Enantioselective alkynylbenzaldehyde cyclizations catalyzed by chiral gold(I) acyclic diaminocarbene complexes containing weak Au-arene interactions

Article abstract of DOI:10.1002/anie.201107789

Hold me close: Highly enantioselective catalysis of tandem acetalization/cycloisomerization reactions of o-alkynylbenzaldehydes has been achieved using gold complexes of chiral acyclic diaminocarbene ligands that have electron-deficient aryl substituents. X-ray crystallography and DFT calculations implicate weak gold-arene interactions - absent in the case of simple phenyl substituents - that define the chirality of the substrate binding site. Copyright

Full text of DOI:10.1002/anie.201107789

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DOI: 10.1002/anie.201107789
Enantioselective Gold Catalysis
Enantioselective Alkynylbenzaldehyde Cyclizations Catalyzed by
Chiral Gold(I) Acyclic Diaminocarbene Complexes Containing Weak
Au–Arene Interactions**
Sachin Handa and LeGrande M. Slaughter*
Catalysis with gold(I) has emerged as a powerful tool for the
synthesis of complex organic structures, owing to the ability of
AuI to activate a variety of unsaturated bonds toward
nucleophilic attack.[1] In efforts to develop enantioselective
catalysts, the linear coordination geometry of AuI presents
challenges because of the remoteness of chiral ligand
substituents and the inability of substrates to adopt a chelate
binding mode that favors asymmetric induction. Nevertheless,
high enantioselectivities have been achieved with gold
catalysts in an impressive number of reactions.[2] In a few
cases, there is evidence that secondary interactions of the
ligand help to overcome the inherent difficulty of achieving
a chiral environment at a AuI center.[3] The seminal example
of asymmetric gold catalysis utilized the electrostatic attrac-
tion between a pendant ammonium group and the substrate
to achieve highly enantioselective aldol reactions.[4] For
bis(gold) complexes of chiral biaryl diphosphines—currently
the most successful chiral catalyst design for AuI—it has been
proposed that p–p interactions of the phosphine substituents
play a key role in creating a chiral pocket around the metals.[5]
In one intriguing report, interactions of electron-rich aryl
groups of chiral phosphoramidite ligands with AuI were found
to greatly enhance the chirality of the substrate binding site.[6]
Herein we report that secondary interactions of AuI with
electron-deficient aryl substituents on chiral carbene ligands
are associated with highly enantioselective catalysis in
a tandem addition/cycloisomerization reaction. These results
suggest a new chiral catalyst design motif that could enable
further advances in enantioselective applications of AuI
catalysts.
been achieved using chelating[11] or bridging[12] chiral bis-
(ADC) ligands. Only recently has an ee value above 90%
been reported for a reaction using an ADC/metal catalyst,
notably a bis(ADC) digold complex.[12b] Only low ee values
have been attained with monodentate ADC ligands,[13] and no
ee value higher than 70% has been reported for a chiral
monodentate NHC/AuI catalyst.[14]
We envisioned creating ADC complexes with chiral
groups near the catalytic site by the addition of bulky
amines to AuI complexes of suitably substituted chiral biaryl
isocyanides. As no chiral monoisocyanides of this type are
known,[15] we devised syntheses of 2-isocyanobinaphthyls 1a–
c (Scheme 1) starting from the corresponding alcohols (see
the Supporting Information). Metalation with Au(THT)Cl
provided the AuI complexes 2a–c, which cleanly converted
into enantiomerically pure ADC complexes 3a–c upon
reaction with diisopropylamine.
X-ray crystal structures of 3a–c revealed that the ADC
ligands adopt conformations with the binaphthyl groups syn
to gold, probably to avoid steric clashes with the iPr groups
(Figure 1).[16] 1H NMR spectra indicated static conformations
in solution at 258C, with no sign of isomerization about the
[10c]
À
carbene C N bonds.
Complex 3a forms dimers with
aurophilic contacts of 3.36 ꢀ, but this packing arrangement
is sterically prohibited by the 2’-aryl groups in 3b and 3c. Two
different rotameric forms of the ADC ligand are present in
Acyclic diaminocarbene ligands (ADC),[7] also known as
nitrogen acyclic carbenes (NAC), are potentially advanta-
geous over the more familiar N-heterocyclic carbenes (NHC)
for enantioselective catalysis,[8] because their wide N-C-N
angles (116–1218) can place chiral substituents closer to the
metal. However, few examples of chiral ADC ligands have
appeared despite growing interest in the use of ADC ligands
in catalysis.[9,10] Significant asymmetric induction has only
[*] S. Handa, Prof. L. M. Slaughter
Department of Chemistry, Oklahoma State University
Stillwater, OK 74078 (USA)
E-mail: lms@chem.okstate.edu
[**] Financial support for this work was provided by the U.S. National
Science Foundation (CAREER Award CHE-0645438). We thank Prof.
Tom Cundari (Univ. of North Texas) for computational advice.
Scheme 1. Synthesis of chiral gold(I) ADC complexes. Reaction con-
ditions: a) Au(THT)Cl, RT, 9 h, CH2Cl2; b) HNR’2, RT, 2 h, CH2Cl2; then
608C, 6–8 h, CH3CN. THT=tetrahydrothiophene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107789.
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Table 1: Catalytic alkynylbenzaldehyde cyclizations.[a]
Entry
5
R2
Catalyst
t [h]
Yield [%][b]
ee [%][c]
1
5a
5a
5a
5a
5a
5a
5b
5a
5a
5a
5b
5c
5a
5a
5a
5b
5c
5c
5a
5a
5a
5b
5c
5a
iPr
cHex
iPr
cHex
iPr
cHex
cHex
tBu
benzyl
nOct
nOct
nOct
nBu
nPr
Me
Me
Me
nOct
nBu
nPr
Me
Me
3a
3a
3b
3b
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
4
37
36
36
36
12
36
37
36
28
36
36
37
36
36
24
18
12
37
36
36
24
24
12
18
12[d]
31
8
2[e]
27
61
43
84
98
98
96
98
99
99
nd[g]
nd[g]
69
-
3
28[d]
5[d]
68
4[e]
5[f]
6[e]
65
70
66
68
87
75
7[e]
8[e]
9[e]
10[e]
11[e]
12[f]
13[e]
14[f]
15[f,h]
16[f,h]
17[f,h]
18
5[d]
12[d]
5[d]
NR[d,i]
37[d]
NR[d,i]
86
59
67
75
75
Figure 1. X-ray crystal structures of a) 3a, b) 3b, and c) 3c. Displace-
À
ment ellipsoids are drawn at the 50% probability level, and non-N H
hydrogen atoms are omitted for clarity. A second, crystallographically
56
-
nonequivalent complex of 3c has a similar geometry but is not shown.
99
92
89
87
92
61
>99
19[e]
20
4
4
4
4
4
4
the structures of 3b and 3c. In 3b, the binaphthyl group is
rotated away from the AuCl unit, thus orienting the 2’-phenyl
substituent away from the coordination sphere. However, 3c
adopts a conformation that places the 3,5-(CF3)2C6H3 group
directly adjacent to gold; this conformation is unexpected
given the larger steric profile of this substituent. The aryl ring
centroid is only 3.4 ꢀ from the Au atom of 3c (3.6 ꢀ for
a second, crystallographically independent complex), thus
suggesting the presence of a weak gold–p interaction.[17]
Although complexes of AuI with arenes are rare,[18] several
reports of ligand aryl groups engaging in similar, longer-range
Au–p interactions orthogonal to the two ligation sites have
recently appeared.[6,19,20] In the case of 3c, the presence of the
aryl group adjacent to Au creates a significantly more
pronounced chiral environment at the metal center compared
with 3a and 3b.
On the hypothesis that the solid-state structural differ-
ences observed in 3a–c might also be relevant in solution, we
investigated whether these differences are reflected in
enantioselective catalysis. The tandem acetalization/cyclo-
isomerization of ortho-alkynylbenzaldehydes (Table 1), orig-
inally reported by Yamamoto using Pd(OAc)2 as a catalyst,[21]
gives 1H-isochromene derivatives that can have useful
medicinal properties.[22] Although several related tandem
processes using different nucleophiles are known,[23] including
AuI-catalyzed examples,[23d–h] no enantioselective version of
this reaction has appeared.[24]
21[h]
22[h]
23[h]
24
Me
iPr
26
70
[a] Reaction conditions: catalyst (5 mol%), LiNTf2 (4.5 mol%), 258C
(except as noted), dichloroethane (DCE, 0.25m). [b] Yield of isolated
product. [c] Determined by HPLC on a chiral stationary phase. [d] 5
recovered. [e] At 608C. [f] Yield not improved at 608C. [g] Not deter-
mined. [h] 1.1 equiv MeOH used. [i] No product detected.
3a gave poor yields and ee values of only 8% and 27% using
iPrOH and cyclohexanol, respectively, as nucleophiles
(Table 1, entries 1 and 2). The ee values increased to 61%
and 43% for catalyst 3b, but the yield was still low (entries 3
and 4). Only with 3c [R = 3,5-(CF3)2C6H3] was a good yield
and high enantioselectivity attained (entries 5 and 6). With 3c
selected as the most promising catalyst, the scope of
enantioselective alkynylbenzaldehyde cyclizations was exam-
ined. Good yield and enantiomeric excess of 96% or higher
were obtained using tertiary (entry 8), secondary (entries 6
and 7), and larger primary alcohols (entries 9–11) as nucleo-
philes with alkyne substrates bearing aryl R1 groups. How-
ever, yield and enantioselectivity fell when the alkyne
contained n-propyl instead of an aryl group (entries 12 and
17) and when alcohols containing shorter n-alkyl chains were
used (entries 13–17). The use of MeOH resulted in no product
formation in two cases (entries 15 and 17).
Initial optimization studies identified LiNTf2 (Tf = tri-
fluoromethanesulfonyl) as a necessary additive to achieve
useful activity (see the Supporting Information).[25,26] Under
optimized conditions, both yield and enantioselectivity in
model cyclizations of substrate 5a depended strongly on the
nature of the R group of the AuI/carbene catalyst. Complex
To address these catalyst limitations, we created a modi-
fied version of 3c using the chiral amine ((S)-PhMeCH)2NH
in place of iPr2NH (4; Scheme 1). We hypothesized that the
chiral amine substituents might augment the chiral environ-
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ment at the metal through steric interactions with the
binaphthyl moiety. The X-ray crystal structure of
4
(Figure 2) revealed an Au–arene interaction similar to that
seen in 3c (distance from Au center to ring centroid: 3.6 ꢀ),
as well as a short phenyl–binaphthyl contact (C54–C26
Figure 2. X-ray crystal structure of 4 with 50% probability ellipsoids.
3.28 ꢀ), which could sterically hinder rotation away from
the Au center. Gratifyingly, catalyst 4 provided cyclization
products in good yield and high enantiomeric excess in most
of the reactions for which 3c was ineffective (Table 1,
entries 18–22), although an example with small alkyl groups
at both R1 and R2 was still problematic (entry 23). Signifi-
cantly higher enantioselectivity was also achieved for a reac-
tion in which comparable yields were obtained using 3c
(entries 5 and 24).
The catalytic results suggest that the Au–arene interac-
tions in 3c and 4 might enhance both the enantioselectivity
and stability of the catalysts. To provide insight into these
interactions, DFT calculations were performed on rotamers
of 3b and 3c.[27] For 3c, an enthalpic preference of 8 kJmolÀ1
was calculated for the crystallographically observed “in”
rotamer over the “out” rotamer, whereas no significant
difference in energy was calculated for 3b (Figure 3). The 3,5-
(CF3)2C6H3 and binaphthyl units of the optimized 3cout do not
show any short contacts with the iPr groups, thus discounting
a steric basis for the preference of 3cin. Interestingly, the
preference for the “in” rotamer with the electron-deficient
aryl group of 3c, but not for Ph, suggests an inversion of the
electrostatic pairing that underlies typical cation–p interac-
tions.[20a]
Figure 3. DFT-calculated relative energies of rotamers of 3b and 3c.
Optimized geometries of the rotamers of 3c are shown at the top. The
calculated distance between the Au center and the ring centroid for
3cin is 3.9 ꢀ.
a different electrostatic profile in our system, perhaps related
to the strong donor ability of the carbene ligand. Future work
will seek to better understand these interactions and to
exploit them in other catalytic enantioselective reactions that
are not currently achievable.
Received: November 4, 2011
Published online: January 4, 2012
Keywords: asymmetric catalysis · carbene ligands ·
.
domino reactions · gold · metal-p interactions
[1] For reviews of gold catalysis, see: a) A. S. K. Hashmi, Chem. Rev.
2007, 107, 3180; b) A. Fꢁrstner, P. W. Davies, Angew. Chem.
2007, 119, 3478; Angew. Chem. Int. Ed. 2007, 46, 3410; c) Z. Li,
C. Brouwer, C. He, Chem. Rev. 2008, 108, 3239; d) D. J. Gorin,
B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351.
[2] For reviews of enantioselective gold catalysis, see: a) N. Bongers,
N. Krause, Angew. Chem. 2008, 120, 2208; Angew. Chem. Int. Ed.
2008, 47, 2178; b) R. A. Widenhoefer, Chem. Eur. J. 2008, 14,
5382; c) A. Pradal, P. Y. Toullec, V. Michelet, Synthesis 2011,
1501.
In summary, we report a new class of chiral AuI/ADC
catalysts, for which enantioselectivity in alkynylbenzaldehyde
cyclization correlates with the presence of Au–arene inter-
actions with an electron-deficient aryl group. The combined
structural, catalytic, and DFTresults suggest a dynamic chiral
pocket in which reversible Au–aryl association, augmented by
intra-ligand sterics in the case of 4, is sufficiently favorable to
influence the enantiodetermining step. Although similar
interactions of electron-rich aryl groups have been implicated
in enantioselective AuI catalysis in another report,[6] the
involvement of electron-deficient aryl substituents hints at
[3] Also of note is the use of ion-pairing interactions between AuI
cations and chiral counterions to engender enantioselectivity.
For example, see: a) K. Aikawa, M. Kojima, K. Mikami, Angew.
Chem. 2009, 121, 6189; Angew. Chem. Int. Ed. 2009, 48, 6073;
b) R. L. LaLonde, Z. J. Wang, M. Mba, A. D. Lackner, F. D.
Toste, Angew. Chem. 2010, 122, 608; Angew. Chem. Int. Ed. 2010,
49, 598.
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[4] a) Y. Ito, M. Sawamura, T. Hayashi, J. Am. Chem. Soc. 1986, 108,
6405; b) M. Sawamura, Y. Ito, T. Hayashi, Tetrahedron Lett.
1990, 31, 2723; c) A. Togni, S. D. Pastor, J. Org. Chem. 1990, 55,
1649.
[5] M. A. Tarselli, A. R. Chianese, S. J. Lee, M. R. Gagnꢂ, Angew.
Chem. 2007, 119, 6790; Angew. Chem. Int. Ed. 2007, 46, 6670.
[6] H. Teller, S. Flꢁgge, R. Goddard, A. Fꢁrstner, Angew. Chem.
2010, 122, 1993; Angew. Chem. Int. Ed. 2010, 49, 1949.
[7] For reviews of ADC ligands, see: a) L. M. Slaughter, Comments
Inorg. Chem. 2008, 29, 46; b) J. Vignolle, X. Cattoꢃn, D.
Bourissou, Chem. Rev. 2009, 109, 3333.
[17] For reviews of relevant gold–p interactions, see: a) H. Schmid-
baur, A. Schier, Organometallics 2010, 29, 2; b) H. G. Rauben-
heimer, H. Schmidbaur, S. Afr. J. Sci. 2011, 107, 31.
[18] a) E. Herrero-Gꢆmez, C. Nieto-Oberhuber, S. Lꢆpez, J. Benet-
Buchholz, A. M. Echavarren, Angew. Chem. 2006, 118, 5581;
Angew. Chem. Int. Ed. 2006, 45, 5455; b) V. Lavallo, G. D. Frey,
S. Kousar, B. Donnadieu, G. Bertrand, Proc. Natl. Acad. Sci.
USA 2007, 104, 13569.
[19] a) F.-B. Xu, Q.-S. Li, L.-Z. Wu, X.-B. Leng, Z.-C. Li, X.-S. Zeng,
Y.-L. Chow, Z.-Z. Zhang, Organometallics 2003, 22, 633; b) Q.-S.
Li, C.-Q. Wan, R.-Y. Zou, F.-B. Xu, H.-B. Song, X.-J. Wan, Z.-Z.
Zhang, Inorg. Chem. 2006, 45, 1888; c) D. V. Partyka, T. J.
Robilotto, M. Zeller, A. D. Hunter, T. G. Gray, Organometallics
2008, 27, 28; d) P. Pꢂrez-Galꢄn, N. Delpont, E. Herrero-Gꢆmez,
F. Maseras, A. M. Echavarren, Chem. Eur. J. 2010, 16, 5324;
e) M. Touil, B. Bechem, A. S. K. Hashmi, B. Engels, M. A.
Omary, H. Rabaꢇ, J. Mol. Struct.: THEOCHEM 2010, 957, 21;
f) R. E. M. Brooner, R. A. Widenhoefer, Organometallics 2011,
30, 3182.
[8] For reviews of chiral NHC ligands, see: a) R. E. Douthwaite,
Coord. Chem. Rev. 2007, 251, 702; b) L. H. Gade, S. Bellemin-
Laponnaz, Top. Organomet. Chem. 2007, 21, 117; c) D. R. Snead,
H. Seo, S. Hong, Curr. Org. Chem. 2008, 12, 1370.
[9] For examples of catalysis with ADC ligands, see: a) A. I.
Moncada, M. A. Khan, L. M. Slaughter, Tetrahedron Lett.
2005, 46, 1399; b) D. Kremzow, G. Seidel, C. W. Lehmann, A.
Fꢁrstner, Chem. Eur. J. 2005, 11, 1833; c) A. I. Moncada, S.
Manne, J. M. Tanski, L. M. Slaughter, Organometallics 2006, 25,
491; d) B. Dhudshia, A. N. Thadani, Chem. Commun. 2006, 668;
e) D. R. Snead, I. Ghiviriga, K. A. Abboud, S. Hong, Org. Lett.
2009, 11, 3274; f) K. V. Luzyanin, A. G. Tskhovrebov, M. C.
Carias, M. F. C. Guedes da Silva, A. J. L. Pombeiro, V. Y.
Kukushkin, Organometallics 2009, 28, 6559; g) E. L. Rosen,
D. H. Sung, Z. Chen, V. M. Lynch, C. W. Bielawski, Organo-
metallics 2010, 29, 250; h) A. S. K. Hashmi, C. Lothschꢁtz, C.
Bçhling, F. Rominger, Organometallics 2011, 30, 2411.
[10] For examples of AuI/ADC catalysts, see: a) C. Bartolomꢂ, Z.
Ramiro, P. Pꢂrez-Galꢄn, C. Bour, M. Raducan, A. M. Echavar-
ren, P. Espinet, Inorg. Chem. 2008, 47, 11391; b) C. Bartolomꢂ,
D. Garcꢅa-Cuadrado, Z. Ramiro, P. Espinet, Organometallics
2010, 29, 3589; c) A. S. K. Hashmi, T. Hengst, C. Lothschꢁtz, F.
Rominger, Adv. Synth. Catal. 2010, 352, 1315; d) H. Seo, B. P.
Roberts, K. A. Abboud, K. M. Merz, Jr., S. Hong, Org. Lett.
2010, 12, 4860.
[20] The long Au–C distances (> 3.0 ꢀ) suggest a largely electrostatic
origin for these Au–arene interactions. For relevant discussions,
see: a) S. Mecozzi, A. P. West, Jr., D. A. Dougherty, J. Am.
Chem. Soc. 1996, 118, 2307; b) M. Mascal, J.-L. Kerdelhuꢂ, A. J.
Blake, P. A. Cooke, R. J. Mortimer, S. J. Teat, Eur. J. Inorg.
Chem. 2000, 485.
[21] N. Asao, T. Nogami, K. Takahashi, Y. Yamamoto, J. Am. Chem.
Soc. 2002, 124, 764.
[22] W. Wang, T. Li, R. Milburn, J. Yates, E. Hinnant, M. J. Luzzio,
S. A. Noble, G. Attardo, Bioorg. Med. Chem. Lett. 1998, 8, 1579.
[23] a) N. T. Patil, Y. Yamamoto, J. Org. Chem. 2004, 69, 5139; b) N.
Asao, C. S. Chan, K. Takahashi, Y. Yamamoto, Tetrahedron
2005, 61, 11322; c) X. Yu, Q. Ding, W. Wang, J. Wu, Tetrahedron
Lett. 2008, 49, 4390; d) X. Yao, C.-J. Li, Org. Lett. 2006, 8, 1953;
e) S. Obika, H. Kono, Y. Yasui, R. Yanada, Y. Takemoto, J. Org.
Chem. 2007, 72, 4462; f) T. Godet, C. Vaxelaire, C. Michel, A.
Milet, P. Belmont, Chem. Eur. J. 2007, 13, 5632; g) M.
DellꢈAcqua, D. Facoetti, G. Abbiatti, E. Rossi, Synthesis 2010,
2367; h) L. Zhou, Y. Liu, Y. Zhang, J. Wang, Beilstein J. Org.
Chem. 2011, 7, 631.
[11] a) Y. A. Wanniarachchi, L. M. Slaughter, Chem. Commun. 2007,
3294; b) Y. A. Wanniarachchi, Y. Kogiso, L. M. Slaughter,
Organometallics 2008, 27, 21.
[12] a) C. Bartolomꢂ, D. Garcꢅa-Cuadrado, Z. Ramiro, P. Espinet,
Inorg. Chem. 2010, 49, 9758; b) Y.-M. Wang, C. N. Kuzniewski,
V. Rauniyar, C. Hoong, F. D. Toste, J. Am. Chem. Soc. 2011, 133,
12972.
[13] D. R. Snead, S. Inagaki, K. A. Abboud, S. Hong, Organometal-
lics 2010, 29, 1729.
[24] For a related enantioselective 5-exo-dig cyclization, see: a) Z.
Chai, Z.-F. Xie, X.-Y. Liu, G. Zhao, J.-D. Wang, J. Org. Chem.
2008, 73, 2947; For enantioselective cyclization of hydroxy-
substituted arylalkynes, see: b) M. Murai, J. Uenishi, M.
Uemura, Org. Lett. 2010, 12, 4788.
[25] NTf2À is known to engender highly electrophilic AuI catalysts by
[14] a) Y. Matsumoto, K. B. Selim, H. Nakanishi, K. Yamada, Y.
Yamamoto, Tetrahedron Lett. 2010, 51, 404; b) K. Wilckens, D.
Lentz, C. Czekelius, Organometallics 2011, 30, 1287; c) W. Wang,
J. Yang, F. Wang, M. Shi, Organometallics 2011, 30, 3859.
[15] 2,2’-Diisocyano-1,1’-binaphthyl and the 3,3’-diphenyl derivative
have been synthesized and used to prepare digold bis(ADC)
complexes, see Ref. [12a].
acting as
a
charge-delocalized, weakly coordinating anion
À
(Ref. [26]). We postulate that NTf2 might also play a role in
proton transfer from the alcohol to the Au C bond subsequent
to attack of the alcohol on an aurated isochromenylium
intermediate (see Ref. [23f]). A 0.9:1 ratio of LiNTf2 to Au is
important, as higher ratios result in racemization of the 1H-
isochromene products.
À
[16] For crystallographic data, catalytic procedures, and other
experimental details, please see the Supporting Information.
CCDC 850130 (3a), 850131 (3b), 850132 (3c), and 850133 (4)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[26] S. Antoniotti, V. Dalla, E. DuÇach, Angew. Chem. 2010, 122,
8032; Angew. Chem. Int. Ed. 2010, 49, 7860.
[27] DFToptimization and energy calculations employed the B3LYP
functional in GAUSSIAN03, using the LANL2DZ basis set for
Au and the 6-31G* basis set for other elements. See the
Supporting Information for details.
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