Abstract
The hydrofluorocarbon 245 isomers, 1,1,1,3,3-pentafluoropropane, 1,1,1,2,2- pentafluoropropane, and 1,1,1,2,3-pentafluoropropane (HFC-245fa, HFC-245cb, and HFC-245eb) were activated through C–F bond activations using aluminium chlorofluoride (ACF) as a catalyst. The addition of the hydrogen source Et3SiH is necessary for the activation of the secondary and tertiary C–F bonds. Multiple C–F bond activations such as hydrodefluorinations and dehydrofluorinations were observed, followed by hydroarylation and Friedel–Crafts-type reactions under mild conditions.
Keywords: aluminum fluoride; C–F bond activation; dehydrofluorination; hydrodefluorination; hydrofluorocarbons
*Original abstract online at https://www.beilstein-journals.org/bjoc/articles/16/213
*Full article online at http://fluoridealert.org/wp-content/uploads/kervarec-2020.pdf
Excerpt
Introduction
Hydrofluorocarbons (HFCs) have been intensively used in daily life, mainly due to their excellent properties in refrigeration applications [1-3]. In the past, HFCs were considered as replacements that do not deplete ozone for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which have been strictly regulated by the Montreal protocol [4-6]. However, due to the high global warming potential (GWP), HFCs have also been included in the Montreal protocol in 2019 (Kigali amendment) and have to be phased out [7-10].
On the other hand, HFCs are valuable starting compounds or intermediate products for the synthesis of hydrofluoroolefins (HFOs), which have been regarded as the next generation of refrigerants, exhibiting zero ozone depletion potential (ODP) and a negligible GWP [11-13]. A considerable amount of studies has been carried out to synthesize HFOs under mild conditions [11,14-16]. Among them are routes to access 2,3,3,3-tetrafluoropropene and 1,3,3,3-tetrafluoropropene (HFO-1234yf and HFO-1234ze), for which numerous patents suggest synthetic pathways and showcase the reactivity [12,13,15]. One possibility for the preparation includes the conversion of pentafluoropropanes (HFC-245 isomers) using chromia-based catalysts, or metal chloride/fluoride (AlF3, MgF2)-supported catalysts at elevated temperatures (350 °C) [11,14,15,17,18]. The group of Lu recently reported the gas-phase transformation of 1,1,1,3,3-pentafluoropropane (HFC-245eb) into 1,3,3,3-tetrafluoropropene (HFO-1234ze) using mesoporous nanoscopic aluminum fluoride-based catalysts [19]. The catalysts were prepared via a sol–gel process in the presence of polyols, allowing for the evolution of a large surface area and improved acidic properties when compared to fluorinated Cr2O3 or traditional ?-AlF3 catalysts. At a reaction temperature set at 280 °C, the conversion of 1,1,1,2,2- pentafluoropropane (HFC-245fa) into the 1,3,3,3-tetrafluoropropene (HFO-1234ze) varied between 50 and 60%, depending on the conditions used to synthesize the catalyst, reaching almost full selectivity. The harsh conditions are in part needed due to the high dissociation energy of C–F bonds, and in general, C–F activation steps are considered to be challenging [20-27]…
*Full article online at http://fluoridealert.org/wp-content/uploads/kervarec-2020.pdf
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