Hydrogenolysis of fluorine

Hydrogenolysis of fluorine and chlorine possible only under exceptionally energetic conditions practically unfeasible. 

Hydrogenolysis of vinylic fluorides is hardly ever done with the aim of removing fluorine finom a molecule. Rather, it occurs as a usually undesirable side reaction, accompanying reductions of other functions in fluorinated compounds. " It frequently takes place during catalytic hydrogenation of fluorinated alkenes. A clear-cut saturation of the double bonds in polyfluorinated alkenes is rare and usually requires mild reaction conditions,and hydrogenolysis of fluorine without reduction of the double bond in catalytic hydrogenation is unknown. Numerous examples are quoted in the review literature, " and a few samples are shown in Table 1. 

Another regioselective hydrogenolysis of fluorine with hydrides takes place in perfluorotoluene, and electrolytically in pentafluorobenzoic acid (equations 25 and 26). ... 

Hydrogenation of (13) over palladium black yields mainly (14) with only partial hydrogenolysis of fluorine to give (15 equation 34). ... 

Catalytic Hydrogenolysis of Carbon-Huonne Bonds it-Bond Participation Mechamsm Hudlicky, M J Fluorine Chem 44, 345-359 58 n o C) S ... 

Reductive removal of fluorme from alk I fluorides requires a potent reducing agent and so is not noimally encountered However, hydrogenolysis of an unacuvated carbon-fluorine bond in, for example, 3 (3-fluorocholestane has been efficiently accomplished in 88% yield with a solution of potassium and dicyclohexyl 18 crown-6 in toluene at 25 C [/] Similarly, sodium naphthaiene in tetrahydrofuran converts 6 tluorohexene-1 and 1-fluorohexane to hydrocarbons in 50% yield at 25 C over a 7-h penod [2]... 

During hydrogenation of vinylic chlorides, both saturation of the double bond and hydrogenolysis of the C-Cl bond take place.433 35 Also, selective hydrogenolysis of chlorine occurs in the presence of fluorine atoms on a Pd/C catalyst (Scheme 4.125).436... 

The conspicuously high speed of hydrogenolysis of allylic and vinylic halogens as compared with those removed further from the multiple bonds or halogens in alkanes implies that the multiple bond participates in a multicenter transition state [63]. Such a mechanism would account even for the surprisingly easy hydrogenolysis of allylic and vinylic fluorine [66, 528, 529, 530]. 

Catalytic Hydrogenolysis of Carbon-Fluorine Bonds rc-Bond Participation Mechanism Hudlicky, M. J. Fluorine Chem. 1989, 44, 345-359. 

Catalytic hydrogenolysis of 1,1-difluorocyclopropanes occurs at the C2-C3 bond with partial retention of fluorine in the cases studied [49] (equation 38) In contrast, reduction of acetoxy 1,1 difluorocytlopropanes with lithium aluminum hydride occurs with loss of fluoride to give (3-fluoroallyl alcohols, often with high stereose lectivity [50] (equation 39) A further variant involving free radical intermediates produces allylidene difluorides after ring opening [5/] (equation 40)... 

There is now substantial evidence that hydrogenolysis of CC12F2 on supported noble metal catalysts, which leads to mixtures of CH2F2 and CH4, occurs via a difluorocarbene intermediate [106,107,108], a pleasing parallel with one of the mechanisms proposed for CC14 fluorination [104], A carbene intermediate is pro-... 

Catalytic hydrogenation of fluoro- and difluOTO-m- and tran -butenedioic acids and their esters frequently affords products only of hydrogenolysis, especially when platinum oxide (Adams catalyst) is used. Hydrogenolysis is favored in polar solvents, and is found to be more effective when rhodium or nickel rather than palladium are used as the catalysts (Table 2). Hydrogenolysis takes place prior to the saturation of the double bonds fluoro- and 2,3-difluoro-succinic acids do not suffer any loss of fluorine under the same reaction conditions. ... 

Selective hydrogenolysis of chlorine in preference to fluorine occurs during catalytic hydrogenation of chlorotrifluoroethylene. In addition to trifluoroethylene, trifluoroethane is formed by reduction of the double bond (equation 10). ... 

Vinylic bromine can be replaced fairly easily without reduction of the double bond. This is a characteristic difference from hydrogenolysis of vinylic chlorine and especially fluorine. Catalytic hydrogenation of ethyl a-methyl-3-bromocrotonate resulted in replacement of bromine and formation of ethyl a-methylcrotonate (equation 15). In 14-bromocodeinone (9), replacement of bromine occurs with a double bond shift to give neopinone (10 equation 16).4... 

Hydrogenolysis of aromatic halides using catalytic hydrogenation takes place easily with rates of replacement of halogen decreasing in the order I > Br > Cl > F. For example, replacement of chlorine in preference to fluorine takes place in chlorofluoropyridines (equation 21)."... 

Hydrogenolysis of polyfluoro and perfluoro aromatics by lithium aluminum hydride has already been shown in equations (24) and (25). Additional examples are replacement of fluorine by hydrogen in pentafluorobenzene, where the predominant product is 1,2,4,5-tetrafluorobenzene (20), resulting from the reduction by lithium aluminum hydride para to the hydrogen (equation 38), and replacement of fluorine in the -position in 1,2,3,4-tetrafluoronaphthalene (equation 39). ... 

The palladium-catalyzed hydrogenolysis of 2-butoxy-l,l-difluorocyclopropanes 3 resulted in the reductive cleavage of the cyclopropyl bond opposite to the electron-deficient carbon atom. Fluorine-hydrogen exchange in the product was observed as a side reaction. 

The main synthetic route to CFC, HCFC and Halons is the Swarts fluorination. Technically this is often achieved by reaction of a chlorinated or brominated precursor with anhydrous hydrofluoric acid in the presence of a solid Lewis acid catalyst, for example chromia. Other important reactions are Lewis acid-catalyzed halogen isomerization and hydrogenolysis of chlorine or bromine. 

X-ray crystallographic analysis for 18 [15]. The fluorinated imines 16, 17, and 18 provided (S, S) amines (20) exclusively on reduction with NaBH4, where hydride attacked from the less hindered face via the preferred conformation 19. Regioselective Pd-catalyzed hydrogenolysis of 20 afforded enantiomerically pure 2-fluoro-, 2,2-difluoro-, and 2,2,2-trifluoro -1 -phenylethylamines [16]. 

Yet another way to obtain a 1 -monohalogenated alkylphosphonic diester is based on the application of the Wadsworth-Emmons adaptation of the Wittig reaction (Scheme 10). Here, the anion from tetraisopropyl (fluoromethyl)bisphosphonate reacts with a carbonyl compound to give the (l-fluoroalk-l-enyl)phosphonic esters 126 as an E-Z mixture (80-95 20-5) hydrogenolysis of these mixtures yields (l-fluoroalkyl)phosphonic esters. In a review of the literature the authors pointed out the widely different results experienced by other workers in their attempts to alkylate carbanions derived from fluorinated alkylphosphonic diesters and, as a result of their own work, advocated the use of alkyl triflates, which appear to react with lithiated carbanions very quickly and cleanly. 

Very recently, a different synthetic approach to fluorinated p-lactams involving catalytic hydrogenolysis of the N-O bond of isoxazohdines appeared in the literature. The method involves the synthesis of isoxazohdines (71) by 1,3-cycloaddition of aromatic nitrones to fluorinated alkenes, followed by N-O bond cleavage by hydrogenolysis using palladium catalyst (Scheme 2.34). ... 

The hydrogenolysis of diethyl (l-fluoro-2-propynyl)phosphonate (obtained by the fluorination of diethyl (l-hydroxy-2-propynyl)phosphonate with Et2NSp3 (DAST))... 

In facE em-difluorocyclopropanes undergo hydrogenolysis much faster than monoflu-orocyclopropanes. Although hydrogenolysis of the C—bond is usually very difQcult, in these instances, loss of one or both of the fluorine atoms was also observed. 

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