Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
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Construction of Vicinal Quaternary Carbon Centers via CobaltCatalyzed Asymmetric Reverse Prenylation Minghe Sun,‡,§ Jia-Feng Chen,†,§ Shufeng Chen,‡ and Changkun Li*,† †
Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China ‡ Inner Mongolia Key Laboratory of Fine Organic Synthesis, Department of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, People’s Republic of China
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ABSTRACT: A highly enantioselective cobalt-catalyzed reverse prenylation of β-ketoester with tertiary allylic carbonate to construct vicinal all-carbon quaternary carbon centers has been developed. By using the cobalt/(S,S)Ph-BPE complex generated in situ with zinc reduction, excellent branched to linear selectivity (>20:1) and up to 92% ee have been obtained.
T
On the other hand, stereoselective construction of vicinal allcarbon quaternary carbon centers is still very challenging in organic synthesis.14 By applying the allylic substitution strategy, very few examples were developed with precious transition metals (Scheme 1).15,16 For reverse prenylation, Trost reported a Pd-catalyzed enantioselective allylation of oxindoles.15a,d Carreira and Stark developed the Ir-catalyzed highly diastereoselective C3 reverse prenylation of indoles with phosphoramidite ligands.15b,c As for earth-abundant base metals, the only example is Plietker’s Fe-catalyzed quaternary carbon construction.7b However, the B/L selectivity is moderate and enantioselectivity is not available. Herein, we report the first highly enantioselective cobalt-catalyzed allylic substitution reaction to construct vicinal all-carbon quaternary carbon centers. We commenced our studies with β-ketoester 1a and α,αdimethyl allyl methyl carbonate 2a as model substrates (Table 1). Several diphosphine ligands were examined using 10 mol % Co(BF4)2 and 10 mol % Zn in acetonitrile at 60 °C. Diphosphine ligands with two-carbon spacers exhibit very high reactivities and regioselectivity (L1 to L3), affording the branched reverse prenylation product 3a exclusively. L1 was chosen to prepare the racemic products. Then different chiral diphosphine ligands with two-carbon spacers were examined. The simple substitutions on the carbon backbone lead to low enantioselectivities (L4 and L5), while the electron-rich (S,S)Ph-BPE (L7, entry 5) gave 75% ee.17 L6 with three-carbon linker between the two phosphorus atoms gave neither good reactivity nor enantioselectivity. Ligands with chiral phosphorus atoms (L8 and L9) do not provide high enantioselectivities. The steric effect of ester on β-ketoester 1 was also checked. By simply replacing the ethyl group with tert-butyl group, 85%
ransition-metal-catalyzed allylic substitution reactions provide efficient ways to construct new carbon−carbon and carbon−heteroatom bonds.1 Palladium plays the most important role in this key transformation.2 However, it usually produces linear products when unsymmetric allylic substrates are used for steric reasons. In the past two decades, noble metals (Ir,3 Ru,4 and Rh5) catalyzed branched-selective allylation reactions were extensively studied and different ligand systems were developed to realize the asymmetric synthesis of chiral allylic compounds. Nevertheless, there are still several problems to be solved, such as the quaternary carbon center formation as well as low reactivity and regioselectivity of aliphatic substituted allylic substrates. From the viewpoints of sustainable chemistry and cost, earth-abundant first-row transition metals catalyzed asymmetric allylations are still very attractive. Cu-catalyzed allylic substitution reactions normally involve the hard organometallic reagents in an SN2′ manner.6 Plietker reported low-valent-iron catalyzed allylic substitution with soft nucleophiles, in which only stereospecific (from chiral substrates to chiral product) catalysis was realized.7 Nickel catalysts can usually produce the linear or stereospecific products.8 Tungsten and molybdenum exhibit similar regioselectivities as iridium does; however, the scope of nucleophiles is limited to carbon.9 Cobalt, another earth-abundant base metal, catalyzed allylic substitution is much less developed,10 although cobalt-catalyzed organic reactions have gained much attention recently.11 Iqbal reported a cobalt-catalyzed allylation of β-ketoesters in moderate yield and selectivity without the support of any ligand.12 To the best of our knowledge, there has not been a cobalt-catalyzed highly enantioselective allylic substitution reaction reported to date.13 Regarding the mechanism, it is still an unexplored question whether cobalt undergoes a fast σ−π−σ allyl equilibrium as palladium or slow equilibrium as rhodium/phosphite5a or iron catalysts.7 © XXXX American Chemical Society
Received: December 18, 2018
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DOI: 10.1021/acs.orglett.8b04030 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
ee was obtained. When the reaction temperature decreases to 40 °C and 5 mol % catalyst was used, the enantioselectivity was further improved to 92% ee. Some other cobalt halide salts were screened, and no obvious counteranion effect was observed. Acetonitrile is the only choice among the solvents examined. It is worthy to mention that it is not necessary to use any base or other additives in this reaction except a catalytic amount of reducing zinc. In all cases, >20:1 branch/ linear selectivity is observed. With the optimized conditions in hand, we investigated the scope of the asymmetric reverse prenylation reaction (Scheme 2). Different groups can be applied in the ester function (3a,
Scheme 1. Noble/Base-Metal-Catalyzed Allylic Substitution Reactions To Construct Vicinal Quaternary Carbon Centers
Scheme 2. Scope of the β-Ketoesters in the Asymmetric Reverse Prenylationa
Table 1. Optimization of Reaction Conditionsa
entry
R
ligand
yield (%)b
ee (%)
1 2 3 4 5 6 7 8 9 10 11c
Et Et Et Et Et Et Et Et Et t-Bu t-Bu
L1 L2 L3 L4 L5 L6 L7 L8 L9 L7 L7
98 90 99 99 89 26 99 98 54 97 85
− − − 23 14 5 75 11 17 85 92
a
Conditions: 0.2 mmol of 1 and 0.3 mmol of 2 in 1 mL of CH3CN were used as the standard conditions. bThe reaction was performed with 10 mol % cobalt salt, ligand and zinc at 40 °C.
a
Conditions: 0.2 mmol of 1 and 0.3 mmol of 2 in 1 mL of CH3CN were used as the standard conditions. bIsolated yield of 3 and above 20:1 B/L for all cases. cThe reaction was performed with 5 mol % cobalt salt, ligand, and zinc at 40 °C.
3b, 3c, and 3d). No decrease in reactivity was observed when the adamantyl group was used (3c). The allyl group is also introduced, and the resulting product can be used in decarboxylative allylation reactions (3d). Halogens on the tetralone moiety are tolerated (3e, 3f, and 3g), although the 7bromo group leads to a lower yield, probably because of the competing oxidative addition with cobalt complex. The electron-donating methoxy group at different positions on the aromatic ring makes the reactions slower (3h, 3i, and 3j), and 10 mol % cobalt complex is required for 3h and 3i to obtain full conversion. A substrate with a phenyl group at the 6-position also reacts smoothly to afford the allylation product 3k in 87% ee. The absolute configuration of 3k was assigned to be R (the Flack parameter is 0.0(5)) by single crystal B
DOI: 10.1021/acs.orglett.8b04030 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
To gain more insight into the reaction mechanism, some control experiments were conducted. A cobalt/dppbz (L1) complex Co(L1)2Cl can be prepared by NaBH4 reduction in ethanol in high yield. This compound was characterized by single crystal X-ray diffraction experiment as shown in Scheme 4 (eq 1).18 In contrast to the chloride-bridged dimer of the
diffraction analysis. The potential coordination-competing alkynyl group does not affect the reaction of allyl carbonate 2a with the cobalt/diphosphine complex, and 3l was isolated in 95% yield and 92% ee. The reaction is sensitive to the ring size of the β-ketoesters. The reactivities of five-membered βketoesters are higher, with the bromo function untouched, but the enantioselectivities are lower (3m and 3n). The sevenmembered 1o is sluggish to give 3o in 52% yield and 86% ee even with 10 mol % cobalt catalyst. The scope of allylic carbonates was further examined in the cobalt-catalyzed regioselective allylation reaction (Scheme 3).
Scheme 4. Proposed Reaction Model for the Stereochemistry Control
Scheme 3. Scope of the Allylic Carbonatesa
a
Conditions: 0.2 mmol of 1 and 0.3 mmol of 2 in 1 mL of CH3CN were used as the standard conditions. bThe reaction was performed at 60 °C. cThe reaction was performed with achiral L3 at 80 °C. d Racemic 4-vinyl-1,3-dioxolan-2-one (2e) as the substrate.
cobalt/diphosohine complex reported by Chirik,17 cobalt(I) chloride coordinates here with two-diphosphine ligand L1 to form an 18-electron complex. This compound adopts a square pyramidal geometry with the four phosphorus atoms and the cobalt center almost in the same plane. Only 8% of product 3a was isolated when Co(L1)2Cl was used as the catalyst in acetonitrile at 60 °C, probably because of the 18-electron nature of the complex (eq 2). To mimic the catalytic reaction conditions, 5 mol % Zn(BF4)2, which can be generated in situ, was added and 100% of 3a could be obtained (eq 3). This experiment suggests that the chloride needs to be abstracted from the cobalt and a cationic Co(I) might be the real catalyst. The possibility that Zn(BF4)2 is used to activate the allyl carbonate 2a can be excluded by the fact that the addition of NaBARF also increases the yield of 3a to 100% (eq 4). To examine whether one L1 leaves the cobalt center to release the coordination sites for the allyl carbonate and nucleophiles, the effect of different ratios for cobalt and L1 was checked. When the reactions were stopped after 3 h, 68% of 3a was isolated with 1:1 Co/L1, while only an 18% yield was obtained for 1:2 Co/L1 (eq 5). The obvious inhibition effect of ligand may suggest that only one L1 binds to the metal center during the catalysis. No conversion was observed when 2a was treated with Co(L1)2Cl and NaBARF at 60 °C (eq 6). A binding of βketoester 1 with the cobalt center might be necessary before with the reaction with allylic carbonate.5b The strong solvent effect of acetonitrile may support the coordination of acetonitrile to the cobalt center during catalysis. Based on the experiments above, a mechanism involving a Co-σ-allyl intermediate and inner-sphere reductive elimination of the Co-
Besides α,α-dimethyl allyl carbonate 2a, cyclic allylic carbonates 2b and 2c can also react to give more hindered products 3p and 3q. However, both of the reactivities and enantioselectivities are lower when L7 was applied. A higher reaction temperature is required to obtain good conversions. Although good yield and regioselectivity are observed with L3 as the ligand, the diastereoselectivity cannot be controlled when two different groups are attached at the α-position in the linalyl carbonates 2d (3r). β-Ketoester 1a was converted to a separable mixture of 3s and diastereomeric 3s′ with a 4.8:1 ratio in 96% yield after the allylic substitution and subsequent intramolecular transesterification with racemic 4-vinyl-1,3dioxolan-2-one (2e) as the substrate. The enantiomeric excess of major product 3s is 94%, which is higher than the other diastereomer 3s′ (70% ee). The structure of 3s was confirmed by X-ray. The reaction of rac-4-vinyl-1,3-dioxolan-2-one (2e) to give a 4.8:1 dr suggests the cobalt-catalyzed allylation reaction is not stereospecific under the standard conditions like rhodium/phosphite or iron and a fast σ−π−σ allyl equilibrium exists. Similar dr and ee were obtained when simple n-pentyl substituted allylic methyl carbonate 2f was used to construct compound 3t with vicinal quaternary and tertiary carbon centers. The reactivity of allyl methyl carbonate is not high, and more reactive allyl trifluoroethyl carbonate 2g is necessary to obtain good conversion. C
DOI: 10.1021/acs.orglett.8b04030 Org. Lett. XXXX, XXX, XXX−XXX
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C.; You, S.-L. Acc. Chem. Res. 2014, 47, 2558−2573. (e) Hethcox, J. C.; Shockley, S. E.; Stoltz, B. M. ACS Catal. 2016, 6, 6207−6213. (f) Qu, J.; Helmchen, G. Acc. Chem. Res. 2017, 50, 2539−2555. For other representative examples, see: (g) Roggen, M.; Carreira, E. M. Angew. Chem., Int. Ed. 2011, 50, 5568−5571. (h) Krautwald, S.; Sarlah, D.; Schafroth, M. A.; Carreira, E. M. Science 2013, 340, 1065− 1068. (i) Rössler, S. L.; Krautwald, S.; Carreira, E. M. J. J. Am. Chem. Soc. 2017, 139, 3603−3606. (j) Meza, A. T.; Wurm, T.; Smith, L.; Kim, S. W.; Zbieg, J. R.; Stivala, C. E.; Krische, M. J. J. Am. Chem. Soc. 2018, 140, 1275−1279. (k) Shockley, S. E.; Hethcox, J. C.; Stoltz, B. M. Synlett 2018, 29, 2481−2492. (4) For a review, see: (a) Begouin, J.-M.; Klein, J.; Weickmann, D.; Plietker, B. Top. Organomet. Chem. 2011, 38, 269−320. For selected examples, see: (b) Trost, B. M.; Fraisse, P. L.; Ball, Z. T. Angew. Chem., Int. Ed. 2002, 41, 1059−1061. (c) Onitsuka, K.; Okuda, H.; Sasai, H. Angew. Chem., Int. Ed. 2008, 47, 1454−1457. (d) Miyata, K.; Kutsuna, H.; Kawakami, S.; Kitamura, M. Angew. Chem., Int. Ed. 2011, 50, 4649−4653. (e) Kawatsura, M.; Uchida, K.; Terasaki, S.; Tsuji, H.; Minakawa, M.; Itoh, T. Org. Lett. 2014, 16, 1470−1473. (5) For selected examples of rhodium-catalyzed asymmetric allylations, see: (a) Turnbull, B. W. H.; Evans, P. A. J. Org. Chem. 2018, 83, 11463−11479. (b) Hayashi, T.; Okada, A.; Suzuka, T.; Kawatsura, M. Org. Lett. 2003, 5, 1713−1715. (c) Kazmaier, U.; Stolz, D. Angew. Chem., Int. Ed. 2006, 45, 3072−3075. (d) Vrieze, D. C.; Hoge, G. S.; Hoerter, P. Z.; Van Haitsma, J. T.; Samas, B. M. Org. Lett. 2009, 11, 3140−3142. (e) Arnold, J. S.; Nguyen, H. M. J. J. Am. Chem. Soc. 2012, 134, 8380−8383. (f) Li, C.; Breit, B. Chem. - Eur. J. 2016, 22, 14655−14663. (g) Tang, S.-B.; Zhang, X.; Tu, H.-F.; You, S.-L. J. J. Am. Chem. Soc. 2018, 140, 7737−7742. (6) For selected recent reviews, see: (a) Alexakis, A.; Bäckvall, J. E.; Krause, N.; Pàmies, O.; Diéguez, M. Chem. Rev. 2008, 108, 2796− 2823. (b) Harutyunyan, S. R.; den Hartog, T.; Geurts, K.; Minnaard, A. J.; Feringa, B. L. Chem. Rev. 2008, 108, 2824−2852. (c) Langlois, J.-B.; Alexakis, A. Top. Organomet. Chem. 2011, 38, 235−268. (d) Hornillos, V.; Gualtierotti, J.-B.; Feringa, B. L. Top. Organomet. Chem. 2016, 58, 1−40. (7) For selected examples, see: (a) Plietker, B. Angew. Chem., Int. Ed. 2006, 45, 1469−1473. (b) Plietker, B. Angew. Chem., Int. Ed. 2006, 45, 6053−6056. (c) Plietker, B.; Dieskau, A.; Möws, K.; Jatsch, A. Angew. Chem., Int. Ed. 2008, 47, 198−201. (d) Holzwarth, M.; Dieskau, A.; Tabassam, M.; Plietker, B. Angew. Chem., Int. Ed. 2009, 48, 7251−7255. (e) Dieskau, A. P.; Holzwarth, M. S.; Plietker, B. J. J. Am. Chem. Soc. 2012, 134, 5048−5051. (8) For selected reviews on Mo- or W-catalyzed AAA reactions, see: (a) Belda, O.; Moberg, C. Acc. Chem. Res. 2004, 37, 159−167. (b) Moberg, C. Top. Organomet. Chem. 2011, 38, 209−234. (9) For selected recent examples of nickel-catalyzed asymmetric allylations, see: (a) Yatsumonji, Y.; Ishida, Y.; Tsubouchi, A.; Takeda, T. Org. Lett. 2007, 9, 4603−4606. (b) Srinivas, H. D.; Zhou, Q.; Watson, M. P. Org. Lett. 2014, 16, 3596−3599. (c) Sha, S.-C.; Jiang, H.; Mao, J.; Bellomo, A.; Jeong, S. A.; Walsh, P. J. Angew. Chem., Int. Ed. 2016, 55, 1070−1074. (d) Kita, Y.; Kavthe, R. D.; Oda, H.; Mashima, K. Angew. Chem., Int. Ed. 2016, 55, 1098−1101. (e) Zhou, Q.; Srinivas, H. D.; Zhang, S.; Watson, M. P. J. J. Am. Chem. Soc. 2016, 138, 11989−11995. (f) Ngamnithiporn, A.; Jette, C. I.; Bachman, S.; Virgil, S. C.; Stoltz, B. M. Chem. Sci. 2018, 9, 2547− 2551. (10) For selected examples of cobalt-catalyzed allylation reactions of hard nucleophiles, see: (a) Reddy, C. K.; Knochel, P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1700−1701. (b) Gomes, P.; Gosmini, C.; Périchon, J. Org. Lett. 2003, 5, 1043−1045. (c) Gomes, P.; Gosmini, C.; Périchon, J. J. J. Org. Chem. 2003, 68, 1142−1145. (d) Yasui, H.; Mizutani, K.; Yorimitsu, H.; Oshima, K. Tetrahedron 2006, 62, 1410− 1415. (e) Qian, X.; Auffrant, A.; Felouat, A.; Gosmini, C. Angew. Chem., Int. Ed. 2011, 50, 10402−10405. For other cobalt catalyzed allylation reactions, see: (f) Chen, Q.-A.; Kim, D. K.; Dong, V. M. J. Am. Chem. Soc. 2014, 136, 3772−3775. (g) Suzuki, Y.; Sun, B.; Sakata, K.; Yoshino, T.; Matsunaga, S.; Kanai, M. Angew. Chem., Int. Ed. 2015, 54, 9944−9947.
enolate is more likely, although no NMR or crystal information about Co-σ-allyl or Co-π-allyl is available currently. In summary, we have reported the first highly regio- and enantioselective cobalt-catalyzed allylic substitution reaction. As expected for the same group metals rhodium and iridium, earth-abundant first row cobalt complexes with small bite-angle diphosphine generate the branched allylic products. Vicinal allcarbon quaternary carbon centers with aliphatic substitutions could be constructed enantioselectively, which is still very challenging in the iridium-catalyzed reactions. The development of suitable less expensive chiral ligands for earthabundant cobalt catalyzed allylic substitutions is under investigation in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b04030. Detailed experimental procedures, characterization data, copies of 1H, 13C NMR spectra, and X-ray crystal structure of 3k, 3s, and Co(L1)2Cl (PDF) Accession Codes
CCDC 1852151−1852152 and 1880186 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shufeng Chen: 0000-0002-7021-2581 Changkun Li: 0000-0002-4277-830X Author Contributions §
M.S. and J.-F.C. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the “Thousand Youth Talents Plan”, National Natural Science Foundation of China (NSFC) (Grant 21602130), and Shanghai Jiao Tong University.
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REFERENCES
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DOI: 10.1021/acs.orglett.8b04030 Org. Lett. XXXX, XXX, XXX−XXX
Letter
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