1-Bromo-1-fluoro-1-alkene

In the chromium-mediated reaction of 1-fluoro-l-bromo-l-alkene with an aldehyde, the ( )-isomer reacts more quickly than the (Z)-isomer. Therefore, when a mixture of stereoisomer 42a was subjected to the reaction with benzaldehyde in the presence of CrCl2 and Ni catalyst, a (Z)-isomer of allylic alcohol 51 was selectively formed [77, 78] (Scheme 21). The reaction proceeds through an alkenylchromium species 52, and the formation of the ( )-alkenylchromium species is much faster than that of the (Z)-isomer. Actually, the reactimi of a pure (Z)-isomer of 1 -fluoro-1 -bromo-1-alkene with aldehyde is sluggish, and the corresponding ( )-isomer of the product was obtained in low yield or not obtained at all. Therefore, a good method for the synthesis of ( )-51 has not yet been reported. 

Z)-2-Fluoro-l-bromo-l-alkene 55a was prepared from 1-bromo-l-alkene by the addition of BrF, followed by treatment with a base. This method is applicable only when R is an aiyl group. The resulting (Z)-55a was used in the Suzuki-Miyaura coupling for the synthesis of the (Z)-fluorostilbene derivative 56 [83] (Scheme 24). 

The rates of phenylchlorocarbene have also been compared with the fluoro and bromo analogs.120 The data show slightly decreased rates in the order Br > Cl > F. The alkene reactivity difference is consistent with an electrophilic attack. These reactions have low activation barriers and the reactivity differences are dominated by entropy effects. 

Alkyl- and aryl-hydrazones of aldehydes and ketones readily peroxidise in solution and rearrange to azo hydroperoxides [1], some of which are explosively unstable [2], Dry samples of the p-bromo- and p-fluoro-hydroperoxybenzylazobenzenes, prepared by oxygenation of benzene solutions of the phenylhydrazones, exploded while on filter paper in the dark, initiated by vibration of the table or tapping the paper. Samples were later stored moist with benzene at —60°C to prevent explosion [3], A series of a-phenylazo hydroperoxides derived from the phenyl-or p-bromophcnyl-hydrazones of acetone, acetophenone or cyclohexanone, and useful for epoxidation of alkenes, are all explosive [4], The stability of several substituted phenylazo hydroperoxides was found to be strongly controlled by novel substituent effects [5],... 

Bromo [ F]fluoride (Section 3.9.1) addition across a double bond was used in the synthesis of fluorine-18-labelled steroids of high specific radioactivity. After addition, the bromine is removed by reduction or by dehydrobromination. [ F]Fluoro-5a-dihydrotestosterone was obtained in about 3% radiochemical yield (Scheme 17) [64] and 6a-p F]fluoroprogesterone in only 0.3% [65]. The yields were quite low but sufficient to allow for animal studies. These reactions had been tested out successfully with simpler model alkenes [66]. 

The substituents fluoro. chloro, bromo, cyano, nitro, alkoxycarbonyl, A.yV-dialkylaminocar-bonyl and dialkylphosphonyl do not interfere with the molybdenum(Vl) fluoride reactions, but hydroxy, alkoxy, amino, dialkylamino and alkene groups do.9,10... 

Phenylmercury derivatives are the best carbene-transfer reagents in this category. 1-Bromo-1,2,2,2-tetrafluoroethyl(phenyl)mercury (1) reacted with alkenes at elevated temperature to give l-fluoro-1-trifiuoromethylcyclopropanes 3 and 4 (X = F). Similarly, thermolysis of 1-bromo-l-chloro-2,2,2-trifluoromethyl(phenyl)mercury (2) in the presence of excess alkene yielded 1-chloro-l-trifluoromethylcyclopropanes 3 and 4 (X = Cl) and occasionally a small amount of 1 -bromo-1 -trifluoromethylcyclopropane 5. 1,1 -Dichloro-2,2,2-trifluoroethyl(phenyl)mer-... 

The thermolysis of a mixture of l-bromo-l,2,2,2-tetrafluoroethyl(phenyl)mercury and an alkene affords a 1-fluoro-l-trifluoromethylcyclopropane. ... 

Reaction of bromofluorophenylmethane (for a preparation see ref 118) and potassium tert-butoxide with an alkene afforded l-fluoro-l-phenylcyclopropanes (Houben-Weyl, Vol.4/3, p233 Vol. E19b, p980). To identify the reacting species, e.g. carbene or carbenoid, the reaction was carried out without and with an equimolar amount of 18-crown-6. ° Alternatively, chlorofluorophenylmethane, in place of the bromo derivative, can be used. Reactions with bromofluorophenylmethane were performed at 25 °C (sealed tube), whilst those with chlorofluorophenylmethane at 60-80 °C. Addition of fluoro(phenyl)carbene to alkenes is at least 98% stereospecific e.g. formation of . ... 

Aryl-1 -fluorocyclopropanes 2 were synthesized by irradiation of a 3-aryl-3-fluoro-3//-diazirine and an alkene (2 > 300 nm, 25 °C) in a screw-top Pyrex Carius tube (Houben-Weyl, Vol. E19b, p980). The starting 3-aryt-3-fluoro-3//-diazirines were prepared from the corresponding 3-aryl-3-bromo-3//-diazirines and anhydrous tetrabutylammonium fluoride. The products were purified by column chromatography on silica gel (purity > 97%). Due to the explosive properties of diazirines, this method is not suitable for large-scale preparations. 

Using mercury derivatives, 1-bromo-l-chlorocyclopropanes are prepared from alkenes of low reaetivity, such as fluoro- and chloroethene, 1,1-difluoro-, tri- and tetrafluoroethene, tetrachloroethene, etc. In the case of some alkenes, C —H insertion of bromochlorocarbene competes effectively with cycloaddition, e.g. formation of 14 and 15. ... 

BROMOFLUORINATION OF ALKENES 1-BROMO-2-FLUORO-2-PHENYLPROPANE (Benzene, (2-bromo-1-fluoro-1 -methylethyl))... 

BROMOFLUORINATION OF ALKENES 1-BROMO-2-FLUORO-2-PHENYL-PROPANE... 

Compounds with a halogen atom bonded to an sp -hybridized carbon atom are called halo-alkanes. Halogens can also bond to an sp -hybridized carbon atom of an aromatic compound or an alkene. However, since the chemistry of these compounds is very different, we will not consider them in this chapter. Compounds with the halogen atom bonded to an sp-hybridized carbon atom, which are very unstable, are seldom encountered. We will primarily focus on the chemistry of chloro and bromo compounds. lodo compounds are less stable. The chemistry of fluoro compounds is somewhat different from the other halogen compounds, and we will not discuss fluoro compounds in this text. 

Scheme 16 Stereoselective iluoroalkene synthesis by cross-coupling reaction using 1-bromo-l-fluoro-l-alkene 42a...

Pannecoucke et al. also reported the selective synthesis of (E)- and (Z)-a-fluoroenone 46 by the Negishi coupling of 1-bromo-1-fluoro-l-alkene 42c with an ethoxyvinylzinc reagent. When a (Z)-isomer of the a-fluoroenone 46 is desired, an excess amount of a mixture 42c was used. Under these conditions, ( )-42c reacted more quickly than the (Z)-isomer, and (Z)-a-fluoroenone 46 was formed selectively, while (Z)-42c remained unchanged. Therefore, pure (Z)-42c could be obtained from the reaction mixture and was used for the synthesis of ( )-46 by reaction with the ethoxyvinylzinc reagent [72] (Scheme 18). 

Scheme 19 Synthesis of (Z)-l-bromo-l-fluoro-l-alkene 42a and its application to cross-coupling...

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