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Letter
In situ-Generated Chiral Co(I)-Catalyst for Asymmetric [2+2+2] Cycloadditions of Triynes Phillip Jungk, Fabian Fischer, and Marko Hapke ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00560 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 10, 2016
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ACS Catalysis
In situ-Generated Chiral Co(I)-Catalyst for Asymmetric [2+2+2] Cycloadditions of Triynes Phillip Jungk†, Fabian Fischer† and Marko Hapke*†,‡ †
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a, D-18059 Rostock (Germany) ‡
Johannes Kepler Universität Linz, Institut für Katalyse, Altenberger Strasse 69, A-4040 (Austria)
ABSTRACT: The development of an in situ catalytic system based on cobalt(II)-salts, chiral P,N-ligands and a reductant for the intramolecular asymmetric cyclization of triynes is described. This system represents the first chiral in situ– generated Co(I)-catalyst system for asymmetric [2+2+2] cycloadditions. The system consists only of commercially available components and leads to high yields and selectivities for the [2+2+2] cycloaddition of triynes in up to 16 examples.
KEYWORDS Asymmetric catalysis, biaryls, P,N-ligands, cobalt, cyclotrimerization
The development and application of transition metalcatalyzed cyclization reactions has led to significant advances in the synthesis of non-aromatic as well as aromatic carbo- and heterocyclic compounds. Axially chiral biaryls are commonly found key structures in biologically active natural products1 and catalysts.2 While prominent roles in the synthesis of natural products have been occupied by cross-coupling reactions3 and the alkenemetathesis,4 cyclotrimerization reactions have proven to be a suitable tool as well. The asymmetric aromatic ring construction via transition metal-catalyzed [2+2+2] cycloaddition is a conceptually new approach to axial chiral biaryls which came to the fore in the last decade.5 Initially Ni-complexes were used for enantioselective [2+2+2] cycloadditions,6 until 2004 several groups reported chiral catalysts of the group 9 metals for this reaction.7 Today the transition metals of group 9 (Co, Rh and Ir) represent the most important class of catalysts in [2+2+2] cycloaddition reactions (Scheme 1).8 The development of asymmetric versions of [2+2+2] cycloadditions with complexes of these metals was initiated by work of Heller et al.,9 who reported a molecularly defined Co(I)-precatalyst, as well as Tanaka et al.10 and Shibata et al.,11 who reported catalytic systems based on Rh(I)- and Ir(I)-complexes respectively. These on the contrary are assembled in situ from metal-olefin precursors and ligands, e.g. chiral bisphosphines.12 For cobalt the applied molecular catalyst is a rarely used, chiral menthyl-substituted indenyl Co(I)complex (1a or 1b, Scheme 1), accessible by an intricate synthesis and only working well under photochemical conditions at low temperatures for the assembly of chiral pyridines.9a,d
Scheme 1. Chiral catalytic systems of group 9 metals for [2+2+2] cycloadditions. However, until today to the best of our knowledge no thermally assisted in situ cobalt catalyst for asymmetric [2+2+2] cycloaddition of alkynes has been reported, which we would like to disclose herein.13 For our initial investigations we chose triyne 2a as a qualified test substrate.11f Since we identified CoCl(PPh3)3 as suitable catalyst for the cyclotrimerization of different triynes,14 we were focusing on designing a straightforward, simple and easily applicable chiral catalytic system with the use of low-cost Co(II)sources. Salts like CoBr2 can be reduced in situ with Zn and ZnI2 to afford the active cobalt(I)-species in the presence of bisphosphine ligands, as described before by Hilt et al.15 and Okamoto et al..16 Applying this system with no ligand at all, we found that the racemic product 3a can be obtained in a reasonably good yield of 73% at 65 °C as reaction temperature. Adding chiral P,P-ligand (aR)BINAP (4) to the catalytic system, triaryl 3a was isolated in 58% yield after a reaction time of 17 hours, however, no enantioselectivity was observed (Scheme 2).17 We then switched to P,N-ligands possessing a different bite angle due to the second coordinating heteroatom being part of the aromatic ring system. Gratifyingly using (aS)QUINAP (5) led to quantitative yields and enantioselec-
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tivities up to 57% ee under the same conditions (Scheme 2).17 preparation of the in situ-catalyst:
adding substrate:
reaction:
O CoBr2 (5 mol%) Zn (10 mol%) ZnI 2 (10 mol%) ligand* (5 mol%)
O
THF
O
THF
65 °C, 5 min-2 h
65 °C , 17-21 h
O
2a
3a R
(aR)-BINAP (4)
(aR)-BINAP (4) 58% (rac) (aS)-QUINAP (5) >95% (57% ee)
N N
N
PPh 2 PPh 2
*
*
PPh 2
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perature by decreasing the temperatures for the cycloaddition as well as reduction step, showing that the catalyst generation as well as the reaction with the substrate can be performed even at 25 °C.19 For further investigations we set out to synthesize an easily applicable precatalyst from ligand 6 and cobalt(II)salts. We were able to isolate the green complex CoBr2{(R,aR)-N-PINAP} (7) in nearly quantitative yield. The molecular structure was elucidated by X-ray crystal structure analysis (Scheme 3).20
PPh2
(aS )-QUINAP (5)
H
N N
THF
(R,aR)-N-PINAP (6) R = N-(R)-phenylethyl
6
CoBr2 [Co]
25 °C, 1 h
Table 1. Screening of chiral ligands in asymmetric cyclotrimerizations. preparation of the in situ-catalyst:
adding substrate:
reaction:
O CoBr2 (5 mol%) Zn (10 mol%) ZnI2 (10 mol%) ligand* (5 mol%)
O
65 °C, 2 h
*
65 °C , t2
7 (98%)
Scheme 3. Synthesis of 7 and ORTEP presentation of its molecular structure (ellipsoids with 30% probability). Gratifyingly we were able to confirm the results obtained by the in situ catalyst system by using complex 7 in the cyclization reaction of 2a under identical conditions (see supporting information for details). The high enantioselectivity was maintained and even higher yields of product 3a were observed. Interestingly addition of a second equivalent of ligand 6 did not lead to a large change in the yield and selectivity, corroborating the assumption that only one ligand is involved in the catalytic cyclization process and excluding a possible inhibition by excessive ligand (Table 2).
*
O
2a
Br Br
Table 2. Asymmetric cyclization with precursor 7 and additional ligand 6
O
THF
THF
Co PPh2
Scheme 2. Ligand screening for in situ-generated chiral cobalt catalysts. The initial rapid screening was conducted in glass reaction vials under “semi-inert” conditions, later under strictly oxygen-free reaction conditions18 in Schlenk tubes. In all cases we observed mostly comparable yields but significantly higher selectivities, possibly caused by sensitivity of the ligand for oxidation (Table 1). Out of all the P,N-ligands (aS)-QUINAP (5) and (R,aR)-N-PINAP (6) showed the highest ee values with 75% and 77% (Table 1, entries 1 and 4). All other ligands gave mostly moderate ee’s.
N
3a
b
t2
yield
[h]
[%]
d/l: a meso
Sel.
#
Chiral ligand
1
(aS)-QUINAP (5)
17
69
1.2:1
(-)77
2
(aR)-QUINAP (5)
23
85
1.2:1
(+)63
3
(R)-PHOX
27
49
1:1.2
(-)30
4
(R,aR)-N-PINAP (6)
27
57
1.6:1
(+)75
[% ee]
5
(R,aS)-N-PINAP
27
57
1:1.2
(-)49
6
(S,aR)-N-PINAP
25
72
1:1.5
(+)57
7
(R,aR)-O-PINAP
25
59
1.2:1
(+)67
8
(R,aS)-O-PINAP
25
67
1:1.4
(-)47
All reactions were performed under strictly oxygen free a conditions in Schlenk tubes. Determined by integration of b resonances in the 1H NMR spectra. Determined by chiral HPLC analysis.
We optimized and evaluated the reaction conditions and parameters like the individual required reaction tem-
Chiral # ligand a [mol%] 7
1
7 + (R,aR)2 N-PINAP (6) [2.5]
T
t1
t2
yield
[°C]
[h]
[h]
25
1
25
1
c
[%]
d/l: b meso
Sel. [% ee]
20
95
1.3:1
(+)84
20
97
1.7:1
(+)85
a
2.5 mol% precursor, 5 mol% Zn and 5 mol% ZnI2, subb strate 2a (see scheme Table 1). Determined by integration 1 c from the resonances in the H NMR spectra. Determined by chiral HPLC analysis.
The investigation of the substrate scope of these catalytic systems with symmetric ether-bridged triynes yielded the resulting biaryls in moderate to very good yields (Table 3) for all tested triynes. The enantioselectivities for the triynes with larger substituents such as 9phenanthrenyl (Table 3, entry 1) as well as 4-Me-1naphthyl (Table 3, entry 2) are much lower compared to the naphthyl-substituted 3a. The heteroarene-substituted triynes 2f and 2g required higher catalyst loadings and higher temperatures for the cyclization with the cobalt-
2
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ACS Catalysis
precursor 7, but provided high yields and comparably good ee values for the symmetrically substituted triynes (Table 3, entries 6 and 7). Noteworthy differences were observed in the reaction of the unsymmetrically substituted triynes 2h and 2i. Triyne 2h was cyclized with both, the precursor 7 as well as the mixture of (aR)-5 and [Co], in quantitative yields. However, the high selectivity of 78% ee for 3h was only observed with 7 (Table 3, entries 8 and 9). The loss of selectivity by changing the ligand to (aR)-QUINAP (5) was reproducible low for this substrate (Table 3, entry 9). A comparable observation was accounted for the triyne 2i (Table 3, entries 10 and 11). Triyne 2l gave the resulting product in good yield and moderate ee value for both pairs of enantiomers in comparison to triyne 2k with a smaller second substituent (Table 3, entries 12 and 13). Table 3. Substrate screening with ether-bridged triynes. O
R'
R
i) (aR)-/(aS)-5 + [Co] or 7 (x) Zn (2x), ZnI2 (2x) THF, 25 °C , t1
O
H O
ii) substrate 25 °C , t2
R
R
1
7 [2.5]
2
7 [2.5]
3
7 [5]
4
7 [2.5]
5
(aS)-5 + [Co] [5]
6
7 [10]
7
8
9
10
7 [10]
7 [2.5] (aR)-5 + [Co] [5] 7 [2.5]
Co
t2
[h]
[h]
9phenanthrenyl 2b
2
21
4-Me-1naphthyl 2c
1
15
2-Me-1-Ph
b
2-COOMe-1Ph 2e
2
15
2
165
1naphthyl
Me 2h
1naphthyl
Me 2h
1naphthyl
Ph 2i
2
1.5
1.5
1
3d 90
b
16
4isoquinolinyl 2g
a
(+)19
1
2f
ee [%]
3c 92
2-COOMe-1Ph 2e
b
Yield [%]
(+)30
16
4-quinolinyl
PPh2
3b 75
2
2d
N
(aR)-/(aS)-QUINAP (5)
7
t1
R’
Br Br
R' 3
5 + [Co] or 7 [x mol%]
N N
PPh2
2
#
N
O
(+)15 3e 42 (+)24 3e 87
b
rac b
(+)46 b
140
15
15
18
3f 81
3g 86 (-)66
d/l: meso 1.8:1 c
1.4:1 c
1.4:1
c
11
12
13
(aR)-5 + [Co] [2.5]
1naphthyl
Ph 2i
1
16
7 [2.5]
2MeO1naphthyl
Ph 2k
1
18
7 [2.5]
1-naphthyl, 2-MeO-1naphthyl 2l
3i 94 (-)18
3k 70 (+)12 e
3l 61 1
14
a
(+)32, (+)39 b
c
Determined by chiral HPLC analysis. 65 °C. d/l:meso d ratio determined by HPLC area. Determined by integration 1 e from the resonances in the H NMR spectra. Total yield of both individual pairs of enantiomers, which were isolated separately after column chromatography.
The substrate scope of this catalyst-system could also be extended to symmetrical and unsymmetrical substituted malonate-bridged triynes (Table 4). For all cyclizations of the malonate-bridged triynes higher temperatures (6595 °C) were required to receive full conversion in comparison to the ether-bridged triynes where fairly mild conditions (25 °C) were used. The best results were obtained with triynes 8a and 8e (Table 4, entries 1, 2 and 6). Starting with 8a the corresponding triaryl 9a was obtained with up to 78% ee and in 53% yield (Table 4, entry 1). Higher temperatures (up to 90°C) led to better yields, albeit lower selectivity (Table 4, entry 2). Using 2methoxy-1-naphthyl substituted triynes still allowed good yields except in one case. The reaction of 8b is remarkable, because of the exclusive formation of the d/l-form of 9b (Table 4, entry 3). Table 4. Substrate screening with malonate-bridged triynes.
2.7:1
d
1:1.3
d
1.2:1
5+ [Co] # or 7 [x mol%]
T2 n R
R’
1.2:1
t2
a
[h]
yield[%] b ee[%]
25
23
9a 53
65
19
(-)78
[°C]
c
7 [2.5]
1
1-naphthyl 8a
(+)78
2 7 [2.5]
1
1-naphthyl 8a
90
40
3h >95
3 7 [2.5]
1
2-MeO-1-naphthyl 8b
25
22
9b 32
60
18
(-)13
(aR)-5 + 4 [Co] [2.5]
1
2-MeO-1naphthyl
Me f 8c
25
17
9c 91
65
25
(-)17
5 7 [10]
2
2-MeO-1-
Me
95
16
9d 63
1
3h >95
(+)7 3i 74 (-)55
9a >95
d
(-)67 e
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6 7 [10]
2
2-MeO-1naphthyl
8d Ph 8e
(+)60 95
16
9e 87 (-)11
a
No reaction observed by TLC control and the temperab c ture was increased. Determined by chiral HPLC analysis. d 2.2:1 (d/l:meso ratio, HPLC area). 1.9:1 (d/l:meso ratio, HPLC e f area). 100:0 (d/l:meso ratio). 2 h at 25 °C first step.
The mechanistic features of [2+2+2] cycloaddition reactions have been subject to an increasingly larger number of theoretical investigations, including Co(I)-catalysts.21 The coordination of the P,N-ligand via both donor atoms to the cobalt center throughout the entire process appears to be crucial. To prove this assumption we oxidized the phosphorus atom of the (R,aR)-N-PINAP ligand (6) with H2O2 and tested the resulting phosphine oxide O=P,N-ligand in the in situ-catalyzed [2+2+2] cycloaddition of triyne 2a under the above used conditions. Our analysis showed good yield of the expected product but the complete loss of enantioselectivity.22 With regard to the missing selectivity using chiral bisphosphine ligands in these Co(I)-catalyzed cyclizations, we assume as a possible explanation for the observed enantioselectivities with the QUINAP-type P,Nligands the coordination ring sizes of the in situgenerated cobalt complexes (Scheme 4).
Scheme 4. Different ring sizes of the respective CoP,N-ligand coordination compounds. This assumption is corroborated by the non-selectivity of the cyclization using BINAP (4) (7-membered coord. ring) or (S,S)-Et-DUPHOS (10) (5-membered coord. ring) as typical chiral ligands. We also investigated other chiral P,N-ligands ((R)-PHOX (11) and (R,S)-Ph-Bn-SIPHOX (12)) which formed in situ a 6- or a 9-membered coordination ring. While (Table 1, entry 3) (R)-PHOX (11) gave a selectivity of 30% ee, the reaction with (aR,S)-Ph-BnSIPHOX (12) proceeded without any selectivity at all. Obviously, the combination of the P,N-donor and the size of the coordination backbone play important roles in the development of thermal asymmetric Co(I)-catalyzed cyclotrimerization reactions. In conclusion we presented the evaluation of the first in situ cobalt-catalyzed asymmetric [2+2+2] cycloaddition reactions by exclusively using commercially available components for the formation of the active catalytic species. With naphthyl-substituted symmetrical and unsymmetrical triynes we obtained the corresponding products in high yields and mostly moderate to high enantioselectivities. The substrate scope of this catalyst system comprises malonate- as well as ether-bridged triynes and
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the synthesis of a defined precatalyst was described as well. Thus, commercially available chiral P,N-ligands were crucial for the development of enantioselective Co(I)mediated [2+2+2] cycloadditions by prior reduction under extremely mild conditions, complementary to the cobalt(I)-systems requiring activation by irradiation.
ASSOCIATED CONTENT Supporting Information Reaction conditions, ligand screening under not strictly oxygen free conditions, screening of catalyst loading, synthesis of the complex 7 are described in the supporting information. The HPLC-Spectra of the received products are available. This material is available free of charge via the Internet at http://pubs.acs.org..
AUTHOR INFORMATION Corresponding Author *E-Mail:
[email protected], Fax: +49 381 1281 51213
Present Address st
† Since November 1 , 2015 also: Institut für Katalyse, Johannes Kepler Universität Linz, Altenberger Str. 69, A-4040 Linz (Austria)
ACKNOWLEDGMENT The work is dedicated to Dr. Barbara Heller on the occasion of her birthday and her contributions to asymmetric cobalt-catalyzed cyclotrimerizations. We thank Prof. Dr. Uwe Rosenthal for his enduring support and helpful discussions and are indebted to Dr. Anke Spannenberg for the X-ray structure analysis. We thank the DFG (HA3511/3-1) and the Leibniz-Gemeinschaft for financial support. The authors thank Dr. Indre Thiel for helpful comments and discussions.
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Schwarzer, M. C.; Frenking, G.; Schmalz, H.-G.; Schäfer, M. J. Org. Chem. 2013, 78, 10485-10493. (16) Goswami, A.; Ito, T.; Okamoto, S. Adv. Synth. Catal. 2007, 349, 2368-2374. (17) See data for the screening in the supporting information. (18) See the supporting information for more details. (19) For the screening of catalyst loading see the supporting information. (20) Crystal data for 7: C42H38Br2CoN3OP, M = 850.47, orthorhombic, space group P212121, 1a = 10.5574(8), b = 18.3392(14), c = 21.5869(16) Å, V = 4179.5(5) Å3, T = 150(2) K, Z = 4, 43051 reflections measured, 10062 independent reflections (Rint = 0.0347), final R values (I > 2σ(I)): R1 = 0.0340, wR2 = 0.0856, final R values (all data): R1 = 0.0415, wR2 = 0.0885, GOF on F2: 1.056, 429 parameters. Data were collected on a Bruker Kappa APEX II Duo diffractometer. The structure was solved by direct methods and refined by full-matrix least-squares procedures on F2 with the SHELXTL software package: Sheldrick, G. M. Acta Crystallogr., 2008, A64, 112-122. XP (Bruker AXS) was used for graphical representation. CCDC 1418399 contains 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. (21) Selected references: (a) Hardesty, J. H.; Koerner, J. B.; Albright, T. A.; Lee, G.-Y. J. Am. Chem. Soc. 1999, 121, 6055-6067. (b) Dahy, A. A.; Koga, N. Bull. Chem. Soc. Jpn. 2005, 78, 781-791. (c) Dahy, A. A.; Suresh, C. H.; Koga, N. Bull. Chem. Soc. Jpn. 2005, 78, 792-803. (d) Agenet, N.; Gandon, V.; Vollhardt, K. P. C.; Malacria, M.; Aubert, C. J. Am. Chem. Soc. 2007, 129, 8860-8871. (22) See the supporting information for the synthesis of the O=P,N-ligand (product SI-I).
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