Fluoride-ion catalysis

The regiochemistry is determined by the regiochemistry of the fluoride ion addition reaction, that is, via the most stable perfluorocarbanion intermediate Von Werner used a similar reaction to prepare silver compounds from perfluoro-2-methyl-2-butene and perfluoro 2 methyl-2-pentene [271] Silver(I) fluoride adds to bis(ttitluoromethyl)ketene in DMF without fluoride ion catalysis [270] The analogous trifluorovinylsulfurpentafluoride reacts similarly to give the isolable pentafluorosulfur derivative [272] (equation 187)... 

A useful route to 2,1,3-benzothiadiazoles is the F -catalyzed cyclization of l-(4-X-C6F4)-3-trimethylsilyl-l, 3-diaza-2-thiallenes [90JFC(50)359]. Fluoride ion catalysis is also used in the formation of heterocycles from pentafluorobenzoyl and -phenoxy compounds (81BCJ3447). Pentafluoro-phenylcarbonimidoyl dichloride with primary amines gave guanidines,... 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

However, 3- or4-methylcyclohexanone is less reactive to nitromethane than cyclohexanone. Although 2-methylcyclohexanone does not react with nitromethane under the conventional conditions, under high pressure and a fluoride ion catalysis the reaction proceeds to give the P-nitro alcohol in moderate yields (Eq. 3.18).29... 

Intramolecular Sakurai reaction. Allylic and propargylic silanes can undergo a Lewis acid catalyzed intramolecular Sakurai reaction.1 In cyclization to hydrin-danones, the stereochemical outcome can differ from that obtained by fluoride ion catalysis (presumably kinetically controlled cyclization), equation (I).2... 

In addition to the above described procedures, another alternative has to be mentioned, the smooth displacement of the silyl moiety by means of fluoride ion catalysis, leaving the oxirane ring intact79. This method enables phenyl-oxirane (109) to be synthesized from E-l-phenyl-2-TMS-oxirane (93) (Scheme 12). For further ring opening reactions see also80,81. ... 

A, A -Bis(trimethylsilyl) enamines are weak nucleophilic reagents and fail to react with electrophiles at the / -carbon. However, under the influence of fluoride ion catalysis,... 

NUro-aldolreaction (9, 444 10, 381). 2-Methylcyclohexanone does not react with nitromethane under usual basic conditions, but does react under high pressure and with fluoride ion catalysis to give the nitro alcohol (I) in moderate yield. 

Fluorooxiranes and fluorosilyloxiranes, obtained by oxidation of the corresponding (E)-or (Z)-alkenes, in protic media react with aldehydes and ketones under fluoride ion catalysis to give diastereomeric alcohols 18 in 48-85% yield. ... 

A different set of mechanistic possibilities comes into play when nucleophilic catalysis is used in place of the electrophilic catalysis induced by Lewis acids. The most common form of this reaction is fluoride ion catalysis of allylsilane reactions, where the carbon nucleophile is probably a hypervalent allylsilane (12), with the fluoride ion coordinated to the silicon, making the allylsilane unit nucleophilic enough to react with aldehydes without acid catalysis. ... 

For several years now we have studied the inter- and intramolecular Michael reactions with allylsilanes. In light of the precedents mentioned above, we expected that allylsilanes would react in 1,6-fashion with polyethylenic electrophiles. However, we observed that the intermolecular condensation of trimethylallylsilane with conjugated dienoates and dienonitriles under fluoride ion catalysis afforded exclusively 1,4-adducts, while Lewis acid catalysis failed to promote reaction (Eq. 3) ... 

We have found that 4-isobutenyl-dienone cyclizations can also produce cycloheptanes under fluoride ion catalysis provided that the C(4) position is substituted only with the allylsilane-containing side chain (Eq. 12). As mentioned earlier, C(3),C(4)-disubstituted cyclohexenones prefer a conformation with the C(4) substitutent orient axially. In this situation the absence of an equatorial C(4) substitutent favors a transoid con-... 

You will recall that our goal was to reduce the number of branch points in the fluoroether system by more effective chain extension of the original fluoroether. With the photolytlc dimerization, we have probably reached the upper molecular weight limits with fluoride ion catalysis. However, we have demonstrated that, by increasing the molecular weight of the fluoroether backbone to 3000 through this awkward process, it is possible to obtain a T (-50 C) consistent with the chain extension that was required. 

Isomerization. (a, a -Dimethylallyl)trimethylsilane can undergo isomerization to (y,)/-dimethylallyl)trimethylsilane with fluoride ion catalysis. ... 

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