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Catalytic Enantioselective Cyclopropanation of #Fluoroacrylates: An Experimental and Theoretical Study Amandine Pons, Vincent Tognetti, Laurent Joubert, Thomas Poisson, Xavier Pannecoucke, André B. Charette, and Philippe Jubault ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00354 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019
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ACS Catalysis
Catalytic Enantioselective Cyclopropanation of -Fluoroacrylates: An Experimental and Theoretical Study. Amandine Pons,† Vincent Tognetti,† Laurent Joubert,† Thomas Poisson,†, ‡ Xavier Pannecoucke,† André B. Charette*§ and Philippe Jubault*† Normandie Univ, INSA Rouen, UNIROUEN, CNRS, COBRA (UMR 6014), 76000 Rouen, France. Institut Universitaire de France, 1 rue Descartes, 75231 Paris, France. § Centre in Green Chemistry and Catalysis, Faculty of Arts and Sciences, Department of Chemistry, Université de Montréal, P.O. Box 6128, Station Downtown, Montréal, Québec, Canada H3C 3J7. Supporting Information Placeholder † ‡
ABSTRACT: Herein, we report the catalytic asymmetric synthesis of functionalized fluorocyclopropanes from -fluoroacrylates. The method using Rh2((S)-TCPTTL)4 allowed the difficult reaction of an in situ generated electrophilic Rh-carbene with an electron poor -fluoroacrylate. The desired fluorocyclopropanes were obtained in good yields, excellent dr and ee. Finally, the mechanism of this transformation was studied by Density Functional Theory (DFT) calculations to explain the particular reactivity of the donor-acceptor diazo compounds with electron deficient -fluoroacrylates.
KEYWORDS: Fluorine, Cyclopropanes, Rhodium, DFT calculations, Asymmetric synthesis. As the smallest cycloalkane, the cyclopropane ring is present in many natural or synthetic bioactive compounds. Indeed, its incorporation into molecules of biological interest increase their biodisponibility and metabolic stability due to its high structural rigidity.1 Besides, the fluorine atom displays singular properties thanks to its high electronegativity and its small size. These specific features induced modifications of the physico-chemical properties of the fluorinated molecules such as acidity or lipophilicity, for instance.2 Hence, fluorinated cyclopropanes are interesting scaffolds, which combine the properties of the cyclopropane ring and the fluorine atom, exhibit promising biological properties.3 Despite their high interest, few enantioselective methods exist for their synthesis. Among them, the Michael-Initiated Ring Closure (MIRC) strategy, developed by Hu (Scheme 1, eq. 1),4 and the Simmons-Smith approach, developed by one of us (Scheme 1, eq. 2),5 displayed high diastereo- and enantioselectivities, but at the cost of the use of a stoichiometric amount of chiral species. As an alternative, the Cu- or Rh-catalyzed reactions of diazo compounds with fluoroalkenes represent a promising approach. The use of tertbutyl -cyanodiazo acetates allowed access to a large scope of fluorocyclopropanes with high ee values, albeit with moderate diastereoselectivities (Scheme 1, eq. 3).6 The reaction with
monoacceptor7 or donor-acceptor8 diazo derivatives is restricted to the cyclopropanation of α-fluorostyrenes, which are rather electron-rich substrates (Scheme 1, eq. 4).9 Thus, to broaden the current portfolio of the available methods to build up functionalized fluorocyclopropanes, we aimed at developing the Rh-catalyzed enantioselective cyclopropanation of -fluoroacrylates with aryldiazoacetates.
Scheme 1. Previous work.
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Rh
Stoechiometric use of a chiral agent: O R1
OMe
N
O NTs S R2 Ph F
R
Base
R1
O
CONMe2
O
CONMe2
F
nBu B
R
OH
I
+
OH
ZnF
eq. 2
H
CO2tBu Chiral [Rh] catalyst N2
R = alkyl
N Rh O ArO2S
Rh
CO2tBu
R
eq. 3 CN F low diastereoselectivity
O
Rh
tBu O
Rh O O
O
Rh
tBu N
Rh O
N
4 Ar = 4-(C12H25)C6H4 A = Rh2((S)-DOSP)4
R
NC
F
eq. 1
H F
Chiral catalyst: R
OMe N
O 2F
O
Page 2 of 5 O F
O F 4
B = Rh2((S)-NTTL)4
O
tBu N
Rh O
F F
C = Rh2((S)-TFPTTL)4
O Cl
O Cl 4
Cl Cl
4
D = Rh2((S)-TCPTTL)4
a Conditions:
1a (1 equiv), [Rh] (2 mol %), 2a (1.5 equiv), solvent (1 M), T °C. 2a was added as a solution over 2 hours. b determined by 19F NMR. c Determined by HPLC analysis on a chiral stationary phase. d 2a was added as a solution over 8 hours. e Isolated yield.
entry
[Rh]
T
Solvent
Yield (%)b
drb
ee (%)c
1
Rh2(OPiv)4
40
DCM
76
89:11
-
2
A
40
DCM
60
78:22
25
3
B
40
DCM
46
84:16
29
4
C
40
DCM
40
87:13
40
5
D
40
DCM
55
93:7
76
To our delight the desired cyclopropane 3a (Table 1) was formed in 76% NMR yield and 89:11 dr, when the reaction was carried out at 40 °C (entry 1).12 Having demonstrated the feasibility of the process, we then moved on the development of the catalytic asymmetric reaction using chiral Rh-catalysts. Rh2((S)-DOSP)4 allowed the formation of 3a with moderate diastereo- and enantioselectivities (entry 2), while the use of Rh2((S)-NTTL)4 led to a slight increase of both diastereo- and enantioselectivity (entry 3). These selectivities were further improved by using Rh2((S)-TFPTTL)4, giving 3a in 40% yield, 87:13 dr and 40% ee (entry 4). Finally, Rh2((S)-TCPTTL)4 proved to be the optimal catalyst for this reaction, over those tested, leading to 3a in 55% yield, 76% ee and 93:7 dr (entry 5). A decrease of the reaction temperature from 40 °C to -20 °C allowed to reach 87% ee and excellent dr (>95:5), albeit with a moderate 37% yield (entry 6). A screening of solvents was performed but no improvement of the yield of the reaction or the enantioselectivity was observed (entries 7-8). Finally, a longer addition time of the 2a solution from 2 to 8 hours provided 3a in 53% isolated yield as a single diastereoisomer with 88% ee (entry 9). With the optimal set of conditions in hand, we then explored the scope of the reaction with benzyl 2-fluoroacrylate 1a and various tert-butyl -aryldiazoacetate 2a (Scheme 2). Aryldiazoacetates bearing halogens at the para position (3b-d) offered the best results in terms of yields (up to 82% NMR yields), enantioselectivities (95-97% ee) and diastereoselectivities (>95:5). Note that in most cases, the isolated yields are significantly lower than the NMR yields due to a tedious purification of the fluorocyclopropanes. For instance, the use of tert-butyl -(parachlorophenyl)diazoacetate 2d yielded the corresponding fluorocyclopropane 3d in 82% NMR yield, as a single diatereoisomer with 97% ee (vs 52% isolated yield). The presence of a fluorine atom (3e) and a trifluoromethyl group (3f) at the para position also allowed performing the reaction with excellent diastereo- (> 95:5) and enantioselectivities (8891% ee), albeit in lower NMR yields (53% and 50%, respectively). The presence of a chlorine atom at the meta position (3g) induced a significant decrease in enantioselectivity (61%), while yield and diastereoselectivity remained excellent.
6
D
-20
DCM
37
>95:5
87
Scheme 2. Scope of the reaction. a
7
D
-20
Et2O
NR
-
-
8
D
-20
toluene
31
>95:5
87
9d
D
-20
DCM
64 (53)e
>95:5
88
Ar
R1
+
CO2R2
Chiral [Cu] or [Rh]
N2
F
R1
Ar
catalyst R = H, Ar
eq. 4
CO2R2
F Only electron-rich fluoroalkenes
This work: CO2Bn
Chiral [Rh]
CO2tBu
Ar
+
N2
F
catalyst
Ar
BnO2C
CO2tBu F electron-deficient fluoroalkenes Highly diastereoselective Highly enantioselective
The use of -fluoroacrylates constitutes a real challenge since these electron-deficient substrates do not seem to be compatible with the Rh-carbene electrophilicity at first glance. Indeed, reports dealing with the cyclopropanation of acrylates using Rh-carbenes are scarce. The unique general method was reported by Davies using Rh2((S)-TCPTAD)4 as a catalyst in the presence of a large excess of acrylates (5 equiv.).10 Note that sporadic examples were also reported on particular substrates or with limited efficiency.11 Herein, we report our investigations using a chiral Rh-catalyst and -fluoroacrylates to build up chiral fluorocyclopropanes. In addition, a careful study of the reaction mechanism using Density Functional Theory (DFT) calculations is depicted to explain the particular reactivity of the Rh-carbene with the electron deficient fluoroacrylates. At the beginning of the project, we studied the racemic cyclopropanation reaction of benzyl 2-fluoroprop-2-enoate 1a with tert-butyl -phenyldiazoacetate 2a using Rh2(OPiv)4 as a catalyst in dichloromethane (DCM).
Table 1. Optimization of the reaction conditions.a CO2Bn
+
Ph
F 1a
CO2tBu
[Rh] (2 mol%)
N2
Ph
BnO2C
solvent, T °C
CO2tBu
F
2a
3a
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ACS Catalysis EWG F
1a-b
+
CO2tBu Rh2((S)-TCPTTL)4 (2 mol%)
Ar N2
DCM, -20 °C
Ar
EWG
CO2tBu
F
2a-g
3a-h
1a, EWG = CO2Bn 1b, EWG = CON(Me)OMe
2a, Ar = C6H5 2b, Ar = 4-I-C6H4 2c, Ar = 4-Br-C6H4 2d, Ar = 4-Cl-C6H4
2e, Ar = 4-F-C6H4 2f, Ar = 4-CF3-C6H4 2g, Ar = 3-Cl-C6H4
I
BnO2C F
CO2tBu
3a, 64%,b 53%c (>95:5)d ee = 88%e Cl
Br
BnO2C
BnO2C
CO2tBu F 3b, 71%,b 43%c (>95:5)d ee = 95%e F
CO2tBu F 3c, 79%,b 57%c (>95:5)d ee = 96%e CF3
BnO2C
BnO2C CO2tBu CO2tBu F F 3d, 82%,b 52%c (95:5)d 3e, 53%,b 31%c (>95:5)d ee = 97%e ee = 91%e
BnO2C
CO2tBu F 3f, 50%,b 28%c (>95:5)d ee = 88%e Br
Cl BnO2C F
Scheme 3. Extension to chloro- and bromoalkenes. a
O CO2tBu
MeO
3g, 71%,b 50%c (>95:5)d ee = 61%e Unreactive diazo compounds
N
F
Indeed, we noticed that aryl bearing an electron-donating (Me and OMe) or strong electron-withdrawing group (CN or NO2) shut down the cyclopropanation reaction and no desired product was formed. First, a rudimental correlation of these results with the Hammett constants was carried out to understand the reaction outcome, but without success.13 Hence, we decided to study more carefully the mechanism of this transformation using DFT calculations to explain the observed results in conjunction with conceptual DFT and atoms-in-molecules theory, following our recent synergetic approach.14 To this aim, we considered the reaction of 1a with four partners: 2a, 2f, 2i, 2k, the two first ones being reactive and the two last ones almost inactive. As no simple reactivity descriptor evaluated on different precursors was successful in discriminating between these two behaviors, the detailed mechanism was then determined using three exchangecorrelation functionals (M06-L, M06-2X, M06-2X-D3) (Scheme 4A).
3h, 62%,b 35%c (77:23)d ee = 99%e R
2h, R = NO2, 0%b 2i, R = CN, 0%b CO2tBu 2j, R = Me, 21%b 2k, R = OMe, 3%b N2
Notably, the presence of strong electron-withdrawing or electron-donating groups at the para position of the aromatic motif hampered the reaction as no formation of the desired fluorocyclopropanes was observed (2h-k). Finally, we demonstrated that the -fluoro Weinreb acrylamide 1b reacted smoothly with tert-butyl -(para-bromophenyl)diazoacetate 2c to afford the desired fluorocyclopropane 3h in 62% NMR yield, 77:23 dr and an excellent 99% ee. We then sought to extend the reaction of aryldiazoacetates to other functionalized haloalkenes as this reaction is restricted to sporadic examples with limited dr and ee (Scheme 3).6 First, -fluoroalkenes substituted at the allylic position by a phthalimide or a sulfone group were reacted with 2c and afforded corresponding fluorocyclopropanes 3i and 3l with excellent yields and ee, but somehow lower diastereoselectivities than those obtained with 1a. Pleasingly, when using the corresponding chloro- and bromoalkenes, excellent dr were obtained, albeit with a slight decrease of the yield and the ee. These last examples demonstrate the influence of the halogen atom on the the alkene (F vs Cl or Br) on the level of diastereoselectivity of the cyclopropanation reaction, ca 60:40 dr for the fluorine atom to more than 95:5 dr with the chlorine or the bromine atom. We have been intrigued by the results obtained when 1a was reacted with various -aryldiazoacetates 2 (Scheme 2).
R +
N2
DCM, -20 °C
2c
1c-h
X
Ar CO2tBu
R
3i-n
Ar = 4-Br-C6H4
1c, X = F, R = NPhth 1f, X = F, R = SO2Ph 1d, X = Cl, R = NPhth 1g, X = Cl, R = SO2Ph 1e, X = Br, R = NPhth 1h, X = Br, R = SO2Ph F
Ar CO2tBu
PhthN 3i,
a
Conditions: 1 (1 equiv), Rh2((S)-TCPTTL)4 (2 mol %), 2 (1.5 equiv), DCM (0.5 M), -20 °C. 2 was added as a solution over 8 hours. b The yield was determined by 19F NMR. c Isolated yield. d The cis:trans ratio was determined by 19F NMR. e The ee of the major diastereoisomer was determined by HPLC analysis on a chiral stationary phase.
CO2tBu Rh2((S)-TCPTTL)4 (2 mol%)
Ar
X
CO2tBu
85%,b
74%c
(64:36)d
3j,
70%,b
ee = 95%, 98%e F
Ar CO2tBu
PhO2S 3l,
80%,b
65%c
(57:47)d
ee = 86%, 91%e
Ar
Cl PhthN
CO2tBu 67%c
(>95:5)d
Br PhthN 3k,
ee = 90%e Cl PhO2S 3m,
60%,b
Ar CO2tBu 54%c
(>95:5)d
ee = 83%e
45%,b
CO2tBu
(>95:5)d ee = 91%e
Br PhO2S 3n
Ar
55%,b
42%c
Ar CO2tBu 55%c
(>95:5)d ee = 81%e
a
Conditions: 1 (1 equiv), Rh2((S)-TCPTTL)4 (2 mol %), 2c (1.5 equiv), DCM (0.5 M), -20 °C. 2c was added as a solution over 8 hours. b The yield was determined by 19F or 1H NMR. c Isolated yield. d The trans:cis ratio, determined either by 19F or 1H NMR. e eetrans, eecis, determined by HPLC analysis on a chiral stationary phase.
Two main steps were identified besides others linked to non-covalent adducts: 1) the generation of the catalytic active Rh-carbene (Int3) through N2 elimination and 2) the formation of the fluorinated cyclopropane (product). Then, two main pathways were investigated (routes “A” & “B”, scheme 4B), leading to the two possible stereoisomers at carbon C1, cis and trans respectively. The corresponding transition states showed coplanar Rh-C and C-C bonds. Interestingly, an “orthogonal” geometry was also found possible, affording unfavorable fivemembered ring byproducts.13 In all cases, the ring formation was occurring concertedly, in one step with a strong asynchronicity. The dual character of the carbon linked to rhodium was also found essential: it is electrophilic in the first part of the cyclization process to form the first carbon-carbon bond, while it is later nucleophilic in order to form the second one (Scheme 4A).13 From a methodological point of view, an important exchange-correlation functional effect was observed, the M06L one, for instance, being not able to account for the outcome difference between 2a and 2k.
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Scheme 4. DFT Calculations – mechanistic studies. a A) Plausible catalytic cycle
C) Turn-over frequencyb vs experimental yields
B) Turn-over frequency determination
M06-2X
tBuO2C Ph
F
2a
Rh2L4 CO2tBu
BnO2C TS2
Rh2L4
Ph
Int5
N2
Rh2L4
2a
TOF = 1.37.105 s-1 TOF = 5.69.103 s-1 TOF = 2.58.103 s-1 TOF = 1.18.103 s-1
2a
Ph Rh2L4
concerted asynchronous
F BnO2C Int4
CO2tBu
M06-2X-D3
2a R = H TOF = 7.8.103 s-1 2f R= CF3 TOF = 7.77.102 s-1 2i R = CN TOF = 1.91.102 s-1 2k R = OMe TOF = 3.4.101 s-1
3a
2f
Int1
2a
2f R CO2tBu
1a
Rh2L4
N2
CO2tBu Ph Int3
N2
CO2tBu
N2
3a
Ph Rh2L4 TS1
Int2
2k
2i
2k 2i
For clarity, L = OAc. b Values of TOF calculated on a two steps simplified mechanism, see reference 12 for details. The two other DFT functionals evidenced that the two main *
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activation barriers were actually comparable, and that, as a consequence, no rate-determining step could be properly Author Contributions defined. All the authors approved the final version of the manuscript. Hence, to rationalize and to theoretically evaluate substituent Notes The authors declare no competing financial interest. effects, the turn-over frequencies were subsequently computed using Kozuch and Shaik’s approach (Scheme 4).15 This ACKNOWLEDGMENT approach was first performed on the reaction of 1a with 2a (Scheme 4B). Then, the study was extended to 2f, 2i and 2k. This work was partially supported by INSA Rouen, Rouen Interestingly, reactive and non-reactive diazo derivatives could University, CNRS, EFRD, Labex SynOrg (ANR-11-LABXthen be differentiated, even if such a model remained 0029), and Région Haute-Normandie (Crunch Network). A.P. thanks the Labex SynOrg (ANR-11-LABX-0029) for a doctoral qualitative (Scheme 4C). In fact, in view of the subtle and fellowship. T.P thanks the Institut Universitaire de France for possibly antagonist effects, an accurate modeling of these support. A.B.C. thanks the Labex SynOrg (ANR-11-LABX-0029) reactions rates is certainly elusive with the standard quantum for a chair of excellence. The CRIANN (Centre Régional chemistry techniques. In summary, the independent analysis of Informatique et d'Applications Numériques de Normandie) is each reaction step was inefficient to explain the observed acknowledged for providing HPC resources. outcome of the transformation. However, a global picture of the reaction, obtained from the determination of the turn-over REFERENCES frequency, allowed to qualitatively rationalize the difference 1 Veber, D. F.; Johnson, S. R.; Cheng, H.-Y.; Smith, B. R.; Ward, of reactivity observed between pretty similar diazo derivatives. K. W.; Kopple, K. D. Molecular Properties That Influence the Oral In conclusion, we reported herein the catalytic Bioavailability of Drug Candidates. J. Med. Chem. 2002, 45, 2615– enantioselective cyclopropanation reaction of 2623. 2 O'Hagan, D. Understanding Organofluorine Chemistry. an fluoroacrylates with -aryl diazoacetates, a difficult process. Introduction to the C–F Bond. Chem. Soc. Rev. 2008, 37, 308–319. The reaction proceeded smoothly giving the desired products 3 (a) David, E.; Milanole, G.; Ivashkin, P.; Couve-Bonnaire, S.; with high dr and ee. The reaction was then extended to Jubault, P.; Pannecoucke, X. Syntheses and Applications of haloalkenes. Finally, the difference of reactivity observed with Monofluorinated Cyclopropanes. Chem. Eur. J. 2012, 18, 14904– various diazo acetates was qualitatively rationalized thanks by 14917. (b) Pons, A.; Poisson, T.; Pannecoucke, X.; Charette, A.; determining overall turn-over frequencies. a
ASSOCIATED CONTENT Supporting Information Experimental procedures, compound characterization data, 1H, 13C, and 19F NMR spectra of the products and HPLC traces (PDF). The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
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ACS Catalysis Highly Functionalized Fluorocyclopropanes. Org. Lett. 2015, 17, 1790–1793. 7 Meyer, O. G.; Fröhlich, R.; Haufe, G. Asymmetric Cyclopropanation of Vinyl Fluorides: Access to Enantiopure Monofluorinated Cyclopropane Carboxylates. Synthesis 2000, 1479– 1490. 8 (a) Hruschka, S.; Fröhlich, R.; Kirsch, P.; Haufe, G. Synthesis of New Enantiopure Fluorinated Phenylcyclopropanecarboxylates – Potential Chiral Dopants for Liquid-Crystal Compositions. Eur. J. Org. Chem. 2007, 141–148. (b) Su, Y.; Bai, M.; Qiao, J. B.; Li, X. J.; Li, R.; Tu, Y. Q.; Gu, P. Diastereo-and Enantioselective Cyclopropanation of Alkyenyl Fluorides with Benzyl Diazoarylacetates. Tetrahedron Lett. 2015, 56, 1805–1807. 9 Note that the cyclopropanation of 1-chloro-1-fluoroethylene has been reported in patents, see: (a) Ebata, Tsutomu; Akiba, Toshifumi; Ikeya, Takanobu; Wakita, Ryuhei; Sasaki, Mikio. PCT Int. Appl. (1997), WO 9708128 A1 Mar 06, 1997. (b) Hatakeyama, Keisuke; Miki, Takashi; Hirata, Norihiko. PCT Int. Appl. (2010), WO 2010005003 A1 Jan 14, 2010. 10 Wang, H.; Guptill, D. M.; Varela-Alvarez, A.; Musaev, D. G.; Davies, H. M. L. Rhodium-Catalyzed Enantioselective Cyclopropanation of Electron-Deficient Alkenes. Chem. Sci. 2013, 4, 2844–2850. 11 For other examples with very limited scope, see for example: (a) Leroy, B. A New and Rapid Access Towards Exo-Methylene-Valerolactones From (Cyclopropyl)Methylstannanes. Tetrahedron Lett. 2005, 46, 7563–7566. (b) Gharpure, S. J.; Shukla, M. K.; Vijayasree, U. Stereoselective Synthesis of Donor−Acceptor Substituted Cyclopropafuranones by Intramolecular Cyclopropanation of Vinylogous Carbonates: Divergent Synthesis of Tetrahydrofuran-3-
One, Tetrahydropyran-3-One, and Lactones. Org. Lett. 2009, 11, 5466–5469. (c) Lindsay, V. N. G.; Fiset, D.; Gritsch, P. J.; Azzi, S.; Charette, A. B. Stereoselective Rh2(S-IBAZ)4-Catalyzed Cyclopropanation of Alkenes, Alkynes, and Allenes: Asymmetric Synthesis of Diacceptor Cyclopropylphosphonates and Alkylidenecyclopropanes. J. Am. Chem. Soc. 2013, 135, 1463–1470. 12 The use of tert-butyl ester is mandatory to ensure high diastereoselectivity. Indeed, methyl ester gave only 59:41 dr and 78% yield. 13 See supporting information for details. 14 (a) Falkowska, E.; Tognetti, V.; Joubert, L.; Jubault, P.; Bouillon, J.-P.; Pannecoucke, X. First efficient synthesis of SF5substituted pyrrolidines using 1,3-dipolar cycloaddition of azomethine ylides with pentafluorosulfanyl- substituted acrylic esters and amides RSC Advances 2015, 5, 6864–6868. (b) Tognetti, V.; Bouzbouz, S.; Joubert, L. A Theoretical Study of the Diastereoselective Allylation of Aldehydes with New Chiral Allylsilanes J. Mol. Model. 2017, 23, 5. 15 (a) Kozuch, S.; Shaik, S. How to Conceptualize Catalytic Cycles? the Energetic Span Model. Acc. Chem. Res. 2011, 44, 101– 110. (b) Kozuch, S.; Shaik, S. A Combined Kinetic−Quantum Mechanical Model for Assessment of Catalytic Cycles: Application to Cross-Coupling and Heck Reactions. J. Am. Chem. Soc. 2006, 128, 3355–3365. (c) Kozuch, S.; Shaik, S. Kinetic-Quantum Chemical Model for Catalytic Cycles: the Haber−Bosch Process and the Effect of Reagent Concentration. J. Phys. Chem. A 2008, 112 (26), 6032– 6041..
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