Hydrodefluorination reaction

Attempts to perform hydrodefluorination on other polyfluoroarenes using 9c met with mixed success (Table 8.4). Pentafluoropyridine proved to be highly active with a turnover number of 13.6, although the distribution of products revealed up to three hydrodefluorination reactions (Table 8.4, entry 4). Substrates with a lower fluorine content, such as CF.CH, and 1,2- or 1,4-CF,H were unreactive. 

Gatalytic G-F activation has been demonstrated in several reactions. Most of the catalytic conversions involving the cleavage of a G-F bond achieve hydrodefluorination reactions (i.e., the replacement of fluorine by hydrogen) or cross-coupling reactions of fluorinated aromatics. Prominent exceptions consist of Murai s rhodium-catalyzed exchange reaction between fluorobenzenes (see Scheme 41) and a disilane as well as the amination of 2-fluoroni-trobenzene. At present, turnover numbers are invariably low and there are no detailed studies of kinetics and mechanism of catalytic G-F activation. 

The transformation of carbon-fluorine bonds into carbon-hydrogen bonds, known as hydrodefluorination reaction, is interesting for environmental chemists since it would help to remove fluorine-containing atmospheric... 

Lentz and coworkers reported that air-stable titanocene difluoride can act as a catalyst for hydrodefluorination reactions in the presence of silane derivatives, giving excellent yields even at low temperature. The authors expanded the scope of the reaction to other substrates such as vinylic and allylic fluorides and a range of catalysts.Mechanistic studies indicated that a titanium(m) hydride is the active species, and the silane derivative is the hydride source. This catalytic system is not only interesting form the environmental point of view, but also for the synthesis of new organic molecules as it represents a new synthetic route to obtain inaccessible aminopyridine derivatives." ... 

Titanium tetrachloride has been studied in the hydrodefluorination reaction of trifluorotoluene derivatives in the presence of lithium aluminium hydride as hydride source, giving excellent yields. Rieger et al. studied group (iv) metallocenes dichlorides in carbon-fluorine activation reactions and they found higher catalytic activities for zirconocene and hafnocene complexes compared to the titanocene complex probably due to the tendency toward reduction of titanium(iv). ... 

Synthesis of fluorinated building blocks by transition metal-mediated hydrodefluorination reactions 13AG(E)3328. 

Scheme 40 Hydrodefluorination reaction of alkyl fluorides catalyzed by trialkylsilylium ions in silanes (anion omitted)...

The exact nature of the catalytically active Ni species in these reactions is yet to be conclusively established. Hydrodechlorination proves optimal with a NHC Ni ratio of 2 1 suggesting that 14-electron Ni(NHC)2 is involved, whereas the 1 1 NHC Ni ratio necessary for hydrodefluorination implies that it is the 12-electron mono-carbene adduct Ni(NHC) which is catalytically active [10]. Smdies by Matsubara et al. revealed that treatment of NKacac) with either one or two equivalents of IMes HCl 1 or SlMes HCl 2 in the presence of NaOHu formed the mono-NHC complex Ni(NHC)(acac)j which, upon reduction with NaH in the presence or absence of carbene, formed Ni(NHC)2 [11]. Density functional theory (DFT) calculations suggest that the strength of the Ni-NHC bond (ca. 50 kcal/mol) makes... 

Similarly to alkyl fluorides, fluorocyclohexane is resistant toward catalytic hydrogenolysis. Hydrides and metals appear to be more efficient in the hydrodefluorination of alicyclic fluorine-containing compounds. Butyllithium in refluxing hexane defluorinates fluorocyclohexane to cyclohexane.20 The complete defluorination of fluorocyclohexane (11) is accomplished by activated magnesium in propan-2-ol, but the reaction leads to the predominant formation of cyclohexene.21... 

We classify the fundamental processes of intermolecular G-F bond activation in the following six categories (i) oxidative addition of fluorocarbon, (ii) M-G bond formation with HF elimination, (iii) M-G bond formation with fluorosilane elimination, (iv) hydrodefluorination of fluorocarbon with M-F bond formation, (v) nucleophilic attack on fluorocarbon, and (vi) defluorination of fluorocarbon (Scheme 4). Table 2 shows the occurrence of these processes for intermolecular G-F activation, classified according to the type of G-F bond. For instance, oxidative addition is a characteristic process for fluoroaromatics but not for fluoroalkanes, while defluorination is characteristic of fluoroalkanes but not fluoroaromatics. We include processes as intermolecular even if they involve coordination of the fluorocarbon in one of the modes in Section 1.26.1.1 prior to G-F bond breaking. Notice that processes (ii), (iii), and (iv) could be described as cr-bond metatheses we have avoided this descriptor, since it has mechanistic connotations that are only appropriate in some cases. It should also be recognized that secondary processes may make some reactions awkward to classify under this scheme. 

Apart from olefin metathesis, the mechanisms of various [(NHC)Ru]-cat-alyzed reactions have been analyzed by DFT computations. Among them, the catalytic hydrodefluorination of fluoroarenes, the coupling of alcohols and amines, and the hydrogenation of ketones and esters have received... 

Isolated reports mentioned the use of NHC-Ru species for the asymmetric Diels-Alder reaction of methacrolein and cyclopentadiene/ cyclopropana-tion of styrene with ethyl diazoacetate/ or dehydrative condensation of Al-(2-pyridyl)benzimidazole and allyl alcohol. Whittlesey and co-workers described the stoichiometric activation of C-F bonds in perfluoroaromatic substrates such as hexafluorobenzene, perfluorotoluene, and pentafluoro-pyridine using the dihydrido complex 49 (Scheme 7.9). Modification of the NHC nitrogen substituents from N-alkyl to N-aryl groups and replacement of the chelating diphosphine with two triphenylphosphine ligands afforded complexes 50 that performed the catalytic hydrodefluorination of polyfluoroarenes with alkylsilanes with reasonable turnover number (up to 200) and frequency (up to 0.86 h ). ... 

Silylium ions have been used to catalyze Diels—Alder (14-15) or Mukaiyama reactions (15) as well as hydrodefluorination of alkyl fluorides (16) or recently defluorinative Friedel—Crafts processes (17). First attempts to use these silylium ions in asymmetric Diels—Alder reaction by Oestreich et al. led so far to modest levels of enantioselectivity (4). 

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