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Catalytic Synthesis of Trifluoromethyl Cyclopropenes and Oligo-Cyclopropenes

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Catalytic Synthesis of Trifluoromethyl Cyclopropenes and Oligo-Cyclopropenes Catalytic synthesis of trifluoromethyl cyclopropenes and oligo-cyclopropenes Uyen P. N. Tran,[a,b] Renè Hommelsheim,[a],+ Zhen Yang,[a],+ Claire Empel,[a] Katharina J. Hock,*[a] Thanh V. Nguyen*[b] and René M. Koenigs*[a] Abstract: The synthesis of trifluoromethylated cyclopropenes is very high catalyst loading was required to facilitate the reactions on a important for applications in drug discovery and functional materials. small substrate scope of only four aromatic alkynes.[2g] A general In this report, we describe the application of readily available, chiral and broadly applicable catalytic approach for the rhodium(II) catalysts in a highly efficient asymmetric enantioselective synthesis of fluorinated cyclopropenes has not cyclopropenation reaction of fluorinated donor-acceptor been described until now and still remains one of the major diazoalkanes using a broad variety of aliphatic and aromatic alkynes. challenges in asymmetric synthesis. Further studies highlight the unique reactivity of fluorinated donor- As part of our ongoing interest in small fluorinated molecules, we acceptor diazoalkanes in the synthesis of oligo-cyclopropenes. became intrigued by cyclopropenation reactions of fluoroalkyl- Subsequent C-H functionalization of trifluoromethyl cyclopropenes substituted donor-acceptor diazo compounds[7] with alkynes and furnishes densely substituted cyclopropene frameworks and also oligo-alkynes. This approach would result in a concise synthesis allows the synthesis of bis-cyclopropenes. of valuable unsaturated trifluoromethyl-cyclopropenes that can be readily functionalized for further synthetic values. Based on our previous study in this field,[8] we envisioned that Cyclopropenes are the smallest carbocycles containing at least commercially available chiral Rh(II) complexes would be suitable one C-C double bond and are a fascinating class of highly catalysts to promote cyclopropenation reactions of fluoroalkyl- strained small molecules with applications in the fields of drug substituted diazo compounds. Such method would not only discovery, catalysis and materials chemistry.[1] Existing methods provide a simple and convenient access to chiral trifluoromethyl- to construct these compounds typically rely on catalytic carbene substituted cyclopropenes, but also be a significant improvement transfer reactions of ester-substituted diazoalkanes and alkynes over existing methods in terms of efficiency and selectivity. using chiral Cu(I), Rh(II), Ir(III) or Au(III) catalysts.[2,3] More recently, light-mediated processes were demonstrated as powerful alternatives to access these important strained this work [4] CF3 carbocycles. Despite tremendous research efforts, the N2 Rh2(S)-BTPCP4 Ar + synthesis of the trifluoromethylated cyclopropene subclass Aryl / Alkyl Ar CF3 remains a challenge in organic synthesis.[3] On the other hand, Aryl / Alkyl small molecules containing at least two cyclopropene units, or Alkyne Trifluoromethyl- Trifluoromethyl oligo-cyclopropenes, have been rarely reported, although their diazoalkanes cyclopropenes oligo-cyclopropane counterparts have been well explored in key features oligocyclopropenes cyclopropene functionalization [5] literature. In 1986, Okamoto and co-workers described a C-H functionalization - aliphatic alkynes of CF3-cyclopropenes synthetic protocol for bis-cyclopropenes by cycloaddition - aromatic alkynes Ph [6a] - high enantioselectivity CF reactions of free carbenes and diaryl-substituted alkynes. Lin F C CF 3 - readily available 3 3 Ar and co-workers subsequently demonstrated that bis-alkynes Rh(II) catalysts Ph - low catalyst loading could be transformed into bis-cyclopropenes with a multi-step Aryl / Alkyl synthesis using ruthenium catalysts.[6b] To the best of our knowledge, there has been no report of a catalytic method for efficient synthesis of oligo-cyclopropenes. Scheme 1. Enantioselective synthesis of trifluoromethyl cyclopropenes. From the perspective of the carbene-transfer reagent, while the majority of research was performed on ester-substituted diazoalkanes, limited examples report on cyclopropenation We set out our investigations by examining different Rh(II) reactions of diazoalkanes with other electron-withdrawing catalysts in the reaction of 5-chloro-pent-1-yne (1a) and (1- substitutions such as nitrile, sulfonyl or fluorinated alkyl diazo-2,2,2-trifluoroethyl)benzene (2) using toluene as solvent.[9] [2,3] groups. In particular, the asymmetric carbene-transfer The phthalimido rhodium series (Rh2NTTL4, Rh2PTAD4, reaction of trifluomethylated diazoalkanes for the Rh2PTTL4, see entries 1-3 in Table 1) only gave reasonable enantioselective synthesis of their cyclopropene derivatives is cyclopropene product yields with a moderate level of enantio- even less investigated. In 2011, Katsuki et al. reported the induction. Contrarily, Rh2BTPCP4 proved to be highly efficient application of a chiral Ir(III) complex in cyclopropenation and excellent stereoselectivity was observed (entry 5). Although reactions of trifluoromethyl-substituted diazoalkanes, though Rh2DOSP4 has been reported to be an excellent catalyst for cyclopropenation reactions of ester-substituted diazoalkanes, as demonstrated by Davies and co-workers,[2c] this catalyst proved [a] Institute of Organic Chemistry, RWTH Aachen University to be inefficient in our cyclopropenation reaction of the aliphatic Landoltweg 1, 52074 Aachen, Germany E-mail: [email protected] alkyne 1a with fluorinated diazoalkane 2 (entry 5). To further E-mail: [email protected] understand this marked difference in reactivity, we investigated [b] School of Chemistry, University of New South Wales the reactivity of aromatic alkynes with Rh2DOSP4 and Sydney, NSW 2052, Australia Rh BTPCP . In the reaction of diazoalkane 2 with p- E-mail: [email protected] 2 4 [+] These authors contributed equally. tolylacetylene, both catalysts proved to be highly efficient and the desired aryl-substituted cyclopropene was obtained with excellent stereoselectivity (entries 6-7), clearly indicating that position gave significantly reduced yields and the cyclopropenes Rh2BTPCP4 is more suitable for wider alkyne substrate scope. could be isolated with only moderate enantioselectivity, which No better results were obtained using ethers or halogenated can be attributed to the steric hindrance imposed by ortho- solvents; higher reaction temperatures resulted in significantly substituents (5l,m). Pyridinyl and non-terminal alkynes proved to reduced product yields (entries 8-12, Table 1). be unreactive in this transformation (4o,p). Table 1. Reaction Optimization. CF N2 Rh2BTPCP4 (0.5 mol-%) 3 1 R CF3 toluene, -78 °C N CF3 1 2 3 R1 N catalyst (1 mol-%) Cl CF 3 CF3 CF3 CF3 1a Cl 2 3a entry[a] catalyst Solvent/T Yield e.r. Cl 3a 3b Br 3c 96 : 4 e.r. 97 : 3 e.r. 94 : 6 e.r. 1 Rh2NTTL4 toluene / –78 °C 44% 67 : 33 93% 98% 76% 2 Rh2PTTL4 toluene / –78 °C 58% 64 : 36 CF3 CF3 CF3 3 Rh2PTAD4 toluene / –78 °C no rct. - 3d 3e 3f MeO2C Ph 4 Rh2DOSP4 toluene / –78 °C no rct. - 93 : 7 e.r. 89 : 11 e.r. 95 : 5 e.r. 58% 75% 81% 5 Rh2BTPCP4 toluene / –78 °C 93% 96 : 4 CF3 CF3 CF3 CF3 [b] 6 Rh2DOSP4 toluene / –78 °C 97% 94 : 6 7[b] Rh BTPCP toluene / –78 °C 97% 96 : 4 PhO 2 4 BnO 3g 3h 3i 3j 92 : 8 e.r. 95 : 5 e.r. 88 : 12 e.r. 8 Rh2BTPCP4 hexane / –78 °C 59% 85: 15 92 : 8 e.r. 62% 61% 64% 82% 9 Rh2BTPCP4 DCM / –78 °C <5% n.d. Scheme 2. Substrate scope of aliphatic alkynes. 10 Rh2BTPCP4 THF / –78 °C <5% n.d. 11 Rh2BTPCP4 toluene / –45 °C 59% 94 : 6 We subsequently decided to apply our newly developed method 12 Rh2BTPCP4 toluene / RT 49% 90:10 to investigate the challenging synthesis of oligo-cyclopropenes from substrates bearing multiple alkyne moieties. These studies [a]Reaction conditions: 1a (2.5 eq.) and catalyst were dissolved in the would a) reveal insights into the chemoselectivity and the appropriate solvent (1 mL) and a solution of 2 (1.0 eq.) in 1 mL solvent was reactivity of oligo-alkynes in this transformation and b) provide added over 3 hours at the given temperature and stirred for 12 h; [b]Reaction with p-tolylacetylene instead of 1. convenient access to the structural class of rare oligo- cyclopropenes. We commenced these studies by investigating With the optimal conditions in hand, we next investigated a the reaction of 1,4-bis(ethynyl)benzene (6, Table 2) with (1- range of aliphatic alkynes with different chain length, halogen- diazo-2,2,2-trifluoroethyl) benzene (2). In principal three different and ester substituents as well as branched aliphatic alkynes in reaction products can be obtained from this transformation, this asymmetric cyclopropenation reaction (Scheme 2). In all namely the mono-cyclopropene 7, and two different cases we obtained the desired trifluoromethylated diastereoisomers of bis-cyclopropene 8. We therefore embarked cyclopropenes with a high level of enantioselectivity and good to to study the effect of different rhodium(II) catalysts and the excellent isolated yields. Surprisingly, benzylic and olefinic stoichiometry of reactants on the outcomes this transformation. substituents (entries 3e and 3j) had a slightly detrimental effect Much to our surprise, the latter had only little influence on the on the enantio-induction. Nevertheless a good level of product distribution. The uses of one or two equivalents of 1,4- enantioselectivity was obtained and exclusive cyclopropenation bis(ethynyl)benzene 6 both resulted in the bis-cyclopropene 8 as was observed for these two substrates. Having established a the only
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