[7783-81-5] · F6U · Uranium(VI) Fluoride · (MW 352.03)
Physical Data: mp 64.8 °C; sublimes at 56.5 °C; vapor pressure at room temperature 115 mmHg.4
Solubility: sol Freon, chloroform, methylene chloride.
Form Supplied in: white crystalline compound. Available in the 235U depleted form as a byproduct of uranium enrichment plants.
Handling, Storage, and Precautions: UF6, depleted of fissionable 235U, contains less than 0.20% 235U. It is corrosive, moisture sensitive, and mildly radioactive (low level of radiation). Caution should be exercised against these potential hazards. UF6 is best handled in all copper apparatus. However, Freon solutions of UF6 are stable and do not attack glass. This reagent should only be handled in a fume hood.
UF6 is a strong oxidant. Its oxidizing ability compared with that of other higher fluorides of f- and d-elements in Groups 5 and 6 is in the following sequence: VF5 > UF6 > MoF6 > WF6 > MoF5.1 Another study shows that, in acetonitrile, UF6 is a stronger oxidant than MoF6, NO+, and Cu2+.5 UF6 reacts with higher alkanes, alkenes, and arenes vigorously to give carbonaceous substances; this is of little synthetic value.1 However, it has been used to oxidize partially oxidized organic compounds selectively. UF6 reacts with benzylic alcohols and bromides readily to form aldehydes or ketones in moderate to good yields (eqs 1 and 2).2 Oxidation of alkyl bromides or iodides is, however, not successful.
Alkyl methyl ethers are oxidatively cleaved to the corresponding carbonyl compounds (eq 3). When alkyl benzyl and alkyl benzhydryl ethers are used in the reaction, the parent alkyl alcohols are obtained together with benzaldehyde and benzophenone, respectively.2 UF6 is also effective in cleaving allyl ethers, but the direction of the cleavage is unpredictable. The intermediates of the oxidation can be intercepted by dithiols to form dithioacetals (eq 4). Tertiary amines (N,N-dimethylamines) react similarly with UF6 to yield the corresponding carbonyl compounds (eq 5).
Hydrazones and oximes react with UF6 to regenerate the parent carbonyl functions (eqs 6 and 7).2 This offers a new alternative for these deprotections.
Oxidation of iodine by UF6 in acetonitrile yields bis(acetonitrile)iodine(I) hexafluorouranate(V).6a In the case of bromine, oxidation generates tris(acetonitrile)bromine(I) hexafluorouranate(V) with a possible 1,3,5-triazine like structure (eq 8); this can be further utilized to brominate arenes.6b
Aliphatic alcohols react with UF6 in the gas phase to form fluoroalkanes, alkenes, and ethers.7 For primary alcohols, fluoroalkanes are the major products (eq 9). This is a valuable reaction, since it is difficult to prepare primary fluorides, especially methyl fluoride.
Both alkyl and aryl aldehydes have been converted to acyl fluorides in moderate yields (eq 10).2 Adamantanone is oxidatively fluorinated to 2,2-difluoroadamantane in 41% yield (eq 11).2 On the other hand, UF6 behaves as a mild Lewis acid towards enolizable ketones, leading to their condensation.8 UF6 reacts with carboxylic acids to form acyl fluorides,1 but in some cases this reaction is accompanied by decarboxylation.9
George A. Olah, G. K. Surya Prakash, Qi Wang & Xing-ya Li
University of Southern California, Los Angeles, CA, USA