Vinylic halogen, replacement

Strongly nucleophilic lithium aluminum hydride and sodium borohydride in diglyme replace vinylic halogens including fluorine [521, 555], sometimes under surprisingly gentle conditions [52/]. Because the carbon atom linked to fluorine is more electrophilic than that bonded to chlorine, fluorine in 1-... 

When halogen atoms are attached to a vinylic carbon and also to one allylic to it, an SN2 process converts the vinylic halogen into an allylic one, while the formerly allylic one is replaced, and a new olefin is formed. Another SK2 attack at the new terminal vinylic carbon would result in the replacement of the original vinylic halogen. The vinylic halide can thus be exchanged in two consecutive SN2 reactions. 

The reaction of tetrahalocyclopropenes with iodide ion leads to replacement of the vinylic halogens by iodine, presumably by an addition — elimination process 20S). 

The various reactions of cyclic polyfluoroalkenes [85] (Figure 7.32) can largely be explained on a similar basis to that described above, although the opportunity for some rather more subtle orientation effects arises. Vinylic fluorine is generally replaced in preference to other vinylic halogens, either in the same molecule or in comparable systems, e.g. in the reaction of 7.32A with a deficiency of aUcoxide ion. In these cases, vinylic substitution is usually preferred over Sn2 processes. 

Based on the data generated from susceptible-TSM, a general structure-activity relationship can be formulated. The rule-of-thumb is to keep the rings and double bond as planar as possible. The structure-activity rules are shown schematically in Figure 2. Replacement of the vinyl halogens with methyl groups does not decrease activity. The vinyl hydrogen can be replaced with fluorine and the molecule retains the activity. Substitution at the four-position of the thiophene does not adversely affect... 

Henne AL (1962) Replacing vinylic halogens with fluorine. US 3024290, CA 56 15333i... 

A soln. of p-chloraniline in methanol-d4 deaerated by dry Ng and irradiated 1.5 hrs. with the quartz-filtered light of a 125 w. high-pressure Hg-lamp p-deuterioaniline. Y 82%. F. e., also replacement of vinylic halogen, s. J. P. H. Muller, H. Parlar, and F. Korte, Synthesis 1976, 524. 

Olsen et al. (330) describe the reaction of Z-P-halo-a-acylaminoacrylic acid derivatives with lithium dialkyl copper reagents. Replacement of the vinyl halogen by alkyl takes place with complete or predominant retention of configuration at the double bond. 

The reaction of )3-substituted vinyl ketones RCOCH=CHY (Y = halogen, OR or NR2) with hydroxylamine hydrochloride has been extensively investigated (63AHC(2)365, 62HC(l7)l). One would anticipate that replacement of hydroxylamine hydrochloride with hydroxylamine in these reactions would result in enhanced regiospecificity and increased... 

Many reactions of fluorinated organics with metal halides result in the replacement of fluorine with halogen A general route to 1,1,1-trichloro- or tribromo-fluoroalkanes involves treating primary fluoroalkyl iodides with aluminum trichloride or aluminum tribromide [74], Benzylic [75, 76] or vinylic [72] fluorine can be exchanged for chlorine when treated with aluminum trichloride... 

Vinyl and phenyl mfluoromethyl groups are reactive in the presence of aluminum chloride [10] Replacement of fluorine by chlorine often occurs Polyfluori-nated trifluoromethylbenzenes form reactive a,a-difluorobenzyl cations in antimony pentafluoride [11] 1 Phenylperfluoropropene cyclizes in aluminum chloride to afford 1,1,3-trichloro 2 fluoroindene [10] (equation 10) The reaction IS hypothesized to proceed via an allylic carbocation, whose fluoride atoms undergo halogen exchange... 

Similar results for the replacement of halogen on an olefinic linkage by phosphorus have been accomplished using dialkyl phosphites with palladium(O) catalysts.4179 Another reaction involving replacement of a vinylic halide by phosphorus utilizes palladium catalysis with a trimethylsilyl-substituted phosphine (Figure 6.19).80... 

Replacement of the halogen by a metal atom, and further reaction with an electrophile E+ (Curtin and Harris, 1951a, b Curtin et al., 1955) also result in a formal vinylic substitution (XVI), but will not be discussed here. 

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