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Download Article (PDF) Main Group Metal Chemistry Vol. 25, No. 10, 2002 (POLYFLUOROORGANO)HALOBORANES AND (POLYFLUOROORGANO)FLUOROBORATE SALTS: PREPARATION, NMR SPECTRA AND REACTIVITY Vadim V. Bardin1 and Hermann-Josef Frohn2 * 1 Ν. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, 630090 Novosibirsk, Russia 2 Institute of Chemistry, Inorganic Chemistry, Gerhard-Mercator-Universität Duisburg, Lotharstr. 1, D-47048 Duisburg, Germany <[email protected]> ABSTRACT New developments concerning the preparation and reactivity of (polyfluoroorgano)haloboranes RFBX2, (RF)2BX (X = F, CI, Br), some of their adducts (RF)NBX,. · Base (n = 1 - 3), and their related fluoroborate salts Μ [(RF)nBF4.„] were reviewed. Significant 'B and F NMR spectroscopic patterns of all three classes were described. CONTENTS 1. Introduction 2. Preparation of (polyfluoroorgano)haloboranes RFBX2 and (RF)2BX (X = F, CI, Br) 2.1. From (polyfluoroorgano)metallic precursors 2.2. From other (polyfluoroorgano)boron compounds 2.3. By various procedures 3. Preparation of (polyfluoroorgano)fluoroborate salts Μ [(RF)nBF4_n] 4. The NMR spectra of (polyfluoroorgano)haloboranes, their complexes with bases and (polyfluoroorgano)fluoroborate salts 5. "The reactivity of (polyfluoroorgano)haloboranes 5.1. Thermal stability 5.2. Reactions with bases and nucleophiles 5.3. Reactions with anhydrous HF 5.4. Conversion of(RF)„BX3.n into (RF)nBF3_n (X = CI, Br) 5.5. Reactions of (polyfluoroorgano)haloboranes with hypervalent fluorine-element-fluorine bonds in xenon fluorides, bromine trifluoride, and organoiodine difluorides 5.6. Reactions of (polyfluoroorgano)haloboranes with selected organoelement compounds. 6. The reactivity of (polyfluoroorgano)fluoroborate salts 7. Conclusions 8. References 1. INTRODUCTION (Organo)dihaloboranes RBX2 and di(organo)haloboranes R2BX (X = F, CI, Br, I) are well-established compounds and their preparations and reactivities were reviewed in fundamental monographs [1, 2]. However, the replacement of all or the majority of hydrogen atoms in the organic group R by fluorine caused significant changes of properties. In fact, the synthetic approaches to the polyfluorinated organoboranes and -borate salts as well as the reactivity of these compounds have a number of peculiarities which combine the specific properties of both roots, organofluorine and organoboron chemistry, and their co-operation. The intensive development of the organofluorine chemistry in 1960s also initiated the research in the field of polyfluorinated organometallics and organo element compounds including the borderline element boron. In 1960 - 1970, the first representatives of perfluorinated alkyl-, alkenyl-, and arylboron compounds were obtained but during the next two decades they remained only "chemical exotics". The situation changed in the 1980s when the high efficiency of (C6F5)3B and of some Μ [(C6F5)4B] salts as co-catalysts in the homogeneous olefin polymerisation was discovered. This caused an "explosive" number of publications on the chemistry of tris(pentafluorophenyl)borane, tetrakis(pentafluorophenyl)borate salts, and related (perfluoroaryl)boron compounds. The application of perfluorinated tri(aryl)boranes as versatile Lewis acids and suitable co-catalysts in the olefin polymerization stimulated the research on this class of acids. Consequently, the tetrakis(polyfluoroaryl)borate anions (often attributed as "non-coordinating anions") attracted interest as counter anions of low nucleophilicity for stabilizing salts with electrophilic cations and similar purposes. The recent advances in these fields (synthesis, reactivity, and practical application) were surveyed in reviews [3,4, 5], actual original papers [6, 7, 8, 9 ], and in the references cited there. Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity The initial achievements in the chemistry of (poiyfluoroorgano)boron compounds were collected in reviews [1, 3, 10, 11] which covered the literature until 1983. Advances in the chemistry of the (trifluoromethyl)boron compounds R(CF3)nBX2_ and Μ [R(CF3)„BX3_„] (η = 1 - 3, X = F, CI, ...) were discussed by Pawelke and Buerger [5], and the preparation of the first tetrakis(trifluoromethyl)borate salts was reported in 2001 [12], When our review had been finished, a paper of Chivers [13] on pentafluorophenylboron halides appeared. The recent advances in the synthesis and reactivity of polyfluorinated mono- and di(organo)haloboranes (Rf^BXj^, their adducts (RF)„BX3_n · Base, and the related salts Μ [(Rf)„F4_„] where RF represents polyfluorinated alkyl-, alk-1-enyl-, and aryl groups, and X fluorine, chlorine, and bromine, are the scope of this review. Poly- and perfluorinated (alkyl)haloboranes RFBX2, (RF)2BX (X = CI, Br, I), (alk-1- ynyl)haloboranes (RFC-C)„BX3_n (X = F, CI, Br, I), the corresponding fluoroborates Μ [(RFC-C)„BF^„] (n = 1,2), and (polyfluoroorgano)haioborates Μ [(RF)nBX4_J (X = CI, Br, I) are still unknown compounds. 2. PREPARATION OF (POLYFLUOROORGANO)HALOBORANES RFBX2 AND (RF)2BX (X = F, CI, Br) 2.1. From (polyfluoroorgano)metallic precursors A common approach to (organo)haloboranes is the reaction of boron trihalides with organotin or organomercury compounds (borodemetalation) [1, 2]. This route was successfully used to prepare some polyfluorinated (aryl)- and (ethenyl)haloboranes. The treatment of trimethylpentafluorophenyltin or dimethylbis(pentafluorophenyl)tin with boron trichloride in excess gave (pentafluorophenyl)dichloroborane in a high yield [14, 15]. In a similar way, (pentafluorophenyl)dibromoborane (low yield) [16] and (pentafluorophenyl)difluoroborane [15] were obtained (Eqs. (1) - (3)). BC13 (excess) C6F5SnMe2R * C6F5BCI2 + Me2RSnCl (1) -20 °C, 12 h R = CH3 (96 %), C6F5 (74 %) BBr3 (excess) (C6Fs)2SnBu2 » C6F5BBr2 + Bu2SnBr2 (2) 90 °C, 3 h 22 % BF3 (excess), CC14 C6F5SnMe3 » C6F5BF2 + FSnMe3 + "[Me3Sn] [BF4]" (3) 20 °C, 60 h The preparation of perfluorinated di(aryl)chloro- and di(aryl)bromoboranes and related compounds required the use of stoichiometric amounts of boron trihalides. The continuous heating of (C6F5)2SnMe2 with BC13 (1 : 1) at 100 to 120 °C led to the formation of bis(pentafluorophenyl)chloroborane (Eq. (4)) [15, 17], A similar replacement of the >SnMe2 fragment by >BBr occurred in the reaction of 9,9-dimethylperfluoro-9- stannafluorene with BBr3 (Eq.(5)) [9]. The reagent (C6F5)4Sn did not react either with boron tribromide (reflux, 24 h) [16] or boron trifluoride (130 °C, 65 h) [18]. This is in contrast to the hydrocarbon analogues, tetraalkyl- and tetraphenyltin, which were easily converted into the corresponding (organo)dihaloboranes when heated with BX3 (X = Br, I) [19]. (C6Fs)2SnMe2 + BC13 • (C6F5)2BC1 + Me2SnCl2 (4) 100 °C, 2 h 36% [15] or 120 °C, 48 h 68 % [17] 590 Main Group Metal Chemistry Vol. 25, No. 10, 2002 benzene SnMs2 + BBr3 BBr (5) -78 to 20 °C 4 h The first (polyfluoroalk-l-enyl)haloboranes, (trifluoroethenyl)difluoroborane, (trifluoroethenyl)dichloro- borane, and bis(trifluoroethenyl)chloroborane, were prepared by borodestannylation of bis(trifluoroethenyl)- dimethyltin with BX3 (X = F, CI). The formation of (trifluoroethenyl)dichloroborane proceeded in a high yield whereas the corresponding difluoroborane was obtained in 18 % yield only (Eq. (6)). The successful synthesis of di(perfluoroethenyl)chloroborane (CF2=CF)2BC1 was performed by heating of (CF2=CF)2SnMe2 with one equivalent of boron trichloride (Eq. (7)) [20]. (CF2=CF)2SnMe2 + 2BX3 2 CF,=CFBX, + Me,SnX, (6) 20 to 70 °C, 1 h X = F(18 %), CI (93 %) (CF2=CF)2SnMe2 + BC13 (CF2=CF)2BC1 + Me2SnCl2 (7) 20 to 70 °C, 1 h 85% In some cases, the isolation of the (polyfluoroorgano)haloboranes from the reaction mixtures was hindered because of the similar boiling points of the boranes and the resulting alkyltin halides. This problem could be overcome by using other (organo)metal precursors, polyfluorinated aryl- or ethenylmercury derivatives. The reaction of the latter with boron trihalides resulted in less or non-volatile organomercury halides. (Pentafluorophenyl)dihaloboranes C6F5BC12 and C6F5BBr2 were obtained from pentafluorophenyl- mercury derivatives C6F5HgR and boron trihalide (Eqs. (8) - (10)) [15,21, 22], BC13 (excess) C6F5HgMe C6F5BC12 + MeHgCl (8) 20 °C, 24 h 84% BBr3, CH2C12 C6F5HgEt C6F5BBr2 + EtHgBr (9) -50 to 20 °C 82% BBr3, toluene C6F5HgBr C6F5BBr2 + HgBr2 (10) reflux, 2 days 80% Borodemercuration of the cyclic trimer f(C6F4)Hg]3 with BBr3 at 120 °C led to 1,2-bis(dibromoboryl)- tetrafluorobenzene in 82 % yield (Eq. (11)) [23], Surprisingly, boron trichloride reacted with [(C6F4)Hg]3 under the same conditions [23] or with l,2-bis(trimethylstannyl)tetrafluorobenzene at 180 eC to give decafluoro-9,10-dichloro-9,10-diboraanthracene (53 % yield) (Eq. (12)) [6] instead of the expected 1,2- bis(dichloroboryl)tetrafluorobenzene. BBr2 BBr3, benzene (11) 120 °C, 1 h BBr2 82% 591 Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity HG ψHg BCI3, benzene 5 120°C, 1 h Hg Φ: "" » 180 B^^^ C, 18 h SnMe3 CI The treatment of bis(trifluoroethenyl)mercury with BC13 smoothly led to (trifluoroethenyl)dichloro- borane (Eq. (13)) [24], xylene (CF2=CF)2Hg + 2 BC13 • 2 CF2=CFBC12 + HgCl2 (13) 4 °C, 1 h 41 % Attempts to use other polyfluorinated organoelement compounds as precursors for polyfluorinated (aryl)haloboranes gave unsatisfactory results. Bochmann et al reported that the fluorine-pentafluorophenyl
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