NHC·Alane Adducts as Hydride Sources in the Hydrodefluorination of Fluoroaromatics and Fluoroolefins

Abstract

We present herein the utilization of NHC-stabilized alane adducts of the type (NHC)·AlH 3 [NHC = Me 2 Im (1), Me 2 Im Me (2), iPr 2 Im (3), iPr 2 Im Me (4), Dipp 2 Im (5)] and (NHC)· AliBu 2 H [NHC = iPr 2 Im (6), Dipp 2 Im (7)] as novel hydride transfer reagents in the hydrodefluorination (HDF) of different fluoroaromatics and hexafluoropropene. Depending on the alane adduct used, HDF of pentafluoropyridine to 2,3,5,6-tetrafluoropyridine in yields of 15-99 % was observed. The adducts 1, 2, and 5 achieved a quantitative conversion into 2,3,5,6-tetrafluoropyridine at room temperature immediately after mixing the reactants. Studies on the HDF of fluorobenzenes with the (NHC)·AlH 3 adducts 1, 3, and 5 and (Dipp 2 Im)·AliBu 2 H (7) showed the decisive influence of the reaction temperature on [a] 4032 Scheme 3. Hydrodefluorination of hexafluorobenzene, pentafluorobenzene, and 1,2,4,5-tetrafluorobenzene using (NHC)·AlH 3 [NHC = Me 2 Im (1), iPr 2 Im (3), Dipp 2 Im (5)] and (Dipp 2 Im)·AliBu 2 H (7)..Eur. J. Inorg. Chem. 2018, 4031-4043 www.eurjic.org

Selective Hydrodefluorination of Hexafluoropropene to Industrially Relevant Hydrofluoroolefins

“…Despite the strongly bound amine ligand, AlH 3 •NMe 3 reacts readily and with the same selectivity as the AlH 3 •NHC system. 14 In contrast, the borane system BH 3 •L shows a strong ligand dependence with L = NMe 3 being ineffectual for HDF and L = THF highly inefficient. The latter borane was used as a THF solution and it is likely that exogeneous THF is acting as an inhibitor in the reaction.…”

“…Aspects of this work are the subject of UK patent application number 1810647.6 filed 28 th June 2018 (one day before the 1 st publication of ref. 14) in collaboration with Imperial Innovations.…”

“…In 2018, it was reported that AlH 3 •NHC (NHC = N-heterocyclic carbene) can selectively hydrodefluorinate HFP to form HFO-1234yf in 79% yield. 14,15 Herein, we detail the hydrodefluorination of HFP to commercially relevant HFO-1234ze and HFO-1234yf. The selectivity can be guided by careful choice of Lewis base and group 13 hydride (EH 3 •L; L = THF, SMe 2 , NMe 3 ; E = B, Al).…”

Copper(I)-Catalyzed Boryl Substitution of 1-Trifluoromethyl Allenes for the Synthesis of 3-Boryl-Substituted 1,1-gem-Difluorodienes

“…Fluorine-containing organoboron compounds are attractive synthons that have great potential for the flexible assembly of an array of structurally diverse organofluorine compounds. , Recently, several groups have reported the selective syntheses of fluorine-containing organoboron compounds through transition metal-mediated C–F bond activation. , We previously reported the copper­(I)-catalyzed enantioselective borylation reactions of allyltrifluorides and allyldifluorides using bis­(pinacolato)­diboron . These reactions presumably proceed through the enantioselective borylcupration of alkenes followed by copper­(I)-β-fluoro elimination to give gem -difluoroallylboronates. , Based on our studies, we anticipated that 3-boryl-1,1- gem -difluorodienes could be efficiently synthesized by the borylation of 1-trifluoromethyl-substituted allenes through the regioselective borylcupration/copper­(I)-β-fluoro elimination sequence (Scheme b). ,, The products would be versatile intermediates to obtain potentially useful gem -difluoroalkenyl compounds, which are difficult to obtain by other methods, through cross-coupling reactions with aryl halides.…”

Regioselective C–F Bond Activation/C–C Bond Formation between Fluoropyridines and Cyclopropyl Groups at Zirconium

“…Activation at position 4 of the pentafluoropyridine is the most common, with activation at position 2 being less frequent. Both early- and late-transition-metal complexes have been used as catalysts for these reactions. − In stark contrast, reactions that lead to the formation of C–E bonds (E ≠ H) and recovery of the coupling product from polyfluoropyridines, whether stoichiometrically or catalytically, are less common and, to our knowledge, the exclusive privilege of late-transition-metal complexes (Ni, Pd, Pt, Co, and especially Rh) with the recent addition of photocatalysis with Ir complexes. , The interest in early-transition-metal chemistry stems in part from a broader range of metal–fluoride bonds, complementary reaction mechanisms, and, consequently, reaction selectivity.…”