Hermann-Josef Frohn

Highly efficient cross-coupling reactions with the perfluoroorganotrifluoroborate salts K [RFBF3] (RF=C6F5, CF2CF)

Potassium pentafluorophenyl- and trifluorovinyltrifluoroborates, both containing electron-poor organo groups, reacted with iodobenzene and p-substituted iodobenzenes in the presence of palladium compounds and stoichiometric amounts of silveroxide to the fluorinated cross-coupling products in very good yields. Substituted bromobenzenes were less efficient substrates.

A new application of (polyfluoroorgano)trifluoroborate salts: the palladium-catalysed cross-coupling reaction with substituted benzenediazonium tetrafluoroborates

Abstract For the first time (polyfluoroorgano)trifluoroborate salts, namely K[C 6 F 5 BF 3 ], K[C 6 HF 4 BF 3 ] (all three isomers), K[3,4,5-C 6 H 2 F 3 BF 3 ], and K[CF 2 CFBF 3 ] were principally applied as reagents to Pd-catalysed cross-coupling reactions. A series of polyfluorinated biphenyls C 6 H 5− n F n –C 6 H 4 X-4′ were obtained from K[C 6 H 5− n F n BF 3 ] and the substrates [4-XC 6 H 4 N 2 ][BF 4 ] in the presence of Pd catalysts. The influence of the number and the position of the fluorine atoms in the (polyfluorophenyl)trifluoroborate salts on the reactivity in the coupling reaction was elucidated. In addition to (polyfluorophenyl)trifluoroborate salts, the cross-coupling was expanded to a first example of a perfluoroalkenyl reagent. K[CF 2 CFBF 3 ] reacted with [4-FC 6 H 4 N 2 ][BF 4 ] and formed CF 2 CF–C 6 H 4 F-4.

Polyfluoroorganoboron-Oxygen Compounds. 2 [1] Base-catalysed Hydrodeboration of Polyfluorophenyl(dihydroxy)boranes†

Polyfluorinated phenyl(dihydroxy)boranes C6H5-nFnB(OH)2 (n = 3 — 5) underwent hydrodeboration (formal replacement of the (dihydroxy)boryl group by hydrogen) in the presence of bases (MeOH, 33 % H2O—MeOH, KOH (1 equiv.)/33 % H2O—MeOH, pyridine and 9 % D2O—pyridine) and formed the fluoroaromatic compounds ArH or ArD, respectively. The rate of reaction depends on the number of fluorine atoms in the phenyl group and on the position of the fluorine atoms, relative to the B(OH)2 substituent. Polyfluororganische Bor-Sauerstoff Verbindungen. 2 [1] Die Hydrodeborierung von Polyfluorphenyl(dihydroxy)boranen unter basischen Bedingungen Polyfluorierte Phenyl(dihydroxy)borane C6H5-nFnB(OH)2 (n = 3 — 5) unterliegen in Gegenwart von Basen (MeOH, 33 % H2O—MeOH, KOH (1 Aquiv.)/33 % H2O—MeOH, Pyridin und 9 % D2O—Pyridin) einer Hydrodeborierungsreaktion (formaler Ersatz der (Dihydroxy)borylgruppe durch Wasserstoff) und bilden die entsprechenden Fluoraromaten ArH oder ArD. Die Reaktionsgeschwindigkeit hangt dabei von der Anzahl der Fluoratome in der Arylgruppe und der relativen Stellung der Fluoratome zum B(OH)2-Substituenten ab.

Polyfluoroorganoboron-Oxygen Compounds. 1 Polyfluorinated Aryl(dihydroxy)boranes and Tri(aryl)boroxins†

A general preparative procedure for polyfluorinated aryl(dihydroxy)boranes C6H5-nFnB(OH)2 (n = 3 — 5) is described. Polyfluorinated aryl(dihydroxy)boranes are easily dehydrated to the corresponding tri(aryl)boroxins (C6H5-nFnBO)3 by thermal or chemical treatment. The property of the acids C6H5-nFnB(OH)2 to condensate depends on the number and on the position of the fluorine atoms in the aryl group. Examples of both classes of boron compounds were isolated as pure individuals and characterized by multinuclear NMR spectroscopy. Polyfluororganische Bor-Sauerstoff Verbindungen. 1 Polyfluorierte Aryl(dihydroxy)borane und Tri(aryl)boroxine Es wird ein allgemeiner Weg fur die Darstellung polyfluorierter Aryl(dihydroxy)borane beschrieben. Polyfluorierte Aryl(dihydroxy)borane lassen sich thermisch und chemisch leicht dehydratisieren und gehen dabei in die entsprechenden Tri(aryl)boroxine uber. Die Neigung zur Kondensation hangt sowohl von der Anzahl als auch von der Stellung der Fluoratome in der Arylgruppe ab. Es werden Beispiele fur beide Klassen von Borverbindungen vorgestellt, die in Reinsubstanz isoliert und durch Multikern-NMR-Spektroskopie charakterisiert wurden.

The palladium-catalysed cross-coupling reaction of lithium polyfluorophenyltrimethoxyborates with 4-fluoroiodobenzene

Abstract Li[C 6 F 5 B(OMe) 3 ], Li[C 6 HF 4 B(OMe) 3 ] (all three isomers) and Li[3,4,5-C 6 H 2 F 3 B(OMe) 3 ] are the first examples of polyfluorophenyltrimethoxyborate salts which have been applied as reagents to Pd-catalysed cross-coupling reactions. A series of polyfluorinated biphenyls C 6 H 5− n F n -C 6 H 4 F-4′ were obtained from Li[C 6 H 5− n F n B(OMe) 3 ] and the model substrate 4-FC 6 H 4 I in the presence of Pd catalysts. The influence of the number and the position of fluorine atoms in the polyfluorophenyltrimethoxyborate salts on the reactivity in the coupling reaction was elucidated.

Trifluoropropynylxenon(II) tetrafluoroborate [CF3CCXe][BF4] – isolation of an alkynylxenon(II) compound for the first time

The salt [CF3CCXe] [BF4] was prepared as neat compound by the reaction of the hitherto unknown alkynyldifluoroborane CF3CCBF2 with XeF2 in 1,1,1,3,3-pentafluoropropane (PFP) at −45 °C in 59% yield. [CF3CCXe] [BF4] was unambiguously characterised by multinuclear NMR spectroscopy in anhydrous HF (aHF) solution.

A Convenient and General Approach to Alkyl-, Alk-1-enyl-, and Aryldifluoroboranes

A simple and general procedure, especially useful for the small scale preparation of alkyl-, alk-1-enyl-, and aryldifluoroboranes RBF2 was elaborated. Suspensions of the easily available and storable K [RBF3] salts undergo fluoride abstraction when treated with boron trifluoride to form solutions of the corresponding difluoroboranes RBF2.

Two New Types of Xenon–Carbon Species: The Zwitterion, 1-(Xe+)C6F4-4-(BF3–), and the Dication, [1,4-(Xe)2C6F4]2+

In the 1:1 reaction of XeF(2) with 1,4-(F(2)B)(2)C(6)F(4) in 1,1,1,3,3-pentafluoropropane the insoluble zwitterion 2,3,5,6-tetrafluorophenylene-1-xenonium-4-trifluoroborate, 1-(Xe(+))C(6)F(4)-4-(BF(3)(-)), was formed as the main product (77%) along with the zwitterion, 1-(Xe(+))-cyclo-1,4-C(6)F(6)-4-(BF(3)(-)), the [BF(4)](-) salts of the dication, [1,4-(Xe)(2)C(6)F(4)](2+), and the cation, [1-Xe-cyclo-1,4-C(6)F(6)-4-H](+). The isolation of pure 1-(Xe(+))C(6)F(4)-4-(BF(3)(-)) was feasible after extraction of the byproduct with 27% aq HF. The zwitterion, 1-(Xe(+))C(6)F(4)-4-(BF(3)(-)), was characterized by multi-NMR and Raman spectroscopy and by DSC. The zwitterion, 1-(Xe(+))C(6)F(4)-4-(BF(3)(-)), reacted with halide nucleophiles (in excess) in 27% aq HF to form [4-HalC(6)F(4)BF(3)](-) (Hal = I, Br, Cl) along with [2,3,5,6-C(6)F(4)HBF(3)](-), whereas with fluoride ions [2,3,5,6-C(6)F(4)HBF(3)](-) was obtained exclusively. Reactions of XeF(2) with 1,4-(F(2)B)(2)C(6)F(4) in the molar ratio 2:1 did not allow for improving the yield of the tetrafluoroborate salt with the dication [1,4-(Xe)(2)C(6)F(4)](2+). Instead, addition of fluorine to the phenylene unit was increased with the formation of 1-(Xe(+))-cyclo-1,4-C(6)F(6)-4-(BF(3)(-)), [1-Xe-cyclo-1,4-C(6)F(6)-4-H](+), and [1,4-(Xe)(2)-cyclo-1,4-C(6)F(6)](2+). After enrichment in an anhydrous hydrogen fluoride extract, the dication, [1,4-(Xe)(2)C(6)F(4)](2+), was characterized by multi-NMR spectroscopy and by means of chemical proof (reaction with an excess of KI to 1,4-I(2)C(6)F(4)).

ELECTROCHEMICAL REDUCTION OF PENTAFLUOROPHENYLXENONIUM, -DIAZONIUM, -IODONIUM, -BROMONIUM, AND -PHOSPHONIUM SALTS

Cyclic voltammetry measurements on pentafluorophenylonium compounds of [C6F5X]+ Y– type with X = Xe, N2, C6F5Br, C6F5I, and (C6F5)3P were carried out. In these series [C6F5Xe]+ shows the lowest and [(C6F5)4P]+ the highest reduction potential. One electron reduction of [C6F5Xe]+ and [C6F5N2]+ followed by the loss of Xe or N2, respectively, leads to the generation of the [C6F5]· radical. Favoured following reactions of the [C6F5]· radical are the abstraction of hydrogen from MeCN or dimerisation. After the first reduction step the other onium cations split off the pentafluorophenyl element molecule such as (C6F5)3P, C6F5Br, or C6F5I, respectively. These molecules undergo further reductions. The low reduction potential of [C6F5Xe]+ is in contrast to former measurements on partially fluorinated or chlorinated phenylxenonium cations. A plausible explaination for the different behaviour of these Xe–C compounds in electrochemical reduction processes is given. Elektrochemische Reduktion von Pentafluorophenylxenonium, -diazonium, -iodonium, -bromonium und -phosphonium-Salzen Cyclovoltametrische Untersuchungen an Pentafluorphenylonium-Verbindungen des Typs [C6F5X]+ Y– mit X = Xe, N2, C6F5Br, C6F5I und (C6F5)3P wurden in MeCN durchgefuhrt. In dieser Reihe weist [C6F5Xe]+ das niedrigste und [(C6F5)4P]+ das hochste Reduktionspotential auf. [C6F5Xe]+ und [C6F5N2]+ spalten nach Aufnahme eines Elektrons Xe bzw. N2 ab und bilden das Radikal [C6F5]·, welches Wasserstoff aus dem Losemittel MeCN abstrahiert oder dimerisiert. Die anderen Oniumionen spalten nach dem ersten Reduktionsschritt ihre Pentafluorphenylstammverbindung (C6F5)3P, C6F5Br oder C6F5I ab. Diese Verbindungen unterliegen weiteren Reduktionen. Das niedrige Reduktionspotential von [C6F5Xe]+ steht im Gegensatz zu dem von teilfluorierten bzw. teilchlorierten Phenylxenonium Kationen in fruheren Messungen. Es wird eine plausible Erklarung fur das unterschiedliche elektrochemische Reduktionsverhalten dieser Xe–C Verbindungen gegeben.

Synthesis and Properties of Perfluoroalkenylboronic Acids and some of their Derivatives

Lithium perfluoroalkenyltrimethoxyborates were prepared and characterised by NMR spectroscopy. These compounds possess the properties inherent in both organic and inorganic boron-containing compounds. Li[XCF=CFB(OMe)3] (X = F, c«-CF3, Irans-CF^) react with I2 to give corresponding 1iodopolyfluoroalkenes in 30-50 % yields. Interaction of Li[XCF=CFB(OMe)3] with ClSiMe3, followed by working up with XCF=CFLi leads to formation of lithium bis(perfluoroalkenyl)dimethoxyborates. Reaction of Li[(XCF=CF)„B(OMe)4-n] with K[HF2] in aqueous HF gives corresponding potassium perfluoroalkenylfluoroborates, K[(XCF=CF)nBF4.n].