Application of HOF*MeCN reagent as an oxidizer of unsaturated compounds.The formation of this route is bound with Appelman name who synthesized HOF in 1973[61], studied its structure [62,63], 19F NMR spectrum and made X-ray analysis of HOF crystals [65,66]. The first investigations of chemical properties of HOF have already shown that this reagent is able not only to be added to olefins to the double bond but also possesses the high oxidizing ability [67]. In this respect it is similar to F2O [68]. Thus, interaction of HOF with sulfides, naphthaline and derivatives of acetylene gives oxidation products [14,69-72].
Meanwhile, this hypofluoric acid (HOF) was well-known earlier [73]. But it was not used in organic chemistry because of its excessive instability. However in recent years a new approach was developed in production and stabilizing this compound. It happened due to the studies by S.Rozen who has developed the new approach in stabilization of this compound by means of the formation a complex of HOF with acetonitrile HOF MeCN [17,70,71]. This work exerted stimulant influence on the process of formation of reagents containing the O-F bonds and on development of processes of fluorination and oxidation of organic molecules. The high oxidizing properties of HOF discovered a new synthetic approach to the possibility to oxidize unsaturated compounds in a simple and extremely effective way. The X-ray analysis of this complex has shown that the H-O-F angle is 90oC and the length of the hydrogen bond between the nitrogen atom and hydrogen is 1.7A [74]. This complex is considerably more stable than HOF and its solution of the concentration up to 1 mole can be produced (for the HOF solution only concentration of ~ 1mmole is permissible at room temperature). The entropy of this complex formation is –14.3kJ [75]. These studies have discovered new synthetic approaches to use HOF as an excellent agent for oxygen “transportation”. Its ability to execute a direct epoxidation of a multiple bond, hydroxylation of the inert C-H bond at tertiary carbon atom and oxidation of aromatic compounds[30], dimethyl dioxirane [42] , fluorine-containing olefins [78], sulfides [77], conversions of aromatic and aliphatic amines [78,79] and alcohols [80,81] to the appropriate nitro-derivatives and ketones , conversions of ethers into aldehydes and acids has already been shown. Appearance of fluorine-containing compounds able to transform organic molecules into compounds without fluorine atoms shows one of remarkable properties of fluorine containing compounds that influenced much development of methods of synthetic organic chemistry. The determination of conditions of epoxidation of olefins is among new methods. Taken into account that HOF is made by passing elemental fluorine through an aqueous solution of acetonitrile, the developed method essentially is a smart and effective way to use fluorine for oxidation of olefins. Thus, as it was shown in [17], passing elemental fluorine through an aqueous solution of acetonitrile forms an oxidizer that converts trans-stilbene to f trans-stilbene oxide in 90% yield for 1 minute at 0oC. (E)-stilbene gives a mixture of benzophenone and threo-2-fluoro-1,2-diphenylethanol [69].
Traditional oxidation of olefins under subjection to H2O2/MeCN proceeds slowly and requires elevated temperatures. But in this case the system of F2/H2O2/MeCN reacts almost immediately at 0oC. The examples of epoxidation reactions are given in table.2. Table 2. Epoxidation of unsaturated compounds under the influence of HOF-MeCN reagent
The high electrophilicity of this reagent, the reaction rate and complete configuration transformation point to the two-stage process that includes
B If the stable
When water is used with the oxygen isotopes ( H2O 18, H2O17) 97% of isotopic mark is found in the epoxy cycle [17].
Fluorosubstituted olefins under subjection to this system give epoxides in good yields [76].
When this reagent reacts with phenylacetylene , 2-fluoro-1-phenylethanone and phenylacetylfluoride are formed [67].
If tret-butyl substituent is at the triple bond(for example, for 3,3-dimethyl –1-ine) then 2-fluoro-3,3 dimethylbutanal is formed. Oxyethylporphyrin is converted to N-oxide of octaethylpophyrin [82]. The hydrogen atoms at the tertiary carbon atoms are successfully hydroxylated [83]. This approach is used to introduce 18O isotope into molecules.
Hypofluoric acid (HOF) in a reaction with some aromatic compounds gives phenol derivatives and an increase in orto-isomers content draws attention [17].
The reaction with naphthalene proceeds extremely slowly to give 1-oxynaphthol in 2.9% yield merely, 2-oxynaphthol in 0.75% yield and 1,4-naphthoquinone in 7.4% yield [69]. In the interaction of HOF-MeCN complex with aromatic and polycyclic compounds the latter are oxidized to quinones ( table 2)[60].
Oxidation of compounds containing a carbonyl group proceeds more readily. Thus, a solution of HOF-MeCN complex interacts with trimethylsilyl ether of 1-indanone enol to give 2 hydroxy-1-indanone at 0oC for 3-4 minutes [84].
R=R1=R2= H 88 % R=R1= OMe, R2= H 92 % R= OMe, R1=R2= H 93 % R=R1= H, R2= Me 98 % R=R1= áĺíçî, R2= H 92 % HOF-MeCN complex currently is the best reagent in organic synthesis for oxygen transportation and effective oxidation of unsaturated organic compounds. It is environmentally appropriate oxidizing reagent. If the benzene ring contains a group certainly inclined to oxidation then the process proceeds under especially mild conditions. For example, when this oxidizers affects anilines, the appropriate nitrobenzenes are formed [78,79].
It should be noted that 4-benzaminic acid and 3-aminophenol give the appropriate nitro-derivatives also, while perbenzoic acid and quinone are not formed in this case in contrast to oxidizers based on peroxides. Aliphatic amines are oxidized to the appropriate nitro-derivatives as well. 3-Aminomethyl-3,5,5-trimethylcyclohexanol converts to the nitro-derivative without involving the OH-group [80].
HOF-MeCN complex is used for oxidation of secondary alcohols and ketones according to the Baeyer-Villiger reaction [80]. Some ethers containing the methyl group are subjected to oxidation also (Table 2) [85,86].In this case ketones, there are formed aldehydes and acids.
Methyl(trans-4-tert-butylcyclohexyl) ether (ester?) under action of HOF MeCN is converted to 4-tert-butylcyclohexanone in 90% yield under mild conditions [81]. It should be noted that in the reaction of oxidation of ketones with this reagent in a solution of acetonitrile the formation of a-oxy ketones is observed. Antipyrin gives 3- and 4-fluoro-derivatives whereas uracil gives 5-fluorouracil under subjection to HOF produced by fluorine interaction in situ [87].
In sulfur-containing heterocyclic compounds containing 2-valence sulfur atom the latter is oxidized to 6-valence one [42]. Thus, benzothiophene and 2,5-substituents of thiophene under the influence of HOF-MeCN give 1,1-dioxides [69].
Six-membered sulfur-containing heterocycles behave similarly.
If 2,5-dibromothiophene with HOF-MeCN gives S,S-dioxide 2,5 dibromothiophene in 95% yield, whereas dimethyl dioxirane is oxidized in 27% yield as maximum. Sulfides containing aryl or perfluoroalkyl groups under the influence of this complex are effectively oxidized to the appropriate sulfones [88,89].
Polyfluorovinyl ethers containing a SR group, for example CF2=CFOCH2CF2CF2SR, are oxidized with replacement of the sulfur atom using HOF-MeCN complex to form CF2SO2H groups that allows producing after polymerization polymers used as membranes and durable films [90]. Amino acids ( ethers of ethylglycine, alanine, valine, leucine etc.) under action of HOF-MeCN are converted to nitro-derivatives [91].
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-oxycarbcation
B 
