Olahs reagent

In an epoxy-ketone system such as 4yS,5j8-epoxy-6y -fluoroandrostane-3,17-dione (430), fluorination (by the Olah reagent ) occurred as in an Sn2 type of mechanism, giving 2y ,6 -difluoroandrost-4-ene-3,17-dione (431). This procedure may be applicable to sugars. 

There is continuous interest in cleavage of epoxides as a general method lor the preparation of fluorohydrins Epoxides are effectively cleaved under mild conditions by a 70% hydrogen fluoride-pyridine complex (Olah s reagent). The... 

Reactions of benzylic a,P-aminoalcohols and a-hydroxyazindines with Olah s reagent provide a highly efficient way to a P-fluoroamines, a-fluoro-azindines, and ot,Y-difluoroamines [dS] (equations 43 and 44)... 

Fluorodediazoniation of 2-fluoro-2-phenylethylamines with sodium nitrite m Olah s reagent (70% anhydrous hydrogen fluoride-30% pyridine) gives high yields of 1,1-difluoro-2-phenylethanes arising from 1,2 migration of phenyl fJ] (equation 2)... 

High-yield deaminative fluorination of a-atmnocarboxylic acids to a-fluoro-carboxylic acids can be effected with sodium nitrite in Olah s reagent [5, 6] (equation 3 and Table 2)... 

Fluoromethylketones are obtained by dediazoniative hydrofluorination with anhydrous hydrogen fluoride [91, 92 93] or Olah s reagent [d] (equation 23)... 

For an improved procedure with amides, see Olah, G.S. Prakash, G.K.S. Arvanaghi, M. Synthesis, 1984, 228. See Martin, R. Romea, R Tey, C. Urpi, R Vilarrasa, J. Synlett, 1997, 1414 for reaction with an amide derived from morpholine and Grignard reagents, whch gives the ketone in good yield. See Kashima, C. Kita, I. Takahashi, K. Hosomi, A. J. Heterocyclic Chem., 1995, 32, 25 for a related reaction. 

For a monograph, see Roberts, R.M. Khalaf, A.A. Friedel-Crafts Alkylation Chemistry Marcel Dekker NY, 1984. For a treatise on Friedel-Crafts reactions in general, see Olah, G.A. Friedel-Crafts and Related Reactions Wiley NY, 1963-1965. Volume 1 covers general aspects, such as catalyst activity, intermediate complexes, and so on. Volume 2 covers alkylation and related reactions. In this volume the various reagents are treated by the indicated authors as follows alkenes and alkanes, Patinkin, S.H. Friedman, B.S. p. 1 ... 

For a review of selectivity in this reaction, that is, which group preferentially attacks when the reagent contains two or more, see Olah, G.A. in Olah, Ref. 240, vol. 1, p. 881. This review also covers the case of alkylation vs. acylation. 

Azido-bromo-addition has also been done with another reagent Olah, G.A. Wang, Q. Li, X. Prakash, G.K.S. Synlett, 1990, 487. 

The structurally simplest silicon reagent that has been used to reduce sulphoxides is the carbene analog, dimethylsilylene (Me2Si ) . This molecule was used as a mechanistic probe and did not appear to be useful synthetically. Other silanes that have been used to reduce sulphoxides include iodotrimethylsilane, which is selective but unstable, and chlorotrimethylsilane in the presence of sodium iodide, which is easy to use, but is unselective since it cleaves esters, lactones and ethers it also converts alcohols into iodides. To circumvent these complications, Olah has developed the use of methyltrichlorosilane, again in the presence of sodium iodide, in dry acetonitrile (equation 8). A standard range of sulphoxides was reduced under mild conditions, with yields between 80 and 95% and with a simple workup process. The mechanism for the reaction is probably very similar to that given in equation (6), if the tricoordinate boron atoms in this reaction scheme are replaced... 

Oxazolidinones have also been used as intermediates in simple transformations utilising their peculiar reactivity. The absolute configuration of WBoc-P-aminoalcohol 213 can be easily inverted via Sn2 cyclisation to oxazolidinone 214 <00TL10071>. Treatment with Olah s reagent (HF-Pyridine) of 4-alkyl-5,5-diphenyl-oxazolidinones 216 afforded the corresponding a-(fluorodiphenylmethyl)alkylamines 217 <00TA2033>. 

Fluorohydrins.1 Both this reagent (1) and Olah s pyridinium poly (hydrogen fluoride) (2) serve as an attenuated source of hydrogen fluoride, and both convert simple epoxides into fluorohydrins, but reactions with 1 require higher temperatures than those with 2. The two reagents can differ in the regioselectivity, with steric factors being more important in reactions of 1 than those of 2. 

Nevertheless, a different selectivity has really been observed83 in bromofluorination reactions of 1,5-cycloalkadienes with NBS/Et3N-3HF or Olah s reagent. The reaction of 1,5-cyclooctadiene (3) with the former reagent yields mainly the 1,2-addition product 52, but when the reaction is carried out with Olah s reagent only compounds 53 and 54, arising from the usual transannular jr-cyclization, are formed (equation 53). 

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