Fluorine Notes, 2001, 19, 1-2
Use of hydrogen fluoride and its complexes with bases for introduction of fluorine atoms into organic moleculesG.G.Furin Novosibirsk Institute of organic chemistry named after N.N.Vorozhtsov Siberian branch of the Academy of Science of Russian Federataion Fax: +7-3822-344752 e-mail: furin@nioch.nsc.ru Annotation The review summarizes and systematizes up-to-date data on fluorinating ability of anhydrous hydrogen fluoride and its complexes with bases of unsaturated organic compounds, alcohols, diazoketones, hydrazones and oximes of ketones, 3,3-dialkyl-1-aryltriazenes, aryldiazosulfides etc.. It contains an analysis of main achievements in use of anhydrous hydrogen fluoride as a fluorinating agent to produce ozone-friendly freons in gas and liquid phases both without catalysts and in the presence of latter. There has been examined factors influencing opening three-membered cycles containing oxygen and nitrogen atoms. The review contains examples of practical application of different groups of fluoroorganic compounds, rational methods of their production and their role in development of modern industry . Contents Introduction. Hydrogen fluoride as a basic stock substance in chemical industry. 1. Fluorination with anhydrous hydrogen fluoride and its complexes with bases of compounds from different classes. 1.1.Hydrofluorination of unsaturated compounds 2. Processes of replacement of functional groups with fluorine atoms. 2.1. Replacement of oxy-group with fluorine under effect of hydrogen fluoride
complexes containing bases. 3. Opening nitrogen- and oxygen-containing three-membered heterocycles 3.1.Opening an epoxy ring by anhydrous hydrogen fluoride and its complexes with
bases. Introduction.Hydrogen fluoride as a basic stock substance in chemical industrySynthesis and application of fluorine organic compounds were caused by demand in new materials stable to oxidants, radiation etc. They have started to be a subject of great attention since the creation of convenient and efficient methods of introduction of fluorine atoms into organic molecules [1-3]. At this point the key role was played by hydrogen fluoride produced from available calcium fluoride [3]. Hydrogen fluoride and hydrofluoric acid are widely used as raw materials for production of various large-tonnage products. Many efforts were spent to design apparatus for these processes because anhydrous hydrogen fluoride is aggressive enough and very toxic. But its availability and cheapness resulted in its use as the main fluorination reagent for many years [4,5]. Hydrofluorination and halofluorination in the presence of catalysts ( mainly Lewis’ acids) became the basic synthesis methods of various chladones. A possibility to carry out electrolysis in a medium of anhydrous hydrogen fluoride resulting in total replacement of all hydrogen atoms of an organic molecule with fluorine atoms made available a significant number of perfluorinated organic compounds. The latter possess unique properties and found wide practical applications in such fields as liquid dielectrics, heat carriers and materials for medicine. These compounds are non-combustible, non-toxic, do not react with aggressive reagents, ozone, concentrated nitric acid etc., that stimulates researchers to look for new methods of synthesis of such compounds. Direct fluorination by elemental fluorine or by other very active fluorinating agent plays an important role. At the same time development and implementation of new approaches to the use of systems modified with hydrogen fluoride is continued. It should be noted that hydrofluorination with anhydrous hydrogen fluoride is followed by side processes connected with generation of intermediate highly active carbcations. At the same time the use of complexes of hydrogen fluoride with bases reduces considerably such side processes that made a foundation for practical application of the complexes in processes of hydrofluorination and halofluorination. It was found possible to obtain biologically active compounds containing one or two fluorine atoms in an organic molecule when a fluorine atom was introduced selectively and complexes of anhydrous hydrogen fluoride with bases were used as a fluorinating agent. A variety of properties of systems containing hydrogen fluoride such as aqueous solutions of hydrogen fluoride, complexes with bases make it possible to carry out processes of introduction of fluorine atoms selectively enough and probability of purposeful introduction of the fluorine atom in the organic molecule increases. Systematization of the review is based on the properties of the fluoride-ion source on the example of anhydrous hydrogen fluoride and its complexes with bases. The main attention is paid to the result obtained i.e. to the way of required fluorine selective introduction. Different methods of the use of anhydrous hydrogen fluoride for effective introduction of one or two fluorine atoms are analyzed in the review as well as side processes and reaction mechanisms. Numerous publications are devoted to application of hydrogen fluoride because it is used in many industrial processes due to its relatively cheapness and easiness in handling, therefore this review does not consider such important processes as use of hydrogen fluoride in electrochemical fluorination, diazotization and fluorination with elemental fluorine and different fluorinating reagents. The properties of hydrogen fluoride are determined in many aspects by its specific physical characteristics. So, anhydrous hydrogen fluoride under normal conditions is a colorless transparent liquid ( within a temperature range from –85 to +19.6oC) with a low freezing point and extreme hygroscopicity. It makes easy to obtain solutions of hydrogen fluoride with water. At the same time hydrogen fluoride changes its physical and chemical properties in such solutions. Conventionally there are used anhydrous hydrogen fluoride, its 40% and 70% aqueous solutions ( their characteristics are given in Table 1). This review pays main attention to anhydrous hydrogen fluoride exclusively. Anhydrous hydrogen fluoride is associated considerably at the expense of hydrogen bonds with the energy of 45.7kJ/mol [7]. Because of that the interval of liquid state of hydrogen fluoride is wider than of analogues of the Periodic System and exceeds 100oC counting from melting point of –83.37oC up to the boiling point of +19.54oC[9]. The studies on the vapor density of anhydrous hydrogen fluoride bring evidence of that its association remains in gas state also, changing with temperature from dimer at 100oC and runs through the maximum at 0oC as a mixture of tetramer and pentamer. According to studies of electron difraction the oligomer molecular associates of hydrogen fluoride have a structure of an open chain. The hydrogen fluoride properties are given in more details in papers[10]. The density of anhydrous hydrogen fluoride changes steadily from 1.21 at the melting point [11] to 0.959 at the boiling point. The viscosity of anhydrous hydrogen fluoride changes from 0.914 at –69oC to 0.24cP at +6oC [12]. The dielectric constant of anhydrous hydrogen fluoride is 174.8 at –73oC and decreases to the value of 83 at 0oC [13]. The high dielectric constant and significant dipole moment determine excellent solvent ability of hydrogen fluoride. A considerable number of nitrogen-, oxygen-, sulfur- and fluorine-containing compounds are soluble in anhydrous hydrogen fluoride. But saturated aromatic compounds, alkylhalogenides and aromatic ethers are weakly soluble in anhydrous hydrogen fluoride and sometimes are insoluble in it. Anhydrous hydrogen fluoride is a strong acid, the acidity constant of hydrofluoric acid pK=3.18+0.02 [14]. It has a stable anion of (HF)nF- type that is its the most important property as a fluorinating agent (2HF=H2F+ + F-). Hydrogen fluoride is polymerized to a great degree at the expense of hydrogen bonds and its properties in liquid and gas phases differ considerably. Intermolecular hydrogen bonds F-H-F (6kcal/mol) result in formation of linear macromolecules. That influences much the properties of hydrogen fluoride. Only strong acceptors of fluoride ion (SbF5, BF3, AsF5) are acids in comparison with hydrogen fluoride whereas other mineral acids, water and fluorides of alkali metals are bases with regard to it. In contrast to anhydrous hydrogen fluoride aqueous solutions of HF possess not great acidity. The ionization constant of diluted HF (6.16 10 -4) is close to the ionization constant of formic acid. Table 1. Physical and chemical properties of hydrogen fluoride and some of its complexes with bases
Though anhydrous hydrogen fluoride is an available and inexpensive reagent, its use as a fluorinating agent was limited due to a low nucleophilicity of fluoride-ion [1-3]. Intermolecular hydrogen bond usually reduces fluorinating ability and the presence of water in hydrogen fluoride often stimulates formation of undesirable side processes[1]. To avoid and suppress these disadvantages anhydrous hydrogen fluoride is used as complexes with bases [15]. But one should have in mind influence of the base nature on the activity of the fluorinating agent. The most important property of anhydrous hydrogen fluoride is its ability to form strong bonds of hydrogen type with various bases ( amines, pyridine, trialkylphosphines, acid amides) that results in formation of polyhydrofluoride stable complexes and the ratio of these reagents is not always equimolar. So, pyridine and triethylamine at usual conditions dissolve 5 equivalents of hydrogen fluoride whereas a complex to be distilled at 160-170oC contains 3 equivalents of hydrogen fluoride. A temperature reduction increases hydrogen fluoride quantity. Thus. the content of hydrogen fluoride at 60oC with organic compounds is 9-10 equivalents at a concentration of 70wt.% of system (HF)n-pyridine [HF/Py]. Such complexes are stable, distilled under usual conditions, stable in storage, easy in handling and allow working in usual laboratory glassware, their properties are close to those of anhydrous hydrogen fluoride and they are used widely as fluorinating agents [15,16].Complex (HF/Py), an Ohle reagent, is used most frequently [15,16]. Table 1 shows some physical and chemical properties of the most often used complexes:HF/Py, triethylamine-hydrogen fluoride (Et3N/3HF). Further examination does not consider separately the properties of anhydrous hydrogen fluoride and properties of its complexes with bases, only in certain cases some specific properties of these two groups of reagents will be mentioned. Anhydrous hydrogen fluoride is a strong protonating solvent with high acidity and regarding the properties as an electrolytic medium it is close to anhydrous sulfuric acid. The measurements of the Hammete acidity function have shown that it is far in the range of its negative values reaching –10, that is only by one unit lower than that for sulfuric acid and by 6 units higher than acidity of trifluoroacetic acid. Three main ways can be chosen for anhydrous hydrogen fluoride application in organic synthesis: 1. as a fluorinating reagent, 2. as a Friedel-Crafts catalyst 3. as an effective polar solvent. To be continued |
Fluorine Notes, 2001, 19, 1-2
