nakagawa, H.; Ohyama, R.; Fukuuchi, T.;
Ohta, S.; Suzuki, T.; Miyata, N., J. Med. Chem. 2007, 50, 5053-5056.
(2) For a review, see: Chauhan, P.; Mahajan, S.; Enders, D., Chem. Commun. 2015, 51, 12890-12907.
(3) (a) Gogoi, S.; Zhao, C.-G., Tetrahedron Lett. 2009, 50, 2252-2255; (b) Yang, Z.; Wang, Z.; Bai, S.; Liu, X.; Lin, L.; Feng, X., Org. Lett.
2011, 13, 596-599; (c) Liao, Y.-H.; Chen, W.-B.; Wu, Z.-J.; Du, X.-L.; Cun, L.-F.; Zhang, X.-M.; Yuan, W.-C., Adv. Synth. Catal. 2010,
352, 827-832; (d) Wang, Z.; Yang, Z.; Chen, D.; Liu, X.; Lin, L.; Feng, X., Angew. Chem. Int. Ed. 2011, 50, 4928-4932; (e) Alba, A.-N. R.;
Zea, A.; Valero, G.; Calbet, T.; Font-Bardía, M.; Mazzanti, A.; Moyano, A.; Rios, R., Eur. J. Org. Chem. 2011, 1318-1325; (f) Wang, Z.;
Chen, Z.; Bai, S.; Li, W.; Liu, X.; Lin, L.; Feng, X., Angew. Chem. Int. Ed. 2012, 51, 2776-2779; (g) Mazzanti, A.; Calbet, T.; Font-Bardia,
M.; Moyano, A.; Rios, R., Org. Biomol. Chem. 2012, 10, 1645-1652; (h) Li, F.; Sun, L.; Teng, Y.; Yu, P.; Zhao, J. C.-G.; Ma, J.-A.,
Chem. Eur. J. 2012, 18, 14255-14260; (i) Wang, H.; Wang, Y.; Song, H.; Zhou, Z.; Tang, C., Eur. J. Org. Chem. 2013, 2013, 48444851; (j) Tao, Z.-L.; Zhang, W.-Q.; Chen, D.-F.; Adele, A.; Gong, L.-Z., J. Am. Chem. Soc. 2013, 135, 9255-9258; (k) Li, J.-H.; Du, D.M., Org. Biomol. Chem. 2013, 11, 6215-6223; (l) Li, J.-H.; Du, D.-M., RSC Advances 2014, 4, 14538-14545; (m) Han, X.; Yao, W.;
Wang, T.; Tan, Y. R.; Yan, Z.; Kwiatkowski, J.; Lu, Y., Angew. Chem. Int. Ed. 2014, 53, 5643-5647.
(4) Gogoi, S.; Zhao, C.-G.; Ding, D., Org. Lett. 2009, 11, 2249-2252.
(5) (a) Zea, A.; Alba, A.-N. R.; Mazzanti, A.; Moyano, A.; Rios, R., Org. Biomol. Chem. 2011, 9, 6519-6523; (b) Liu, L.; Zhong, Y.; Zhang,
P.; Jiang, X.; Wang, R., J. Org. Chem. 2012, 77, 10228-10234; (c) Liang, J.; Chen, Q.; Liu, L.; Jiang, X.; Wang, R., Org. Biomol. Chem.
2013, 11, 1441-1445; (d) Chen, Q.; Liang, J.; Wang, S.; Wang, D.; Wang, R., Chem. Commun. 2013, 49, 1657-1659; (e) Geng, Z.-C.;
Chen, X.; Zhang, J.-X.; Li, N.; Chen, J.; Huang, X.-F.; Zhang, S.-Y.; Tao, J.-C.; Wang, X.-W., Eur. J. Org. Chem. 2013, 4738-4743; (f)
Zhang, J.-X.; Li, N.-K.; Liu, Z.-M.; Huang, X.-F.; Geng, Z.-C.; Wang, X.-W., Adv. Synth. Catal. 2013, 355, 797-808; (g) Chauhan, P.;
Mahajan, S.; Loh, C. C. J.; Raabe, G.; Enders, D., Org. Lett. 2014, 16, 2954-2957; (h) Rassu, G.; Zambrano, V.; Pinna, L.; Curti, C.;
Battistini, L.; Sartori, A.; Pelosi, G.; Casiraghi, G.; Zanardi, F., Adv. Synth. Catal. 2014, 356, 2330-2336; (i) Yetra, S. R.; Mondal, S.;
Mukherjee, S.; Gonnade, R. G.; Biju, A. T., Angew. Chem. Int. Ed. 2016, 55, 268-272.
(6) (a) Mandha, S. R.; Siliveri, S.; Alla, M.; Bommena, V. R.; Bommineni, M. R.; Balasubramanian, S., Bioorg. Med. Chem. Lett. 2012, 22,
5272-5278; (b) Elfahham, H. A.; Abdel-Galil, F. M.; Ibraheim, Y. R.; Elnagdi, M. H., J. Heterocycl. Chem. 1983, 20, 667-670; (c) Khan,
M. A.; Cosenza, A. G.; Ellis, G. P., J. Heterocycl. Chem. 1982, 19, 1077-1085.
(7) (a) Azzam, S. H. S.; Pasha, M. A., Tetrahedron Lett. 2012, 53, 6834-6837; (b) Kuo, S. C.; Huang, L. J.; Nakamura, H., J. Med.
Chem. 1984, 27, 539-544.
(8) Enders, D.; Grossmann, A.; Gieraths, B.; Düzdemir, M.; Merkens, C., Org. Lett. 2012, 14, 4254-4257.
(9) Jiang, X.; Liu, L.; Zhang, P.; Zhong, Y.; Wang, R., Angew. Chem. Int. Ed. 2013, 52, 11329-11333.
(10) (a) Zhang, H.-M.; Lv, H.; Ye, S., Org. Biomol. Chem. 2013, 11, 6255-6257; (b) Yetra, S. R.; Mondal, S.; Suresh, E.; Biju, A. T., Org.
Lett. 2015, 17, 1417-1420.

5

(11) For reviews, see: (a) Jiang, X.; Wang, R., Chem. Rev. 2013, 113, 5515-5546; (b) Pellissier, H., Tetrahedron 2009, 65, 2839-2877;
(c) Jørgensen, K. A., Angew. Chem. Int. Ed. 2000, 39, 3558-3588; (d) Boger, D. L., Chem. Rev. 1986, 86, 781-793.
(12) Jiang, H.; Albrecht, Ł.; Jørgensen, K. A., Chem. Sci. 2013, 4, 2287-2300.
(13) (a) Albrecht, Ł.; Dickmeiss, G.; Weise, C. F.; Rodríguez-Escrich, C.; Jørgensen, K. A., Angew. Chem. Int. Ed. 2012, 51, 1310913113; (b) Weise, C. F.; Lauridsen, V. H.; Rambo, R. S.; Iversen, E. H.; Olsen, M.-L.; Jørgensen, K. A., J. Org. Chem. 2014, 79, 35373546.
(14) Gu, J.; Ma, C.; Li, Q.-Z.; Du, W.; Chen, Y.-C., Org. Lett. 2014, 16, 3986-3989.
(15) (a) Albrecht, Ł.; Dickmeiss, G.; Acosta, F. C.; Rodríguez-Escrich, C.; Davis, R. L.; Jørgensen, K. A., J. Am. Chem. Soc. 2012, 134,
2543-2546; (b) Albrecht, Ł.; Jiang, H.; Jørgensen, K. A., Chem. Eur. J. 2014, 20, 358-368; (c) Qi, L.-W.; Yang, Y.; Gui, Y.-Y.; Zhang, Y.;
Chen, F.; Tian, F.; Peng, L.; Wang, L.-X., Org. Lett. 2014, 16, 6436-6439.
(16) For selected examples, see: (a) Bertelsen, S.; Marigo, M.; Brandes, S.; Dinér, P.; Jørgensen, K. A., J. Am. Chem. Soc. 2006, 128,
12973-12980; (b) Han, B.; He, Z.-Q.; Li, J.-L.; Li, R.; Jiang, K.; Liu, T.-Y.; Chen, Y.-C., Angew. Chem. Int. Ed. 2009, 48, 5474-5477; (c)
Li, J.-L.; Kang, T.-R.; Zhou, S.-L.; Li, R.; Wu, L.; Chen, Y.-C., Angew. Chem. Int. Ed. 2010, 49, 6418-6420; (d) Song, A.; Zhang, X.;
Song, X.; Chen, X.; Yu, C.; Huang, H.; Li, H.; Wang, W., Angew. Chem. Int. Ed. 2014, 53, 4940-4944; (e) W. Li, J. Wei, Q. Jia, Z. Du, K.
Zhang, J. Wang, Chem. Eur. J. 2014, 20, 6592-6596; (f) Orue, A.; Uria, U.; Reyes, E.; Carrillo, L.; Vicario, J. L., Angew. Chem. Int. Ed.
2015, 54, 3043-3046; for reviews, see: (g) Ramachary, D. B.; Reddy, Y. V., Eur. J. Org. Chem. 2012, 865-887; (h) Li, J.-L.; Liu, T.-Y.;
Chen, Y.-C., Acc. Chem. Res. 2012, 45, 1491-1500; (i) Jurberg, I. D.; Chatterjee, I.; Tannert, R.; Melchiorre, P., Chem. Commun. 2013,
49, 4869-4883.
(17) For examples where the substitution at C4 is important, see: (a) Li, Y.; Liu, X.; Yang, Y.; Zhao, G., J. Org. Chem. 2007, 72, 288-291;
(b) Itoh, T.; Ishikawa, H.; Hayashi, Y., Org. Lett. 2009, 11, 3854-3857; (c) Caruana, L.; Kniep, F.; Johansen, T. K.; Poulsen, P. H.;
Jørgensen, K. A., J. Am. Chem. Soc. 2014, 136, 15929-15932.
(18) X-ray structure deposited under CCDC 1438957.

6