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Efficient Synthesis of New Fluorinated Building Blocks by Means of Hydroformylation

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Efficient Synthesis of New Fluorinated Building Blocks by Means of Hydroformylation Fluorine Chemistry CHIMIA 2014, 68, Nr. 6 371 doi:10.2533/chimia.2014.371 Chimia 68 (2014) 371–377 © Schweizerische Chemische Gesellschaft Efficient Synthesis of New Fluorinated Building Blocks by means of Hydroformylation Lidia Fanfonia, Lisa Diaba, Tomas Smejkal*b, and Bernhard Breit*a Abstract: Hydroformylation of fluorinated alkenes is an efficient method for the preparation of fluorinated functionalized building blocks for the synthesis of biologically active target structures. In this article we summarize known hydroformylation reactions of fluorinated olefins and we add new results from our research groups. Particular attention is paid to the remarkable influence of organofluorine substituents on catalyst activity, regio- and stereoselectivity of the hydroformylation reaction. Keywords: Aldehydes · Alkenes · Hydroformylation · Organofluorides · Rhodium 1. Introduction Fluorine is a key element commonly used in modern pharma and agrochemical products. As a result of the unique elec- tronic and steric properties, fluorinated substituents can beneficially influence the physico-chemical properties, improve bio- logical activity and modulate metabolism Scheme 1. Efficient synthesis of the herbicide Prosulfurone. of active ingredients.[1] Therefore, new technologies which allow the selective and Hydroformylation is a relatively un- 2. Substrates Fluorinated at the cost-efficient synthesis of various fluori- derutilized reaction regarding the use of Double Bond nated intermediates are highly desirable. fluorinated olefins so far. This is surpris- During the last few years, particularly the ing considering the fact that it could poten- 2.1 Vinyl Fluoride toolbox of transition metal-catalyzed reac- tially provide synthetically highly versatile The hydroformylation of vinyl fluoride tions for late stage introduction of a single intermediates – fluorinated aldehydes. (VF) was studied by Ojima and found to fluorine atom or small fluorinated moieties Moreover, hydroformylation is an example furnish 2-fluoropropanal (2-FPA) exclu- (-CF , -CF H, -SCF ,…) has been expand- 3 2 3 of a particularly atom-economic catalytic sively irrespective of the catalyst applied ing rapidly.[2] reaction, which is well established for the (see Scheme 2, Table 1).[7] The high regi- An alternative strategy exploits de- synthesis of bulk chemicals on a multiton oselectivity for the branched aldehyde is rivatization of fluorine-containing sub- scale.[5] most likely caused by the electronegative strates.[3] In this context we became in- The aim of this article is to summarize fluorine substituent which stabilizes the terested in transition metal-catalyzed known hydroformylations of fluorinated reactions of fluorinated olefins which are alkenes. Particular attention is paid to the readily available starting materials (large remarkable influence of organofluorine amounts of fluorinated monomers are uti- substituents on catalyst activity, regio- and lized by the polymer industry). stereoselectivity – all factors which are de- One prominent example of this ap- cisive for possible industrial applications. proach is the elegant synthesis of the Some new results from our groups are dis- herbicide Prosulfuron using 3,3,3-trifluo- cussed as well.[6] ropropene in a one-pot reaction sequence of a palladium-catalyzed Heck reaction Scheme 2. Regioselective hydroformylation of followed by hydrogenation (Scheme 1).[4] vinyl fluoride. Table 1. Results of hydroformylation of vinylfluoride. Entry Cat. CO/H [atm] Substr/Metal T [°C] l/b yielda 2 [%] a b 1 Rh (CO) 68 2000 80 0/100 81 *Correspondence: Prof. Dr. B. Breit , Dr. T. Smejkal 4 12 aInstitut für Organische Chemie 2 HRh(CO)(PPh ) 68 500 80 0/100 52 Albert-Ludwigs-Universität Freiburg 3 3 Albertstrasse 21, 79104 Freiburg i. Bg., Germany 3 Ru (CO) 68 100 80 0/100 46 E-mail: [email protected] 3 12 bSyngenta Crop Protection Münchwilen AG 4 Co (CO) 110 100 100 0/100 30 2 8 Schaffhauserstrasse, CH-4332 Stein, Switzerland E-mail: [email protected] Conditions: vinyl fluoride 20 mmol, toluene, 18h. aisolated product after distillation. 372 CHIMIA 2014, 68, Nr. 6 Fluorine Chemistry adjacent rhodium-carbon bond formed during the alkene hydrometalation step. 2.2 1,1-Difluoroethene The first and only evidence of a suc- cessful hydroformylation applying Scheme 3. Cobalt-catalyzed hydroformylation Scheme 4. Rhodium-catalyzed hydrofor- 1,1-difluorothene as the substrate dates of 1,1-difluoroethene. mylation of 1,1-difluoroethene. back to a German patent from Hoechst in 1953.[8] With Co (CO) as the precatalyst 2 8 under harsh reaction conditions (180 atm of syngas and 120–140 °C reaction tem- perature) the main reaction product ob- served was 3,3-difluoro-1-propanol which is formed through hydrogenation of the corresponding aldehyde, with a conversion of 49% (Scheme 3). The aldehyde hydrate of 3,3-difluoropropanal was found as a by- product. Scheme 5. Best rhodium catalyst for hydroformylation of 1,1-difluoroethene. Since rhodium catalysts show an in- trinsic higher activity and chemo- selectivity in the hydroformylation of olefins we speculated that it should be possible to develop a hydrofor- mylation of 1,1-difluoroethylene under milder reaction conditions (Scheme 4). However, after screening of a large number of rhodium complexes together with a series of monodentate and bidentate ligands with different steric and electronic properties as well as exploiting a range of reaction conditions only very low hydrofor- mylation activity was observed. Notably, the best result in terms of conversion and regioselectivity was obtained applying the electron-rich para-anisylphosphine ligand. With this catalyst system and the condi- tions indicated in Scheme 5 the hydrate of the linear aldehyde was obtained (10% conversion, TOF = 0.22 h–1, TON = 5.2). In order to understand why the hydro- Fig. 1. Comparison between the energy of the transition states in ethylene and 1,1-difluoroethene formylation of this particular substrate hydroformylation considering rhodium catalyst bearing PPh as modifying ligand (phenyl substitu- 3 is so difficult we performed high level ents omitted for clarity). ab initio calculations (CCSD(T)/aug-cc- pwCVD(Q)Z//BP86/aug-cc-pVTZ) of the Scheme 6. rate and selectivity determining energetic Hydroformylation of span of the hydroformylation reaction.[9] tetrafluoroethylene. For triarylphosphine-modified rhodium catalysts this is the energy difference start- ing from the catalyst resting state, the rhodium-dicarbonyl hydrido diphosphine complex to the transition state of the hy- drometalation step. Hence the energetic span for ethylene as well as for 1,1-difluo- of tetrafluoroethylene has been reported markable dependence of the regioselectiv- roethylene were calculated and compared to occur under harsh reaction conditions ity of this hydroformylation on the nature (Fig. 1). (Scheme 6). Also here the product is the of the catalyst was found. While rhodium Indeed, it was found that the ∆∆GTS alcohol which is derived from a subsequent catalysts furnished the branched aldehyde for these two substrates is 4.74 kcal/mol. hydrogenation of the corresponding alde- selectively, using dicobalt octacarbonyl a In other words, the hydroformylation of hyde.[8] However, the isolated yield was regioselectivity switch in favor of the lin- 1,1-difluoroethene is 10000 times slower low (7%). ear aldehyde was noted. With a chiral plat- than that of ethylene. Presumably, the pres- ence of a ligand able to raise the electronic density at the metal might lower the energy 3. Substrate Fluorinated alpha to of this transition state facilitating the reac- the Double Bond tion. 3.1 3,3,3-Trifluoropropene (TFP) 2.3 1,1,2,2-Tetrafluoroethene The hydroformylation of TFP was first Scheme 7. Hydroformylation of 3,3,3-trifluoro- A cobalt-catalyzed hydroformylation studied by Ojima et al.[7] Interestingly, a re- propene. Fluorine Chemistry CHIMIA 2014, 68, Nr. 6 373 inum catalyst the branched aldehyde was Table 2. Effect of different precatalysts and ligands in the hydroformylation of TFP. obtained in good enantioselectivity (ee > 90%). However, the regioselectivity was [10] rather low (Scheme 7). O PPh2 Building on these earlier experiments O O N PPh2 H O PPh2 with the goal to identify more active and PPh2 PPh2 selective catalysts we screened a series Xantphos 6-DPPon Diop of different precatalysts and ligands in Entry Catalyst CO/H Substr/Metal T [°C] time l/ba 2 the hydroformylation of TFP (Table 2). [bar] [h] Our self-assembling 6-diphenylphosphi- [11] 1 Rh(CO) acac/6-DPPon 20 1000 70 24 5/95 nopyridone ligand (6-DPPon) together 2 with Rh(CO) acac furnished an extremely b 2 2 Rh (CO) /PPh 110 1200 80 20 3/97 competent catalyst (entry 1). This com- 6 16 3 3 PtCl (Xantphos)/SnCl 96 100 80 48 96/4 plex delivered the branched aldehyde with 2 2 4b PtCl (diop)/SnCl 130 100 100 471/29 nearly perfect regioselectivity in excellent 2 2 yield. In addition, conditions much milder 5 Co (CO) 45 100 100 24 96/4 (70 °C, 20 bar) and very low catalyst load- 2 8 6b Co (CO) 130 50 100 20 93/7 ings could be used. Interestingly, when 2 8 we modified the platinum system with Conditions: ligands 10%, toluene. Conversion > 90%. adetermined by HRGC. bref. [7]. Xantphos ligand (entry 3) high regioselec- tivity in favour of the linear aldehyde was noted. F3C F3C COOH NH2 All fluoro-aldehydes obtained in the H2N COOH hydroformylation of TFP, described above, AcNH 2 AcNH2 CO, H 2 77% CO, H2 80% may serve as excellent intermediates for Co (CO) 2 8 Co2(CO)8 [12] CO Et CF3 the synthesis of fluoro-amino acids and N 2 CF3 S ,Et N S various other useful heterocycles such as 1) CHCl2CO2Et 8 3 S LDA, THF F3C F3C O DMF,67% H2N substituted 2-aminothiophene-3-carboxyl- CO2Et H2N N 2) S 1 CO2Et ate (1 in Scheme 8)[13] or 2-aminothiazole O 4 H2N NH2 [14] 1) Br2,DCM derivatives (2, 3 and 4 in Scheme 8).
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