2-(Perfluoroalkyl)ethyl Triflates, Building Blocks for the Synthesis of Bis(polyfluoroalkylated) Cyclopentadienes

Abstract

Fluorinated building blocks, 2-(perfluorohexyl)ethyl triflate (11a) and 2-(perfluorooctyl)ethyl triflate (11b) were prepared from the corresponding fluoroalkanols and triflic anhydride. Fluorinated triflates 11 were employed for the preparation of bis(polyfluoroalkylated) cyclopentadienes 9, which are potential ligands for homogeneous catalysts used in fluorous biphase systems. Fluorotriflates 11 are also excellent substrates for the preparation of various polyfluorinated compounds, e.g. nitriles or azides, by nucleophilic substitution.Homogeneous catalysis in biphase systems containing fluorous phase belongs to the most attractive topics in recent organofluorine chemistry. 1,2 An essential part of these systems consists of a homogeneous catalyst with significant fluorophilic properties, which are introduced into the molecule by means of fluorophilic ligands. These catalysts found also excellent use in reactions in supercritical CO 2 . 3 Sufficient fluorophilic properties of the catalysts are achieved provided that the overall fluorine content in the molecule is high. 4 This results in the need of fluorophilic ligands with multiple fluorinated chain. Phosphines with multiple fluorinated chains can be synthesized relatively easily and are hence mostly used for fluorophilic catalysts.Homogeneous catalysts with cyclopentadiene based ligands are very common and therefore it is surprising that only little attention has been paid to the synthesis of cyclopentadiene derivatives with fluorophilic properties. 5,6 Known cyclopentadienes contain only one fluorinated chain and hence their fluorophilic properties are insufficient. We were therefore interested in developing a strategy for the synthesis of cyclopentadienes with two attached fluorinated chains.Our first strategy for the preparation of bis(polyfluorinated) cyclopentadienes 9 started from cyclic epoxides. The epoxide ring in 1 was opened with vinylmagnesium bromide, then radical addition of a perfluoroalkyl iodide, iodide reduction and final dehydration gave 4 in 5.4% yield. 7 Although this strategy worked well for the model compound, 6-oxabicyclo[3.1.0]hexene (cyclopentene-1,2-oxide, 1), we obtained unexpected side-product 7 when attempting to prepare the target fluorocyclopentadienes 9 from 3,7-dioxatricyclo[4.1.0.0 2,4 ]-heptane 5 (see Scheme 1).We therefore turned our attention to methods which employ reactions of cyclopentadiene based carbanions with polyfluoroalkylated electrophiles. This approach was used earlier for the preparation of cyclopentadienes substituted by one polyfluoroalkyl chain (8). 6 We first prepared monopolyfluorinated cyclopentadiene 8 (mixture of 1-and 2-substituted isomers in about 1:1 ratio) according to the literature. 6 Lithiation by BuLi and then reflux of the carbanion formed with 2-(perfluorohexyl)ethyl iodide gave the bis-polyfluoroalkylated cyclopentadiene 9 in very poor yield, mostly because of low conversion of the reaction (Scheme 2). Scheme 1 i CH 2 = CHMgBr, CuI, THF, 0°C, 3 h, 54%; ii C 8 F 17 I, AIBN, 100°C, 1 h, 64%; iii...

Electrophilic polyfluoroalkylating agents based on sulfonate esters

“…With lithium phenylacetylide or tetrabutylammonium difluorodimethylphenylsilicate, polyfluorinated phenylacetylene 60 and fluoride 61 were formed in low yields, 28 and 14%, respectively. This reflects that triflate can be successfully reacted even with nucleophiles of moderate strength [20,21] (Scheme 21). 2-(Perfluoroalkyl)ethyl triflate was used for the N-alkylation of 3-(4-pyridyl)propan-1-ol (62).…”

“…With strong nucleophiles such as azide or cyanide, polyfluoroalkyl triflates undergo substitution and the corresponding polyfluorinated azide 58 and cyanide 59 was obtained in moderate yield 55 and 51%, respectively [20,21]. With lithium phenylacetylide or tetrabutylammonium difluorodimethylphenylsilicate, polyfluorinated phenylacetylene 60 and fluoride 61 were formed in low yields, 28 and 14%, respectively.…”

“…For the more reactive sulfonates such as triflates or nonaflates, this work up technique is not recommended, but it has been successfully used for the separation extraction between aqueous and organic system [23,24]. Alternatively, they can be separated by simple filtration through a short column of silica gel and eluted with dichloromethane [12,20,21].…”

“…These esters are also mentioned in patents [61,62]. In 2001, it was described their synthesis on a laboratory scale [20,21] from the reaction of 2-(perfluoroalkyl)ethyl alcohols with the triflic anhydride (Scheme 3) in dichloromethane and hexane with pyridine as a base. The solution of the alcohol, dichloromethane/hexane and pyridine was slowly dropped to the stirred solution of triflic anhydride at À10 8C.…”

“…In the second, the compounds are added in reversed sequence. A solution of the alcohol in dichloromethane in the presence of a base is added to the cooled mixture of sulfonic acid anhydride in dichloromethane [20,21].…”